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The 
Physiology 

of 

Flowering 


Biology 


►►► 


Studies 


THE    PHYSIOLOGY 
OF    FLOWERING 


These  Studies  are  designed  to  inform  the  mature 
student— the  undergraduate  upperclassman  and  the 
beginning  graduate  student— of  the  outstanding  ad- 
vances made  in  various  areas  of  modern  biology. 
The  books  will  not  be  treatises  but  rather  will  briefly 
summarize  significant  information  in  a  given  field 
and  interpret  it  in  terms  of  our  current  knowledge 
of  the  rapidly  expanding  research  findings  within 
the  life  sciences.  Also  it  is  hoped  that  the  Studies 
will  be  of  interest  to  teachers  and  research  workers. 


BIOLOGY 
STUDIES 


Bell,  Ultrasound  in  Biology 

Carlquist,  Comparative  Plant  Anatomy 

Carpenter,  Fossil  Insects  and  Evolution 

Crafts,   Translocation  in  Plants 

Deevey,  Historical  Ecology 

Delevoryas,  Morphology  and 
Evolution  of  Fossil  Plants 

Hillman,  The  Physiology  of  Flowering 

Slobodkin,  Growth  and 
Regulation  of  Animal  Populations 

Sutton,  Genes,  Enzymes,  and 
Inherited  Diseases 


CO* 


William  S.  Hillman 

Yale  University 


THE    PHYSIOLOGY 
OF    FLOWERING 


Holt,  Rinehart 
and  Winston 

New  York    •    Chicago    •    San  Francisco 
Toronto    •    London 


4     5     6     7     8     9 

COPYRIGHT  ©   1 962  BY  HOLT,  RINEHART  AND  WINSTON,  INC. 

LIBRARY  OF  CONGRESS  CATALOG  CARD  NUMBER!  62 8419 

ALL  RIGHTS  RESERVED 

2109452 
PRINTED  IN  THE  UNITED  STATES  OF  AMERICA 


► 


preface    t 


To  the  botanist  flowering  is  of  interest  as  the  means  of  sexual 
reproduction  in  the  higher  plants,  and  because  the  processes  leading 
to  it  provide  experimental  systems  for  the  study  of  environmental 
and  internal  controls  of  development— problems  of  basic  significance 
throughout  biology.  To  the  rest  of  mankind,  which  often  has  more 
pressing  problems  to  consider,  flowering  is  nevertheless  of  the  great- 
est practical  importance  since  agriculture  is  based  on  the  control  of 
flowering  and  its  resultant  fruits  and  seeds.  Flowering  has  been 
studied  with  both  attitudes  for  many  centuries;  only  during  the 
past  few  decades,  however,  has  a  large  body  of  knowledge  about 
flowering  been  accumulated.  It  is  the  purpose  of  this  book  to  survey 
this  knowledge.  The  major  emphasis,  which  simply  reflects  the 
direction  of  most  research,  will  be  on  processes  affecting  the  initia- 
tion and  early  development  of  flowers  rather  than  on  associated  or 
subsequent  events.  Historical  details  are  omitted  except  when  they 
are  required  to  clarify  current  concepts. 

I  have  tried  to  write  for  several  kinds  of  readers,  from  graduate 
students  in  botany  and  other  branches  of  biology  to  laymen  with 
some  formal  training  in  science.  Inevitably,  then,  any  given  reade? 
will  find  some  passages  too  elementary  or  others  insufficiently  ex- 
plained. As  for  the  relatively  small  group  of  professional  plant 
physiologists  who  specialize  in  the  study  of  flowering,  I  hope  this 
book  will  serve  as  a  useful  review  for  them.  They  should  not  expect 
to  find  much  new  in  it,  except  perhaps  another  point  of  view,  and 
there  are  as  many  of  these  as  there  are  specialists. 

This  question  of  point  of  view,  particularly  in  presentation,  has 

v 


vi    ■    Preface 

not  been  an  easy  one  to  resolve.  There  is  much  to  be  said  for  the 
practice  of  sketching  the  broad  lines  of  a  topic  with  a  few  intel- 
lectually satisfying  concepts  and    not   burdening  the   student   im- 
mediately with  exceptions  and  difficulties.  If  I  have  avoided  this 
procedure— and  surely  the  bewildered  reader  of  Chapter  Five  will 
agree  that  I  have-it  is  because  1  am  afraid  it  can  be  fundamentally 
misleading.  My  intention  is  to  introduce  the  reader  to  the  field  and 
if  possible  to  give  him  the  "feel"  of  it,  bringing  him  close  to  the 
position  of  the  research  workers  themselves.  Since  in  my  opinion 
science  progresses,  like  all  endeavors,  by  fumbling,  backing  out  of 
dead  ends,  and  now  and  then  taking  a  few  steps  forward,  it  is  often 
easy   to  believe  in   a  clear  pattern  of  conceptually  clean   "break- 
throughs" after  some  time  has  passed,  but  it  is  harder  to  do  so  as 
the  work  becomes  more  recent.  Or,  at  least,  I  doubt  my  own  ability 
at  this  sort  of  judgment.  The  alternative,  then,  is  to  stress  the  phe- 
nomena, the  empirical  observations:  these  are  not  so  likely  to  be 
subjectively  distorted,  and  it  is  these  that  must  be  lived  with,  ex- 
amined, correlated,  and  finally  understood. 

All  this  is  of  course  no  excuse  for  a  mere  random  collection  of 
'[acts,"  and  the  reader  will  find  nothing  of  the  kind.  It  is,  however, 
the  justification  for  bringing  in  exceptions  almost  simultaneously 
Avith  the  tentative  rules,  for  employing  an  often  deliberate  vague- 
ness in  terminology— since  words  used  in  a  systematic,  authorita- 
tive way  can  often  conceal  ignorance— and  for  stressing,  above  all, 
the  kinds  of  experiments  and  results  rather  than  merely  the  con- 
cepts they  may  or  may  not  illustrate.  I  can  think  of  no  better  way 
to  convey  the  extreme  openness  of  the  subject,  the  way  in  which  few 
if  any  principles  are  irrevocably  established.  It  is  all  a  question  of 
how  much  confusion  is  necessary  to  provide  a  true  picture  of  the 
present  state  of  things;  I  have  tried  to  avoid  an  excess,  but  not  to 
exclude  it  entirely. 

A  general  outline  of  the  way  in  which  I  have  grouped  various 
topics  for  consideration  is  provided  by  the  table  of  contents,  and 
requires  no  further  comment  here.  However,  some  remarks  on  the 
bibliography  and  the  manner  in  which  papers  are  cited  may  be 
useful. 

The  proportion  of  general  reviews  to  original  experimental 
articles  cited  is  relatively  high,  and  I  have  made  no  attempt  to 
include  all  the  revelant  literature.  Frequently  a  paper  is  considered 


Preface    •    vii 

not  because  it  is  the  first  or  most  important  of  its  kind  but  simply 
because  it  provides  a  particularly  good  example  of  a  problem  under 
discussion.  The  great  preponderance  of  English-language  refer- 
ences is  simply  a  concession  to  the  convenience  of  both  reader  and 
writer,  and  does  not  reflect  the  frequency  or  importance  of  publi- 
cations in  other  languages.  Fortunately  for  the  English-speaking 
world  most  of  the  work  from  other  countries  is  reviewed,  and  much 
is  even  reported,  in  English  by  the  original  workers  themselves. 

I  have  adopted  the  following  convention  with  regard  to  cita- 
tions in  the  text.  If  a  statement  is  followed  simply  by  author  (s) 
and  date,  for  example,  Hamner  (1940) ,  the  paper  cited  has  original 
data  on  the  point  in  question.  Directions  to  see  a  paper,  on  the  other 
hand,  indicate  reviews  or  other  discussions  from  which  further 
references  may  be  obtained.  All  plants  are  referred  to  for  the  first 
time  by  both  common  (if  any)  and  scientific  names.  Thereafter,  the 
practice  adopted  is  arbitrary,  but  the  index  can  always  be  used  to 
establish  one  from  the  other. 

In  summary,  I  have  tried  to  treat  the  field  in  a  manner  not 
quite  like  that  to  be  expected  from  a  technical  review  or  article,  but 
in  such  a  way  that  the  previously  uninformed  reader  will  afterward 
be  able  to  read  any  of  these  with  understanding  and  enjoyment;  and 
then,  best  of  all,  perhaps  try  his  own  hand  at  the  game. 

W.  S.  H. 

New  Haven,  Connecticut 
September,  1961 


acknowledgments    t 


During  the  time  this  book  was  written,  the  author  was  sup- 
ported entirely  by  research  grants  from  the  National  Science 
Foundation. 

The  patient  cooperation  of  Violet  Esdaile  and  Margaret  Wark 
in  typing  successive  stages  of  the  manuscript  has  been  of  great  value. 

Discussions  and  correspondence  with  numerous  investigators 
have  contributed  greatly  to  this  survey,  but  particular  thanks  are 
due  Dr.  Bruce  A.  Bonner  and  Dr.  Ian  M.  Sussex  for  critically  read- 
ing the  manuscript. 


\  in 


contents 


► 


chapter  one         ►  Background 


MORPHOLOGY  OF   FLOWERING  1 

NATURAL    HISTORY    OF    FLOWERING 
THE    MEASUREMENT    OF    FLOWERING 


chapter  two 


►  Photoperiodism:  An  Outline  10 

DEFINITIONS  OF  PHOTOPERIODISM  10 

HISTORICAL  NOTE  11 

GIANT  TOBACCO  AND  SEPTEMBER  SOYBEANS  12 

KINDS   OF   PHOTOPERIODIC   FLOWERING 

RESPONSES  13 

THE  ROLE   OF    LEAVES   IN   PHOTOPERIODISM. 

PHOTOPERIODIC    INDUCTION  17 

THE    CENTRAL   ROLE   OF   THE   DARK   PERIOD  18 

REQUIREMENTS  FOR  HIGH-INTENSITY  LIGHT  20 

MUTUAL  INTERACTIONS  OF  LIGHT  AND  DARK 

PERIOD    LENGTHS  22 

INTERACTION    OF    DIFFERENT    PHOTOPERIODIC 

cycles:     FRACTIONAL    INDUCTION    IN    LDP 

AND   LONG-DAY    INHIBITION    IN    SDP  24 

PHOTOPERIODISM     AND     TEMPERATURE  25 

PHOTOPERIODISM  AND  VEGETATIVE  GROWTH  27 

LITERATURE  29 


chapter  three      ►  Photoperiodism:   Attempts  at  Analysis 

A.  Photoperiodism   and   light   quality         30 

ACTION     SPECTRA    FOR     LIGHT-BREAKS  30 


30 


IX 


Contents 

the  red,  far-red  reversible  system        34 
the  red,   far-red  system   in 

photoperiodism         35 
nature  and  function  of  the  red,  far-red 

PIGMENT  39 

PROLONGED    EXPOSURES    TO    LIGHT    OF 
DIFFERENT    COLORS  40 

B.  Time  relations  and  endogenous  rhythms 
in  photoperiodism         42 

ENDOGENOUS    C1RCADIAN    RHYTHMS    IN 

PLANTS  44 

ENDOGENOUS    CIRCADIAN    RHYTHMS    AS    THE 

BASIS    OF    PHOTOPERIODISM  46 

CIRCADIAN    RHYTHMS    AND    THE    ACTION    OF 

LIGHT-BREAKS  47 

FLOWERING  IN  LIGHT-DARK  CYCLES  OF 

DIFFERENT  LENGTHS;   TEMPERATURE 

INTERACTIONS  5 1 

ENDOGENOUS    CIRCADIAN    RHYTHMS    AND 

THE    RED,    FAR-RED    SYSTEM  52 

CONCLUDING   REMARKS    ON    CIRCADIAN 

RHYTHMS    AND    PHOTOPERIODISM  52 


chapter  four        ►  Temperature  and  Flowering  54 

vernalization:  cold  treatments  and 
flowering         54 

Vernalization  in  winter  rye Vernalization 

in  other  plants 

DEVERNALIZATION  59 

RELATIONS    BETWEEN    VERNALIZATION    AND 

PHOTOPERIODISM  61 

THE   SEMANTICS  OF   VERNALIZATION! 

FURTHER    EFFECTS    OF    TEMPERATURE    ON 

FLOWERING  62 

TEMPERATURE    AND    FLOWERING    IN    BULB 

PLANTS  64 


chapter  five 


chapter  six 


chapter  seven 


chapter  eight 


Contents    •    xi 

►  Floral  Hormones  and  the  Induced  State  67 

DEFINITIONS    AND    BACKGROUND!    AUXINS   AS 

PLANT    HORMONES  67 

PRELIMINARY    EVIDENCE   FOR  THE  EXISTENCE 

OF  FLOWERING   HORMONES  69 

TRANSLOCATION  OF  FLOWERING  HORMONES  72 

TRANSLOCATION   RATE  77 

FLOWER  PROMOTION  OR  FLOWER  INHIBITION? 

THE  SPECIFICITY  OF  FLOWERING  STIMULI  78 

VERNALIN  AND  METAPLASIN  82 

PERMANENCE  AND  LOCATION   OF  THE 

INDUCED  STATE  83 

THE   BIOCHEMISTRY    OF    INDUCTION  88 

CONCLUDING  REMARKS  94 

►  Chemical  Control  of  Flowering  99 

THE  GIBBERELLINS  100 

AUXINS,    AUXIN    ANTAGONISTS,    AND   OTHER 

GROWTH  REGULATORS  106 

PLANT    EXTRACTS    OF    VARIOUS   KINDS  109 

MINERAL  NUTRITION;  MAJOR  ELEMENTS  1 1 1 

HEAVY  METALS  AND  FLOWERING  113 

►  Age  and  Flowering 

AGE  AND  FLOWERING  IN  HERBACEOUS 

PLANTS  117 

FLOWERING    IN    WOODY    PLANTS  120 

►  A  Miscellany  127 

ANTHESIS  127 

THE  SEX  EXPRESSION  OF  FLOWERS  1  30 

GENETICS  OF  FLOWERING  RESPONSES  134 

FLOWERING  AND  DEATH  136 

PROSPECTS  137 


116 


Bibliography 

Index  of  Plant  Names 

Subject  Index 


141 
159 
161 


L 


Extreme  modification  of  development  by  photoperiodism  in  the  common  weed 
Chenopodium  rubrum.  Right,  a  plant  germinated  and  grown  under  8-hour  photo- 
periods;  much  of  the  bulk  of  the  3-week-old  seedling  consists  of  flower  parts. 
Left,  a  plant  germinated  and  grown  under  20-hour  photoperiods;  after  more 
than  3  months,  it  remains  completely  vegetative.  (Right-hand  photograph  from 
Cumming  [1959],  by  permission  of  the  editors  of  Nature;  both  photographs 
courtesy  of  Dr.  B.  G.  Cumming  of  the  Canada  Department  of  Agriculture.) 


► 
► 
► 
► 


chapter  one  £  Background 


Experimental  work  is  the  main  concern  of  this  study,  but  some 
purely  descriptive  information  on  flowering  should  be  helpful.  This 
chapter  considers,  first,  the  structure  and  origin  of  flowers  as  dealt 
with  by  morphologists.  The  natural  history  of  certain  flowering 
habits  will  then  be  briefly  described,  and  an  outline  of  some  of  the 
methods  used  to  "measure"  or  evaluate  flowering  concludes  the 
chapter. 

MORPHOLOGY  OF  FLOWERING 

The  word  "flower"  is  commonly  used  for  structures  of  the 
greatest  variety,  from  those  of  the  elm,  simple  and  inconspicuous, 
to  the  showy,  complex  blossoms  of  orchids  or  sunflowers.  Morphol- 
ogists use  the  term  "flower"  to  mean  a  determinate  sporogenous 
shoot  bearing  carpels.  Determinate  means  of  strictly  limited 
growth;  sporogenous,  bearing  the  reproductive  microspores  (male) 
or  megaspores  (female).  The  key  portion  of  this  definition,  how- 
ever, is  the  presence  of  carpels. 

The  carpel,  characteristic  organ  of  the  angiosperms,  or  "flower- 
ing" plants,  is  an  organ  bearing  and  enclosing  the  ovules;  the 
ovules,  in  turn,  contain  the  megaspores.  Under  this  definition  of  a 
flower,  the  sporogenous  axes  of  gymnosperms— pine  cones,  for  ex- 
ample—cannot be  considered  flowers;  the  absence  of  true  carpels 
is  one  of  the  major  characteristics  setting  off  the  gymnosperms— 
conifers,  cycads,  and  the  like— from  the  angiosperms.  Strict  use  of 
this  definition  of  a  flower  of  course  also  eliminates  those  structures, 

1 


2    •    Background 

borne  by  many  true  angiosperms,  which  are  commonly  called  "male 
flowers"— that  is,  structures  containing  only  the  pollen-bearing 
stamens  and  without  even  rudimentary  carpels.  In  practice,  the 
restriction  to  carpel-bearing  structures  need  not  apply  here.  Studies 
of  flowering  in  gymnosperms  such  as  pines  have  been  conducted 
and  are,  for  physiological  purposes  at  any  rate,  analogous  to  studies 
on  angiosperms.  For  these  purposes,  then,  flowering  can  be  taken 
to  mean  the  production  of  sporogenous  shoots  by  either  angio- 
sperms or  gymnosperms;  the  term  flower  in  its  common  usage  will 
not  be  misleading. 

The  parts  of  "typical"  flowers— such  as  those  found  in  botany 
texts— are  usually  described  as  follows:  the  floral  axis  is  more  or 
less  shortened  as  compared  with  that  of  a  vegetative  shoot,  and 
bears  successive  whorls  of  parts  arranged  around  it.  The  structure 
on  which  the  flower  parts  are  placed  is  the  receptacle,  and  the 
stalk  bearing  the  flower  is  the  pedicel.  The  lowest  or  outermost 
parts  are  the  sepals,  commonly  enclosing  the  bud;  within  and  above 
are  the  petals.  Sepals  and  petals  are  collectively  the  perianth. 
Within  this  are  the  stamens,  each  consisting  of  a  filament  bearing 
a  pollen-producing  anther.  The  upper  or  innermost  flower  parts 
are  the  carpels,  which,  either  singly  or  united,  give  rise  to  one  or 
more  ovaries,  containing  the  ovules,  and  to  a  pollen-receptive  sur- 
face, the  stigma.  Stigma  and  ovary  together,  whether  derived  from 
one  or  more  carpels,  are  called  the  pistil. 

Many  individual  flowers  often  occur  on  a  single  simple  or 
complex  axis  as  in  the  sunflower  (Helianthus)  or  in  grasses;  such  a 
group  of  flowers  is  an  inflorescence.  Flowers  may  also  be  solitary, 
each  borne  on  a  separate  pedicel  attached  to  the  vegetative  axis. 
Flowers  or  inflorescences  may  be  terminal  (at  the  ends  of  shoots)  or 
lateral,  or  both,  and  may  also  be  enclosed  or  accompanied  by  leafy 
or  scaly  bracts. 

There  are  great  differences  between  various  plants  in  the  num- 
ber, arrangement,  shape,  size,  color,  degree  of  fusion,  and  even 
presence  or  absence  of  the  various  flower  parts.  In  spite  of  this, 
there  is  a  good  area  of  agreement  among  botanists  both  on  the 
phylogeny,  or  evolutionary  origin  of  the  flower,  and  on  its  ontogeny, 
or  development  from  the  vegetative  axis  in  the  individual  plant. 

The  definition  of  a  flower  as  a  particular  kind  of  determinate 
shoot   already   implies   an    interpretation   of   both   phylogeny   and 


Morphology  of  Flowering    •    3 

ontogeny.  The  evidence  suggests  that  the  various  flower  parts,  from 
sepals  to  carpels,  are  homologous  with  ordinary  foliage  leaves. 
That  is,  they  bear  essentially  the  same  anatomical  and  morpho- 
logical relation  to  the  axes  on  which  they  are  borne  as  do  the 
leaves.  This  does  not  necessarily  mean  that  the  flower  parts  have 
been  derived  from  foliage  leaves,  even  though  the  flower  parts  of 
many  plants,  particularly  those  considered  more  primitive,  may 
show  distinctly  leaflike  characteristics.  Probably  the  most  widely 
accepted  view  is  that  both  leaves  and  flower  parts  were  evolu- 
tionarily  derived  from  similar  structures.  These  may  have  been 
fused  branch  systems,  some  of  them  entirely  sterile  and  represented 
in  our  present  leaves,  some  of  them  sporogenous  and  represented 
in  modern  carpels  and  anthers,  and  still  others  with  functions 
accessory  to  the  sporophylls  and  represented  in  modern  sepals  and 
petals.  While  the  details  of  such  questions  remain  speculative  for 
the  present  since  the  ancestry  of  the  angiosperms  is  not  really 
known,  the  homology  between  leaves  and  flower  parts  is  generally 
accepted  and  may  be  of  some  importance  physiologically;  it  is  at 
least  implicitly  challenged,  however,  by  some  of  the  work  on 
flower  ontogeny  to  be  considered  next. 

The  flower,  like  the  leaves  and  the  shoot  itself,  is  derived 
from  the  apical  meristem.  This  is  a  region  of  relatively  small, 
undifferentiated,  more  or  less  actively  dividing  cells  located  at  the 
very  apex  of  the  shoot.  Meristems  in  general  are  the  sources  of  new 
growth  in  all  higher  plants,  and  this  has  given  rise  to  the  concept 
that  plants,  unlike  animals,  show  a  "continuing  embryogeny." 
Relatively  little  is  known  about  the  mechanism  of  the  formation 
of  new  organs  by  such  embryonic,  seemingly  slightly  organized 
groups  of  cells.  The  central  problem  of  the  physiology  of  flowering 
might  be  stated  as  the  question  of  how  various  factors  affecting 
flowering,  be  they  environmental  or  genetic,  are  translated  by  the 
plant  into  physico-chemical  "signals"  to  the  meristem,  and  how 
these  determine  whether  the  meristem  will  produce  flowers.  The 
major  morphological  question  on  which  there  is  disagreement  is 
whether  the  meristematic  activity  that  produces  flower  primordia— 
recognizably  distinct  structures  that  will  develop  into  flowers  under 
favorable  conditions— is  qualitatively  different  from  that  which 
produces  leaf  initials,  which  develop  into  leaves. 

According  to  the  majority  of  recent  workers  there  is  no  essen- 


4    •    Background 

tial  difference  between  the  organization  of  a  meristem  producing 
only  leaves  and  one  producing  flowers.  Gross  differences  of  course 
exist  between  floral  and  vegetative  apices  in  a  given  plant.  These 
differences  appear  to  be  correlated  with  the  vegetative  and  in- 
florescence structures  of  the  particular  plant  involved,  and  no 
generalizations  true  for  all  plants  can  be  made.  But  the  question 
of  essential  organization  goes  beyond  this,  which  is  largely  a  matter 
of  shape  and  size. 

The  organization  of  many  vegetative  shoot  apices  can  be  ex- 
pressed loosely  in  terms  of  the  tunica,  or  outer  layers  of  cells,  and 
the  corpus,  or  inner  core  of  cells,  the  developmental  functions  of 
which  may  be  somewhat  different.  Most  recent  investigators  have 
observed  that  where  this  organization  is  present  it  continues  with 
no  sharp  change  into  the  floral  meristems,  which  are  thus  not 
qualitatively  different  from  the  vegetative.  See,  for  examples,  Wet- 
more,  Gifford,  and  Green  (1959);  Stein  and  Stein  (1960);  and 
Tucker  (1960).  However,  according  to  a  minority  of  investigators 
working  chiefly  in  France,  floral  development  is  the  exclusive 
function  of  a  "waiting  meristem"  (meristeme  d'attente)  that  re- 
mains inactive  until  the  onset  of  flowering,  whereas  leaf  production 
and  purely  vegetative  growth  are  carried  on  by  an  "initial  ring" 
(annean  initial)  surrounding  it.  This  work  is  reviewed  by  Buvat 
(1955).  In  this  view,  then,  reproductive  and  vegetative  development 
are  quite  different,  originating  in  different  meristem  regions, 
whereas  the  majority  view  is  that  there  are  not  two  sorts  of  develop- 
ment but  merely  a  continuum  with  extremes. 

The  view  of  no  essential  difference  seems  to  be  supported  by 
experimental  work,  to  be  described  later,  showing  that  certain 
plants  (Cosmos,  Kalanchoe),  given  a  treatment  insufficient  to  in- 
duce flowering  but  having  some  effect  in  that  direction,  may  re- 
spond by  producing  a  series  of  structures  intermediate  between 
normal  inflorescences  and  leafy  shoots  (see  Fig.  1-1).  Although 
one  can  interpret  such  "vegetative  flowering"  as  the  interaction  of 
two  fairly  distinct  meristematic  activities,  the  majority  view  appears 
to  involve  less  difficulty. 

Descriptive  morphology  of  the  meristem  has  little  more  to  tell 
the  student  of  flowering  physiology,  although  experimental  (oper- 
ative) morphological  studies  may  well  do  so  in  the  future.  The  reader 
should  bear  in  mind  that,  in  general,  experiments  on  the  physiol- 


Morphology  of  Flowering    •   5 

ogy  of  flowering  have  been  more  concerned  with  the  conditions 
bringing  about  the  production  of  flower  primordia— with  flower 
initiation,  as  it  is  called— than  with  subsequent  flower  development, 
although  in  practice  both  are  studied. 


-■  — 


B 


D 


Fig.  1-1.  Intermediate  conditions  between  full  flowering  and  vegetative  habits 
in  Kalanchoe  blossfeldiana,  from  (A)  normal,  fully  developed  inflorescence  through 
(B)-(D)  increasingly  vegetative  forms,  to  (E)  a  fully  "vegetative  inflorescence" 
in  which  there  are  no  flowers  at  all,  but  a  branching  habit  still  unlike  that  in  the 
normal  vegetative  state.  The  sequence  (A)-(E)  reflects  decreasing  amounts  of 
short-day  treatment.  (Photographs  from  Harder  [1948],  by  permission  of  the 
company  of  Biologists,  Ltd.,  and  courtesy  of  Dr.  R.  Harder,  University  of 
Gottingen.) 


A  concept  occasionally  found  in  the  experimental  literature 
is  that  of  ripeness-to-flower.  In  the  development  of  many  plants 
from  seed,  there  may  be  a  stage  before  which  flower  initiation  can- 
not occur,  at  least  in  response  to  conditions  that  would  bring  it 
about  in  older  plants.  A  plant  which  has  passed  this  stage  is  said 


6    •    Background 

to  be  ripe-to-flower.  This  concept  will  be  considered  in  connection 
with  work  requiring  it,  notably  in  Chapter  Seven,  but  by  itself  it 
explains  little  about  the  physiological  events  taking  place  and 
seems  not  to  reflect  any  basic  morphological  conditions  common  to 
all  plants. 

For  more  detailed  treatments  of  the  topics  discussed  here,  see 
Lawrence  (1951),  Esau  (1953),  and  Foster  and  Gifford  (1959). 


NATURAL  HISTORY  OF  FLOWERING 

Most  of  what  is  known  about  flowering  is  based  on  work  done 
either  with  plants  native  to  the  temperate  zone  or  with  cultivated 
plants.  Flowering  times  and  habits  particularly  have  been  studied 
more  thoroughly  in  the  higher  latitudes  than  in  the  tropics.  This 
limitation  should  be  kept  in  mind  in  any  discussion  of  flowering 
habits  and  physiological  mechanisms.  The  general  state  of  igno- 
rance on  flowering  in  the  tropics,  and  particularly  its  seasonal 
aspects,  is  well  summarized  by  Richards  (1957,  pp.  199-204). 

Plants  are  often  classified  as  annual,  biennial,  or  perennial. 
Under  these  familiar  terms  a  plant  either  germinates,  flowers,  and 
dies  within  a  single  season,  germinates  and  develops  during  one 
season  and  flowers  and  dies  in  the  next,  or  persists  for  many  years 
flowering  repeatedly.  Such  classifications  are  not  always  physiolog- 
ically meaningful,  although,  as  will  appear  later,  many  biennials 
can  be  regarded  as  annuals  in  which  a  low-temperature  treatment 
is  required  for  flower  initiation.  But  many  plants  commonly  called 
annuals  do  not  die  after  flowering  and  fruiting  in  all  climates; 
they  may  be  tropical  perennials  able  to  survive  or  cultivated  as 
annuals  in  cooler  regions. 

There  might  be  more  meaning,  both  ecological  and  physiolog- 
ical, to  a  classification  into  two  groups— the  first  being  perennials, 
defined  as  above,  and  the  second,  a  group  called  monocarpic  plants. 
Under  this  term  can  be  classified  true  annuals,  such  as  the  edible 
pea  (Pisum  sativum),  biennials,  and  certain  others,  all  having  in 
common  the  behavior  of  flowering  only  once,  with  fruiting  fol- 
lowed by  death.  This  group  then  would  include  plants  such  as  the 
century  plant  (Agave)  that  may  develop  from  five  to  twenty  or 
more  years  before  flowering,  and  many  tropical  bamboos,  with  life 
spans   from   two   to  perhaps   over  fifty   years.   Such   plants  clearly 


The  Measurement  of  Flowering    •    7 

differ  somehow  from  typical  perennials  that  flower  and  fruit  over 
tens  or  even  hundreds  of  years  without  evincing  any  ill  effects. 

Many  studies  of  flower  initiation  and  development  under  nat- 
ural temperate-zone  conditions  have  been  made  on  individual 
species.  A  survey  of  a  large  number  of  species  in  Britain  was  re- 
ported by  Grainger  (1939).  By  determining  the  times  of  flower 
initiation,  bud  development,  and  subsequent  anthesis  (flower  open- 
ing), Grainger  distinguished  three  classes  of  temperate-zone  plants. 
Direct-flowering  plants  are  those  in  which  development  through 
anthesis  follows  on  initiation  without  interruption;  this  is  perhaps 
the  commonest  type  of  flowering  behavior,  found  in  both  mono- 
carpic  and  perennial  plants.  Initiation  and  anthesis  may  occur 
either  together  with  the  maximum  vegetative  growth,  as  for  ex- 
ample in  bluebells  (Campanula)  and  mint  (Mentha),  or  at  the 
period  of  minimum  vegetative  growth  (winter  or  early  spring)  as 
in  Saxifraga.  A  second  class,  indirect-flowering  plants,  contains 
those  species  in  which  a  distinct  period  of  rest  intervenes  at  some 
stage  between  initiation  and  anthesis.  Here  again,  initiation  may 
coincide  with  the  period  of  maximum  vegetative  growth,  as  in 
many  fruit  trees  (Pyrus,  Prunus)  and  in  Anemone,  or  with  the 
period  of  minimum  vegetative  growth,  as  in  many  bulb  flowers 
(Tulipa,  Narcissus)  that  initiate  flower  primordia  in  summer  after 
the  leaves  wither.  A  third  class,  cumulative-flowering  plants,  form 
primordia  over  a  long  period  of  time,  in  regular  succession,  but 
anthesis  of  all  occurs  in  a  brief  period.  A  number  of  weed  species, 
notably  dandelion  (Taraxacum),  are  in  this  class.  Grainger  distin- 
guished a  fourth  class,  climax-flowering  plants,  not  found  in  the 
temperate  zone  but  including  long-lived  monocarpic  plants  such  as 
the  bamboos  mentioned  above. 

Most  experimental  studies  of  flowering  have  been  conducted 
on  plants  of  Grainger's  first  class— direct-flowering  plants  initiating 
in  the  period  of  maximum  vegetative  growth.  Other  types  have 
been  studied,  however,  as  will  appear  in  the  succeeding  chapters. 
Unfortunately,  but  for  obvious  reasons,  there  has  been  little  if  any 
experimentation  on  long-lived  monocarpic  plants. 

THE  MEASUREMENT  OF  FLOWERING 

The  general  structure  of  experiments  on  flowering  is  obvious- 
groups  of  plants  given  various  treatments  are  kept  under  observa- 


8    •    Background 

tion  until  the  effects  on  flowering  can  be  ascertained.  The  situations 
may  be  complicated  by  the  fact  that  conditions  bringing  about 
initiation  are  not  always  the  same  as  those  favoring  bud  develop- 
ment, and  these  in  turn  may  differ  from  those  required  for  anthesis. 
As  mentioned  earlier,  experimentalists  have  been  most  concerned 
with  initiation;  since,  however,  flower  primordia  in  their  earliest 
stages  are  detectable  only  by  dissection  and  microscopy,  the  data 
in  many  studies  have  been  based  on  the  appearance  of  macroscopic 
buds  or  flowers. 

Within  this  general  framework,  methods  of  evaluating  the 
results  quantitatively  are  less  obvious  and  vary  considerably.  The 
crudest  method  is  simply  to  record  the  time  required  for  the  first 
appearance  of  the  designated  floral  stage  in  the  various  treatments. 
This  of  course  will  vary  between  individuals  given  the  same  treat- 
ment, so  averages  are  used.  Alternative  but  related  data  are  the 
percentage  of  plants  in  each  treatment  showing  the  designated 
stage  at  a  given  time  after  the  start  of  the  experiment.  There  are 
also  plants,  such  as  soybean  (Glycine),  in  which  flowering  may 
occur  at  a  number  of  nodes,  and  the  effectiveness  of  treatments  can 
be  estimated  by  establishing  the  average  number  of  nodes  with 
flowers  per  plant  after  a  given  time.  Still  another  related  method 
is  that  of  assigning  arbitrary  number  values  to  various  stages  in 
the  development  of  flower  or  inflorescence  primordia.  With  a  scale 
so  established  and  an  appropriate  time  for  evaluation  chosen,  the 
plants  in  each  treatment  are  dissected  or  examined  and  the  result- 
ing values  averaged.  A  danger  of  this  method  lies  in  the  subjective 
judgments  involved  in  assessing  stages  and  assigning  values  to 
them. 

These  procedures  are  all  related  in  that  the  major  independent 
variable,  other  than  the  nature  of  the  treatments  given,  is  time. 
That  is,  in  a  graph  of  results  so  obtained,  each  flowering  value, 
however  stated,  is  a  function  of  time  in  or  after  treatment.  A  draw- 
back of  such  methods  is  that  if  the  treatments  differ  in  their  effects 
on  overall  growth,  and  the  times  involved  are  (as  is  usual)  a  week 
or  longer,  differences  in  flowering  values  may  simply  reflect  differ- 
ences in  growth  rate  of  the  entire  plant.  For  example,  a  10°  C 
increase  in  temperature  might  double  the  rate  of  vegetative  growth 
and  also  that  of  the  appearance  of  buds.  But  in  such  a  case  the 
rate  of  bud  appearance  relative  to  vegetative  growth  is  unchanged, 
although   time-based   data   would    indicate   more   rapid    flowering. 


The  Measurement  of  Flowering    •    9 

This  sort  of  danger  is  widely  recognized  and  it  is  usually  avoided 
by  careful  workers.  One  way  of  doing  this  depends  on  the  possi- 
bility, which,  as  will  appear,  is  often  present,  of  using  treatments 
of  short  duration  followed  by  a  return  of  all  the  plants  to  the 
same  conditions  where  the  same  rate  of  growth  will  be  maintained. 
Or  treatments  may  be  found  that  have  demonstrably  little  direct 
effect  on  vegetative  growth  rate.  Another  method,  often  combined 
with  one  of  these,  is  to  avoid  the  use  of  time  as  a  variable. 

Instead  of  time,  some  index  of  the  rate  of  vegetative  growth 
can  be  used  as  an  independent  variable.  The  most  common  such 
index  is  simply  the  number  of  new  leaves  or  nodes  produced  in  or 
after  treatment  before  the  designated  floral  stage  appears.  The  node 
or  leaf  index  can  be  substituted  for  the  time  scale,  and  systems 
can  be  produced  that  are  analogous  to  those  using  time.  These 
matters  of  scale  are  not  trivial.  For  instance,  an  experiment  on  a 
time  scale  might  show  that  treatment  A  caused  45  percent  flower- 
ing and  treatment  B  95  percent  flowering  after  20  days;  the 
same  results  on  a  nodal  scale  (also  after  20  days)  might  be:  A, 
100  percent  flowering  by  the  third  new  node;  and  B,  10  percent 
flowering  by  the  third  new  node.  Results  that  "differ"  as  much  as 
this  are  not  uncommon  and  require  care  in  interpretation.  The 
reader  may  find  it  instructive  to  invent  reasonable  data  from 
which  such  values  could  arise. 

Naturally,  the  choice  of  scale  depends  on  the  intention  of  the 
experimenter.  For  practical  agricultural  or  horticultural  purposes, 
emphasis  is  often  placed  on  flowering  time.  Investigations  on  more 
fundamental  questions  however,  such  as  the  existence  or  non- 
existence of  flower-inducing  hormones,  are  bound  to  be  concerned 
with  flower  initiation  or  development  relative  to  vegetative  growth. 
In  the  best  practice,  results  are  reported  in  sufficient  detail  so  that 
the  entire  developmental  situation  can  be  assessed.  Very  few  factors 
affect  flowering  exclusively,  without  modifying  vegetative  growth. 
Whether  the  changes  are  brought  about  indirectly,  as  a  result  of 
flowering,  or  directly,  by  the  factors  causing  flowering,  a  plant 
which  is  flowering  frequently  differs  from  a  vegetative  one  of  the 
same  age  in  height,  branching,  leaf  shape,  or  pigmentation  (to 
name  only  a  few  characteristics),  and  not  simply  in  the  production 
of  flowers.  Such  changes  may  provide  clues  to  the  mechanisms 
underlying  flower  initiation,  or  they  may  be  effects  of  flower 
development  itself;  in  the  cases  studied  so  far,  it  is  not  clear  which. 


chapter  two 


Photoperiodism: 
An  Outline 


For  obvious  reasons,  flowering  has  been  studied  largely  in 
plants  in  which  it  is  controllable  by  environmental  factors  that 
in  turn  are  easily  controlled  by  the  plant  physiologist.  Chief 
among  such  factors  is  the  photoperiod,  or  daily  length  of  illumina- 
tion. Whether  or  not  it  eventually  turns  out  to  be  as  significant 
for  the  flowering  of  most  plants  as  it  is  for  many  that  have  been 
studied,  the  following  three  general  statements  can  be  made  with 

certainty. 

The  phenomenon  to  be  defined  as  photoperiodism  is  observed 
not  only  among  plants  but  in  many  animals  as  well,  and  is  a  wide- 
spread mechanism  in  the  seasonal  regulation  of  biological  processes, 
particularly  reproduction.  Although  it  was  first  discovered  through 
its  connection  with  flowering,  photoperiodism  controls  other  plant 
processes  also,  even  when  it  does  not  affect  flowering.  Finally, 
part  of  the  basic  mechanism  involved  in  plant  photoperiodism 
occurs  in,  and  can  modify  the  growth  of,  most  higher  plant  cells 
and  tissues. 

DEFINITIONS  OF  PHOTOPERIODISM 

Photoperiodism  has  been  variously  defined  as  a  response  to 
the  daylength,  photoperiod,  or  daily  duration  of  illumination;  as 
a  response  to  the  relative  lengths  of  day  and  night,  or  light  and 
darkness;   or,   in  view  of  later  information,  as   a  response   to  the 

10 


Historical  Note    •    II 

nightlength  or  daily  duration  of  darkness.  These  definitions  all 
convey  the  general  idea,  but  they  may  be  misleading.  A  more 
general  definition  is  that  photoperiodism  is  a  response  to  the  dura- 
tion and  timing  of  the  light  and  dark  conditions.  Total  light 
quantity,  even  light  intensity  above  a  certain  threshold  level,  is 
of  secondary  importance  in  photoperiodism,  although  it  may  be 
a  modifying  factor.  The  relative  length,  or  ratio  of  the  lengths  of 
dark  and  light  exposures,  is  also  secondary.  It  is  the  time  relations 
in  which  light  and  darkness  succeed  each  other  that  appear  to 
be  crucial. 

Under  natural  conditions  of  a  24-hour  day-night  cycle,  of 
course,  the  duration  and  timing  of  light  exposure  cannot  be 
changed  without  a  complementary  change  in  the  dark  exposure, 
but  cycle  lengths  totaling  more  or  less  than  24  hours  have  been 
used  to  study  photoperiodism  experimentally,  as  have  brief  light 
(or  dark)  interruptions  of  extended  dark  (or  light)  periods.  Results 
from  this  sort  of  work  have  led  to  the  definition  given  above.  In 
nature,  however,  the  lengths  of  day  and  night  change  seasonally 
except  on  the  equator,  and  it  is  evident  that  photoperiodism  might 
be  expected  to  have  some  relation  to  the  seasonal  changes  in 
biological  events.  In  fact,  it  was  observations  on  the  relation 
between  seasonal  daylengths  and  flowering  that  led  to  the  discovery 
of  photoperiodism. 

HISTORICAL  NOTE 

Like  many  important  phenomena,  photoperiodism  was  observed 
frequently  before  being  finally  "discovered."  References  to  early 
observations  by  workers  such  as  Tournois,  Klebs,  and  others  can 
be  found  in  Murneek  and  Whyte  (1948),  a  volume  recommended 
to  those  interested  in  the  history  and  early  development  of  flower- 
ing physiology.  Such  observations  suggested  that  flowering  in 
plants  such  as  hops  (Humulus)  or  houseleek  (Sempervivum)  could 
be  brought  about  by  artificially  shortening  or  lengthening  their 
daily  exposure  to  light.  It  remained,  however,  for  Garner  and 
Allard,  plant  physiologists  in  the  U.S.  Department  of  Agriculture, 
to  show  that  such  effects  were  not  isolated  curiosities.  It  was  their 
early  papers  (1920,  1923)  that  attracted  other  workers  to  the  field 


12    •    Photoperiodism:  An  Outline 

and  in  which  the  term  "photoperiodism"  first  appeared,  although 
the  definition  favored  above  is  not  their  original  one.  These  papers 
are  among  the  classics  of  plant  physiology;  not  only  do  they  outline 
many  of  the  major  problems  still  facing  students  of  photoperiodism, 
but  they  are  also  models  of  the  critical,  at  first  almost  reluctant, 
demonstration  of  what  then  seemed  a  revolutionary  concept. 
Although  there  is  no  intention  here  to  maintain  a  historical 
approach,  a  brief  outline  of  two  practical  problems  faced  and 
explained  by  Garner  and  Allard  will  serve  as  a  concrete  introduc- 
tion to  photoperiodism. 

GIANT  TOBACCO  AND  SEPTEMBER  SOYBEANS 

The  preceding  heading  might  well  have  been  used  by  Garner 
and  Allard  to  summarize  the  problems  that  led  to  their  dis- 
covery. The  tobacco,  Nicotiana  tabacum,  was  a  mutant  named 
Maryland  Mammoth  since  it  grew  over  10  feet  high  in  an  experi- 
mental plot  at  Beltsville,  Maryland.  It  nevertheless  remained 
vegetative,  thus  frustrating  its  growers  who  wanted  to  use  it  in 
breeding  experiments.  Propagated  by  cuttings  and  grown  in  the 
greenhouse  in  the  winter,  however,  the  mammoth  flowered  and  set 
seed  when  less  than  five  feet  high.  Equally  puzzling  was  the 
behavior  of  the  Biloxi  variety  of  soybean,  Glycine  (or  Soja)  max. 
When  successive  sowings  were  made  at  two-week  intervals  from 
early  May  through  July,  all  of  them  showed  their  first  flowers  in 
September,  so  that  the  earliest  planted  had  taken  some  120  days 
to  flower  and  the  latest  about  60.  It  was  as  if  all  were  waiting  for 
some  signal  at  which  to  start  flowering,  irrespective  of  their  age 
from  germination— an  improbable  notion  that  turned  out  to  be 
correct. 

After  eliminating  other  factors  such  as  temperature  variations, 
nutrition,  and  light  intensity,  Garner  and  Allard  concluded  that 
the  length  of  day  was  controlling  flowering  in  both  situations. 
Both  Biloxi  soybean  and  Maryland  Mammoth  tobacco  are  short- 
day  plants,  a  term  introduced  by  Garner  and  Allard.  Neither  will 
iflower  unless  the  daylength  is  shorter  than  a  certain  critical  number 
of  hours  (which  happens  to  be  different  for  the  two  plants).  On 
sufficiently  short  days,  flowering  takes  place.  Thus  Maryland 
Mammoth  flowered  in  the  greenhouse  in  winter  under  the  naturally 


Kinds  of  Photoperiodic  Flowering  Responses    •    13 

short  days  of  that  season,  but  merely  vegetated  and  grew  large  in 
the  field  in  summer  and  fall.  Biloxi  soybeans,  no  matter  when 
they  were  planted,  would  not  flower  until  the  sufficiently  short  days 
of  late  summer.  Garner  and  Allard  were  able  to  show  all  this 
experimentally  both  by  artificially  shortening  the  summer  days 
(placing  the  plants  in  light-tight  sheds  or  cabinets  at  various  times) 
or  artificially  lengthening  winter  or  fall  days  even  with  dim 
incandescent  lights.  They  also  examined  the  effects  of  various 
daylengths  on  other  plants  and  discovered  various  kinds  of  flower- 
ing responses,  as  well  as  many  other  effects.  Work  on  photoperiodism 
soon  became  world-wide  and  has  remained  so,  with  major  contribu- 
tions coming  from  Britain,  France,  Germany,  Italy,  Japan,  the 
Netherlands,  Russia,  the  United  States,  and  elsewhere. 

KINDS  OF  PHOTOPERIODIC  FLOWERING  RESPONSES 

The  flowering  responses  of  various  plants  to  different  day- 
lengths  in  a  normal  24-hour  cycle  can  be  roughly  grouped  into 
the  following  classes,  of  which  the  first  two  are  those  commonly 
studied. 

1.  Short-Day  Plants:  The  abbreviation  SDP  will  be  adopted 
for  these  hereafter.  Flower  initiation  in  SDP  is  promoted  by  day- 
lengths  shorter  than  a  particular  value,  the  so-called  critical  day- 
length,  which  differs  widely  from  species  to  species.  It  is  probably 
actually  the  nightlength  that  is  the  most  critical  factor  in  such 
plants;  hence,  they  have  been  described  as  "long-night  plants." 
Much  more  work  has  been  done  with  SDP  than  with  the  other 
classes.  Examples  are  Maryland  Mammoth  tobacco  and  Biloxi 
soybeans,  discussed  above,  also  the  common  cocklebur,  Xanthium, 
and  the  succulent  Kalanchoe  blossfeldiana.  See  the  illustration 
facing  page  1  and  Fig.  2-1  for  two  examples  of  SDP. 

2.  Long-Day  Plants:  The  abbreviation  LDP  will  be  used  for 
these.  Flower  initiation  is  promoted  by  daylengths  longer  than  a 
particular  value,  the  critical  daylength,  which  differs  from  species 
to  species.  Again,  such  plants  have  also  been  described  as  "short- 
night  plants."  Examples  are  the  Black  Henbane,  Hyoscyamus  niger, 
and  some  varieties  of  barley,  Hordeum  vulgare. 

3    and    4.  Short-Long-    and    Long-Short-Day    Plants:    Flower 


14 


Photoperiodism:  An  Outline 


initiation  in  a  relatively  few  plants  appears  to  be  promoted  by 
successive  exposures  to  the  kinds  of  conditions  promoting  it  in 
classes  1  and  2,  in  an  order  depending  upon  the  particular  species. 
Each  requirement  in  a  given  species  may  have  its  own  critical 
daylength.  Such  plants  have  been  little  studied  but  may  be  valuable 


Fig.  2-1.  Short-day  response  in  morning  glory  (Ipomoea  hederacea  var.  Scarlett 
O'Hara).  Plants  are  about  8  weeks  old,  all  grown  with  8  hours  of  sunlight  per 
day.  In  addition,  the  plant  to  the  right  received  a  further  8  hours  per  day  of  dim 
(40  foot  candles)  incandescent  light  for  a  total  photoperiod  of  16  hours.  (Photo- 
graph from  Hendricks  [1956],  American  Scientist,  44:  229-247,  by  permission  of 
the  board  of  editors  of  the  American  Scientist  and  courtesy  of  Drs.  H.  A.  Borthwick 
and  S.  B.  Hendricks,  U.  S.  Department  of  Agriculture.) 


in  analyzing  the  photoperiodic  mechanism.  Some  varieties  of  wheat, 
Triticum  vulgare,  and  rye,  Secale  cereale,  may  be  short-long-day 
plants;  some  Bryophyllum  species  and  the  night-blooming  jasmine, 
Cestrum  nocturnurn,  are  long-short-day  plants. 

5.  Day-Neutral  or  Day length-lndifj event  Plants:  These  simply 
flower  after  reaching  a  certain  age  or  size  and  apparently  irre- 
spective of  daylength.   Other  processes,   however,  may   be  photo- 


Kinds  of  Photoperiodic  Flowering  Responses    ■    15 

periodically  controlled.  Flowering  in  such  plants,  which  may 
constitute  the  majority,  has  been  relatively  little  studied.  Com- 
mon examples  are  tomato,  Lycopersicon  esculentum,  and  many 
varieties  of  peas,  Pisum  sativum. 

Note  that  in  this  classification  the  distinction  between  SDP 
and  LDP  is  based  not  on  the  absolute  values  of  the  critical  day- 
lengths  (which  may  range  from  four  to  over  18  hours  for  LDP, 
for  example);  the  distinction  is  whether  flowering  is  promoted 
by  photoperiods  shorter  or  longer  than  the  critical.  The  critical 
daylength  for  Xanthium,  for  example,  is  about  \ol/2  hours,  and 
that  for  Hyoscyamus  about  11  hours.  Yet  the  former  is  properly 
classified  as  an  SDP  since  it  flowers  on  photoperiods  shorter  than 
its  critical  value,  whereas  the  latter  is  an  LDP,  requiring  photo- 
periods longer  than  its  critical.  It  is  necessary  to  belabor  this 
distinction  since  it  is  possible  to  find  textbooks  that  should  know 
better  implying  that  LDP  flower  with  more  hours  of  light  per  day 
than  SDP.  Such  statements  miss  the  point.  Both  Xanthium  and 
Hyoscyamus  flower  with  14  hours  of  light  per  day.  The  daylength 
in  which  a  plant  flowers  is  no  indication  of  its  response  class  in 
the  absence  of  further  information. 

In  addition  to  the  classes  of  response  described,  the  following 
considerations  should  be  recognized  before  proceeding  further. 
There  are  plants  in  which  the  appropriate  photoperiodic  treatment 
is  an  absolute  requirement  for  flowering  under  all  naturally 
occurring  conditions.  Neither  Xanthium  nor  Hyoscyamus,  for 
example,  ever  flowers  unless  exposed  to  the  proper  photoperiodic 
conditions.  Such  plants  are  referred  to  as  having  a  qualitative 
photoperiodic  response,  or  requirement.  In  other  plants,  differing 
photoperiodic  conditions  merely  hasten  or  delay  but  do  not  abso- 
lutely determine  flower  initiation.  Such  plants  have  a  quantitative 
response  to  photoperiod.  There  are  also  plants  in  which  qualitative 
or  quantitative  photoperiodic  responses  are  observed  only  under 
particular  conditions  of  temperature  or  some  other  environmental 
factors;  these  would  be  conditional  photoperiodic  responses.  Still 
other  plants  may  require  one  photoperiodic  condition  for  flower 
initiation  but  a  markedly  different  one  for  flower  development. 
Finally,  there  are  many  species  in  which  the  photoperiodic  response 
may  change  with  age;   such  changes  are  usually  in   the  direction 


16    •    Photoperiodism:  An  Outline 

of  day-neutrality  from  an  initial  qualitative  or  quantitative  long-  or 
short-day  response. 

A  particularly  clear  example  of  the  last  sort  of  behavior  is 
shown  by  a  variety  of  sunflower,  Helianthus  annum,  recently 
studied  by  Dyer  et  al.  (1959).  Seedlings  raised  under  12-hour 
daylengths  all  showed  inflorescences  after  40  days,  while  seed- 
lings raised  under  16-hour  daylengths  showed  no  detectable  flower 
primordia  at  the  time.  Over  90  percent  flowering  occurred  on 
both  12-  and  16-hour  photo  periods  in  experiments  carried  to  130 
days,  however,  and  even  20-hour  photoperiods  gave  over  70  percent 
flowering.  In  other  words,  young  plants  had  a  qualitative  short-day 
response  with  a  critical  daylength  between  12  and  16  hours,  but 
older  plants  were  either  day-neutral  or  showed  a  weak  quantitative 
short-day  response. 

While  this  brief  list  by  no  means  exhausts  the  ways  in  which 
photoperiodic  responses  may  differ  within  the  overall  classification, 
and  examples  will  appear  frequently  in  what  follows,  there  do 
appear  to  be  limits  on  such  variation.  Although  varieties  of  the 
same  species  often  differ  in  critical  daylength  and  frequently  show 
a  range  from  day-neutrality  to  a  qualitative  long-  or  short-day 
requirement,  the  writer  knows  of  no  species  with  both  LDP  and 
SDP  varieties;  it  is  even  relatively  unusual  to  find  both  types 
within  a  single  genus.  The  range  of  variation  that  can  be  caused 
by  age  or  environmental  conditions  is  also  apparently  limited  in 
the  same  way  as  that  within  a  species;  that  is,  no  experimental 
treatment  yet  found  will  convert  an  LDP  to  an  SDP,  or  vice  versa. 
Such  an  effect  would  obviously  be  very  valuable  for  studies  of  the 
mechanism  involved.  Aside  from  these  generalizations,  however, 
the  responses  of  species  and  varieties  within  a  given  class  are 
extremely  various,  and  there  is  no  evident  correlation  between 
photoperiodic  response  classes  and  any  taxonomic  or  ecological 
category.  Thus,  although  much  of  this  discussion  will  proceed  by 
considering  some  of  the  results  from  a  few  well-studied  plants,  let 
the  reader  beware:  the  country  is  large,  and  the  map,  so  far,  is 
small.  For  many  variations  and  modifications  in  photoperiodic 
response  that  have  not  been  studied  systematically,  see  Chouard 
(1957). 


Leaves  in  Photoperiodism    •    17 


THE  ROLE  OF  LEAVES  IN  PHOTOPERIODISM. 
PHOTOPERIODIC  INDUCTION 

Neither  of  these  topics  will  be  considered  in  detail  until 
Chapter  Five  where  the  discussion  is  on  the  nature  of  the  flower- 
ing stimulus,  since  both  are  more  germane  to  that  question  than 
to  photoperiodism  proper.  Brief  summaries  are  given  here  simply 
to  render  the  rest  of  this  chapter  intelligible. 

In  almost  every  plant  studied,  it  is  the  leaf  blades  that  perceive 
the  photoperiodic  treatment.  This  has  been  shown  in  several  ways. 
Photoperiodic  treatments  given  to  all,  or  in  some  cases  one  or  a 
few,  leaf  blades  on  a  plant  will  have  the  same  effects  as  though 
the  entire  plant  had  been  treated.  Defoliated  plants,  with  rare 
exceptions,  are  photoperiodically  unresponsive.  Photoperiodic 
treatment  of  the  apices  or  other  meristematic  areas  is  usually  in- 
effective, although  the  meristems  are  the  actual  sites  of  the  change 
from  vegetative  to  reproductive  growth.  One  can  conclude  that 
the  primary  photoperiodic  effect  occurs  in  the  leaves  and  that  the 
leaves  somehow  communicate  its  results  to  the  meristems. 

Certain  plants  require  more  or  less  constant  exposure  to 
appropriate  photoperiodic  cycles,  at  least  until  flower  primordia 
can  be  easily  detected,  in  order  to  flower  successfully.  In  many 
others,  however,  exposure  to  only  a  few  such  cycles  will  cause 
flowering  even  when  the  plants  are  returned  to  unfavorable  photo- 
periodic conditions.  Such  plants  are  said  to  be  induced  by  the 
photoperiodic  treatment;  photoperiodic  induction  is  an  aftereffect 
of  favorable  photoperiods  which  will  result  in  flowering  or  at  least 
considerable  primordium  development,  even  on  unfavorable  photo- 
periods.  An  induced  plant  indicates  clearly  by  this  behavior  that 
some  change  has  taken  place  and  persists,  but  no  anatomical  or 
morphological  changes  can  usually  be  detected  after  the  few  induc- 
tive cycles  required  in  such  plants.  Naturally,  not  only  is  induction 
of  great  theoretical  interest  but  it  is  also  experimentally  useful. 
One  of  the  major  reasons  for  the  widespread  use  of  Xanthium 
in  photoperiodic  studies  is  that,  under  favorable  conditions,  a 
single  short-day  cycle  (even  given  to  a  single  leaf)  will  lead  to 
flowering  in  plants  kept  the  rest  of  the  time  on  noninductive  long 
days.  This  sensitivity  to  a  single  cycle  is  unusual,  but  is  not  unique 


18    •    Photoperiodism:  An  Outline 

to  Xanthium;  it  has  been  reported  also  in  the  Japanese  morning 
glory,  Phcrbitis  (or  Ipomoea)  nil  (Imamura  and  Takimoto,  1955a), 
a  duckweed,  Lemna  perpusilla  (Hillman,  1959a),  and  pigweeds, 
Chenopodium  (Gumming,  1959),  all  SDP.  Many  other  SDP  also 
can  be  induced  by  2  to  10  days  of  the  appropriate  photoperiodic 
treatment.  Induction  by  a  very  few  cycles  is  perhaps  less  common 
among  LDP,  although  at  least  dill,  Anetham  graveolens  (A.  W. 
Naylor,  1941),  and  mature  plants  of  the  grass  Lolium  temulentum 
(Evans,  1960)  are  both  inducible  by  one  long-day  cycle. 

THE  CENTRAL  ROLE  OF  THE  DARK  PERIOD 

While  the  terms  "short-day"  and  "long-day"  plant  have  been 
maintained  by  constant  usage,  probably  the  most  important  single 
difference  between  these  two  response  classes  is  in  their  reactions 
to  the  nightlength,  or  dark  period.  In  general,  flowering  in  SDP 
is  promoted  by  certain  reactions  taking  place  during  the  dark 
periods,  and  the  "critical  daylength"  actually  represents  the  maxi- 
mum daylength  that  will  allow  a  dark  period  of  sufficient  length 
in  a  normal  24-hour  cycle.  In  LDP,  on  the  other  hand,  dark  periods 
have  an  inhibitory  effect  on  flower  initiation,  and  the  critical 
daylength  is  thus  the  minimum  which  in  a  24-hour  cycle  will  keep 
the  dark  period  short  enough  to  allow  flowering.  These  generaliza- 
tions are  supported  by  the  fact  that  LDP  usually  flower  best  on 
continuous  light,  so  that  apparently  the  entire  role  of  the  dark 
period  is  inhibitory  (A.  W.  Naylor,  1941;  see  Lang,  1952).  Several 
SDP,  on  the  contrary,  flower  in  continuous  darkness  if  they  are 
given  sucrose  (see  Doorenbos  and  Wellensiek,  1959;  Hillman, 
1959a),  suggesting  that  light  is  unnecessary  if  its  photosynthetic 
function  is  replaced  by  another  source  of  carbohydrate.  However, 
at  least  one  LDP,  spinach,  Spiiuicia  oleracea,  also  flowers  in  total 
darkness  when  supplied  with  sucrose  (GentschefT  and  Gustaffson, 
1940)  so  that  reliance  on  this  sort  of  evidence  alone  is  undesirable. 

Hamner  and  Bonner  (1938)  were  able  to  show  that  in 
Xanthium  the  critical  time  for  an  appropriate  photoperiodic 
treatment  lay  in  the  dark  period  length.  When  24-hour  cycles  of 
light  and  darkness  were  used,  these  plants  flowered  with  dark 
periods  of  8%  hours  or  longer.  Thus  the  critical  daylength  was 
15%  hours.  No  flowering  occurred  on  schedules  of  16  hours  light- 


The  Central  Role  of  the  Dark  Period    •    19 

8  hours  darkness.  To  determine  whether  it  was  actually  the  day- 
length  or  nightlength  that  was  critical  in  this  schedule,  Hamner 
and  Bonner  performed  several  kinds  of  experiments. 

Using  artificial  light  when  necessary,  they  exposed  some  plants 
to  schedules  of  4  hours  light-8  hours  darkness.  None  of  these 
flowered,  although  each  light  period  was  far  shorter  than  the  critical 
daylength  of  15 y>  hours.  On  the  other  hand,  all  plants  flowered 
rapidly  under  cycles  of  16  hours  light-32  hours  darkness,  even 
though  each  light  period  was  longer  than  the  critical  daylength. 
Two  conclusions  come  from  such  data.  First,  it  seems  to  be  the 
length  of  the  dark  period,  not  that  of  the  light  period,  that  is 
important  for  Xanthium.  Second,  the  relative  length  of  day  and 
night  is  clearly  not  the  critical  factor  since  the  ratio  of  light  to 
darkness  was  the  same  in  both  schedules  used. 

Perhaps  the  best  evidence  concerning  the  role  of  the  dark 
period  in  both  LDP  and  SDP  can  be  obtained  by  interrupting 
these  dark  periods  with  brief  light  exposures.  Hamner  and  Bonner, 
for  example,  showed  that  the  inductive  effects  of  9-hour  dark  periods 
could  be  completely  annulled  by  interrupting  each  one  in  the 
middle  with  a  minute  of  relatively  dim  (150  foot  candles)  incandes- 
cent light.  This  "light-break"  effect  is  widespread  among  both 
response  classes,  and  the  general  situation  can  be  summarized  as 
follows  (see,  for  example,  Borthwick,  Hendricks,  and  Parker,  1956). 

In  order  to  be  photoperiodically  effective  in  either  SDP  or 
LDP,  a  dark  period  of  sufficient  length  has  to  be  uninterrupted. 
Total  light  energies  (100-1000  kiloergs/cm2)  that  are  very  low 
compared  to  those  of  daylight,  even  given  in  a  few  minutes,  are 
sufficient  to  constitute  an  effective  interruption.  In  SDP  such  as 
Xanthium  or  Biloxi  soybeans,  light-breaks  in  otherwise  inductive 
dark  periods  will  completely  inhibit  flowering.  In  LDP  such  as 
Hyoscyamus  or  the  Wintex  variety  of  barley,  Hordeum  vulgare, 
light-breaks  in  otherwise  noninductive  periods  (that  is,  in  schedules 
with  daylengths  less  than  the  critical)  bring  about  flowering  as 
though  the  plants  had  been  on  an  adequate  long-day  schedule. 
As  will  become  evident  later  on,  light-break  experiments  have 
proved  very  useful  for  further  studies  on  the  mechanism  of  photo- 
periodism.  At  this  juncture,  however,  they  are  simply  presented  as 
evidence  for  the  role  of  the  dark  periods  as  the  single  most  im- 
portant controlling  factor  in  photoperiodism.  Similarly,  brief  "dark- 


20    •    Photoperiodism:  An  Outline 

breaks"  during  main  light  periods  have  essentially  no  effect  on 
the  process. 

The  evidence  reviewed  above  should  make  clear  the  reason  lor 
emphasizing  duration  and  timing  of  light  (and  darkness)  rather 
than  total  energy  in  the  definition  of  photoperiodism.  It  has 
also  resulted  in  the  term  "critical  nightlength"  replacing  "critical 
daylength"  in  some  reviews  and  articles  on  the  subject,  in  order 
to  stress  the  relative  importance  of  light  and  dark  periods.  However, 
as  will  be  shown,  light  also  plays  a  role,  although  perhaps  less 
important,  in  the  normal  time  requirements  of  photoperiodism,  so 
that  the  second  terminology  is  only  slightly  more  accurate  than  the 
first.  Either  will  be  used,  as  occasion  demands. 

Ancillary  evidence  for  the  more  crucial  role  of  the  dark  periods 
has  also  been  derived  from  experiments  in  which  temperature  is 
varied,  some  of  which  will  be  considered  elsewhere. 

REQUIREMENTS  FOR  HIGH-INTENSITY  LIGHT 

The  effects  of  brief  or  prolonged  exposures  to  low-intensity 
light,  nullifying  dark  periods,  will  be  considered  in  detail  in  the 
next  chapter.  Meanwhile,  after  setting  up  generalizations  that  dark- 
ness plays  the  major  role  in  photoperiodism  and  that  the  total 
light  energy  during  a  treatment  or  cycle  is  relatively  unimportant, 
it  is  now  necessary  to  consider  what  role,  if  any,  is  played  by  the 
high-intensity  light  periods  which,  at  least  in  nature,  normally 
alternate  with  dark  periods. 

1.  Short-Day  Plants:  Early  work  with  SDP  soon  showed  that 
in  spite  of  the  critical  role  of  the  dark  periods,  the  main  light 
periods  also  had  to  include  at  least  a  certain  amount  of  high- 
intensity  light  for  optimum  (lowering  to  occur  in  many  plants. 
An  elegant  demonstration  of  this  was  given  by  Hamner  (1910). 
using  Xantliium. 

It  was  obviousl)  not  reasonable  to  study  the  effect  of  a  dark 
period  preceded  by  a  dark  period,  since  the  two  together  simply 
add  up  to  a  longer  one.  Hamner  made  use  ol  the  light-break  tech- 
nique, however,  in  the  following  manner.  Xanthium  plants  can  be 
kept  vegetative  on  cycles  of  3  minutes  Light-3  hours  darkness. 
After  a   lew  such  cycles,  a  single  dark  period  of   12  hours,  which 


Requirements  for  High-Intensity  Light    •    21 

would  normally  cause  flowering  if  the  plants  were  subsequently 
placed  on  long-day  conditions,  was  entirely  ineffective.  Before  such 
a  dark  period  could  be  effective,  the  plants  had  to  be  exposed  to 
at  least  a  few  hours  of  high-intensity  light;  within  limits,  the 
effectiveness  of  the  dark  period  was  then  directly  related  to  the  light 
energy  given  before  it.  This  "high-intensity  light  reaction"  clearly 
differs  from  the  low-intensity  reaction  sufficient  to  interrupt  a  dark 
period,  since  it  requires  light  energies  some  10,000  times  higher  for 
maximum  effect.  It  has  since  been  shown  that  C02  must  be  present 
for  the  high-intensity  light  to  have  its  effect;  in  addition,  feeding 
the  leaves  with  carbohydrates  or  organic  acids  can  at  least  partially 
replace  the  high-intensity  light  requirement  (see  Liverman,  1955). 
Such  results  suggest  that  this  requirement  is  largely  a  requirement 
for  products  of  photosynthesis. 

Another  high-intensity  light  requirement  has  also  been  reported 
in  Xanthium.  To  be  maximally  effective,  an  inductive  dark  period 
must  be  followed  as  well  as  preceded  by  a  period  of  high-intensity 
light.  Lockhart  and  Hamner  (1954),  for  example,  found  that  if 
only  a  brief  light  flash  was  given  to  end  the  inductive  dark  period 
and  this  was  then  followed  by  another  dark  period  before  the 
plants  were  replaced  in  long-day  conditions,  flowering  was  com- 
pletely or  partially  inhibited.  A  period  of  high-intensity  light  given 
before  the  second  (inhibitory)  dark  period  rendered  it  ineffective, 
but  low-intensity  light  did  not.  Both  auxin  (see  Chapter  Six)  and 
high  temperature  increased  the  effect  of  the  second  dark  period. 
Subsequently,  Carr  (1957)  found  that  sucrose  given  to  the  leaf 
during  the  second  dark  period  almost  nullified  the  inhibition, 
allowing  flowering  to  take  place.  He  thus  suggested  that  the  "second 
high-intensity  light  requirement,"  like  the  first,  is  a  requirement 
for  photosynthetic  products. 

While  experiments  of  this  sort  show  that  high-intensity  light 
periods  can  have  profound  modifying  effects  on  photoperiodic 
induction,  these  are  probably  due  to  effects  of  photosynthate  as  an 
energy  source  and  on  the  translocation  of  the  flowering  stimulus 
(see  Chapter  Five)  rather  than  on  photoperiodism  proper.  Even 
Xanthium,  on  which  the  most  detailed  work  of  this  kind  has  been 
done,  can  eventually  initiate  flowers  in  total  darkness  (Hamner, 
1940).  Thus  the  primary  role  of  the  dark  period  in  photoperiodism 
is  not  contradicted  by  these  data. 


22    •    Photoperiodism:  An  Outline 

The  interpretation  of  high-intensity  light  requirements  in  SDP 
as  basically  photosynthetic  is  not  entirely  secure.  Kalanchoe  bloss- 
feldiana  is  an  SDP  incapable  of  flowering  in  continuous  darkness. 
It  will,  however,  initiate  flowers  if  it  receives  one  one-second  flash 
of  light  in  every  24  hours  (see  Harder,  1948;  Schwabe,  1959). 
Although  COo  is  indeed  required  during  the  light  flash,  it  is  not 
likely  that  a  great  deal  of  photosynthesis  takes  place  during  that 
time,  so  that  a  more  specific  requirement  is  at  least  suggested. 

Even  the  generalization  that  the  photoperiodic  responses  of  SDP 
are  generally  promoted  by  at  least  some  exposure  to  high-intensity 
light  does  not  hold  for  the  widely  studied  Perilla.  Using  Perilla 
crispa,  de  Zeeuw  (1953)  found  that  the  critical  daylength  becomes 
longer  (dark  requirement  becomes  shorter)  as  the  main  light  period 
intensity  is  lowered;  with  sufficiently  low  light  intensities,  flower 
initiation  occurs  under  continuous  light.  A  set  of  experiments  on  the 
complex  interactions  of  bright  and  dim  light  periods  on  Kalanchoe 
has  been  published  by  Krumwiede  (1960),  who  also  provides  a 
thorough  bibliography  on  the  question.  It  seems  clear  that  probably 
more  factors  than  photosynthesis  are  involved  in  the  effects  of 
bright  light. 

2.  Long-day  Plants:  Since,  in  general,  the  longer  the  light 
period  the  better  for  flowering  in  LDP,  analyses  of  the  kind  de- 
scribed above  have  attracted  little  interest.  A  number  of  LDP  are 
nevertheless  known  to  flower  more  rapidly  in  either  continuous 
light  or  long  photoperiods  if  at  least  part  of  each  light  exposure 
is  at  high  intensity  (see  Bonner  and  Liverman,  1953).  Much  of  the 
work  on  the  main  light  periods  of  LDP,  like  some  of  that  on  SDP, 
has  been  on  the  effects  of  various  light  colors,  and  will  be  considered 
in  the  next  chapter. 

MUTUAL  INTERACTIONS  OF  LIGHT  AND 
DARK  PERIOD  LENGTHS 

Extremely  complex  interactions  between  light  and  dark  period 
lengths  have  been  observed  in  both  LDP  and  SDP,  to  the  extent 
that  the  critical  values  of  either  light  or  dark  periods  are  markedly 
aflected  by  the  lengths  of  the  complementary  periods. 

Claes  and  Lang  (1947)  studied  the  effects  of  various  light  and 
dark  schedules  on  the  rapidity  with  which  the  LDP  Hyoscya?nus 


Interactions  of  Light  and  Dark  Period  Lengths    •    23 

niger  would  initiate  flowers.  As  long  as  the  light-dark  cycles  totaled 
24  hours,  flowering  occurred  with  at  least  1 1  hours  of  light  per 
cycle,  and  was  most  rapid  with  15-16  hours.  When  cycles  totaling 
48  hours  were  used,  however,  flowering  occurred  with  as  few  as 
9  hours  light  per  cycle,  and  reached  its  maximum  rapidity  with 
13  hours  per  cycle.  Thus  longer  total  cycle  lengths  actually  reduced 
the  "critical  daylength"  by  at  least  two  hours,  in  spite  of  the  fact 
that  the  shorter  daylength  was  active  with  a  much  longer  dark 
period. 

Differing  but  equally  complex  results  were  obtained  by 
Takimoto  (1955)  in  experiments  in  which  he  exposed  the  LDP 
Silene  armeria  to  10-day  treatments  of  cycles  composed  of  various 
durations  of  light  and  darkness.  Flower  initiation  was  most  rapid 
in  continuous  light.  In  cycles  with  light  periods  of  12  hours  or 
shorter,  initiation  occurred  only  when  the  associated  dark  periods 
were  shorter  than  13  hours;  in  cycles  with  light  periods  of  14  or  16 
hours,  however,  even  dark  periods  of  24  or  32  hours  duration  failed 
to  prevent  initiation.  Some  of  the  interactions  between  light  and 
dark  periods  in  the  SDP  Biloxi  soybeans  were  studied  by  Blaney 
and  Hamner  (1957).  Only  a  few  of  the  results  will  be  mentioned 
here,  but  this  paper  provides  one  of  the  best  examples  of  the  com- 
plexity of  such  interactions  and  resultant  difficulty  of  reaching  any 
general  conclusions  on  the  problem  at  present.  The  Biloxi  soybean, 
like  most  SDP,  requires  several  cycles  of  appropriate  photoperiodic 
treatment  to  initiate  flowers.  When  plants  were  given  7  cycles  of 
8  hours  fluorescent  light  and  16  hours  darkness,  then  placed  on 
long-day  greenhouse  conditions,  high  flowering  values  were  ob- 
tained. Hence  8-hour  light  periods  and  16-hour  dark  periods 
together  constitute  an  inductive  cycle.  However,  when  each  portion 
of  such  an  inductive  cycle  was  examined  separately,  the  following 
results  were  obtained.  Seven  cycles  of  8  hours  light  alternating  with 
24-hour  or  26-hour  dark  periods  resulted  in  no  induction  at  all. 
Seven  cycles  of  16-hour  dark  periods  alternating  with  light  periods 
either  4  hours  or  shorter,  or  longer  than  12  hours,  also  resulted  in  no 
induction.  For  further  results  and  tentative  conclusions  the  original 
paper  should  be  consulted.  The  concept  of  a  minimum  critical 
dark  period  requirement  was  still  supported  since  induction  was 
never  brought  about  by  any  cycle  with  less  than  a  10-hour  dark 
period,  no  matter   what   the   associated   light  period;    however,   it 


24    •    Photoperiodism:  An  Outline 

also  did  not  occur  on  cycles  containing  16-hour  light  periods,  no 
matter  what  the  dark  period. 

The  generalization  that  crucial  events  in  photoperiodism  take 
place  during  the  dark  period  is  evidently  not  annulled  by  results 
such  as  those  presented  in  this  section.  The  precise  values  of 
"critical  nightlengths,"  however,  arc  markedly  dependent  upon  the 
lengths  of  the  associated  light  periods,  and  in  a  manner  which 
conforms  to  no  simple  pattern. 

INTERACTION  OF  DIFFERENT  PHOTOPERIODIC  CYCLES: 

FRACTIONAL  INDUCTION  IN  LDP  AND 

LONG-DAY  INHIBITION  IN  SDP 

In  all  the  experiments  so  far  considered,  not  more  than  one 
particular  kind  of  light-dark  cycle  was  used  for  each  experimental 
treatment,  although  such  cycles  might  be  repeated  several  times. 
It  is  desirable  to  examine  some  results  of  using  more  than  one  kind 
of  cycle  in  a  given  treatment.  Most  such  experiments  have  been 
concerned  with  the  effects  of  intercalating  noninductive  between 
inductive  cycles,  and  have  naturally  been  conducted  largely  with 
plants  requiring  more  than  one  cycle  for  induction.  The  responses 
of  LDP  and  SDP  to  such  treatments  differ  fairly  consistently  from 
each  other,  but  show  considerable  regularity  within  each  class. 

Most  LDP  studied  are  susceptible  to  "fractional  induction." 
This  is  best  illustrated  by  an  example  reported  by  Snyder  (1948). 
Plants  of  the  plantain  Plantago  lanceolata  showed  100  percent 
inflorescences  after  exposure  to  25  long-day  cycles  (18  hours  light- 
6  hours  darkness).  Exposure  to  only  10  such  cycles  resulted  in  no 
flowering  when  followed  by  exposure  to  short-day  cycles  (8  hours 
light- 16  hours  darkness).  However,  if  10  long-day  cycles  were  given 
and  followed  by  20  short-day  cycles,  only  15  more  long-day  cycles 
were  required  for  100  percent  inflorescence  formation.  Thus  the 
effect  of  the  first  10  inductive  cycles,  though  insufficient  to  cause 
flowering  by  itself,  persisted  throughout  the  short-day  treatment 
so  that  only  15  more  long-day  cycles  gave  the  effective  total  of  25. 
This  remarkably  accurate  "memory"  is  apparently  not  unusual  in 
fractional  induction.  It  implies  that,  in  such  LDP  at  least,  non- 
inductive  cycles  play  a  merely  passive  role  and  do  not  oppose  the 
effects  of  inductive  cycles. 


4 

Photoperiodism  and  Temperature    ■    25 

In  several  SDP,  on  the  other  hand,  noninductive  cycles  have 
a  clearly  inhibiting  action  on  induction.  Schwabe  (1959)  has  shown 
that  for  Perilla  ocymoides,  Chenopodium  amaranticolor,  and  Biloxi 
soybean,  noninductive  cycles  intercalated  between  inductive  cycles 
positively  inhibit  the  effects  of  the  latter.  Each  long-day  cycle,  in 
fact,  appears  capable  of  counteracting  the  effect  of  two  short-day 
cycles.  A  long-day  cycle  probably  acts  by  annulling  the  effectiveness 
of  the  short  days  immediately  following  it,  rather  than  by  destroy- 
ing the  effect  of  the  short  days  preceding  it.  Such  a  conclusion  agrees 
with  the  results  of  Harder  and  Biinsow  (1954)  who  had  found  that 
the  number  of  flowers  formed  by  Kalanchoe  blossjeldiana  after  a 
given  number  of  short-day  cycles  was  inversely  related  to  the 
daylength  used  in  the  noninductive  cycles  on  which  the  plants  were 
kept  previous  to  short-day  treatment.  However,  Carr  (1955)  obtained 
fractional  induction  in  a  number  of  SDP,  including  some  of  the 
same  plants  used  by  Schwabe,  above.  Carr  also  cites  other  results 
that  oppose  the  generalization  that  only  LDP  exhibit  the  phe- 
nomenon, holding  instead  that  it  shows  no  particular  correlation 
with  response  type  but  rather  is  an  individual  species  characteristic. 

Possibly  the  ability  of  Xanthium  and  a  few  other  SDP  to 
flower  in  response  to  one  short-day  cycle  is  due  to  the  lack,  or 
weaker  operation,  of  inhibitory  long-day  effects.  Even  in  Xanthium, 
of  course,  flowering  intensity  increases  proportionately  to  the 
number  of  short-day  cycles  over  a  considerable  range  (see  Chapter 
Five)  so  that  the  phenomenon  may  be  quite  general. 

PHOTOPERIODISM  AND  TEMPERATURE 

Temperature  enters  into  the  physiology  of  flowering  in  numerous 
ways,  many  of  which  will  be  considered  later.  A  few  interactions  of 
temperature  with  photoperiodism  will  be  mentioned  now,  but  with 
the  cautionary  note  that  the  results  of  such  studies  tend  to  defy 
generalization  more  completely  than  any  other  aspect  of  the  field. 
For  a  major  treatment  of  the  effects  of  temperature  on  plant 
growth,  see  Went  (1957). 

Temperatures  differing  slightly  from  one  another  may  strongly 
modify  the  effects  of  daylength  on  flower  initiation.  For  example, 
Roberts  and  Struckmeyer  (1938)  found  that  both  Maryland  Mam- 
moth tobacco  and  Jimson  weed,  Datura  stramonium,  were  SDP  only 


26    •    Photoperiodism:  An  Outline 

at  24°  C  or  higher,  but  tended  toward  day-neutrality  at  about 
13°  C.  Strawberry,  Fragaria  virginiana  x  chiloensis,  shows  a  virtually 
identical  response  (Went,  1957,  Chap.  11).  The  requirement  of 
at  least  a  flash  of  bright  light  for  induction  of  Kalanchoe,  men- 
tioned previously,  has  been  confirmed  by  Oltmanns  (1960)  at  20° 
or  25°,  but  apparently  is  no  longer  present  at  15°  C,  since  Kalanchoe 
will  initiate  flowers  in  total  darkness  at  that  temperature. 

The  critical  daylength  for  certain  LDP  is  reduced  at  low 
temperatures.  Hyoscyamus  niger  grown  at  28.5°  C  requires  at  least 
1  1  Vo  hours  of  light  per  day  to  flower,  whereas  at  15.5°  the  critical 
daylength  is  reduced  to  8 ]/2  hours  (see  Melchers  and  Lang,  1948). 
However,  the  LDP  Rudbeckia  bicolor  will  flower  at  relatively  high 
temperatures  (about  32°  C)  under  photoperiods  too  short  to  permit 
flowering  under  cool  conditions;  Rudbeckia  speciosa,  a  similar 
species,  remains  a  true  LDP  under  both  conditions  (Murneek, 
1940). 

Most  effects  of  this  kind  have  been  ascribed  primarily  to  dark 
period  rather  than  light  period  temperatures  (see  Lang,  1952), 
but  unusual  temperatures  can  modify  both  light  and  dark  period 
processes.  Two  of  the  early  papers  on  Xanthium  illustrate  this 
point. 

Long  (1939)  found  that  Xanthium  required  at  least  six  cycles 
of  9  hours  light- 15  hours  darkness  for  induction  if  the  dark  period 
temperature  was  5°  C,  even  though  the  light  periods  were  given 
at  21°  C.  Further  experiments  showed  that  when  plants  were 
grown  at  21°  light  temperature  and  5°  dark  temperature,  the 
( ritical  nightlength  was  increased  to  about  1 1  hours  compared  with 
8%  hours  for  plants  held  constantly  at  21°.  Long  concluded  that 
"variations  in  temperature  greatly  affect  the  length  of  the  critical 
dark  period,"  although  his  work  has  been  cited,  in  a  context  to  be 
discussed  later,  as  showing  a  "relatively  temperature-independent 
time  measurement  of  nightlength"  (Pittendrigh  and  Bruce,  1959). 

The  light  period  processes  in  Xanthium  also  are  temperature- 
sensitive,  at  least  when  they  are  made  relatively  limiting  (Mann, 
1940).  At  least  four  hours  of  bright  light  (over  2000  foot  candles) 
are  required  for  the  optimum  action  of  a  subsequent  dark  period 
if  the  light  is  given  at  10°  C,  but  only  about  one  half  hour  of  light 
is  required  at  30°  for  the  same  effect. 

The  sensitivity  of  light  or  dark  periods  to  temperature  changes 


Photoperiodism  and  Vegetative  Growth    •    27 

has  been  studied  extensively  in  connection  with  the  possible  rhyth- 
mic components  of  photoperiodism  (see  Chapter  Three).  The  paper 
by  Blaney  and  Hamner,  previously  cited,  also  contains  data  on  the 
interactions  of  temperature  with  the  various  light-dark  cycles  used. 
A  simpler  example  of  such  work  is  a  paper  by  Schwemmle  (1957) 
reporting  the  effects  on  the  SDP  Kalanchoe  blossfeldiana  of  brief 
exposures  to  30°  C  during  various  portions  of  12-hour  dark  periods 
alternated  with  12-hour  light  periods  (inductive  for  Kalanchoe), 
the  temperature  otherwise  being  20°.  Such  exposures  promoted 
flowering  significantly  when  given  for  the  first  three  hours  of  each 
dark  period,  but  inhibited  it  completely  when  given  for  the  last 
three  hours.  Full  12-hour  exposures  to  30°  during  the  night  also 
inhibited  completely. 

One  of  the  most  striking  temperature  effects  reported  recently 
deals  again  with  Xanthium,  which  will  apparently  flower  on  a 
16  hours  light-8  hours  darkness  schedule,  completely  noninductive 
at  23°  C,  if  the  first  8  hours  of  each  light  period  are  given  at  4°. 
Low  temperatures  during  the  second  half  of  each  light  period,  or 
during  the  dark  period  itself,  do  not  cause  flowering,  nor  does 
flowering  occur  on  continuous  light  with  any  alternation  of  tem- 
peratures used  (Nitsch  and  Went,  1959);  see  Fig.  2-2.  The  SDP 
Pharbitis  can  be  brought  to  flower  even  under  continuous  light 
by  low-temperature  treatments  (Ogawa,   1960). 

On  the  basis  of  some  experiments  with  Hyoscyamus  and  the 
SDP  Chenopodium,  as  well  as  other  results  in  the  literature, 
Schwemmle  (1960)  has  suggested  in  a  brief  paper  that,  in  a  physio- 
logical sense,  high  temperatures  may  be  equivalent  to  light  and 
low  temperatures  to  darkness  in  their  effects  on  photoperiodism. 
Whether  this  generalization  will  withstand  critical  examination 
remains  to  be  seen.  So  far,  all  that  can  be  said  with  certainty  is 
that  high  or  low  temperatures  can  modify  both  dark  and  light 
processes  in  photoperiodism  in  a  manner  varying  widely  with  the 
temperatures,  species,  specific  cycle,  and  portion  of  light  or  dark 
period  chosen. 

PHOTOPERIODISM  AND  VEGETATIVE  GROWTH 

Structures  and  processes  of  all  kinds  can  be  affected  by  photo- 
periodism,   and    such    results    are    widespread    in    the    literature, 


28    •    Photoperiodism:  An  Outline 


Fig.  2-2.  Photoperiodic  control  of  flowering  in  cocklebur  {Xanthium  pennsyl- 
vanicum)  as  modified  by  low  temperature.  Growing  points  of  plants  of  the  same 
age — with  all  except  terminal  leaves  removed  to  show  development — photo- 
graphed after  13  days  of  the  following  treatments:  (A)  8-hour  days  at  23°  C 
(flowering);  (B)  16-hour  days  at  23°  C  (vegetative);  (C)  16-hour  days  as  in  (B) 
but  with  4°  during  first  8  hours  of  each  light  period;  (D)  24-hour  (continuous) 
days  at  23°  except  4°  during  8  hours  of  each  day.  (Photographs  from  Nitsch  and 
Went  [1959],  by  permission  of  the  American  Association  for  the  Advancement 
of  Science  and  courtesy  of  Dr.  J.  P.  Nitsch,  Le  Phytotron,  Gif-sur-Yvette,  France.) 

starting  with  Garner  and  Allard.  Some  of  the  characteristics 
frequently  under  photoperiodic  control  even  when  flowering  is  not 
are  stem  elongation,  leaf  shape  and  size,  branching,  pigmentation, 
tuberization,  and  pubescence  (see,  for  example,  Nay  lor,  1953). 
Effects  on  these  have  been  studied  far  less  than  the  flowering 
responses,  but  the  data  at  hand  suggest  that  they  are  less  likely 
to  be  inductive.  That  is,  when  the  photoperiodic  conditions  are 
changed,  the  new  vegetative  growth  quickly  reflects  the  new  con- 
ditions. This  may  even  be  true  when  the  vegetative  change  would 
normally  be  associated  with  a  truly  inductive  effect  on  flowering. 
In  Murneek's  work  on  Rudbeckia  bicolor,  for  example,  continuous 
treatment  with  long  days  (longer  than  12  hours)  caused  both 
flowering  and  stem  elongation.  Exposure  to  only  25  long  days  still 
brought  about  flowering,  both  normal  and  abnormal,  but  the  plants 
remained  in  a  semirosette  stage. 

Many  papers  on  responses  of  all  types  make  it  difficult  to  decide 
whether  they   are   truly   photoperiodic  or  not.   Paradoxically,   this 


Literature    •    29 

is  more  often  true  in  very  recent  research,  since  air  conditioning 
now  makes  it  possible  to  grow  plants  entirely  under  high  intensi- 
ties of  artificial  light.  This  frequently  results  in  comparisons  between 
plants  grown,  for  example,  in  8  and  16  hours  of  light  per  day, 
comparisons  with  the  implicit  or  explicit  assumption  that  the 
operative  difference  between  treatments  is  in  light  duration,  even 
though  the  total  light  energies  also  differ  proportionately  (see, 
for  example,  Galston  and  Kaur,  1961;  also  portions  of  Went,  1957). 
It  would  help  clarify  the  literature  if  the  term  photoperiodic  were 
properly  restricted  to  effects  that  have  been  concurrently  or  pre- 
viously shown  to  be  controlled  by  light  and  dark  duration  and 
timing,  as  indicated  by  light-breaks  or  low-intensity  supplementary 
illumination.  Any  other  use  of  the  term  only  results  in  confounding 
photoperiodism  with  the  effects  of  greatly  increased  or  decreased 
photosynthesis,  or  other  light  actions. 

LITERATURE 

The  literature  on  photoperiodism  is  vast.  Some  of  the  most  use- 
ful reviews  are  by  Lang  (1952),  Naylor  (1953),  Bonner  and  Liver- 
man  (1953),  Borthwick,  Hendricks,  and  Parker  (1956),  and  Door- 
enbos  and  Wellensiek  (1959).  A  volume  edited  by  the  late  R.  B. 
VVithrow  (1959)  contains  many  valuable  reviews  and  original  re- 
ports on  photoperiodism  and  related  phenomena  in  both  plants 
and  animals. 


► 
► 


chapter  three    t  Photoperiodism: 

Attempts  at  Analysis 


Faced  with  the  various  phenomena  of  the  previous  chapter, 
many  investigators  of  photoperiodism  have  naturally  tried  to  dis- 
cover characteristics  common  to  the  various  response  classes,  and 
particularly  to  look  for  indications  of  whatever  cellular  and  bio- 
chemical mechanisms  might  be  involved.  Two  major  lines  of  such 
research,  by  no  means  completely  separate,  are  the  subject  of  this 
chapter. 

A.    PHOTOPERIODISM    AND    LIGHT    QUALITY 

So  far,  photoperiodism  has  been  considered  simply  in  terms 
of  white  light  versus  darkness,  but  experiments  with  light  quality— 
different  colors  or  wavelengths  of  light— have  proved  very  valuable. 
They  have  opened  up  photoperiodism  itself  to  further  manipula- 
tion and  linked  it  to  a  biochemical  system,  still  incompletely 
known,  that  is  probably  universal  among  plants  except  perhaps  for 
the  bacteria  and  fungi.  The  main  point  of  departure  for  this  work 
was  the  effectiveness  of  relatively  brief,  low-energy  "light-breaks" 
in  opposing  the  flower-promoting  or  flower-inhibiting  (for  LDP) 
effects  of  appropriate  dark  periods. 

ACTION  SPECTRA  FOR  LIGHT-BREAKS 

In  order  to  act  on  any  process,  light  must  first  be  absorbed. 
Compounds,  called  pigments,  that  absorb  visible  light  are  generally 

30 


Photoperiodism  and  Light  Quality    •    31 

complex  organic  compounds,  although  many  inorganic  salts  are 
highly  colored.  The  absorption  spectrum  of  a  given  pigment,  by 
which  is  meant  a  curve  indicating  the  relative  degree  to  which  it 
absorbs  various  wavelengths  of  light,  is  characteristic  of  that  com- 
pound alone,  or  at  least  of  a  small  class  of  similar  substances.  Thus 


Fig.  3-1.  Method  of  holding  single  leaves  (these  are  soybean  leaflets)  in  the 
image  plane  of  a  spectrograph  for  subsequent  irradiation  with  various  wave- 
lengths of  light.  (Photograph  from  Hendricks  and  Borthwick,  Proc.  First  Int. 
Photobiol.  Cong.  [1954],  courtesy  of  Dr.  H.  A.  Borthwick,  U.  S.  Department  of 
Agriculture.) 


the  action  spectrum  for  any  process  affected  by  light— a  curve 
indicating  the  relative  effectiveness  of  different  wavelengths  on  the 
process— may  provide  information  as  to  the  nature  of  the  com- 
pound or  compounds  by  which  the  light  is  absorbed.  For  example, 
part  of  the  evidence  for  the  role  of  chlorophyll  in  photosynthesis 
is  the  observation  that  the  light  most  active  in  that  process— blue, 


32    •    Photoperiodism:  Attempts  at  Analysis 

wavelengths  4000-4500  A  (Angstrom  units),  and  red,  6200-6800 
A— is  also  the  light  most  strongly  absorbed  by  chlorophyll  solutions. 
That  is,  the  action  spectrum  for  photosynthesis  resembles  the  ab- 
sorption spectrum  of  chlorophyll  solutions. 

In  principle,  this  seems  simple  enough;  in  fact,  the  accurate 
determination  and  evaluation  of  absorption  and  action  spectra  is  a 
complex,  still-developing  branch  of  physics  and  chemistry,  as  well 
as  biology;  for  some  references,  see  articles  in  Hollaender  (1956) 
and  Withrow  (1959).  For  present  purposes,  however,  it  should  be 
evident  that  the  action  spectra  for  light-break  elfects  in  various 
plants  might  indicate  whether  or  not  these  effects  are  mediated  by 
the  same  pigment  and  what  that  pigment  might  be. 

Much  of  the  work  on  this  question  has  been  done  by  Garner 
and  Allard's  successors,  a  group  at  the  U.S.  Department  of  Agricul- 
ture, Beltsville,  Maryland,  and  many  reviews  by  the  original 
workers  are  in  the  literature  (see,  for  example,  Borthwick,  Hen- 
dricks, and  Parker,  1956;  Borthwick,  1959;  Hendricks,  1958,  1959). 
Their  procedures  are  basically  simple,  though  not  technically  easy. 
Stating  the  situation  more  quantitatively  than  before,  an  action 
spectrum  can  be  represented  either  as  a  graph  of  varying  responses 
brought  about  by  equal  energies  of  light  of  given  wavelengths,  or 
as  a  graph  of  the  energy  which  must  be  given  at  each  wavelength 
to  cause  a  particular  degree  of  response.  Thus  it  is  necessary  to 
measure  the  effect  of  each  wavelength  chosen  at  several  energv 
levels,  and  on  a  considerable  number  of  plants;  this  requires  light 
of  considerable  intensities  but  in  relatively  pure  wavelength  bands 
spread  out  over  considerable  areas.  For  this  purpose,  the  Beltsville 
group  built  a  large  spectrograph,  in  which  high-intensity  white 
light  could  be  passed  through  a  prism  and  projected  as  a  spectrum. 
They  then  took  advantage  of  the  fact  that  in  the  plants  chosen 
photoperiodic  treatments  need  only  be  given  to  a  single  leaf  if 
the  other  leaves  were  removed.  The  single  leaf  could  be  placed  so 
as  to  receive  light  of  a  particular  color  and  energy  at  the  optimal 
time  for  dark  period  interruptions;  after  main  such  experiments, 
the  relative  effectiveness  of  the  various  colors  can  be  calculated. 
(See  Figs.  3-1  and  3-2.) 

From  1946  on,  action  spectra  for  light-break  responses  were 
obtained  in  both  SDP  and  LDP,  including  Xanthium,  Biloxi  soy- 
bean,   Hyoscya/nu.s.    and    Wintex    barley.    All    these   spectra    seem 


Photoperiodism  and  Light  Quality 


33 


substantially  alike;  the  most  effective  wavelengths  are  in  the 
orange-red  range,  6000-6800,  with  a  maximum  at  6400-6600  and 
a  steep  drop  beyond  6800  A.  Blue  light  is  much  less  effective  and 
green  is  almost  completely  ineffective.  Such  results  indicated  that 


Fig.  3-2.  Effects  of  various  amounts  of  light  given  as  dark-period  interruptions 
on  inflorescence  primordium  development  in  the  LDP  barley  (Hordeum  vulgare 
var.  Wintex).  Three-week-old  plants  were  grown  for  9  days  with  12  H-hour 
dark  periods  interrupted  in  the  middle  with  various  energies  of  light,  then 
allowed  to  grow  for  19  days  with  uninterrupted  dark  periods.  These  dissections 
show  the  apices  greatly  magnified ;  that  at  the  far  right  was  about  3  mm  high. 
Relative  energies  used  for  the  night  interruptions  ranged  from  none  (extreme 
left)  through  25  (middle)  to  400  (extreme  right)  foot-candle  minutes  of  white 
light.  The  study  of  similarly  graded  responses  to  various  energies  at  various 
wavelengths  indicated  the  effectiveness  of  the  wavelengths  tested.  (Photograph 
from  Borthwick,  Hendricks,  and  Parker  [1948],  Bot.  Gaz.,  110:  103-1 18,  courtesy 
of  Dr.  H.  A.  Borthwick,  U.  S.  Department  of  Agriculture.) 


light-breaks  inhibiting  the  flowering  of  SDP  were  probably  ab- 
sorbed by  the  same  pigment  as  those  promoting  flowering  in  LDP. 
The  nature  of  the  pigment  remained  a  subject  of  speculation  since 
no  known  pigment  in  higher  plants  had  an  absorption  spectrum 
with  a  peak  only  in  the  red  region.  Further  information  came  from 
outside  photoperiodism  proper,  and  it  is  therefore  necessary  to 
digress. 


34    •    Photoperiodism:  Attempts  at  Analysis 


THE  RED,  FAR-RED  REVERSIBLE  SYSTEM 

It  had  been  known  for  a  long  time,  in  a  general  way,  that  the 
germination  of  many  seeds  was  affected  by  light.  Flint  and  Mc- 
Alister  (1935,  1937)  had  found  that  the  germination  of  lettuce, 
Lactuca  sativa,  was  promoted  by  red  light.  If  seeds  previously  ex- 
posed to  enough  red  to  cause  subsequent  germination  were  exposed 
to  either  blue  or  near-infrared  (7000-8000  A)  light,  germination  was 
inhibited.  This  work  was  taken  up  again  by  the  Beltsville  group 
(Borthwick  et  al.,  1952a,  1954).  They  determined  an  action  spec- 
trum for  the  promotion  by  red,  which  showed  maximum  activity 
at  about  6500  A  and  resembled  the  light-break  action  spectra,  and 
also  an  action  spectrum  for  the  infrared  (now  called  far-red)  inhibi- 
tion, which  showed  a  maximum  around  7350  A.  More  important, 
however,  were  observations  leading  them  to  postulate  the  existence 
of  what  is  now  known  as  the  red,  far-red  reversible  pigment  system. 

Some  data  taken  from  the  1954  paper  illustrate  what  is  meant 
by  red,  far-red  reversibility.  Groups  of  lettuce  seeds  were  allowed 
to  imbibe  water  in  darkness  at  20°  C  for  three  hours,  subjected  to 
various  brief  light  treatments,  then  kept  in  darkness  at  20°  C  for 
two  days,  after  which  the  number  germinating  in  each  lot  was 
counted.  The  light  treatments  were  either  1  minute  of  red  (R)  or 
4  minutes  of  far-red  (F)  at  previously  established  intensities,  or 
combinations  of  these  in  immediate  succession.  In  typical  results, 
treatment  R  alone  caused  70  percent  germination,  and  the  treat- 
ment RF  (red  followed  immediately  by  far-red)  caused  7  percent, 
almost  the  same  as  germination  in  darkness.  Such  alternations 
could  be  carried  much  further:  the  treatment  RF,  RF,  RF,  R  gave 
81  percent,  and  the  treatment  RF,  RF,  RF,  RF,  7  percent  again. 
The  germination  depended  simply  on  whether  R  or  F  was  given 
last,  as  if  a  switch  were  thrown  one  way  or  the  other  by  the 
different  radiations.  Any  red  effect  was  reversed  by  far-red  given 
immediately  after,  and  vice  versa.  Similar  results  could  be  obtained 
even  when  the  seeds  were  chilled  to  6°  C  during  the  period  of  light 
treatments.  This  temperature-independence  and  a  number  of  other 
observations  led  to  the  suggestion  that  the  two  opposed  light 
effects  might  be  mediated  by  the  same  pigment.  The  basic  assump- 
tion is  that  the  pigment  can  exist  in  two  forms,  a  red-absorbing 


Photoperiodism  and  Light  Quality    •    35 

form  (or  form  with  greater  red  than  far-red  absorption)  and  a 
far-red-absorbing  form.  These  two  forms,  call  them  PR  and  PF, 
would  be  photochemically  interconvertible,  thus: 

red  light 

Pr  -v  —  I  p» 

far-red  light 

and  the  final  physiological  result  would  then  depend  on  whatever 
form  remained  after  the  last  illumination,  or  on  the  ratio  of  the 
two. 

THE  RED,  FAR-RED  SYSTEM  IN  PHOTOPERIODISM 

Evidence  for  the  red,  far-red  reversibility  of  photoperiodic 
light-breaks  was  presented  first  by  Borthwick  et  al.  (1952b),  using 
Xanthium.  Following  this,  Downs  (1956)  showed  that  the  effects  of 
light-breaks  were  also  far-red  reversible  in  the  LDP  Hyoscyamus 
niger  and  Wintex  barley  and  the  SDP  Amarantlnis  caudatus  and 
Biloxi  soybean,  and  was  able  to  demonstrate  repeated  reversibility, 
like  that  in  lettuce  seeds,  in  both  Xanthium  and  soybeans.  A  more 
concrete  account  of  some  of  these  results  may  be  illustrative  at  this 
point. 

By  this  time,  simpler  light  sources  than  the  spectrograph  had 
been  developed.  The  red  source  was  simply  white  fluorescent  light 
(about  1000  foot  candles  at  plant  level)  with  an  interposed  filter  of 
two  sheets  of  red  cellophane.  Far-red  was  obtained  by  filtering 
either  sunlight  (8000  foot  candles)  or  incandescent  light  (800  foot 
candles)— both  rich  in  far-red  compared  to  fluorescent  light- 
through  two  layers  each  of  red  and  blue  cellophane.  These  cut  out 
almost  all  radiation  of  wavelengths  shorter  than  7000  A  but  allow 
far-red  to  pass.  Using  these  sources,  Downs  then  conducted  a  more 
detailed  investigation  of  the  time  and  energy  relations  of  these 
effects  on  Xanthium.  Groups  of  plants  were  given  various  experi- 
mental treatments  for  three  24-hour  cycles.  They  were  all  then 
placed  under  noninductive  long-day  conditions  and  allowed  to 
develop  for  some  days,  after  which  the  flowering  response  was 
scored  as  an  inflorescence-stage  index  from  0  (vegetative)  to  7 
(maximum  response). 

The  effect  of  red  light  in  the  middle  of  each  dark  period  of 
three  successive  12  hours  light-] 2  hours  dark  cycles  was  propor- 
tional to  the  duration  of  exposure,  that  is,  to  total  energy  given. 


36    •    Photoperiodism:   Attempts  at  Analysis 

Uninterrupted  controls  had  a  mean  flowering  stage  of  6.0;  10 
seconds  red  gave  a  value  of  about  4.8,  20  seconds  brought  it  to 
about  2.5,  and  30  seconds,  to  0.  One  minute  of  sun-source  far-red 
was  sufficient  to  reverse  the  effects  of  two  minutes  of  red  if  given 
immediately  after,  returning  the  value  to  6,  but  twelve  minutes  of 
far-red  brought  it  down  again  to  nearly  4;  such  "overreversals,"  in 
which  long  exposures  to  far-red  act  more  like  red,  occur  in  other 
plants  as  well,  and  will  be  discussed  later. 

Downs  next  studied  the  effect  of  interposing  a  brief  period  be- 
tween the  red  and  far-red  treatments.  In  one  experiment,  far-red 
immediately  after  red  gave  a  value  of  6.5  compared  with  the  un- 
interrupted controls  of  7.0.  With  a  20-minute  dark  period  before 
the  same  far-red  treatment,  the  value  was  only  3.8,  and  with  a 
40-minute  dark  period,  0.5.  Thus  the  far-red  treatment  had  to  be 
given  soon  after  the  red  to  be  effective;  the  simplest  explanation  is 
that  when  most  of  the  pigment  is  in  the  far-red-absorbing  form 
(after  the  red),  a  series  of  reactions  inhibitory  to  induction  is  started 
and  reaches  such  a  stage  after  40  minutes  that  even  changing  the 
pigment  will  no  longer  change  the  result.  If  the  plants  are  held  at 
5°  C  during  the  intervening  dark  period,  this  "escape  from  photo- 
chemical control"  occurs  much  more  slowly.  With  a  40-minute 
dark  period,  for  example,  the  red  effect  was  still  almost  completely 
reversible  at  this  temperature,  precisely  as  would  be  expected  under 
the  explanation  given.  The  escape  from  photochemical  control  also 
explains  wThy,  under  ordinary  conditions,  repeated  reversals  cannot 
be  carried  on  indefinitely  and  the  red  effect  eventually  predomi- 
nates. 

Downs's  results  typify  the  kind  of  control  exerted  by  the  red, 
far-red  system  in  photoperiodism,  but  by  no  means  exhaust  the 
subject.  Evidence  was  obtained,  first  in  lettuce  seed  (Borthwick 
et  al.,  1952a)  and  later  elsewhere,  that  the  conversion  from  the 
far-red-absorbing  to  the  red-absorbing  form  takes  place  not  only  on 
exposure  to  far-red  but  also,  more  slowly,  in  darkness  by  some 
thermal  (temperature-dependent)  process.  This  revises  the  relation 
previously  written  to: 

red 


far-red   PF. 

^dark,  thermal"" 


Photoperiodism  and  Light  Quality    •    37 

Certain  data  on  flowering  further  suggested  that  this  dark  conver- 
sion might  determine  the  length  of  the  critical  dark  period.  Borth- 
wick  et  al.  (1952b)  reported  that  if  Xanthium  plants  were  given  a 
brief  far-red  exposure  at  the  beginning  of  a  dark  period  (end  of 
the  high-intensity  white  light),  less  than  7  hours  of  darkness  were 
required  for  induction.  If  they  received  a  brief  red  treatment 
instead,  9  hours  of  darkness  were  required,  compared  with  the 
Sy2  sufficient  with  no  treatment  after  the  white  light.  Downs  (1959) 
has  also  shown  that  the  quantitative  SDP  millet,  Setaria  italica, 
which  flowers  rapidly  with  12-hour  nights  but  very  slowly  with 
8-hour  nights,  will  also  flower  rapidly  with  8-hour  nights  if  a  brief 
far-red  treatment  is  given  at  the  beginning  of  each.  This  far-red 
promotion  of  flowering  is  reversed  by  red,  and  red  alone  has  no 
effect  at  the  start  of  the  dark  periods.  (See  Fig.  3-3.) 

At  this  point  one  may  well  wish  for  the  solace  of  a  theory 
unifying  all  these  data.  Such  a  theory  exists  (see  Borthwick,  Hen- 
dricks, and  Parker,  1956)  and  can  be  briefly  summarized.  At  the 
end  of  a  long  white-light  period,  the  pigment  is  almost  completely 
in  the  far-red-absorbing  form;  evidence  for  this  is  that  red  given 
then  has  little  or  no  effect,  and  far-red  a  much  larger  one.  It  is 
this  far-red-absorbing  form  that  brings  about  the  inhibition  of 
induction  in  SDP  and  the  promotion  of  induction  in  LDP.  Thus 
SDP  require  a  dark  period  long  enough  to  allow  thermal  conver- 
sion of  the  far-red-absorbing  form  and  its  continued  absence  for 
some  time,  whereas  LDP  are  inhibited  by  too  long  a  dark  period 
since  this  conversion  and  absence  are  unfavorable.  Hence  red  (or 
white)  light-breaks  inhibit  SDP  induction  and  promote  LDP 
induction  by  returning  the  pigment  to  the  far-red-absorbing  form. 
This  theory  takes  into  account  all  the  data  so  far  presented,  and 
even  fits  the  observation  (Chapter  Two)  that  the  dark  period  for 
Xanthium  has  to  be  longer  if  the  temperature  is  lowered,  since 
thermal  conversion  to  the  red-absorbing  form  will  be  slowed.  The 
only  difficulty  is  that  it  does  not  fit  the  equally  valid  data  to  be 
considered  next. 

According  to  the  theory,  far-red  given  to  LDP  at  the  start  of  a 
dark  period  barely  short  enough  to  allow  induction  should  inhibit 
induction.  Yet  in  at  least  two  LDP,  Hyoscyamus  and  dill,  it  pro- 
moted induction.  Also,  flowering  in  the  SDP  Chrysanthemum 
morifolium  is  not  promoted  by  far-red  at  the  start  of  the  dark 


38 


Photoperiodism:  Attempts  at  Analysis 


period,  as  it  is  in  Xanthium  and  millet  (see  Borthwick,  1959).  Still 
more  complicated,  yet  confirmed  now  by  the  Beltsville  group  whose 
theory  it  confounds,  is  the  response  of  the  Japanese  morning  glory, 
Pharbitis  nil. 


Fig.  3-3.  Effect  of  far-red  supplement  at  the  end  of  the  light  period  on  the  SDP 
millet  (Setaria  italica).  All  plants  were  grown  with  16  hours  of  light;  at  the  end 
of  each  light  period  the  following  treatments  were  given,  represented  by  the 
plants  from  left  to  right:  no  further  radiation;  five  minutes  of  far-red;  five 
minutes  of  far-red  followed  by  five  minutes  of  red.  (Photograph  from  Downs 
[1959],  by  permission  of  the  American  Association  for  the  Advancement  of 
Science,  and  courtesy  of  Drs.  R.  J.  Downs  and  H.  A.  Borthwick,  U.  S.  Depart- 
ment of  Agriculture.) 


Pharbitis  seedlings  grown  at  about  26°  C  can  be  induced  to 
flower  by  one  or  more  16-hour  dark  periods,  and  red  light-breaks 
(perceived  by  the  cotyledons)  8  or  10  hours  after  the  start  of  the 
dark  period  completely  inhibit  induction.  This  is  a  typical  SDP 
response.  But  the  effects  of  red  light  are  not  reversed  by  far-red; 
far-red  itself  inhibits  flowering  when  given  during  the  dark  period. 
Far-red  even  inhibits  when  given  at  the  start  of  the  dark  period, 
and  this  effect  is  reversed  by  red.  Thus  the  red,  far-red  reversible 
system  is  present  and  active,  but  in  a  way  unlike  that  suggested  by 


Photoperiodism  and  Light  Quality    •    39 

the  theory  (Nakayama,  1958).  However,  all  this  is  true  only  when 
the  cotyledons  are  the  light-responsive  organs.  Older  plants,  in 
which  the  true  leaves  perceive  the  light,  respond  in  the  same  way 
as  Xanthium  (Nakayama,  Borthwick,  and  Hendricks,  1960).  These 
observations  provide  an  opportunity  for  studying  the  precise  ways 
in  which  the  red,  far-red  system  may  be  linked  to  flowering,  if  the 
operative  differences  between  the  cotyledons  and  the  true  leaves 
can  be  discovered. 

A  point  requiring  further  comment  is  that  white  light  acts 
more  or  less  like  red.  This  is  not  surprising  for  fluorescent  light 
sources  since  their  far-red  emission  is  very  low,  but  both  incandes- 
cent light  and  sunlight  have  a  high  proportion  of  far-red.  Their 
action  as  red  light  is  probably  due  in  part  to  the  proportion  of  red 
to  far-red,  in  part  to  the  relative  sensitivities  of  the  two  forms,  and 
also  to  the  fact,  mentioned  previously,  that  prolonged  exposures  to 
far-red  may  have  an  action  more  like  red  than  short  exposures.  The 
latter  has  been  explained  (see  Borthwick,  1959)  as  being  due  to  the 
maintenance  of  a  small  amount  of  the  far-red-absorbing  form  in 
equilibrium  with  the  red-absorbing  form  during  far-red  radiation, 
since  the  absorption  spectra  of  the  two  forms  must  overlap.  Thus 
darkness  following  the  far-red  treatment  is  needed  to  allow  the 
conversion  to  the  red-absorbing  form  to  be  completed  by  the 
thermal  process.  It  is,  however,  not  strictly  true  that  all  white  light 
sources  are  equivalent  for  photoperiodism.  Fluorescent  and  in- 
candescent light  differ  considerably  in  their  effects  on  both  flower- 
ing and  vegetative  growth  when  used  to  lengthen  light  periods,  and 
the  differences  can  be  ascribed  to  the  different  far-red  emissions 
of  the  two  sources  (Downs,  1959;  Downs  et  al.,  1959). 

NATURE  AND  FUNCTION  OF  THE 
RED,  FAR-RED  PIGMENT 

Many  effects  of  low-intensity  red  light  on  plants  are  now  known 
to  be  reversible  by  far-red,  but  a  discussion  of  the  red,  far-red 
control  of  vegetative  growth— so-called  photomorphogenesis— would 
occupy  too  much  space  here.  References  to  the  abundant  literature 
on  it  are  to  be  found  in  most  reviews  on  photoperiodism;  a 
particularly  good  introduction  is  Withrow's  own  article  in  Withrow 
(1959).    Much   speculation   and   calculation   has   in    the   past   been 


40    •    Photoperiodism:  Attempts  at  Analysis 

devoted  to  the  possible  nature  and  metabolic  function  of  such  a 
reversible  pigment  system,  on  the  assumption,  of  course,  that  it 
existed  and  was  not  a  misinterpretation  of  two  separate  light 
effects.  The  assumption  has  since  been  justified,  and  the  specula- 
tions may  soon  give  way  to  data.  Workers  at  Beltsville  (Butler 
et  al.,  1959),  using  relatively  sophisticated  spectrophotometric 
techniques,  have  shown  that  intact  tissues  and  properly  prepared 
extracts  of  etiolated  (dark-grown)  seedlings  of  various  species,  such 
as  corn,  Zea  mays,  contain  a  pigment  with  the  predicted  reversible 
changes  in  absorption  characteristics  in  the  red  and  far-red.  The 
pigment  is  present  in  very  low  concentrations— the  etiolated  tissue 
in  which  it  was  observed  was  nearly  white— and  is  either  a  protein 
or  closely  bound  to  a  protein.  The  development  of  better  extrac- 
tion and  purification  techniques  should  soon  make  it  possible  to 
characterize  the  pigment  further  and  aid  in  establishing  its  imme- 
diate biochemical  function.  The  rapid  developments  which  should 
ensue  may  make  further  discussion  on  these  points  obsolete  when 
printed. 

Even  discovery  of  the  immediate  biochemical  function  of  the 
pigment,  no  easy  matter  in  itself,  will  not  completely  clarify  its 
role  in  photoperiodism.  Much  more  physiological  work  is  still 
required  on  this  question.  The  only  generalization  that  will  hold 
at  present  is  that  the  red,  far-red  system  mediates  the  low-intensity 
light  effects  and  may  also  be  involved  in  the  critical  time-require- 
ments. There  is  no  clear  evidence,  however,  as  to  the  precise  way 
in  which  the  pigment  is  linked  to  subsequent  events  in  the  induc- 
tion process,  and  the  relation  may  well  differ  from  species  to  species 
even  within  a  given  response  class. 

The  pigment  has  been  dubbed  "phytochrome"  by  its  dis- 
coverers (see  Borthwick  and  Hendricks,  1960).  Though  the  name 
is  unfortunate  both  because  it  is  general  (Greek  for  "plant"  plus 
"color"  or  "pigment")  and  because  it  is  liable  to  be  confused  when 
spoken  with  the  cytochromes,  so  significant  in  the  biochemistry  of 
respiration,  it  will  undoubtedly  be  perpetuated. 

PROLONGED  EXPOSURES  TO  LIGHT  OF 
DIFFERENT  COLORS 

In  the  1930's  and  1940's,  Funke  (see  Funke,  1948)  used  sunlight 
filtered  through  white,  red,  or  blue  glass  to  lengthen  photoperiods 


Photoperiodism  and  Light  Quality    •    41 

for  both  LDP  and  SDP.  Red  and  white  were  the  only  effective 
photoperiod-lengthening  conditions  for  many,  with  blue  equivalent 
to  darkness.  For  a  second  large  class,  both  red  and  blue  were  effec- 
tive, as  well  as  white.  For  a  third  very  small  class,  only  white  was 
effective,  but  neither  red  nor  blue.  Funke's  "Class  IV"  has  attracted 
the  most  interest;  these  were  all  of  the  Cruciferae  (Mustard  family) 
and  almost  all  LDP,  in  which  the  blue  and  white,  but  not  the  red, 
were  effective  in  lengthening  photoperiod. 

Since  Funke,  there  has  been  a  great  deal  of  work,  most  of  it  in 
the  Netherlands,  on  the  vegetative  development  and  flowering  of 
plants  grown  with  relatively  high  energies  (high  intensities,  long 
exposures,  or  both)  of  various  colors  of  light.  For  reviews  of  this 
work,  see  Wassink  and  Stolwijk  (1956),  Wassink  et  al.  (1959),  Meijer 
(1959),  and  Van  der  Veen  and  Meijer  (1959).  Although  many  inter- 
esting phenomena  have  been  observed,  such  work  is,  almost  without 
exception,  extremely  difficult  to  evaluate  for  at  least  two  reasons. 
First  is  the  immense  technical  difficulty  of  obtaining  high  energies 
of  light  in  pure  spectral  bands  and  over  large  enough  areas  to  grow 
groups  of  whole  plants.  Often  the  sources  have  been  more  or  less 
impure,  as  Funke's  must  have  been,  so  that  what  appear  to  be 
high-energy  effects  of  the  main  wavelength  region  may  include 
low-energy  effects  of  other  wavelengths.  Such  contaminations  have 
been  gradually  reduced  (see  Wassink  et  al.,  1959)  but  may  still  be 
present.  The  second  problem  is,  if  anything,  worse.  Consider,  for 
example,  the  effects  of  long  exposure  to  high-intensity  blue  light, 
no  matter  how  pure.  The  light  may  be  affecting  at  least  three  sys- 
tems simultaneously.  The  red,  far-red  system  itself  and  photosyn- 
thesis are  already  obvious,  but  one  must  also  consider  whatever 
pigments  mediate  phototropism— the  orientation  of  plant  parts  with 
respect  to  the  direction  of  light— since  blue  light  is  the  most  effec- 
tive in  this  process.  In  addition,  fluorescence  of  chlorophyll  and 
other  compounds  caused  by  the  blue  may  expose  the  cells  internally 
to  longer-wave  radiations.  The  difficulties  of  disentangling  such 
effects  and  reaching  satisfactory  interpretations  can  hardly  be  over- 
estimated. Nevertheless,  some  of  this  work  is  of  considerable 
interest. 

The  unexpected  promotion  of  Hyoscyamus  flowering  by  far- 
red  at  the  start  of  the  dark  period,  mentioned  above,  was  first 
reported  by  Stolwijk  and  Zeevaart  (1955)  who  also  observed  that  this 
LDP  entirely  failed  to  flower  when  grown  in  continuous  red  light, 


42    •    Photoperiodism:  Attempts  at  Analysis 

although  it  flowers  rapidly  in  continuous  white  light.  However, 
small  amounts  of  far-red  given  with  the  continuous  red  brought 
about  flowering,  as  did  also  blue  light.  Nine  hours  of  blue  once 
every  third  day  would  permit  flowering  under  otherwise  continuous 
red  light.  There  is  some  question  as  to  whether  the  slight  far-red 
contamination  in  the  blue  might  be  responsible  for  the  original 
effect  reported,  but  it  has  since  been  repeated  with  much  purer 
sources  (Wassink  et  al,  1959).  Thus,  in  Hyoscyamus,  blue  and 
far-red    may    be    physiologically    equivalent    for    flower    initiation. 

Meijer  (1959)  has  reported  a  number  of  complex  experiments 
on  flower  initiation  in  the  SDP  Salvia  occidentalis.  One  of  the 
most  interesting  results  is  that  a  standard  15-minute  red  light- 
break  during  an  inductive  dark  period  does  not  inhibit  flowering 
if  the  main  (8-hour)  light  period  is  of  red  or  green  light.  It  does 
inhibit,  however,  if  the  main  light  period  is  of  blue  (all  main 
light  periods  being  of  the  same  energy)  or  if  the  red  or  green 
periods  are  supplemented  with  far-red.  It  should  also  be  noted 
that  Salvia  occidentalis,  like  Perilla  crispa  (Chapter  Two)  will 
flower  even  in  continuous  white  light  of  sufficiently  low  intensities; 
at  higher  or  even  lower  intensities,  it  again  behaves  like  a  proper 
SDP  by  failing  to  flower.  Even  more  complex  work  on  Hyoscyamus 
has  been  recently  reported  by  De  Lint  (1960),  to  whose  extensive 
work  the  reader  should  go  for  further  details. 

Work  of  this  kind  has  certainly  indicated  that  light  quality 
and  intensity  have  more  effects  on  flower  initiation  and  other 
aspects  of  development  than  can  readily  be  explained  through 
what  is  known  of  the  red,  far-red  system  at  present.  Unfortunately, 
even  the  effects  of  blue  on  this  particular  system  are  not  under- 
stood; there  is  evidence  that,  in  various  organisms,  blue  (at  high 
energies)  may  act  like  either  red  or  far-red.  Whether  this  is  a 
direct  action  on  the  red,  far-red  reversible  pigment  itself  or  an 
indirect  one,  through  other  pigments  or  metabolic  systems,  is  un- 
certain. Due  to  the  difficulties,  already  mentioned,  of  interpreting 
such  studies,  the  only  suggestions  at  present  are  purely  speculative. 

B.    TIME    RELATIONS    AND    ENDOGENOUS 
RHYTHMS    IN    PHOTOPERIODISM 

The  characteristic  defining  aspect  of  photoperiodism  is  the 
importance  of  the  time  relations  of  light  and  dark  conditions.  The 


Time  Relations  and  Endogenous  Rhythms    •    43 

response  to  this  timing  is  sometimes  surprisingly  precise;  Xanthium 
can  distinguish  clearly  between  a  dark  period  of  8  hours  (non- 
inductive)  and  one  of  8  hours,  40  minutes  (inductive)  (Long,  1939). 
On  the  reasonable  assumption  that  the  main  survival  value  of 
photoperiodism  in  an  organism  is  in  the  seasonal  timing  of  devel- 
opment that  it  affords,  Withrow  (1959)  has  calculated  that  to  be 
accurate,  the  timing  mechanism  must  detect  daylength  differences 
of  14  to  44  minutes  within  a  week  in  temperate  latitudes.  In  addi- 
tion, it  should  be  relatively  insensitive  to  random  changes  in  light 
intensity  and  temperature  brought  about  by  local  weather.  Insensi- 
tivity  to  intensity  changes  is  provided  by  the  fact  that  low  intensi- 
ties are  sufficient  to  bring  about  most  photoperiodic  responses,  but 
insensitivity  to  temperature  is  more  difficult  to  understand. 
Although  both  the  accuracy  and  the  temperature-insensitivity  (see 
Chapter  Two)  of  the  photoperiodic  control  of  flowering  are,  in  the 
writer's  opinion,  often  exaggerated,  it  is  true  that  certain  aspects 
of  photoperiodism  are  less  temperature-sensitive  than  most  plant 
processes. 

The  effects  of  low  temperature  in  lengthening  the  critical  dark 
period  in  Xanthium,  discussed  earlier,  indicate  that  a  drop  of 
about  16°  C  increased  the  dark  period  required  by  only  about  3 
hours,  or  less  than  40  percent  (Long,  1939).  This  contrasts  with  the 
general  observation  that  the  rates  of  most  ordinary  chemical  re- 
actions, and  thus  of  growth  or  other  processes  in  most  biological 
systems,  are  at  least  doubled  by  a  10°  C  rise  in  temperature  within 
a  fairly  wide  range.  If  the  series  of  events  constituting  the  dark 
period  "timing  mechanism"  in  Xanthium  responded  in  this  fashion, 
one  would  expect  the  16°  drop  in  temperature  to  bring  about  at 
least  a  20-  or  24-hour  dark  requirement,  but  it  does  not.  This  and 
similar  evidence,  although  there  is  not  a  great  deal  of  it,  suggest 
that  the  photoperodic  timing  mechanism  is  not  a  simple  linear 
series  of  ordinary  reactions,  but  may  be  more  complex. 

Neither  timing  nor  temperature-insensitivity  are  peculiar  to 
photoperiodism.  In  mammals  and  birds,  of  course,  a  self-regulated 
temperature  could  obviously  permit  the  accurate  timing  of  re- 
sponses and  metabolic  events  by  simple  chemical  means  alone,  but 
it  is  now  well  established  that  probably  all  plants  and  animals- 
even  unicells,  excluding  perhaps  the  bacteria— have  accurate  timing 
mechanisms  that  are  temperature-insensitive,  more  so,  in  fact,  than 
most  photoperiodic  phenomena.   Several  groups  of  workers  have 


44    •    Photoperiodism:  Attempts  at  Analysis 

thus  suggested  that  photoperiodism,  in  both  plants  and  animals, 
is  merely  a  special  case  of  a  general  rhythmic  mechanism  by  which 
all  organisms  can  register  the  passage  of  time. 

ENDOGENOUS  CIRCADIAN  RHYTHMS  IN  PLANTS 

Most  of  the  recent  data  on  rhythmic  processes  in  higher  plants 
have  come  either  from  Erwin  Biinning  and  his  co-workers  in  Ger- 
many or  from  work  done  elsewhere  to  test  their  hypotheses.  Biin- 
ning's  concepts  (see  Biinning,  1956,  1959)  have  developed  from  a 
number  of  basic  observations,  some  antedating  his  own  work. 

Most  plant  processes  exhibit  a  diurnal  rhythm  in  phase  with 
the  daily  alternations  of  light  and  darkness.  This  rhythm  is  not 
simply  a  passive  response  to  external  conditions  since  as  expressed 
in  various  processes— the  nocturnal  "sleep"  movements  of  legume 
leaves,  for  example— it  persists  for  at  least  a  few  days  after  the 
plants  are  placed  in  a  constant-temperature  dark  room.  In  fact, 
periodic  light-dark  alternations  are  not  necessary  to  initiate  such  a 
rhythm.  The  classic  example  is  the  behavior  of  bean,  Phaseolus, 
seedlings  germinated  and  grown  in  constant-temperature  darkness. 
The  movements  of  the  young  leaves,  which  can  be  recorded  with  a 
suitable  apparatus,  are  small,  more  or  less  random,  and  unsynchro- 
nized  among  the  population  of  seedlings.  After  a  single  flash  of 
light  the  movements  become  larger,  synchronized  among  all  the 
seedlings,  and  exhibit  a  marked  periodicity,  with  the  leaves  return- 
ing to  the  same  position  about  once  every  24  hours.  The  move- 
ments become  weaker  after  several  days  and  finally  die  out,  but 
maintain  their  periodicity  until  they  do.  In  Biinning's  view,  such 
results  provide  evidence  of  "endogenous  daily  rhythms"  in  plants. 

By  "endogenous"  Biinning  means  that  the  period,  or  length 
of  a  complete  oscillation  in  such  rhythms,  is  determined  by  the 
plant  and  not  imposed  by  external  conditions.  There  are  at  least 
three  kinds  of  evidence  for  this  in  experiments  with  the  leaf  move- 
ments of  bean  seedlings.  First,  of  course,  the  movements  are  evoked 
by  a  single  exposure  to  light,  not  by  a  repeated  light-dark  schedule. 
Second,  the  phase  of  the  rhythm— as  indicated  by  the  position  of  a 
leaf  at  any  given  time— is  not  affected  by  the  solar  time  of  day,  but 
depends  only  on  the  time  at  which  the  light  flash  was  given.  A 
group  of  plants  given  a  flash  12  hours  before  a  second  group  will 


Time  Relations  and  Endogenous  Rhythms    •    45 

show  movements  12  hours  out  of  phase  with  the  second  group. 
Finally,  and  perhaps  most  important,  the  rhythm  of  such  move- 
ments is  not  exactly  daily,  not  precisely  24  hours  long.  It  may  be 
from  20  to  30  hours;  different  varieties  have  rhythms  with  char- 
acteristic period-lengths,  so  that  this  is  a  genetically  controlled  and 
thus  endogenous  property.  The  term  "circadian"  (Latin:  circa, 
about,  and  dies,  day)  has  been  coined  for  such  rhythms  with  period- 
lengths  of  close  to  24  hours. 

The  relation  of  the  bean  circadian  rhythm  to  temperature  is 
shown  by  data  from  Biinning  (1959a).  In  darkness  (after  a  light 
flash)  the  period  is  28.3  hours  at  constant  15°  C  and  28.0  hours  at 
constant  25°  C.  Thus  a  10°  difference  in  ambient  temperature  has 
no  effect.  However,  a  change  in  temperature  does  have  an  effect. 
Seedlings  moved  from  20°  to  15°  had  a  period  of  29.7  hours,  and 
those  moved  from  20°  to  25°  had  a  period  of  23.7  hours,  for  the 
first  day  or  so  after  a  change.  Later,  compensation  occurred  and 
the  periods  in  the  two  temperatures  became  similar.  Thus  it  is  not 
strictly  true  to  call  such  circadian  rhythms  temperature-insensitive, 
but  they  are  clearly  temperature-compensated  and  arrive  at  the 
same  period  in  different  constant  temperatures. 

In  general,  the  phase  and  amplitude  of  circadian  rhythms  in 
various  organisms  are  greatly  affected  by  the  environment  but  the 
basic  period-length  can  only  be  changed  within  narrow  limits.  An 
organism  with  a  rhythm  of  20  or  30  hours  will  adapt  its  period  to 
a  normal  24-hour  day,  but  may  either  revert  to  its  endogenous 
rhythm  or  exhibit  highly  disorganized  activity  under  light-dark 
cycles  totaling  12  hours  in  length.  Not  only  light  flashes  but  transi- 
tions from  light  to  darkness  and  abrupt  temperature  shocks  as  well 
can  reset  the  phase  or  initiate  circadian  rhythms,  but  it  seems  clear 
that  they  do  not  cause  them. 

Many  processes  in  an  organism  generally  exhibit  the  same  cir- 
cadian rhythm,  probably  manifesting  the  activity  of  a  single  "clock" 
mechanism.  This  "clock"  may  be  a.  basic  property  of  the  organiza- 
tion of  most  cells  or  a  particular  unknown  process,  but  there  is  no 
general  agreement  even  as  to  its  possible  nature.  A  major  investi- 
gator (Brown,  1959)  has  recently  abandoned  the  hypothesis  of  a 
completely  endogenous  origin,  and  suggests  that  organisms  may 
register  the  passage  of  time  by  perceiving  certain  unknown  geo- 
physical periodicities,  although  the  way  in  which  such  an  exogenous 


46    •    Photoperiodism:  Attempts  at  Analysis 

clock  may  be  used  would  still  vary  greatly  from  organism  to  organ- 
ism. Most  other  workers,  however,  consider  the  clock  truly  endog- 
enous. For  summaries  of  the  state  of  this  field  with  particular 
reference  to  animals  and  microorganisms,  see  Pittendrigh  and 
Bruce  (1959)  and  Brown  (1959);  a  recent  symposium  also  covers  the 
field  in  great  detail  (Biological  Clocks,  1960).  Only  experiments 
directly  concerned  with  photoperiodism  and  flowering  will  be  con- 
sidered below. 

ENDOGENOUS  CIRCADIAN  RHYTHMS 
AS  THE  BASIS  OF  PHOTOPERIODISM 

In  the  view  of  Bunning  and  co-workers,  the  endogenous 
circadian  rhythm  of  plants  passes  through  two  phases  of  more  or 
less  opposite  sensitivity  to  light:  a  "photophile"  (light-liking)  phase 
in  which  development  is  favored  by  light  and  a  "scotophile"  (dark- 
liking)  phase  in  which  light  is  unfavorable.  These  phases  are  said 
to  be  distinguishable  by  leaf  movements  as  well  as  by  differences 
in  rates  of  respiration,  photosynthesis,  cell  division,  and  other 
processes.  As  phases  of  a  circadian  rhythm  they  are  affected  but  not 
caused  by  light-dark  alternations;  they  are  the  means  by  which 
the  plant  can  ''time"  the  light  or  dark  exposures  it  receives.  A 
particular  version  of  this  view,  now  considerably  modified  by 
Bunning  (1948,  1959b),  has  provided  the  stimulus  for  much  of  the 
work  on  the  problem.  It  relates  SDP  and  LDP  specifically  by  pro- 
posing that  in  both  types  each  phase  of  the  rhythm  is  about  12 
hours  long,  but  whereas  in  SDP  the  photophile  normally  starts 
soon  after  illumination,  in  LDP  it  starts  only  some  8  to  12  hours 
after  the  start  of  light.  Thus  long  photoperiods  give  the  SDP 
excessive  light  in  its  scotophile,  whereas  short  photoperiods  give 
LDP  most  of  the  light  in  the  scotophile  and  little  in  the  photophile. 

An  example  of  the  kind  of  evidence  supporting  this  proposal 
is  from  Bunning  and  Kemmler  (1954).  They  found  that  flowering 
in  the  LDP  dill  occurred  on  a  daily  schedule  of  17%  hours  light- 
6*4  hours  darkness,  but  was  more  rapid  if  a  2-hour  dark  period 
was  given  3  hours  after  the  start  of  each  main  light  period  (making 
the  schedule  3  hours  light-2  hours  dark-12%  hours  light-61/.  hours 
dark).  This  observation  is  consistent  with  the  idea  that  dill  has  a 
scotophile   phase   that  occurs  shortly  after   the  start  of   the  main 


Time  Relations  and  Endogenous  Rhythms    •    47 

light  period,  and  thus  darkness  during  this  time  promotes  flower- 
ing. However,  the  effect  was  not  detected  in  the  LDP  Plantago  and 
spinach. 

Evidence    has    also    been    obtained    from    leaf   movements,    a 
particularly    impressive    case   being   that   of   Madia   elegans.    This 
desert  composite  was  first  studied  by  Lewis  and  Went  (1945)  who 
found  that  it  flowered  rapidly  with  8,  18,  or  24  hours  of  light  per 
day,  but  slowly  with  12  or  14  hours  of  light.  This  unusual  bimodal 
sensitivity,  with  intermediate  daylengths  less  effective  than  long  or 
short,  is  apparently  reflected  in  the  leaf  movements.  Bunning  (1951) 
was  able  to  show  that  these  movements  corresponded  to  what  his 
hypothesis  would  predict  for  a  plant  with  two  photophile  phases 
within  each  circadian  period,  and  he  explained  the  peculiar  photo- 
periodic   response    on    this    basis.    Indeed,    leaf   movements    have 
generally  been  used  as  the  chief  indication  of  the  postulated  phase 
changes.    Those    in    various    soybeans,    for    example,    can    indicate 
whether  a  given  variety   will   show   SDP   or   daylength-indifferent 
flowering  responses  (Bunning,   1955).  Although  leaf  movements  in 
Kalanchoe  are  difficult  to  detect,  Schwemmle  (1957)  has  found  that 
the  effects  of  high  temperature  given  at  various  times  during  in- 
ductive dark  periods  (see  Chapter  Two)  are  well  correlated  with  the 
effects  of  similar  treatments  on   the  rhythmic  movements  of  the 
petals  of  plants  in  flower.  Not  all  the  leaf-movement  work  is  so 
favorable,   however;    there   is   apparently  no   significant  difference 
between    the    rhythmic    leaf   movements   of    the    qualitative    SDP 
Coleus  frederici  and  Coleus  frederici  x  blumei  and  those  of  the 
quantitative  LDP  Coleus  blumei  (Kribben,  1955).  At  best,  of  course, 
correlatory  evidence  is  merely  circumstantial,  whether  favorable  or 
unfavorable. 

CIRCADIAN  RHYTHMS  AND  THE  ACTION 
OF  LIGHT-BREAKS 

The  most  widely  used  tool  in  assessing  the  relation  of  circadian 
rhythms  to  photoperiodism,  as  in  the  study  of  low-intensity  light 
processes,  has  been  the  light-break.  Here,  instead  of  quality  and 
intensity,  the  timing  of  the  light-breaks  and  the  length  of  the  dark 
periods  are  the  factors  varied.  It  was  tacitly  assumed  during  the 
preceding  sections  that  light-breaks  are  most  effective  when  given 


48    •    Photoperiodism:  Attempts  at  Analysis 

in  the  middle  of  the  dark  period.  This  is  very  approximately  true 
in  ordinary  24-hour  cycles,  but  rarely  so  under  other  conditions,  as 
such  work  has  made  evident.  Under  the  rhythm  hypothesis,  light- 
breaks  act  not  by  merely  breaking  each  long  dark  period  into  two 
short  ones,  but  by  supplying  light  in  the  scotophile  (for  SDP)  or 
photophile  (for  LDP)  phases.  This  has  been  tested  extensively. 

When  Claes  and  Lang  (1947)  examined  the  effects  of  48-hour 
cycles  on  Hyoscyamus  (Chapter  Two),  they  found  that  cycles  of  7 
hours  light-41  hours  darkness  were  noninductive.  A  2-hour  light- 
break  would  promote  flowering  if  given  not  long  after  the  start  or 
before  the  end  of  each  long  dark  period,  but  was  ineffective  in  the 
middle.  The  times  of  maximum  effectiveness  were  about  16  and 
40  hours,  respectively,  after  the  start  of  each  main  light  period. 
These  results  were  consistent  with  the  idea  that  the  photophile- 
scotophile  alternation  continued  through  the  dark  period  with  the 
first  photophile  maximum  (typical  of  LDP)  16  hours  after  the  start 
of  the  main  light  period  and  the  second  about  24  hours  after  the 
first.  Yet  there  was  an  equally  reasonable  alternative  explanation 
not  depending  on  rhythms.  Suppose  that  the  light-break  could  act 
together  with  the  main  light  period  nearest  it  (either  before  or 
after)  to  constitute  a  long  light  period  interrupted  (without  effect) 
by  darkness.  On  this  alternative  the  light-break  was  ineffective  in 
the  middle  of  the  long  dark  period  not  because  it  fell  in  the  scoto- 
phile, as  in  the  rhythm  explanation,  but  because  it  was  too  far  from 
a  main  light  period.  Claes  and  Lang  favored  the  second  view. 

An  experiment  designed  to  avoid  this  ambiguity  was  reported 
by  Carr  (1952),  who  used  the  SDP  Kalanchoe  grown  in  72-hour 
cycles  of  12  hours  light-60  hours  darkness.  On  the  Biinning  theory, 
light-breaks  during  the  dark  period  should  show  three  times  of 
maximum  effectiveness  in  inhibiting  flowering  and  causing  the 
correlated  changes  in  vegetative  growth,  whereas  on  the  Claes  and 
Lang  alternative  there  should  be  only  two,  close  to  either  end  of 
the  dark  period.  Carr's  results  indeed  showed  three  maxima,  about 
24  hours  apart,  although  the  middle  one  was  not  as  well  defined 
as  one  might  wish.  Carr  concluded  that  "the  theory  of  Biinning 
.  .  .  must  therefore  be  regarded  as  finally  and  decisively  proved," 
thereby  illustrating  the  partisan  vigor  that  at  least  enlivens  if  not 
clarifies  the  question. 

Schwabe   (1955a)   repeated   Carr's   results   but   noted   that   the 


Time  Relations  and  Endogenous  Rhythms    •    49 

crucial  differences  (evidence  for  the  second,  or  middle,  maximum) 
were  very  small,  and  reached  opposite  conclusions  on  other  grounds 
(see  below);  but  very  clear  data  confirming  Carr's  results  were  later 
published  by  Melchers  (1956).  Meanwhile,  Hussey  (1954)  had  shown 
that  the  LDP  Anagallis  arvensis  grown  in  72-hour  cycles  with  long 
dark  periods  showed  only  two  maxima  for  the  promotion  of  flower- 
ing by  light-breaks  instead  of  the  three  that  would  correspond  to 
Carr's  results.  With  Hyoscyamus,  however,  Clauss  and  Rau  (1956) 
were  able  to  show  three  optima  in  similar  experiments,  thus  sup- 
porting Carr  and  Biinning.  The  quantitative  LDP  Arabidopsis 
thaliana  was  studied  twice,  with  ambiguous  results  each  time  (Hussey, 
1954;  Clauss  and  Rau,  1956).  The  SDP  Coleus  blumei  x  frederici 
disagreed  with  all  others,  since  the  time  for  maximum  light-break 
inhibition  (72-hour  cycle)  was  in  the  middle  of  the  long  dark  period, 
with  no  sign  of  three  or  even  two  optima  (Kribben,   1955). 

Other  work  besides  that  on  72-hour  cycles  suggests  Carr's 
quoted  conclusion  may  have  been  hasty.  Wareing  (1954)  voiced 
strong  opposition  to  the  idea  that  endogenous  alternation  of  photo- 
phile  and  scotophile  phases  determines  the  action  of  light-breaks. 
He  presented  experiments  with  Biloxi  soybeans  grown  on  9  hours 
light-39  hours  darkness  (48-hour  cycles),  or  on  9  hours  light-51 
hours  darkness  (60-hour  cycles),  testing  the  effects  of  light-breaks 
at  various  times  during  the  long  dark  periods.  In  both  cycles  light- 
breaks  about  6  to  8  hours  before  or  after  the  main  light  periods 
were  maximally  inhibitory,  whereas  they  promoted  flowering  in 
the  middle  of  the  dark  periods.  Since  the  dark  periods  used  in  the 
two  cycles  differed  by  12  hours,  one  would  not  expect  these  results 
if  the  inhibitory  action  of  light-breaks  was  due  to  a  more  or  less 
unchanged  circadian  rhythm.  One  would  expect  them,  however,  if 
light-breaks  interact  with  the  main  photoperiod  when  it  is  close 
enough,  thus  providing  a  total  photoperiod  that  exceeds  the  "limit- 
ing value"  for  soybean  flowering  (see  Chapter  Two).  Further  evi- 
dence for  this  view  was  that  in  cycles  totaling  48  hours,  light-breaks 
given  either  3  or  6  hours  before  the  main  light  period  were  inhibi- 
tory when  the  latter  was  9  hours  long,  whereas  only  a  light-break 
6  hours  before  was  effective  with  a  6-hour  main  photoperiod. 

Wareing  also  reported  experiments  with  Xanthium  in  which 
light-breaks  toward  the  end  of  a  long  dark  period  were  not  inhibi- 
tory.  Since   this  plant,  unlike  soybeans,   has   no   "limiting  photo- 


50    •    Photoperiodism:  Attempts  at  Analysis 

period,"  these  results  were  consistent  with  the  explanation  pro- 
posed. The  inhibition  of  Xanthium  induction  by  light-breaks  given 
early  in  long  dark  periods  was  explained  as  due  to  a  direct  nul- 
lification of  dark  processes  leading  to  flowering  plus  the  fact  that, 
after  the  light-break,  the  high-intensity  light  process  (Chapter  Two) 
is  left  unsatisfied.  The  induction  of  Xanthium  by  a  critical  dark 
period,  regardless  of  length  of  the  preceding  photoperiod,  was 
also  cited  by  Wareing  against  Bunning's  theory,  since  the  latter 
appeared  to  hold  that  the  phase  of  the  rhythm  was  regulated  by 
the  start  of  each  main  light  period.  Thus  the  effect  of  a  dark  period 
should  depend  on  how  long  the  light  continued. 

Biinning  responded  to  all  this  in  considerable  detail.  As  to  the 
Xanthium  results,  leaf-movement  studies  (Biinning,  1955)  indicated 
that  in  this  plant  the  phase  of  the  circadian  rhythm  is  indeed  regu- 
lated by  the  light-to-dark  rather  than  the  dark-to-light  transition, 
thus  refuting  Wareing's  evidence  based  on  the  opposite  assumption. 
A  light-break  given  early  in  the  dark  period  falls  in  the  scotophile 
induced  by  the  transition  to  darkness  and  thus  inhibits,  but  a  light- 
break  late  in  a  long  dark  period  falls  in  the  photophile  that  endog- 
enously  follows  and  thus  does  not  inhibit.  The  results  with  soy- 
beans may  also  be  clarified,  according  to  Biinning  (1954),  by 
attention  to  the  actual  course  of  the  circadian  rhythm  as  shown  by 
leaf  movements.  These  indicate  that  the  rhythm  continues  for 
about  30  hours  in  darkness,  after  which  a  period  of  "dark  rigor" 
(Dunkelstarr)  sets  in.  A  light-break  during  dark  rigor  brings  about 
a  new  photophile  phase  which  is  then  followed  endogenously  by  a 
scotophile.  Wareing's  observation  that  the  effect  of  a  light-break 
toward  the  end  of  a  long  dark  period  depended  not  on  the  length 
of  the  dark  period  but  on  the  light-break's  relation  to  the  following 
main  light  period  is  then  due  to  the  fact  that  the  main  light  period 
and  the  scotophile  phase  of  the  newly  reinitiated  rhythm  now  over- 
lap, with  resultant  inhibition.  In  addition,  Biinning  pointed  out 
that  his  observations  on  leaf  movements  would  also  predict  the 
existence  and  optimum  times  for  the  light-break  promotions  of 
flowering  observed  by  Wareing.  To  Wareing's  position  that  light- 
break  effects  are  due  to  interaction  with  nearby  light  periods, 
Biinning  thus  retorted:  "Yes,  that  is  so— because  of  the  endogenous 
daily  rhythm."1 

i  "Ja,  das  ist  so,  und  es  beruht  auf  der  endogenen  Tagesrhythmik." 


Time  Relations  and  Endogenous  Rhythms    •    51 


FLOWERING  IN  LIGHT-DARK  CYCLES  OF  DIFFERENT 
LENGTHS;  TEMPERATURE  INTERACTIONS 

If  a  circadian  rhythm  regulates  photoperiodic  responses, 
normal  flowering  should  depend  upon  light-dark  alternations  of 
about  24  hours.  Schmitz  (1951)  using  Kalanchoe  and  Schwabe 
(1955a)  using  Kalanchoe,  Xanthium,  and  an  SDP  variety  of  Im- 
patiens  balsamina,  concluded  against  Bunning's  theory  on  the 
grounds  that  cycles  with  total  lengths  ranging  from  15  to  50  hours 
proved  inductive,  with  any  failures  to  flower  attributable  to  the 
length  of  either  the  dark  or  light  periods  but  not  to  the  periodicity 
of  the  cycles.  Schwabe  also  criticized  the  extensive  use  of  leaf- 
movements  as  indicators  of  the  endogenous  rhythm,  since  the 
photoperiodic  response  is  often  insensitive  to  conditions  which 
may  completely  obscure  the  leaf  movements.  Calling  attention  to 
the  remarkable  plasticity  of  both  the  endogenous  rhythm  and 
Bunning's  theory  based  on  it,  Schwabe  questioned  the  value  of  the 
latter  in  explaining  photoperiodism  and  asked  Bunning  to  "define 
the  sort  of  experimental  result  which  he  would  regard  as  in- 
compatible with  it." 

In  contrast  to  the  results  of  Schmitz  and  Schwabe,  cycle-length 
experiments  show  clear  quantitative  effects  on  the  flowering  of 
soybeans  (Blaney  and  Hamner,  1957;  Nanda  and  Hamner,  1958, 
1959).  Cycles  totaling  24,  48,  or  72  hours  in  length  are  far  more 
favorable  to  flowering  than,  for  example,  36-  or  60-hour  cycles, 
although  neither  of  these  most  unfavorable  cycles  are  completely 
inhibitory.  This  certainly  supports  the  concept  of  a  circadian 
rhythm  in  sensitivity  to  light  and  darkness.  Finn  and  Hamner 
(1960)  have  also  published  a  group  of  experiments  with  Hyoscya- 
mus  in  which  the  total  length  of  the  light-dark  cycle  appears  to  be 
a  major  controlling  factor.  For  example,  with  a  10-hour  light 
period,  flowering  was  most  rapid  with  a  total  cycle  length  of  18 
hours  (with  an  8-hour  dark  period),  slowest  or  absent  with  a  total 
cycle  length  of  24-30  hours  (14-  or  20-hour  dark  period),  and  faster 
again  with  a  42-hour  cycle  length  (32-hour  dark  period).  Such 
results  may  also  be  used  to  support  a  rhythm-based  theory  of 
photoperiodism. 

Further  experiments  with  soybeans  (Blaney  and  Hamner,  1957) 


52    •    Photoperiodism:  Attempts  at  Analysis 

indicate  that  the  phase  of  the  rhythm  can  be  shifted  by  low 
temperatures  during  part  of  the  cycles  used.  A  recent  paper  by 
Oltmanns  (1960)  suggests  that  the  interactions  between  tempera- 
ture, light,  and  rhythmic  phemonena  in  the  flowering  of  Kalanchoe, 
and  by  implication  in  the  flowering  of  any  other  plant,  are  not  yet 
sufficiently  understood  to  be  described  by  any  simple  hypothesis. 

ENDOGENOUS  CIRCADIAN  RHYTHMS 
AND  THE  RED,  FAR-RED  SYSTEM 

There  appears  to  be  a  relationship  between  the  red,  far-red 
system,  unquestionably  involved  in  photoperiodism,  and  endog- 
enous circadian  rhythms  in  plants.  Red  is  the  most  effective  light 
in  initiating  the  movements  of  etiolated  bean  seedlings,  previously 
discussed,  and  this  effect  is  far-red  reversible  (see  Bunning,  1959a). 
More  directly  related  to  photoperiodism  is  the  observation  by 
Konitz  (1958)  that  far-red  given  as  an  interruption  of  the  main 
light  period  of  Chenopodium  amaranticolor  (SDP)  inhibits  the 
effectiveness  of  inductive  cycles,  just  as  does  red  given  in  the  dark 
period.  Since  rhythms  in  plants  demonstrably  affect  many  processes 
under  certain  circumstances,  the  particular  closeness  of  their  con- 
nection with  the  red,  far-red  system  is  hard  to  judge,  even  from 
these  results.  Engelmann  (1960)  has  found  that  when  red  light  is 
given  to  Kalanchoe  at  various  times  during  a  62-hour  dark  period, 
it  inhibits  induction  in  what  would  be  predicted  to  be  the  scoto- 
phile  phases  and  promotes  it  in  the  photophile  phases.  Far-red, 
however,  does  not  show  an  inverse  pattern,  but  simply  inhibits 
during  the  first  half  (30  hours)  of  each  dark  period  and  inhibits 
less  during  the  second  half. 

CONCLUDING  REMARKS  ON  CIRCADIAN  RHYTHMS 

AND  PHOTOPERIODISM 

If  the  reader  is  now  confused,  he  is  in  good  company;  no  aspect 
of  flowering  physiology  has  given  rise  to  more  complex  experi- 
ments, tenuous  interpretations,  and  heated  controversy.  The  contro- 
versy is  not  over  the  existence  of  rhythms  in  plants,  which  is  not 
seriously  questioned,  but  over  their  usefulness  and  relevance  in 
understanding  photoperiodism.  In  this  situation,  even  more  obvi- 


Time  Relations  and  Endogenous  Rhythms    •    53 

ously  than  in  most,  appeals  to  expert  opinion  are  useless,  since 
there  are  accomplished  and  respected  investigators  on  both  sides. 
The  writer  is  frankly  of  two  minds  on  the  subject.  On  the  one  hand, 
the  existence  of  rhythms  and  their  influence  in  many  processes 
recommend  them  as  the  underlying  mechanism  of  the  more  particu- 
lar time-dependent  response,  photoperiodism.  Yet  hypotheses  on 
the  precise  relationship  tend  to  seem  vague,  or  easily  disproved, 
or  ad  hoc  elaborations  full  of  special  exceptions.  It  has  understand- 
ably been  argued  that  they  simply  confuse  the  issue,  explaining  the 
relatively  simple  response  of  photoperiodism  in  terms  of  an  equally 
unexplained  set  of  more  complex  phenomena.  Yet,  if  photoperiod- 
ism is  indeed  a  special  case  of  a  basic  biological  process,  it  would  be 
a  pity  not  to  recognize  it  as  such.  So  far,  the  evidence  on  both  sides 
consists  largely  of  correlations  or  the  lack  of  correlations,  and  these 
differ  from  plant  to  plant.  Certainly  endogenous  circadian  rhythms 
are  at  least  modifying  factors  in  photoperiodism;  whether  they  are 
more  than  that,  time  will  undoubtedly  tell. 


chapter  four 


Temperature 
and  Flowering 


Temperature  affects  all  plant  processes,  and  some  temperature 
interactions  with  photoperiodism  have  already  been  mentioned. 
There  are  many  plants  in  which  flowering  is  either  qualitatively  or 
quantitatively  dependent  upon  exposure  to  near-freezing  tempera- 
tures, and  it  is  largely  with  these  that  this  chapter  will  deal.  A  few 
other  less  well-defined  relationships  between  temperature  and 
flowering  will  also  be  considered. 

VERNALIZATION:   COLD  TREATMENTS 
AND  FLOWERING 

It  is  evident  from  Chapter  Two  that  photoperiodism  provides 
not  only  a  convenient  method  lor  controlling  and  studying  flower- 
ing in  many  plants,  but  also  a  basis  for  the  explanation  of  many 
seasonal  phenomena.  The  same  is  true  of  low-temperature  effects, 
which  play  an  important  role  in  the  life  cycles  of  many  temperate- 
zone  plants.  Among  the  monocarpic  plants,  both  biennials  and 
winter  annuals  are  forms  in  which  a  cold  treatment  is  required 
before  flowering  can  take  place  with  optimum  rapidity;  in  winter 
annuals  it  can  be  given  during  germination  to  very  young  seedlings, 
whereas  biennials  must  first  have  made  substantial  growth.  Many 
perennials  also,  both  woody  and  herbaceous,  require  cold  treat- 
ments each  season  to  continue  flowering.   The  ecological  and  adap- 

54 


Vernalization:   Cold  Treatments  and  Flowering    •    55 

tive  significance  of  such  behavior  in  regions  with  a  period  of  winter 
cold,  itself  unfavorable  to  growth,  need  not  be  belabored. 

The  cold  treatment  of  germinating  seeds  in  order  to  hasten 
subsequent  flowering  has  come  to  be  known  as  vernalization.  This 
is  a  translation  of  the  Russian  yarovizatsya,  and  both  words  com- 
bine the  term  for  "spring"  (Russian,  yarov;  Latin,  ver)  with  a 
suffix  implying  "to  make"  or  "become,"  reflecting  the  ability  of 
such  cold  treatments  to  convert  "winter"  strains  of  cereals  to  the 
"spring"  habit  by  satisfying  their  cold  requirement.  Winter  cereals 
must  normally  be  planted  in  late  fall  or  winter  in  order  to  flower 
and  produce  a  crop  in  the  subsequent  year,  whereas  spring  varieties 
may  be  planted  in  the  spring  of  the  year  in  which  the  crop  is 
expected.  The  terms  vernalization  or  yarovizatsya  both  actually 
postdate  the  first  observations  of  such  effects  by  many  years,  but  it 
was  Russian  attention  to  the  possible  practical  values  of  the  process, 
particularly  in  the  1930's,  that  brought  it  generally  to  world-wide 
notice.  For  the  history  of  early  work  on  vernalization,  see  McKinney 
(1940)  and  Whyte  (1948). 

Vernalization  is  probably  the  only  aspect  of  plant  physiology 
that  ever  became  involved  in  political  ideology.  The  agronomic 
use  of  vernalization  in  the  Soviet  Union  was  popularized  by  T.  D. 
Lysenko,  who  viewed  the  effect  as  an  actual  inheritable  conversion 
from  winter  to  spring  habit;  later  he  even  claimed  the  conversion  of 
one  species  of  wheat  into  another.  Lysenko's  theory  eventually  led 
to  the  establishment  of  a  Marxist  form  of  Lamarckism-an  old 
and  thoroughly  discredited  view,  which  holds  that  changes  pro- 
duced by  the  environment  are  directly  inherited  by  the  offspring  of 
the  changed  organism— as  the  Soviet  dogma  in  biology.  The  adopt- 
ing of  this  view  by  the  Soviets  was  probably  partly  due  to  simple 
opportunism  on  Lysenko's  part,  as  he  was  its  chief  interpreter.  Some 
of  the  finest  biologists  in  the  U.S.S.R.  refused  to  support  the  official 
line  and,  as  a  result,  simply  disappeared  or  were  demoted.  This 
unfortunate  episode  in  the  history.,  of  science  has  been  recounted 
and  analyzed  by  Huxley  (1949)  and  Zirkle  (1949)  but  does  not 
appear  to  have  run  its  course  even  yet,  so  that  Soviet  biology 
still  labors  under  a  disadvantage.  Ironically,  vernalization  has  not 
proved  to  be  of  great  agronomic  importance,  since  the  breeding 
of  varieties  suitable  for  particular  climates  and  uses  has  been  far 
more  successful.  At  present,  the  chief  practical  applications  of  an 


56    •    Temperature  and  Flowering 

understanding   of   such   low-temperature   effects   are   in   relatively 
small-scale  horticultural  and  floricultural  practices. 

Vernalization  in  winter  rye 

Although  accounts  of  the  effects  of  chilling  seeds  and  seedlings 
abound  in  the  literature,  there  have  been  relatively  few  extensive 
studies  of  vernalization.  The  work  of  F.  G.  Gregory,  O.  N.  Purvis, 
and  their  collaborators  in  England  since  about  1931,  on  the  effects 
of  vernalization  and  photoperiodism  on  flower  initiation,  develop- 
ment, and  vegetative  growth  of  spring  and  winter  strains  of  the 
Petkus  variety  of  rye,  Secale  cereale,  is  by  far  the  most  thorough. 

The  spring  strain  is  a  typical  quantitative  LDP.  Under 
sufficiently  long  days,  flower  initiation  begins  after  approximately 
seven  leaves  have  differentiated,  whereas  under  short  days  (10 
hours  light)  it  occurs  only  after  at  least  22  leaves  have  been  pro- 
duced. The  winter  strain,  when  germinated  at  relatively  high  tem- 
peratures (for  example,  18°  C) ,  is  not  an  LDP,  but  flowers  equally 
slowly— again  after  about  22  leaves— under  both  long  and  short 
days.  However,  if  the  germinating  winter  strain  is  vernalized  by 
holding  it  at  1°  C  for  several  weeks  before  planting,  it  subsequently 
responds  to  long  days  in  the  same  way  as  does  the  spring  strain 
(Purvis,  1934).  The  effect  of  vernalization  is  thus  to  render  the 
seedling  sensitive  to  long  days;  early  flower  initiation  does  not  take 
place  as  a  result  of  vernalization  alone,  or  vernalization  followed 
by  short  days. 

The  effect  of  vernalization  is  proportional,  within  limits,  to  the 
duration  of  the  cold  treatment.  Four  days'  exposure  is  sufficient  to 
increase  the  subsequent  relative  growth  rate  of  the  stem  apex,  but 
has  no  effect  on  either  the  number  of  days  from  planting  to  full 
anthesis  or  the  number  of  leaves  preceding  flower  initiation.  Both 
values  are  reduced  to  a  minimum  (under  subsequent  long  days)  by 
increasing  the  length  of  the  cold  treatment  up  to  14  weeks  (Purvis 
and  Gregory,   1937). 

To  determine  what  portion  of  the  germinating  seed  perceives 
the  cold  treatment,  Gregory  and  Purvis  (1938a)  and  Purvis  (1940) 
studied  the  effects  of  low  temperature  on  excised  intact  embryos 
and  parts  of  embryos.  Not  only  the  intact  embryo  itself,  separated 
from  the  rest  of  the  seed,  but  even  its  isolated  apex  alone  are 
susceptible   to  vernalization,  giving  rise  to  plants  responding  op- 


Vernalization:  Cold  Treatments  and  Flowering    •    57 

timally  to  long  days.  Thus  the  site  of  vernalization  is  in  the  meri- 
stem  itself,  and  the  results  of  vernalization  are  somehow  maintained 
throughout  the  development  of  the  plant  derived  from  the  few 
cells  originally  exposed.  The  technique  of  vernalizing  isolated 
embryos  also  made  it  possible  to  show  that  vernalization  requires 
a  carbohydrate  source,  presumably  as  an  energy  supply  for  the 
process  involved.  Rapid  flowering  takes  place  only  if  the  embryos 
are  cold-treated  on  a  medium  containing  sucrose,  although  sub- 
sequent vegetative  growth  is  excellent  even  if  the  medium  consists 
of  mineral  salts  alone  (Gregory  and  DeRopp,  1938). 

Oxygen  is  also  required  during  vernalization,  confirming  the 
suggestion  that  the  process  requires  a  considerable  amount  of 
energy.  For  example,  Gregory  and  Purvis  (1938b)  found  that  germi- 
nating seeds  held  at  1°  C  for  9  weeks  would  eventually  produce 
inflorescences  after  the  eighth  leaf  if  the  cold  treatment  was  given 
in  air,  but  only  after  the  twenty-third,  as  in  the  unvernalized 
controls,  if  the  treatment  was  in  nitrogen.  As  little  as  1/500  of  the 
normal  air  concentration  of  oxygen  allowed  some  vernalization  to 
take  place,  but  not  the  maximum  effect. 

Before  proceeding  further,  one  should  bear  in  mind  that 
confusion  occasionally  arises  between  vernalization  and  the  favor- 
able effects  of  chilling  on  seed  germination  in  many  species.  The 
former  has  relatively  specific  effects,  inductive  in  the  sense  that  they 
lead  to  subsequent  changes  in  the  flowering  response  of  the  plants. 
Mere  cold  treatment  to  hasten  germination  is  not  necessarily  ver- 
nalization. It  may  indeed  result  in  earlier  flowering,  but  the  use  of 
developmental  criteria  (number  of  leaves  before  the  inflorescence, 
for  example)  can  usually  indicate  whether  a  genuine  hastening  of 
flowering  relative  to  vegetative  growth  has  occurred. 

Vernalization  in  other  plants 

The  flowering  not  only  of  winter  cereal  strains,  but  of  many 
other  plants,  can  be  hastened  by  vernalization.  Certain  varieties  of 
peas,  Pisum  sativum,  can  be  made  to  produce  their  first  flower  at 
an  earlier  node.  In  the  variety  Zelka,  the  eighteenth  or  nineteenth 
nodes  are  the  first  to  bear  flowers  if  germination  and  growth  take 
place  at  about  20°  C,  but  if  the  germinating  seeds  are  kept  at  7° 
for  30  days  before  planting,  flowers  occur  beginning  with  the 
fourteenth  or  fifteenth  nodes.  The  physiological  stage  susceptible 


58    •    Temperature  and  Flowering 

to  vernalization  appears  to  be  very  brief.  If  the  germinating  seeds 
are  kept  at  20°  for  3  clays  or  at  26°  for  1  or  2  days,  they  can 
no  longer  be  vernalized,  even  though  no  new  nodes  have  developed 
during  the  short  time  involved  (Highkin,   1956). 

The  term  vernalization  has  been  extended  to  cover  similar 
effects  of  low  temperature  given  not  to  germinating  seeds  but  to 
already  developed  plants.  Such  effects  are  typically  found  in  bien- 
nials and  many  perennials,  and  are  at  least  formally  similar  to 
those  obtained  with  the  very  young  plants  used  in  "true"  vernal- 
ization. One  plant  frequently  studied  is  the  biennial  strain  of 
Hyoscyamus  niger,  previously  introduced  as  an  LDP.  The  strain 
discussed  in  Chapters  Two  and  Three  was  the  annual,  from  which 
the  biennial  appears  to  differ  only  in  having  a  cold  (vernalization) 
requirement.  After  this  requirement  is  satisfied,  it  responds  to 
davlength  in  the  same  way  as  the  annual  strain,  but  it  cannot 
flower  otherwise.  It  thus  shows  a  qualitative  vernalization  require- 
ment, unlike  the  plants  so  far  discussed. 

Some  of  Lang's  (1951)  results  with  biennial  Hyoscyamus 
illustrate  how  vernalization  depends  on  both  the  temperature  and 
duration  of  exposure.  Plants  were  exposed  to  temperatures  from 
3°  to  17°  C  under  8-hour  day  conditions  for  varying  periods  of 
time,  after  which  they  were  placed  in  16-hour  days  at  23°  C.  The 
vernalizing  effectiveness  of  the  various  temperature  treatments  was 
then  expressed  by  the  time  required  under  long  days  before  flower 
initiation  was  detectable;  the  shorter  the  time,  the  more  effective 
the  vernalization.  With  a  vernalizing  time  of  105  days,  all  tempera- 
tures from  3°  to  14°  were  highly  effective:  flower  initiation  was 
detected  after  8  days  under  the  long-day  conditions.  With  only 
15  days  of  vernalization,  10°  was  the  most  effective  temperature, 
giving  23  days  to  initiation  as  compared  to  the  35  days  given  by  3° 
and  the  28  days  given  by  14°.  With  an  intermediate  vernalizing 
time  of  42  days,  both  3°  and  6°  allowed  initiation  alter  10  long 
days;  17°  gave  initiation  after  20,  and  the  values  for  the  other 
temperatures  lay  in  between  these.  Thus  die  temperature  optimum 
for  vernalization  shifts  considerably  depending  on  the  length  of 
exposure  (10°  for  15  days,  3  to  6°  for  42  days),  but  ceases  to  exist 
if  the  exposure  is  long  enough. 

As  in  the  rye  embryos,  cold  given  to  the  apex  alone  is  sufficient 
to  vernalize  Hyoscyamus  and  many  other  biennials.  The  gcrminat- 


Devernaijzation    •    59 

ing  seeds,  however,  are  not  vernalizable;  this  distinction  between 
biennials  and  winter  annuals  is  not  always  clear-cut,  but  in 
Hyoscyamus  at  least  it  is  clear  that  seedlings  are  not  sensitive  to 
vernalization  before  10  days  of  age,  and  not  maximally  sensitive 
until  they  are  30  days  old  (Sarkar,  1958).  Work  on  the  vernalization 
of  Hyoscyamus  has  been  reviewed  by  the  original  workers,  Melchers 
and  Lang  (1948)  and  Lang  (1952).  Evidence  for  the  existence  of  a 
translocatable  product  of  vernalization  has  also  been  put  forward 
and  will  be  discussed  in  Chapter  Five. 

An  exception  to  the  observations  that  vernalization  is  per- 
ceived by  the  stem  apex  is  found  in  Streptocarpus  wendlandii 
(Oehlkers,  1956),  in  which  the  leaf  appears  to  be  the  receptive 
region  and  neither  embryo  nor  stem  apex  can  be  vernalized  at  all. 

Several  varieties  of  ornamental  Chrysanthemum  (Chrysanthe- 
mum morifolium)  require  vernalization.  Here  again  the  apex  is  the 
site  of  vernalization,  and  all  the  laterals  subsequently  derived  from 
it  over  a  long  period  of  time  show  the  vernalized  condition 
(Schwabe,  1954).  While  most  of  the  vernalizable  plants  studied 
require  the  treatment  in  order  to  respond  as  LDP,  or  are  daylength- 
indifferent,  vernalized  Chrysanthemum  is  a  quantitative  SDP  for 
both  flower  initiation  and  development.  Three  or  four  weeks  at 
4  to  5°  C  has  an  optimum  vernalizing  effect.  Low  temperature  is 
effective  even  if  given  discontinuously,  and  a  particular  total 
number  of  hours  given  during  each  dark  period  is  more  effective 
than  the  same  number  of  hours  given  only  during  light  periods, 
at  least  under  short-day  conditions.  Chrysanthemum  is  a  perennial, 
and  yet  requires  renewed  vernalization  each  year  (Schwabe,  1950), 
a  situation  probably  characteristic  of  many  such  plants.  This  brings 
up  the  general  topic  of  "devernalization,"  which  has  been  observed 
in  a  number  of  plants. 

DEVERNALIZATION 

Vernalized  seeds  of  Petkus  winter  rye  can  be  devernalized 
simply  by  drying  them  and  holding  them  in  the  dry  condition  for 
several  weeks.  However,  only  the  effects  of  vernalization  on  the 
subsequent  flowering  response  (to  long  days)  are  so  reversed;  the 
effects  on  vegetative  growth  are  more  complex.  This  is  well  illus- 
trated by  some  data  from  Gregory  and  Purvis  (1938a).  Their  unver- 


60    •    Temperature  and  Flowering 

nalized  controls  in  this  experiment  produced  about  4.7  tillers 
(lateral  branches  from  the  base)  per  plant,  and  a  flowering  "score" 
of  19.  The  "score"  is  an  arbitrary  scale  adopted  to  indicate  the 
intensity  and  earliness  of  flowering.  Vernalized  seed  held  dry  for 
one  day  only  (which  has  essentially  no  effect)  gave  a  score  of  51 
and  about  2.7  tillers  per  plant— vernalization  typically  decreases  the 
number  of  tillers.  Seed  devernalized  by  being  dry  for  20  weeks, 
however,  gave  a  score  of  20  and  about  13.7  tillers  per  plant;  the 
promotion  of  flowering  was  completely  reversed,  but  the  number 
of  tillers  was  much  higher  than  in  either  vernalized  or  unvernalized 
plants.  Thus  devernalization  here  is  not  a  simple  reversal  of  vernal- 
ization but  a  conversion  of  its  effects  to  a  different  physiological 
expression.  Like  vernalization  itself,  it  is  proportional,  within  limits, 
to  the  duration  of  exposure  to  the  condition  bringing  it  about. 

Even  spring  Petkus  rye,  which  may  be  regarded  as  already 
genetically  vernalized,  can  be  devernalized  to  some  extent.  The 
leaf  number  preceding  flowering  (in  long  days)  is  increased  from 
an  average  of  6.8  to  8.3  by  a  three-week  germination  period  under 
anaerobic  conditions,  and  this  effect  is  removed  by  a  subsequent 
three-week  vernalization  treatment  (Gregory  and  Purvis,  1938b). 

The  devernalization  of  vernalized  biennial  Hyoscyamus  is 
brought  about  by  relatively  high  temperatures.  Vernalized  plants 
may  be  kept  under  short-day  conditions  for  at  least  several  weeks 
at  about  23°  C  and  still  retain  their  capacity  to  respond  as  LDP. 
Ten  days  at  about  38°  will  completely  remove  this  capacity,  if 
started  immediately  after  the  vernalization  treatment;  if  even  a 
lew  days  of  moderate  temperature  intervene  between  vernalization 
and  the  high  temperature,  however,  the  vernalized  condition 
becomes  stabilized  and  can  no  longer  be  removed  (Lang  and 
Melchers,  1947).  In  general,  studies  of  various  plants  indicate  that 
the  more  complete  the  original  vernalization  and  the  greater  the 
length  of  the  treatment,  the  more  difficult  devernalization  becomes. 
Revernalization  after  devernalization  is  also  possible  in  certain 
plants. 

As  the  only  perennial  studied  in  any  detail,  Chrysanthemum 
again  appears  unusual  in  that  devernalization  is  not  brought  about 
by  high  temperatures  alone,  but  requires  several  weeks  of  low  light 
intensity  (or  darkness)  as  well  as  temperatures  of  23°  to  28°  C. 
The  mechanism  of  this  effect  is  unknown.  It  is  not  due  simply  to 


Vernalization  and  Photoperiodism    •    61 

starvation  for  carbohydrates  since  defoliation  of  the  plants  does 
not  have  the  same  effect,  nor  does  sucrose  feeding  during  treatment 
reduce  devernalization  (Schwabe,  1955b,  1957).  Whether  the  de- 
vernalization  that  occurs  in  the  natural  yearly  cycle  is  actually  due 
to  high  temperatures  and  low  light  intensities  (at  the  underground 
growing  points)  is  still  uncertain. 

RELATIONS  BETWEEN  VERNALIZATION  AND 

PHOTOPERIODISM 

Many  of  the  plants  studied,  and  also  work  with  the  gibberellins 
(Chapter  Six),  may  be  used  to  support  the  idea  of  a  close  relation- 
ship between  vernalization  and  long-day  requirements,  but  the 
situation  is  probably  more  complex  than  this,  varying  greatly  from 
plant  to  plant. 

Petkus  winter  rye  and  biennial  Hyoscyamus  niger  are  "typical" 
vernalizable  plants  in  which  the  cold  treatment  brings  about 
quantitative  or  qualitative  LDP  responses.  In  other  plants,  vernal- 
ization can  even  substitute  partially  or  completely  for  a  long-day 
requirement.  Vernalization  of  spinach  seeds,  for  example,  reduces 
the  critical  daylength  for  flowering  from  14  to  about  8  hours 
(Vlitos  and  Meudt,  1955),  whereas  cold  treatments  given  to  seed- 
lings of  certain  strains  of  subterranean  clover,  Trifolium  subter- 
raneum,  can  completely  remove  any  marked  dependence  on  day- 
length  (Evans,   1959). 

Floral  induction  and  development  in  several  grasses  depend 
upon  both  photoperiod  and  vernalization.  Plants  of  orchard  grass, 
Dactylis  glomerata,  studied  by  Gardner  and  Loomis  (1953)  require 
low  temperatures  and  short  days  (less  than  13  hours  light)  for 
floral  induction,  followed  by  higher  temperatures  and  long  days 
for  optimum  flower  development.  The  short-day  and  vernalization 
requirements  for  induction  can  be  satisfied  separately  but  only  in 
that  order,  not  in  the  reverse.  In  a  sense,  then,  Dactylis  glomerata 
is  one  of  the  short-long-day  plants  (SLDP)  mentioned  in  Chapter 
Two,  except  that  a  period  of  low  temperature  must  occur  between 
the  two  photoperiodic  treatments  or  together  with  the  first. 

In  some  plants,  short-day  treatments  can  substitute  partially  or 
completely  for  vernalization,  making  them  SLDP.  Petkus  winter 
rye  itself  shows  a  response  of  this  kind,  although  the  situation  is 


62    •    Temperature  and  Flowering 

complicated  by  the  fact  that  both  short  days  and  continuous  light 
favor  flower  initiation  more  than  do  long  days  in  unvernalized 
plants  (Gott  et  al,  1955).  A  more  clear-cut  example  of  a  vernalizable 
SLDP  is  Campanula  medium  (see  Doorenbos  and  Wellensiek, 
1959),  which  has  a  qualitative  requirement  for  either  low  tempera- 
ture or  short  days  before  it  can  respond  to  long  days. 

Although  even  in  the  above  plants,  vernalization  generally  has 
to  be  followed  by  exposure  to  long  days,  CJirysanthemiim  is  not 
the  only  plant  in  which  it  promotes  a  response  to  short  days. 
Junges  (1958)  found  that  short  days  following  the  vernalization  of 
a  strain  of  Kohlrabi,  Brassica  oleracea  var.  gongyloides,  a  biennial, 
promoted  the  subsequent  flowering  in  long  days  and  high  tempera- 
tures. Such  results  make  it  unwise  to  regard  vernalization  require- 
ments as  necessarily  linked  to  any  other  environmental  response. 


THE  SEMANTICS  OF  VERNALIZATION: 

FURTHER  EFFECTS  OF  TEMPERATURE 

ON  FLOWERING 

A  restricted  definition  of  vernalization  was  given  earlier,  but 
it  is  now  time  to  acknowledge  its  fluidity.  For  one  thing,  the  term 
is  so  often  misapplied  to  the  breaking  of  bud  or  embryo  dormancy 
by  low  temperatures  that  it  has  become  a  mere  jargon  substitute 
for  "cold  treatment";  this  is  deplorable,  but  perhaps  too  late  to 
mend.  Even  if  one  restricts  its  usage  to  effects  on  flowering,  how- 
ever, difficulties  arise.  It  is  clear  enough  how  certain  effects  of  near- 
freezing  temperatures  on  biennials  and  perennials  are  similar  to 
those  on  germinating  winter  annual  seeds,  and  why  the  term 
vernalization  may  well  be  used  for  both.  As  long  as  one  is  dealing 
with  an  obviously  inductive  action  on  flowering  of  temperatures 
low  enough  to  prevent  growth,  the  phenomena  seem  relatively 
clear-cut.  But  when  the  same  or  very  similar  effects  occur  at 
temperatures  high  enough  to  allow  rapid  growth,  or  are  not  induc- 
tive, or  interact  with  the  conditions  of  light  and  darkness  during 
exposure,  are  they  still  vernalization?  This  is  not  simply  a  matter 
of  semantics;  the  point  is  that  the  influences  of  temperature  on  all 
aspects  of  development  are  so  manifold  that  "typical"  vernali/a- 
tion,   as  in  rye  or  Hyoscyumus,  probably  is  an   extreme  case  of  a 


The  Semantics  of  Vernalization    •    63 

very  general  situation.  If  so,  then  perhaps  the  erosion  of  the  word 
vernalization  is  fortunate. 

The  plasticity  of  some  vernalization  requirements  is  illustrated 
by  celery,  Apium  graveolens  var.  dulce.  If  the  plants  are  kept  at 
usual  vernalizing  temperatures  (about  7°  C)  for  a  month,  they  will 
flower  rapidly  when  transferred  to  cool  (10-16°)  or  moderate 
(16-21°)  but  not  warm  (about  24°)  conditions.  The  initial  vernaliza- 
tion is  not  absolutely  necessary  for  flowering,  which  will  also  take 
place  eventually  under  constant  cool  conditions,  or  under  the 
moderate  conditions  after  two  weeks  under  cool  conditions.  No 
temperature  pretreatment  of  any  kind  will  permit  flower  initiation 
under  the  warm  conditions  (Thompson,  1953).  In  short,  vernaliza- 
tion is  only  weakly  inductive  and  can  take  place  at  temperatures 
high  enough  to  allow  growth.  The  latter  of  course  is  true  to  a  lesser 
extent  even  of  Hyoscyamus,  and  one  can  still  see  in  celery  the 
occurrence  of  vernalization  and  devernalization  in  the  Hyoscyamus 
sense,  but  the  effective  temperatures  are  considerably  closer 
together. 

The  flowering  response  of  stocks,  Matthiola  incana,  as  sum- 
marized by  Kohl  (1958),  represents  a  situation  in  which  it  is  uncer- 
tain whether  the  term  vernalization  can  be  applied  or  not.  Neither 
germinating  seeds  nor  seedlings  can  be  induced  by  low  tempera- 
tures, but  maturing  plants  require  at  least  three  weeks  at  10  to 
16°  C  for  flower  initiation.  If  the  temperature  rises  above  19°  for 
as  little  as  6  hours  per  day,  initiation  is  completely  inhibited;  the 
plants  must  remain  at  the  favorably  low  temperatures  until  full 
differentiation  of  floral  primordia  has  occurred.  After  this,  however, 
they  remain  induced  and  produce  new  flower  primordia  even  at  the 
higher  temperatures.  This  behavior  can  of  course  be  regarded  as 
vernalization  with  a  very  low  degree  of  induction  and  a  small 
difference  between  vernalizing  and  devernalizing  temperatures,  but 
speaking  simply  of  optimum  and  maximum  temperatures  for 
flower  initiation  seems  to  be  as  accurate.  Many  plants  probably 
respond  in  a  similar  fashion,  with  optima  and  maxima  varying 
widely  depending  on  the  species. 

Also  relevant  here  is  another  temperature  effect  on  plants, 
thermoperiodism.  This  term  indicates  the  responses  of  plants  to 
differing  day  and  night  temperatures— growth  and  development  in 
mo6t  of  those  tested  are  favored  by  night  temperatures  markedly 


64    •    Temperature  and  Flowering 

lower  than  those  optimal  during  the  light  period  (see  Went,  1957). 
Work  on  this  question  will  not  be  dealt  with  here,  since  relatively 
little  of  it  directly  concerns  flower  initiation.  In  addition,  the 
interactions  of  temperature  changes  with  high-intensity  light 
periods  of  different  lengths  are  extremely  complex  and  have  not 
been  carefully  analyzed.  Many  of  the  data  do  suggest,  however, 
that  "typical"  vernalization,  the  effects  of  moderately  low  tempera- 
tures, the  effects  of  varying  day  and  night  temperatures,  and  the 
interactions  of  temperature  with  photoperiod  (Chapter  Two)  all 
intergrade. 

Recall  in  this  connection  the  observation  of  Schwabe  (1955b, 
1957)  that  discontinuous  vernalizing  cold  treatments  were  more 
effective  on  Chrysanthemum  when  given  during  each  night  rather 
than  in  the  day.  This  sounds  very  much  like  thermoperiodism. 
Note  also  that  the  tomato,  Lycopersicon  esculentum,  in  which 
major  effects  of  temperature  have  been  studied  as  thermoperiodism, 
is  quantitatively  vernalizable;  exposure  of  the  seedlings  to  tempera- 
tures near  10°  C  soon  after  cotyledon  expansion  significantly  de- 
creases the  number  of  leaves  formed  before  the  first  inflorescence 
and  increases  the  number  of  flowers  in  that  inflorescence  (Wittwer 
and  Teubner,  1956).  Since  one  effect  of  low  night  temperatures  is 
also  to  increase  the  number  of  flowers  per  inflorescence  (Went, 
1957,  Chap.  6),  vernalization  in  the  tomato,  as  in  Chrysanthemum, 
is  perhaps  not  completely  distinguishable  from  thermoperiodism. 

A  further  expansion  of  the  phenomena  that  need  to  be  con- 
sidered in  connection  with  vernalization  is  suggested  by  some 
work  of  Guttridge  (1958).  By  the  definition  previously  given, 
vernalization  results  in  the  promotion  of  flowering.  However,  a 
cold  treatment  affects  certain  varieties  of  strawberry  (Fragaria)  in 
the  opposite  fashion,  inductively  bringing  about  a  condition  in 
which  flower  initiation  is  delayed  and  runner  production  promoted 
when  the  plants  are  subsequently  transferred  to  conditions  that 
would  otherwise  make  for  continued  flowering  and  low  vegetative 
growth.  This  effect  is  certainly  formally  similar  to  vernalization, 
though  inverse  in  result. 

TEMPERATURE  AND   FLOWERING  IN   BULB   PLANTS 

Among  the  most  detailed  studies  yet  done  on  temperature 
and  flowering  are  those  of  Blaauw,   Hartsema,  Luyten,  and  their 


Temperature  and  Flowering  in  Bulb  Plants    •    65 

collaborators  in  the  Netherlands,  particularly  in  the  period  1920- 
1935,  on  the  initiation  and  development  of  flowers  in  bulb  plants. 
This  work  is  largely  recorded  in  Dutch  but  has  been  reviewed  by 
Went  (1948),  from  whom  this  account  is  taken.  The  basic  pro- 
cedure was  to  store  bulbs  at  different  temperatures  for  different 
lengths  of  time  and  determine,  by  anatomical  studies,  the  optimum 
temperature  for  the  various  developmental  events  taking  place 
within  them. 

After  the  current  year's  foliage  has  died,  the  next  year's  apical 
meristem  within  the  tulip  (Tulipa)  bulb  already  has  several  leaf 
primordia.  Flower  initiation,  including  differentiation  of  all  the 
flower  parts,  then  takes  about  three  weeks  at  20°  C,  the  optimal 
temperature  for  this  process.  If  further  flower  development  is  to 
take  place  (still  entirely  within  the  bulb),  the  temperature  must 
now  drop  and  remain  at  about  9°  C  for  13  to  14  weeks.  After  this 
low-temperature  period  the  optimal  temperatures  for  leaf  and  stalk 
elongation  are  successively  higher,  reaching  20°  and  above  for  com- 
plete anthesis.  This  increase  in  optimal  temperature  for  the  final 
stages  of  flowering  is  more  or  less  gradual,  but  it  appears  to  be 
characteristic  of  tulip  and  certain  other  plants  that  flower  initiation, 
favored  by  relatively  high  temperatures,  must  be  followed  quite 
abruptly  by  low  temperatures  for  the  best  subsequent  development. 
In  the  hyacinth  (Hyacinthus)  bulb,  on  the  other  hand,  the  changes 
in  temperature  optima  are  not  as  abrupt  as  in  the  tulip,  though  they 
are  similar,  and  all  the  values  lie  somewhat  higher. 

Such  studies  have  since  been  conducted,  in  the  Netherlands 
and  elsewhere,  on  many  plants  having  bulbs,  rhizomes,  or  other 
fleshy  organs  that  can  be  stored  for  a  considerable  part  of  the  year. 
The  detailed  results  of  course  differ  from  plant  to  plant,  but  are 
usually  of  great  practical  value  since  they  make  it  possible  to 
control  development  or  arrest  it  at  desired  stages  to  suit  almost  any 
shipping  and  planting  schedule.  Tulips  and  hyacinths,  for  example, 
can  be  held  completely  dormant  without  injury  for  weeks  by 
storage  at  35°  C.  As  soon  as  further  development  is  required,  the 
temperature  can  again  be  lowered  to  the  optimal  level  for  the  stage 
previously  attained.  Recent  references  to  this  sort  of  work  can 
be  found  in  journals  and  textbooks  on  horticulture. 

It  needs  to  be  stressed  that  this  sort  of  temperature  response  is 
not  characteristic  of  all  bulb  plants,  but  merely  of  those  adapted  to 
temperate  climates  with  a  well-defined  winter.  The  tropical  bulb 


66    •    Temperature  and  Flowering 

Hippeastrum,  for  example,  also  studied  by  Blaauw  (see  Went, 
1948),  has  no  such  requirement  for  a  long  period  of  low  tempera- 
ture, and  flowers  several  times  a  year  at  high  or  moderate 
temperatures.  The  similarity  between  the  cold  requirement  in  a 
plant  such  as  the  tulip  and  typical  vernalization  should  also  be 
noted.  Here  of  course  the  effect  is  on  flower  development,  not 
induction  or  initiation,  but  the  conditions  involved  and  the  final 
results  are  the  same,  although  the  underlying  physiological  con- 
ditions are  unknown  in  any  case. 

Unlike  light  or  certain  chemical  factors,  temperature  cannot  be 
given  or  withheld  but  only  changed,  and  it  ailects  essentially  all  bio- 
chemical processes.  This  makes  it  at  once  the  most  important  single 
factor  in  development  and  the  most  difficult  to  study  in  any  de- 
limited way.  Hence  it  is  not  surprising  that  terms  such  as  vernali- 
zation are  almost  meaningless  except  to  indicate  a  particular  kind  ol 
manipulation,  and  may  not  designate  any  single  specific  physio- 
logical process.  The  brevity  of  this  discussion  relative  to  those  on 
other  factors  affecting  flowering  should  be  taken  to  reflect  not  a 
lesser  importance  of  its  problems,  but  only  how  little  is  known 
about  them  in  any  fundamental  sense.  See  Went  (1953,  1957)  for 
a  much  more  thorough  treatment  of  the  effects  of  temperature  on 
all  aspects  of  plant  growth;  a  review  by  Chouard  (1960)  emphasizes 
the  complexity  of  vernalization  and  related  low-temperature  effects. 


► 
► 


chapter  five  t  Floral  Hormones 

and  the  Induced  State 


Even  before  the  effects  of  light  and  temperature— the  major 
natural  environmental  influences  on  flowering— were  known,  the 
question  of  what  internal  changes  lead  to  flowering  was  of  obvious 
importance;  photoperiodism  and,  to  a  lesser  extent,  vernalization 
made  experimental  approaches  to  it  more  feasible.  The  next  three 
chapters  are  largely  concerned  with  this  question  in  one  way  or 
another;  the  present  will  examine  the  nature  and  origin  of  sub- 
stances controlling  flowering  and  transmissible  from  one  part  of  a 
plant  to  another  or  from  plant  to  plant  by  grafting. 

DEFINITIONS  AND  BACKGROUND:  AUXINS  AS 

PLANT  HORMONES 

Hormones  can  be  defined  as  substances  produced  in  one  part 
of  an  organism  and  acting  in  another,  and  active  in  very  low  con- 
centrations. Action  at  a  distance  from  the  site  of  production  is  the 
most  crucial  characteristic  of  a  hormone;  activity  in  low  concen- 
trations simply  serves  to  distinguish  it  from  substances  furnishing 
energy  or  structural  materials  and  used  in  large  quantities.  Sugars, 
for  example,  are  produced  in  aerial  parts  of  the  plant  and  used 
in  the  roots  (as  well  as  elsewhere)  but  cannot  be  considered  hor- 
mones. 

67 


68    •    Floral  Hormones  and  the  Induced  State 

The  idea  that  the  formation  of  flowers,  and  of  other  organs  as 
well,  is  controlled  by  hormones  specific  for  each  type  of  organ- 
"  organ-forming  substances"-was  favored  in  the  nineteenth  century 
by  Julius  Sachs,  the  so-called  "father  of  plant  physiology."  Evidence 
at  the  time  was  almost  nonexistent;  more  recent  evidence,  at  least 
for  flowering  hormones,  will  be  considered  below.  First,  however, 
it  is  useful  to  describe  briefly  a  different  and  better  known  class  of 
plant  hormones,  the  auxins.  Research  on  these  substances,  starting 
in  the  1920's,  has  had  a  strong  influence  on  the  less  successful 
investigations  on  possible  flowering  hormones;  in  addition,  auxins 
may  play  at  least  a  minor  role  in  the  control  of  flowering. 

If  the  tip  of  a  growing  shoot  is  removed,  the  elongation  of  the 
remaining  stump  generally  ceases  rapidly.  If  the  tip  is  replaced,  the 
stump  may  resume  and  continue  elongating  for  some  time,  although 
not  necessarily  as  fast  as  in  the  intact  plant.  This  effect  of  the  tip 
may  even  occur  if  it  is  separated  from  the  stump  by  a  thin  layer  of 
agar  or  gelatin.  In  such  cases,  elongation  can  be  brought  about 
simply  by  placing  on  the  stump  a  piece  of  gelatin  or  agar  on  which 
the  cut  surface  of  the  tip,  or  several  similar  tips,  have  rested  for 
some  time.  Such  results  indicate  that  a  substance  or  substances  that 
can  move  out  of  the  tip  and  into  or  through  gelatin  are  required  for 
the  continued  elongation  of  the  tissue  below.  Such  substances  are 
termed  auxins.  It  is  now  known  that  low  concentrations  of  many 
substances,  both  natural  and  synthetic,  can  promote  the  elongation 
of  shoot  tissue  deprived  of  its  natural  auxin  sources.  Most  of  them 
are  relatively  simple  organic  compounds,  such  as  indole-3-acetic 
acid;  those  occurring  naturally  are  clearly  plant  hormones  since 
they  are  produced  in  shoot  tips  (or  other  young,  actively  growing 
regions)  and  affect  tissues  elsewhere.  The  action  of  auxins  is  not 
confined  to  causing  the  elongation  of  shoot  cells,  however;  depend- 
ing on  the  concentration,  they  may  either  promote  or  inhibit 
many  plant  processes,  including  root  initiation,  leaf  abscission,  and 
cell  division.  Space  forbids  further  discussion  of  auxins  as  such, 
but  they  will  figure  in  a  number  of  the  topics  to  be  considered. 
For  additional  information  on  the  general  topic  of  auxin  physi- 
ology, which  has  a  voluminous  literature,  see  Audus  (1959), 
Leopold  (1955),  or  the  recent  volume,  Plant  Growth  Regulation 
(1961). 


Evidence  for  Flowering  Hormones    •    69 


PRELIMINARY  EVIDENCE  FOR  THE  EXISTENCE 
OF  FLOWERING  HORMONES 

The  clearest  early  investigations  indicating  the  existence  of 
floral  hormones  were  by  Chailakhyan  in  Russia.  One  of  his  major 
experiments  (1936a)  showed  that  if  the  upper  portion  of  the  SDP 
Chrysanthemum  indicum  were  defoliated,  it  would  initiate  flowers 
if  the  lower  (leafy)  portion  received  short  days,  even  if  the  de- 
foliated part  were  kept  on  long  days.  With  the  conditions  reversed 
—if  the  upper  defoliated  part  were  kept  on  short  days  and  the 
lower  leaves  on  long  days— no  flowering  occurred.  He  interpreted 
these  results  as  indicating  that  under  the  proper  photoperiodic 
conditions  the  leaves  could  form  a  hormone  that  moved  to  the 
apex  and  brought  about  flowering.  From  subsequent  work  he 
concluded  also  that  this  hormone,  which  he  named  "florigen" 
(flower-maker),  could  move  either  up  or  down  the  stem  and  could 
be  transferred  from  one  plant  to  another  through  grafts  (Chail- 
akhyan, 1936b,  1936c,  1937). 

Several  investigators  at  first  obtained  data  suggesting  that 
florigen,  like  the  auxins,  could  pass  through  a  nonliving  connec- 
tion, but  these  proved  to  be  illusory.  Moshkov  (1939),  for  example, 
soon  reported  his  inability  to  repeat  his  own  earlier  experiment  in 
which  the  Chrysanthemum  floral  stimulus  had  apparently  passed, 
through  a  thin  film  of  water,  and  he  concluded  that  such  move- 
ment could  take  place  only  through  living  tissue.  A  similar  en- 
couraging but  false  start  was  made  by  Hamner  and  Bonner  (1938). 
They  showed  that  a  photoperiodically  induced  Xanthium  plant 
grafted  to  a  noninduced  plant  could  bring  about  flowering  in  the 
latter.  They  further  observed  that  interposition  of  a  piece  of  fine 
lens  paper  between  the  stock  and  scion  would  still  permit  this 
effect.  This  suggested  that  florigen  could  move  from  the  induced 
plant  (the  donor)  to  the  noninduced  plant  (the  receptor)  without, 
direct  tissue  contact.  When  this  work  was  repeated  by  Withrow  and 
Withrow  (1943),  using  various  kinds  of  membranes  including  lens 
paper  between  the  cut  surfaces  of  donor  and  receptor,  it  appeared 
that  the  original  interpretation  was  mistaken.  Anatomical  studies 
showed  that  tissue  union  could  occur  by  the  growth  of  cells  through 
the  lens  paper;  the  transmission  of  florigen  took  place  only  when 


70    •    Floral  Hormones  and  the  Induced  State 

there   was   such    union,    and   all   membranes    that   would    prevent 
actual  "taking"  of  the  graft  also  prevented  transmission. 

Chailakhyan  (1937)  had  already  concluded  from  experiments 
with  Perilla  and  Chrysanthemum,  and  the  Withrows  (1943)  con- 
firmed with  Xanthium,  that  florigen  movement  occurred  only 
through  the  "bark"— the  phloem  and  cortical  tissue.  If  this  was 
removed  in  ringing  or  girdling  experiments,  no  movement  of  the 
floral  stimulus  across  the  girdle  was  observed,  although  water  con- 
tinued to  pass  through  the  xylem  (wood)  and  the  shoots  remained 
healthy.  Presumably  the  major  route  of  transport  is  the  phloem 
itself,  in  which  most  organic  substances  are  transported;  but  we 
will  return  to  this  question  later. 

Questions  obvious  from  the  start  of  this  kind  of  research  are 
whether  the  florigen  of  one  kind  of  plant  is  effective  on  another 
and,  more  particularly,  whether  that  of  an  SDP  will  act  on  an  LDP 
and  vice  versa.  Auxins  are  not  species-specific,  but  such  questions 
are  more  difficult  to  answer  with  respect  to  flowering  hormones, 
transmissible  from  plant  to  plant  only  by  grafting.  Successful  grafts 
are  generally  possible  only  between  closely  related  plants  so  that  no 
completely  general  answer  can  be  given.  Within  these  limitations, 
however,  the  floral  stimulus  produced  by  one  species  is  often 
effective  on  other,  closely  related  species. 

Maryland  Mammoth  tobacco  and  annual  Hyoscyamus  niger 
are  members  of  the  same  family  (Solanaceae)  and  can  be  success- 
fully grafted.  In  such  a  graft  partnership,  the  LDP  Hyoscyamus 
will  flower  under  short-day  conditions  if  the  SDP  tobacco  is  also 
kept  under  short  days,  but  not  if  the  tobacco  is  exposed  to  long 
days.  That  is,  under  short  days  the  tobacco  is  itself  induced  and 
serves  as  the  donor  of  stimulus  of  florigen  to  Hyoscyamus.  Con- 
versely, the  tobacco  can  be  made  to  flower  under  long-day  condi- 
tions if  the  Hyoscyamus  is  induced  by  also  being  kept  under  long 
days,  but  not  if  the  Hyoscyamus  receives  short  days.  Here  Hyoscy- 
amus becomes  the  donor  and  Maryland  Mammoth  the  receptor 
(Lang  and  Melchers,  1947;  see  Lang,  1952).  The  simplest  conclu- 
sion is  of  course  that  the  florigens  produced  by  Hyoscyamus  in 
long  days  and  by  Maryland  Mammoth  tobacco  in  short  days  are 
physiologically  equivalent  if  not  identical. 

There   are  many  similar  experiments   in    the   literature.   The 
SDP  Xanthium,  lor  example,  can  be  made  to  (lower  on  long  days 


Evidence  for  Flowering  Hormones    •    71 

when  grafted  to  any  of  several  LDP  members  of  its  family,  the 
composites,  such  as  species  of  Erigeron  or  Rudbeckia  (Okuda, 
1953).  Using  members  of  the  family  Crassulaceae,  Zeevaart  (1958) 
found  that  the  LDP  Sedum  ellacombianum  or  Sedum  spectabile 


Fig.  5-1.  Transfer  of  flowering  stimulus  between  LDP  and  SDP  by  grafting, 
showing  role  of  leaves.  (/I)  Induction  of  flowering  in  an  LDP  (Sedum  spectabile)  in 
short  days  by  grafting  onto  an  SDP  (Kalanchoe'  blossfeldiana) .  In  the  graft  to  the  right, 
the  Kalanchoe  (below)  was  kept  defoliated.  Photograph  made  96  days  after  grafting. 
(B)  Induction  of  flowering  in  an  SDP  (Kalanchoe)  in  long  days  by  grafting  onto  an 
LDP  (Sedum) — the  reciprocal  of  the  experiment  in  (.4).  Again,  in  the  graft  to  the 
right,  the  Sedum  was  kept  defoliated.  Photograph  made  130  days  after  grafting. 
(Photographs  from  Zeevaart  [1958],  courtesy  of  Dr.  J.  A.  D.  Zeevaart,  Agricultural 
Institute,  Wageningen.) 


could  flower  under  short  days  when  grafted  onto  the  SDP  Kalan- 
choe blossfeldiana,  whereas  the  latter  would  flower  under  long  days 
when  grafted  to  the  Sedians  (see^  Fig.  5-1).  Such  effects  can  be 
turned  to  practical  use.  Many  varieties  of  the  cultivated  sweet 
potato,  Ipomoea  batatas,  flower  irregularly  if  at  all,  no  matter 
what  the  environmental  conditions,  which  is  a  distinct  hindrance 
to  breeding  programs.  This  recalcitrance  can  be  overcome  by 
grafting  shoots  to  any  of  several  free-flowering  (SDP)  genera  of  the 


72    •    Floral  Hormones  and  the  Induced  State 

same  family  (Convolvulaceae— morning  glories)  and  then  inducing 
the  latter  (Lam  et  al.,  1959). 

The  occurrence  of  transmissible  flowering  stimuli  is  not  con- 
fined to  photoperiodic  plants.  This  is  of  course  implicit  in  the 
fact,  noted  earlier,  that  many  plants  are  only  quantitatively  photo- 
periodic, or  are  photoperiodic  only  under  certain  conditions, 
whereas  some  are  completely  daylength-indifferent;  the  processes 
leading  to  flowering  may  or  may  not  be  under  photoperiodic 
control  and  still  have  the  same  end  result.  Lang  (1952)  has  reviewed 
work  in  which  daylength-indifferent  plants  can  serve  as  donors  of 
a  flowering  stimulus  to  closely  related  LDP  or  SDP. 

Not  all  results  on  the  transmission  of  flowering  stimuli  have 
been  straightforward,  and  before  proceeding  further  it  is  well  to 
keep  the  fundamental  difficulty  in  mind.  Auxins  can  be  obtained 
from  plants  either  by  diffusion  from  cut  tissues,  as  previously 
described,  or  by  extraction.  They  can  then  be  reapplied  and  will 
cause  growth  in  responsive  tissue.  This  makes  possible  not  only  the 
identification  and  quantitative  assay  of  naturally  occurring  auxins 
but  also  the  study  of  the  biochemistry  of  their  origin  and  function. 
Not  so  for  the  hypothetical  florigen— which  remains  hypothetical 
for  the  very  reason  that,  with  one  possible  exception,  no  work  to 
date  has  successfully  isolated  it  from  the  living  plant;  attempts  to 
do  so  will  be  discussed  in  the  following  chapter.  Thus  it  has  not 
been  possible  to  study  flowering  hormones  chemically,  and  all  the 
evidence  is  necessarily  circumstantial.  Hence  the  use  in  this  chapter 
of  all  sorts  of  terms-florigen,  floral  hormones,  flowering  stimuli,  and 
so  on— to  avoid  implying  a  precision  that  does  not  exist.  We  must 
now  pay  closer  attention  to  the  experimental  systems  involved  in 
such  work— the  plants  themselves— following  which  we  can  return 
more  critically  to  the  question  of  whether  floral  hormones  actually 
exist. 

TRANSLOCATION  OF  FLOWERING  HORMONES 

The  conclusion  that  florigen  moves  only  through  living  tissues 
is  based  on  observations  besides  those  already  presented.  Borthwick, 
Parker,  and  Heinze  (1941)  showed  that  a  soybean  plant  defoliated 
to  only  a  single  leaf  could  flower  in  short  days,  but  not  if  the  petiole 
was  chilled  to  3C  C.  This  was  true  even  if  another  leaf  was  left  on 


Translocation  of  Flowering  Hormones    •    73 

the  plant,  below  the  first,  and  exposed  to  long  days  without  any 
other  treatment.  Hence  the  inhibition  of  flowering  was  not  due 
simply  to  lack  of  carbohydrate  transport  through  the  chilled  petiole, 
since  carbohydrates  were  still  supplied  by  the  long-day  leaf,  but  to 
the  inhibition  of  the  transport  of  the  stimulus  specifically  from  the 
short-day  leaf.  These  and  similar  results  indicate  that  transport  is 
the  result  of  cellular  activity.  Further  circumstantial  evidence  im- 
plicates the  phloem  in  florigen  transport  by  indicating  that  the 
latter  is  associated  with  the  movement  of  carbohydrates  in  the 
plant.  This  evidence  is  not  unequivocal,  and  is  based  largely  on 
experiments  dealing  with  the  effects  of  noninduced  leaves  on  the 
flowering  response. 

Note  that  in  Chailakhyan's  experiment  with  Chrysanthemum, 
discussed  earlier,  the  upper  portion  of  the  plant  was  defoliated  in 
order  to  demonstrate  the  movement  of  a  flowering  stimulus  from 
the  lower  leaves  on  short  days.  Many  observations,  including  those 
of  Chailakhyan,  indicate  that  in  some  plants  translocation  can  only 
be  demonstrated  in  this  manner.  A  technique  often  used  to  study 
this  sort  of  question  involves  the  use  of  two-branched  plants, 
produced  by  removing  the  apical  portion  of  seedlings  and  allowing 
two  approximately  equal  lateral  branches  to  develop.  One  branch 
can  then  be  exposed  to  inductive  conditions,  making  it  the  donor 
of  flowering  stimulus,  and  the  other,  on  noninductive  conditions, 
is  used  as  the  receptor.  When  Biloxi  soybeans  are  used  in  this  way, 
the  receptor  (long-day)  branches  flower  only  if  they  are  defoliated 
but  not  if  the  leaves  are  left  in  place,  even  though  the  donor 
branch  flowers  well  whether  or  not  the  receptor  has  leaves  (Borth- 
wick  and  Parker,  1938b).  Similar  results  have  been  obtained  in 
other  plants  but  are  by  no  means  universal.  In  the  SDP  Amaranthus 
caudatus,  defoliation  of  the  receptor  (long-day)  branches  greatly 
inhibits,  rather  than  promotes,  their  flowering,  which  is  otherwise 
almost  as  rapid  as  that  of  the  donor  branch  itself  (Fuller,  1949). 

Noninduced  leaves  can  be  kept  in  total  darkness,  rather  than 
removed,  in  order  to  avoid  their  inhibiting  transmission.  This 
observation  was  actually  first  made  by  Garner  and  Allard  in  1925; 
the  only  reason  they  are  not  generally  credited  with  the  discovery 
of  the  translocatable  effects  of  photoperiodism  is  that  they  them- 
selves stressed  the  localization  of  such  effects  in  Cosmos,  the  SDP 
they  chose  for  work  on   this  question.  In  this  as  in  many  other 


74    •    Floral  Hormones  and  thf.  Induced  State 

plants,  flowering  in  normal,  intact  individuals  is  confined  to  the 
area  exposed  to  induction.  A  portion  of  the  plant  kept  in  total 
darkness,  however,  will  exhibit  a  flowering  response,  provided  an 
adjacent  portion  is  kept  on  inducing  (short-day)  conditions.  An 
elegant  experiment  by  Stout  (1945)  illustrates  the  same  situation 
for  an  LDP,  the  sugar  beet  (Beta  vulgaris).  Plants  with  three  shoots 
were  made  by  root  grafts.  If  one  of  these  shoots  was  exposed  to 
long  days,  it  flowered  and  also  brought  about  flowering  in  a  second 
shoot  kept  in  darkness.  The  third  shoot,  however,  maintained  on 
short  days,  remained  vegetative. 

As  suggested  earlier,  most  experiments  of  this  kind  can  be 
interpreted  as  indicating  that  florigen  moves  in  the  prevailing 
direction  of  carbohydrate  movement.  In  this  view,  darkening  or 
removing  leaves  from  a  noninduced  part  of  the  plant  results  in  a 
lower  carbohydrate  production  in  that  part,  so  that  carbohydrate 
(and  florigen)  movement  in  its  direction  is  increased.  There  are 
alternative  explanations,  however,  as  will  become  evident  later.  In 
some  cases,  darkened  leaves  may  inhibit  translocation;  this  has 
been  interpreted  as  a  "diversion"  of  the  movement  into  such  leaves 
(see  Lang,  1952). 

Interesting  evidence  on  the  translocation  of  floral  hormones 
and  the  effects  of  noninduced  leaves  comes  from  work  on  the  SDP 
Kalanchoe  blossfeldiana  reported  by  Harder  (1948).  With  a  mini- 
mal short-day  treatment,  development  of  the  complex,  branching 
inflorescence  is  slow  and  "vegetative";  that  is,  the  flowers  are  small 
or  abortive  and  the  bracts  among  them  overdeveloped  and  leaflike. 
If  only  a  single  leaf  receives  short-da y  treatment,  inflorescence 
development  may  be  normal,  provided  the  treatment  continues 
long  enough;  but  commonly  it  is  notably  asymmetrical,  being  more 
normal  on  the  side  directly  above  the  induced  leaf  and  vegetative 
on  the  side  away  from  it  (see  Fig.  5-2).  Examination  of  the  vascular 
svstem  shows  that  this  is  consistent  with  the  idea  that  florigen 
simply  moves  in  the  phloem:  the  lateral  connections  in  Kalanchoe 
are  relatively  minor,  so  that  little  lateral  movement  of  the  effect 
would  be  expected. 

Experiments  on  the  effects  of  noninduced  leaves  in  Kalanchoe 
depend  on  its  decussate  leaf  arrangement;  that  is,  each  pair  of 
opposite  leaves  is  at  right  angles  to  the  pair  above  or  below  it. 
Thus  looking  down  on  the  plant  one  sees  four  ranks  of  leaves  at 


Translocation  of  Flowering  Hormones    •    75 

right  angles  to  each  other.  If  all  but  two  leaves  (of  different  pairs) 
are  removed  from  the  plant,  and  the  lower  is  given  short  days  and 
the  upper  long  days,  several  different  results  can  be  obtained.  I! 
the  long-day  leaf  is  in  the  same  rank  with  (directly  above)  the 
long-day  leaf,  flowering  is  prevented.  If  the  long-day  leaf  is  in  the 
rank  opposite  that  of  the  short-day  leaf,  flowering  is  the  same  as  if 


Fig.  5-2.  Localization  of  flower- 
ing stimulus  in  Kalanchoe.  A  single 
leaf  situated  on  the  left-hand  side 
was  repeatedly  exposed  to  short 
days  whereas  the  rest  of  the  plant 
received  long  days.  (Photograph 
from  Harder  [1948],  by  permis- 
sion of  the  company  of  Biologists, 
Ltd.,  and  courtesy  of  Dr.  R. 
Harder,  University  of  Gottingen.) 


the  long-day  leaf  were  absent.  Finally,  if  the  long-day  leaf  is  in 
either  of  the  two  ranks  at  right  angles  to  that  of  the  short-day  leaf, 
some  inhibition  of  flowering  is  evident.  In  this  sort  of  experiment, 
the  transport  of  florigen  is  evidently  upward  from  leaf  to  growing 
point,  but  appropriately  trimmed  plants  can  be  used  for  similar 
studies  on  the  transport  downward  from  a  short-day  leaf  to  an 
axillary  shoot.  Here  again,  a  long-day  leaf  between  the  short-day 
leaf  and  the  shoot  inhibits  most  effectively  if  it  is  in  the  same  rank, 
and  least  effectively  if  it  is  in  the  rank  opposite.  In  short,  whether 
movement  is  up  or  down,  the  inhibition  only  occurs  if  the  non- 
induced  leaf  lies  effectively  between  the  induced  leaf  and  the 
growing  point  in  question.  This  is  apparently  true  for  many  plants 
besides  Kalanchoe  and  is  again  consistent  with  the  postulated 
movement  of  florigen  with  the  carbohydrate  stream.  In  addition, 


76    •    Floral  Hormones  and  the  Induced  State 

however,  it  is  also  consistent  with  the  idea  that  the  flowering 
hormone  might  be  taken  up  by  the  noninduced  tissue  and  de- 
stroyed by  it. 

The  latter  interpretation  is  also  suggested  by  analogous  experi- 
ments in  which  parts  of  a  single  leaf  are  subjected  to  long-day  or 
short-day  treatments.  If  the  basal  part  of  the  leaf  is  given  short 
days  and  the  apical  long  days,  flowering  occurs,  but  if  the  situation 
is  reversed,  the  flowering  is  weak  or  absent.  This  is  not  due  to  the 
inability  of  the  apical  portion  to  respond  to  short  days  and  lead 
to  flowering,  since  it  does  so  if  the  entire  basal  part  is  trimmed  off 
as  long  as  the  vascular  connection  to  the  stem  is  left  intact.  Here 
again,  noninduced  tissue  evidently  inhibits  flowering  when  it  is 
situated  between  induced  tissue  and  the  growing  point,  and  possibly 
does  so  by  destroying  the  floral  stimulus.  Earlier  experiments  by 
Chailakhyan  with  Perilla  leaves  also  lead  to  the  same  conclusion 
(seeNaylor,  1953). 

The  most  thorough  recent  studies  of  the  interactions  of  various 
parts  of  the  plant  on  the  effectiveness  of  localized  inducing  treat- 
ments are  those  of  Lincoln,  Raven,  and  Hamner  (1956,  1958),  using 
Xanthium.  The  first  paper  bears  most  directly  on  translocation. 
With  two-branched  plants,  the  intensity  of  flowering  in  the  re- 
ceptor branch  (long  days)  is  inversely  proportional  to  the  amount 
of  mature  tissue  left  on  it.  If,  however,  a  carbohydrate  deficiency 
is  produced  in  the  receptor  by  heavy  shade,  the  inhibition  by  the 
long-day  leaves  is  greatly  reduced.  Conversely,  shading  the  donor 
(short-day)  branch,  which  would  produce  a  carbohydrate  deficit  in 
it,  reduces  flowering  in  the  receptor.  So  also  does  removing  the 
receptor's  young  leaves,  which  are  responsible  for  a  great  portion 
of  its  carbohydrate  uptake.  Although  these  results  are  consistent 
with  the  carbohydrate-flow  hypothesis,  several  others  suggest  a 
more  complex  situation.  The  inhibiting  effect  of  mature  leaves  on 
the  receptor  is  not  simply  proportional  to  the  amount  of  light  they 
receive  but  depends  on  its  timing;  that  is,  the  effect  is  photo- 
periodic. For  example,  the  inhibition  caused  by  leaves  given  12 
hours  light-12  hours  dark  cycles  is  much  greater  if  each  night  is 
interrupted  by  three  evenly  spaced  10-minute  light-breaks  than  if 
interrupted  only  once,  in  the  middle,  by  a  30-minute  light-break. 
If  only  carbohydrate  production  were  involved  in  the  inhibition, 
such  results  would  not  be  expected. 


Translocation  Rate    •    77 

In  certain  plants,  such  as  the  SDP  Piqueria  trinervia  (stevia), 
the  effect  of  inductive  treatment  remains  relatively  localized  no 
matter  what  manipulations  are  performed  (Zimmerman  and  Kjen- 
nerud,  1950).  Thus  the  only  summary  statement  that  can  be  made 
about  the  movement  or  apparent  movement  of  flowering  hormones 
is  that  it  takes  place  in  living  tissue,  probably  through  the  phloem; 
that  it  can  be  either  acropetal  (base  to  apex)  or  basipetal;  that  it 
may  be  localized  or  systemic  depending  on  the  plant,  the  structure 
of  its  vascular  system,  and  the  condition  of  noninduced  portions. 
There  is  evidence  that  noninduced  leaves  act  in  an  inhibitory 
fashion  primarily  but  not  exclusively  by  affecting  the  predominant 
direction  of  carbohydrate  movement,  with  which  the  florigen  may 
be  carried. 

TRANSLOCATION  RATE 

There  are  very  few  studies  on  the  rate  of  movement  of  floral 
stimuli,  again  because  of  the  difficulty  that  only  the  final  response, 
not  the  postulated  hormone,  can  be  measured.  Early  work  by 
Chailakhyan  suggested  values  of  about  2  cm  in  24  hours  in  Perilla, 
but  it  is  doubtful  whether  conditions  were  optimal  (see  Lang, 
1952).  Some  ingenious  experiments  by  Imamura  and  Takimoto 
(1955b)  provide  the  best  data  so  far  available. 

Plants  of  the  SDP  Pharbitis  nil  (Japanese  morning  glory)  can 
be  reduced  to  a  stem  with  a  single  leaf,  and  then  decapitated  so 
that  the  bud  in  the  axil  of  the  leaf  will  start  to  grow.  The  position 
of  the  first  flower  on  the  axillary  shoot  will  then  depend  on  the 
time  between  the  start  of  growth  (decapitation)  and  the  start  of  a 
single  16-hour  inductive  dark  period  given  to  the  leaf.  In  one 
experiment,  for  example,  if  the  dark  period  was  started  imme- 
diately after  decapitation,  the  average  position  of  the  first  flower 
on  the  axillary  shoot  was  at  node  2.8  (that  is,  node  2  in  some 
plants,  node  3  in  most).  If  24  hours  elapsed  between  decapitation 
and  the  dark  period,  the  average  position  was  node  3.5,  and  so  on. 
Such  differences  are  developmental  expressions  of  the  amount  of 
time  during  which  the  axillary  bud  (shoot)  was  growing  before  the 
flowering  stimulus  reached  it.  Parallel  experiments  can  be  done  at 
the  same  time  with  plants  in  which  the  distance  between  the  single 
leaf  and  the  receptor  bud  is  greater-for  example,  by  having  the 


78    •    Floral  Hormones  and  the  Induced  State 

latter  not  in  the  axil  of  the  leaf  but  on  the  opposite  branch  of  an 
otherwise  debudded  two-branched  plant.  The  rate  of  stimulus 
translocation  can  then  be  calculated  by  the  difference  in  the  first- 
llowering-node  values  of  the  shoots  in  the  plants  with  the  receptor 
buds  close  and  the  receptor  buds  far  from  the  induced  leaf.  An 
example  from  one  experiment  may  make  this  clear.  In  the  "close" 
series,  in  which  the  average  distance  from  leaf  to  bud  (mainly 
through  petiole  tissue)  was  90  mm,  the  mean  first  flowering  node 
was  3.4  if  the  dark  treatment  was  given  24  hours  alter  decapitation. 

4.5  with  it  given  48  hours  after,  and  5.3  with  it  given  72  hours 
after.  In  the  "far"  series,  the  distance  between  leaf  and  bud  was 
about  235  mm,  through  both  branch  and  petiole  tissue.  Here, 
inductive  treatment  started  immediately  after  decapitation  gave  a 
first-flowering-node  average  of  4.6.  By  interpolation  from  the  pre- 
ceding figures,  it  is  as  if  the  inductive  treatment  for  the  "close" 
series  had  been  delayed  some  55  hours.  Since  the  difference  be- 
tween "far"  and  "close"  is  about  145  mm,  this  difference  of  55 
hours  represents  the  movement  of  the  stimulus  at  145/55,  or  about 

2.6  mm  per  hour. 

Such  experiments  of  course  give  an  average  value  for  the 
transport  through  a  petiole,  then  both  down  and  up  a  branch: 
other  experiments  suggested  that  upward  transport  may  be  faster 
than  downward.  Also,  the  transport  rate  in  plants  so  mutilated 
may  well  differ  from  that  in  intact  plants.  In  any  case,  all  experi- 
ments with  Pharbitis  gave  values  of  the  order  of  3  mm  per  hour. 
This  represents  a  considerably  slower  movement  than  that  observed 
for  carbohydrates  in  phloem  tissue  (often  exceeding  200  mm  per 
hour),  but  rates  of  virus  transport  in  the  phloem  sometimes  fall 
in  this  low  range  (see  Esati  et  ai,  1957). 


FLOWER  PROMOTION  OR  FLOWER   INHIBITION? 
THE  SPECIFICITY  OF  FLOWERING  STIMULI 

The  florigen  hypothesis  in  its  simplest  form  postulates  a  single 
substance,  common  at  least  to  many  plants,  uniquely  responsible 
lor  flower  initiation.  Much  of  the  evidence  so  far  presented  is 
consistent  with  this  hypothesis,  but  some  investigators,  on  the  con- 
trary, have  concluded  that  flowering  is  controlled  by  an  inhibitory 


Flower  Promotion  or  Flower  Inhibition?    •    79 

substance    or    substances    that    prevent    initiation    until    they    are 
removed  by  the  proper  conditions. 

It  may  be  surprising  that  most  of  the  very  evidence  presented 
in  the  preceding  section  for  movement  of  a  florigen  can  be  reinter- 
preted as  indicating  simply  deinhibition  (see  von  Denffer,  1950). 
Under  this  interpretation,  noninduced  leaves  constantly  produce  a 
flowering  inhibitor  that  moves  to  the  growing  point  along  with  the 
products  of  photosynthesis;  induced  leaves  no  longer  produce  this 
inhibitor.  Hence  the  removal  or  darkening  of  noninduced  leaves 
often  promotes  flowering  not,  as  under  the  florigen  hypothesis,  by 
preventing  interference  with  the  carbohydrate  stream  in  which  the 
florigen  moves,  but  by  reducing  still  further  the  sources  of  the 
inhibitor;  flowering  thus  occurs  simply  as  a  result  of  sufficient 
quantities  of  inhibitor-free  assimilates.  It  has  been  suggested,  on 
the  basis  of  work  to  be  discussed  later,  that  the  inhibitor  in 
question  might  be  an  auxin,  and  the  general  form  of  this  hypothesis 
fits  some  of  the  experimental  data  well  enough.  At  least,  it  often 
fits  no  worse  than  the  other  hypothesis,  as  a  brief  reconsideration 
will  show. 

Stout's  (1945)  work  with  "three-headed"  beet  plants  indicated 
that  the  presence  of  a  shoot  on  short-day  conditions  did  not  inhibit 
the  response  of  a  darkened  receptor  shoot  to  the  long-day  donor 
shoot;  thus  if  the  noninduced  (short-day)  shoot  produces  an  inhib- 
itor, it  is  not  detectable.  This  does  not  help  the  inhibitor  hypoth- 
esis. On  the  other  hand,  the  further  result  that  even  4  hours  of 
light  per  day  (compared  with  17  hours  for  the  donor)  prevents  a 
shoot  from  being  an  effective  receptor  also  does  not  help  the  simple 
fiorigen-movement-with-carbohydrates  hypothesis,  since  it  is  un- 
likely that  the  predominant  direction  of  carbohydrate  movement 
would  be  reversed  under  these  conditions.  Another  ambiguous 
situation  is  of  course  that  the  inhibitory  effect  of  noninduced 
Xanthium  leaves  appears  to  be  a  photoperiodic  phenomenon  in  its 
own  right,  not  simply  a  matter^ of  affecting  carbohydrate  (and 
florigen)   flow. 

The  florigen  hypothesis  can  be  saved  from  many  difficulties, 
including  these,  by  the  suggestion  that  noninduced  leaves  act  not 
by  producing  an  inhibitor  but  by  destroying  florigen.  On  balance, 
the  simple  inhibitor  hypothesis  is  probably  less  satisfactory;  the 
strongest  argument  against  it  is  the  effectiveness  of  small  amounts 


80    •    Floral  Hormones  and  the  Induced  State 

of  induced  leaf  tissue,  of  which  there  are  many  examples  in  the 
literature.  Xanthium  is  striking  in  this  regard.  Several  double- 
branched  plants  grafted  together  in  series  can  all  be  brought  to 
flower  by  short-day  treatment  of  a  single  leaf  on  one  of  them  (see 
Naylor,  1953).  Khudairi  and  Hamner  (1954a)  found  that  a  total 
leaf  area  of  less  than  one  square  centimeter  was  enough  to  bring 
about  flowering  from  a  single  16-hour  dark  period.  Xanthium  may 
be  more  extreme  in  this  regard  than  most  species,  but  the  idea 
that  induced  leaves  simply  supply  an  inhibitor-free  stream  of 
assimilates  is  hard  to  reconcile  with  such  results.  However,  some 
form  of  inhibitor  hypothesis  is  still  favored  by  certain  investiga- 
tions, of  which  a  few  should  be  considered. 

In  annual  Hyoscyamns  (LDP),  removal  of  all  the  leaves  brings 
about  flower  formation,  which  then  takes  place  at  the  same  rate 
irrespective  of  light  or  dark  conditions.  Presumably,  then,  the 
effect  of  long  days  on  an  intact  plant  is  to  prevent  an  inhibition 
of  flowering  exerted  by  the  leaves  under  short-day  conditions.  Since 
clearly  in  the  defoliated  plant  the  floral  stimulus  is  present  or  can 
be  formed  in  the  stem  or  roots,  leaves  on  short  days  apparently  not 
only  fail  to  produce  it  themselves,  but  also  destroy  it,  or  inhibit  its 
production,  or  produce  an  inhibitor  of  flowering.  The  latter  hypoth- 
esis can  be  avoided  either  by  adopting  the  first  or  by  suggesting 
a  mechanism  for  the  second— for  example,  that  the  short-day  leaves 
remove  some  substance  that  could  otherwise  act  as  a  precursor  for 
production  of  the  stimulus.  So  far,  there  is  no  clear  evidence  in  any 
direction  (see  Lang,  1952)  .  Whatever  the  explanation,  such  effects 
may  be  responsible  for  some  of  the  ambiguous  results  obtained 
from  grafting  experiments,  as  in  the  following  example  taken  from 
Zeevaart   (1958). 

Defoliated  scions  of  the  LDP  Nicotiana  sylvestris  grafted  on 
stocks  of  the  SDP  Maryland  Mammoth  will  flower  on  short  days, 
suggesting  florigen  transfer  from  the  induced  stock.  However,  such 
scions  also  flower  on  long  days,  noninductive  for  Maryland  Mam- 
moth, although  similarly  defoliated  but  ungrafted  Nicotiana  sylves- 
tris fails  to  flower  on  long  days.  Does  Maryland  Mammoth  then 
produce  florigen  under,  for  it,  noninductive  conditions?  The  ex- 
planation may  be  that  defoliated  Nicotiana  sylvestris,  like  Hyoscya- 
mus,  has  the  capacity  to  flower  if  sufficient  assimilates  are  present. 
In  Hyoscyamus  these  come  from  the  large  storage  root,  whereas  in 


Flower  Promotion  or  Flower  Inhibition?    •    81 

the  Nicotiana  sylvestris  experiment  they  are  supplied  by  Maryland 
Mammoth  whether  on  long  or  short  days. 

Flower  initiation  in  strawberries,  Fragaria,  requires  short  days, 
at  least  under  certain  conditions.  Hartmann  (1947)  showed  that 
daughter  plants  would  initiate  flowers  in  long  days  if  the  adult 
plant,  to  which  they  were  still  connected  by  runners,  was  exposed 
to  short  days;  he  interpreted  these  results  in  the  conventional 
"florigen"  manner.  Guttridge  (1959)  has  since  performed  experi- 
ments suggesting  the  opposite— that  flowering  occurs  when  the 
level  of  a  flowering  inhibitor,  which  also  promotes  vegetative 
growth,  is  sufficiently  reduced.  This  postulated  substance  would  be 
produced  in  long  but  not  in  short  days,  and  might  even  be 
destroyed  in  the  latter.  The  evidence  is  analogous  to  that  on  the 
translocation  of  flowering  hormones. 

Plants  kept  on  long  photoperiods  (using  light-breaks)  promote 
vegetative  growth  and  inhibit  flowering  in  runner-attached  plants 
under  short  photoperiods.  This  is  favored  by  earlier  daily  illumina- 
tion of  the  plants  on  long  days,  although  earlier  illumination  itself, 
without  light-breaks  to  create  an  effective  long  photoperiod,  has  no 
effect.  These  results  of  course  again  suggest  translocation  of  the 
substance  in  question— this  time  the  flower-inhibiting,  growth- 
promoting  substance— in  the  predominant  direction  of  carbohydrate 
movement.  Experiments  with  radioactive  phosphorus  as  a  tracer 
confirmed  the  postulated  direction  of  assimilate  movement.. 
Guttridge's  results  are  thus  more  consistent  with  the  "simple 
inhibitor"  hypothesis  than  with  "florigen";  here  the  "donor"  is 
vegetative,  the  "receptor"  potentially  flowering. 

The  earliness  of  flowering  in  certain  pea  varieties— by  which 
is  meant  whether  the  first  flower  appears  at  a  lower  or  higher  node 
—can  be  influenced  in  several  ways  other  than  (in  some  varieties) 
photoperiod  or  cold  treatment.  These  include  removing  the 
cotyledons,  making  cuttings  from  the  young  seedlings,  grafting  of 
early  onto  late  varieties  or  vice  versa,  or  even  grafting  stock  and 
scion  of  the  same  variety.  The  situation  is  complicated  by  the  fact 
that  certain  treatments,  which  can  be  broadly  described  as  in- 
hibitory, may  inhibit  vegetative  growth  more  than  flowering  so 
that  the  latter  actually  occurs  at  an  earlier  node,  though  no  sooner 
in  time.  Haupt  (1958)  has  concluded  on  the  basis  of  his  own 
experiments  and  those  of  others  that  transmissible  flower-promoting 


82    •    Floral  Hormones  and  the  Induced  State 

and  flower-inhibiting  substances  both  play  a  part  in  these  effects, 
but  their  nature  is  unknown. 

Resentle  (1959)  also  supports  the  concept  that  flowering 
generally  depends  on  a  change  in  a  complex  balance  rather  than 
on  either  simple  flower-promoting  or  flower-inhibiting  substances, 
since  his  experiments  with  the  Crassulaceae  (Bryophyllum, 
Kalanchoe,  Bryokalanchoe  species)  have  indicated  all  degrees  of 
transfer  of  the  "floral  state"  or  "vegetative  state"  from  one  plant  to 
another  by  grafting.  Further  discussion  on  the  merits  of  various 
hypotheses  will  be  deferred  until  the  concluding  section  of  the 
chapter. 

VERXALIN  AND  METAPLASIN 

In  addition  to  florigen  and  flowering  inhibitors,  the  participa- 
tion of  other  transmissible  substances  in  flowering  or  processes 
related  to  it  has  been  suggested.  With  regard  to  vernalization, 
Melchers  (see  Melchers  and  Lang,  1948;  Lang,  1952)  has  assumed 
the  existence  of  a  substance  called  "vernalin"  on  the  basis  of 
experiments  with  biennial  Hyoscyamus.  If  two  of  these  Hyoscyamus, 
one  previously  vernalized  and  one  unvernalized,  are  grafted 
together,  both  will  flower  in  response  to  long  days,  although  an 
unvernalized  plant  alone  will  not.  This  might  indeed  be  due  to 
transfer  of  vernalin  from  the  vernalized  to  the  unvernalized  plant, 
but  it  can  be  equally  interpreted  as  a  movement  of  floral  stimulus 
from  the  vernalized,  long-day  treated  plant  to  the  other  that, 
unvernalized,  cannot  respond  to  long  days.  The  "vernalin"  inter- 
pretation is  based  on  the  additional  observation  that  unvernalized 
biennial  Hyoscyamus  grafted  to  Maryland  Mammoth  tobacco  will 
flower  in  long  days,  in  which  the  tobacco  itself  is  not  induced. 
The  tobacco  is  visualized  as  a  donor  of  vernalin— produced  without 
vernalization  in  a  non-cold-requiring  plant— enabling  the  unver- 
nalized biennial  to  respond  to  long  days.  In  this  view,  vernalin 
is  either  a  direct  biochemical  precursor  of  florigen  or  makes  its 
synthesis  possible. 

The  difficulties  of  interpreting  grafting  experiments  with 
tobacco  (Nicotiana)  species,  some  of  which  were  mentioned 
earlier,  make  this  evidence  less  than  completely  convincing.  To 
the  writer's  knowledge,  there  has  never  been  any  clear  demonstra- 


Permanence  and  Location  of  the  Induced  State    •    83 

tion  of  the  transmission,  by  grafting  or  otherwise,  of  a  stimulus 
resulting  from  vernalization  alone  rather  than  vernalization  fol- 
lowed by  long  days;  such  a  demonstration  would  be  necessary  to 
establish  the  existence  of  vernalin. 

In  the  course  of  work  on  Kalanchoe,  Harder  (1948)  concluded 
that  short-day  treatment  caused  the  production  not  only  of  flower- 
ing hormones  but  also  of  "metaplasin,"  a  substance  responsible  for 
the  large  and  easily  measured   changes   in  vegetative   habit   (par- 
ticularly leaf  succulence)   accompanying  flowering.  Studies  on   its 
transport,  analogous  to  those  on  the  floral  hormones  in  Kalanchoe, 
did  not  permit  any  separation  of  one  from  the  other.  The  entire 
evidence  for  the  existence  of  metaplasin  as  a  separate  entity  is  this: 
subjecting  the  upper  portion  of  a  plant  on  short  days  to  a  prolonged 
chloroform  treatment  that  will  strongly  inhibit  flowering  has  no 
influence   on    the   vegetative   effects   of   the   photoperiod.   This   is 
hardly  unequivocal  proof  that  short  days  result  in  the  production 
of  two  different  substances,  one  specific  for  flowering  and  one  for 
the  vegetative  changes.  It  is  equally  reasonable  to  assume  that  the 
processes  leading  to  flowering  are  in  some  way  different  and  more 
sensitive  to  this  inhibition  than  those  controlling  vegetative  growth, 
but  it  does  not  follow  that  the  initiating  conditions  or  substance 
brought  about  by  photoperiodic  treatment  is  necessarily  multiple. 

If  the  conclusion  at  present  must  be  that  vernalin  and  meta- 
plasin may  be  myths,  they  nevertheless  serve  a  purpose  here.  They 
remind  us,  to  whom  these  particular  errors  may  seem  obvious,  that 
the  difficulties  of  analyzing  the  responses  of  complex  organisms, 
coupled  with  the  desire  to  achieve  simple  interpretations,  may  lead 
even  some  foremost  investigators  astray. 

PERMANENCE  AND  LOCATION  OF  THE 
INDUCED  STATE 

As  indicated  in  the  preceding  chapters,  the  effect  of  a  par- 
ticular treatment,  temperature  or  photoperiodic,  may  persist  and 
be  expressed  in  flowering  response  later,  even  though  no  anatomical 
changes  are  evident  when  the  treatment  is  stopped.  Induction,  as 
this  aftereffect  is  called,  is  widespread  though  not  universal,  and 
differs  considerably  in  both  permanence  and  location  within  the 
plant.  Confining  this  discussion  first  to   the  photoperiodically  in- 


84    •    Floral  Hormones  and  the  Induced  State 

duced  state,  we  find  that  it  is  transient  in  certain  plants— that  is, 
they  may  require  almost  continuous  exposure  to  the  appropriate 
photoperiod  in  order  to  flower— and  remarkably  long-lived  in  others 
(see,  for  example,  Doorenbos  and  Wellensiek,  1959;  Chouard,  1957). 
Probably  most  plants  are  at  neither  extreme  but,  like  Biloxi  soy- 
bean, revert  to  vegetative  growth  after  flowering  over  a  period  pro- 
portional to  the  previous  photoperiodic  treatment  (Borthwick  and 
Parker,  1938a;  Hamner,  1940).  For  obvious  reasons,  however,  the 
induced  state  has  been  studied  chiefly  in  a  few  plants  in  which  it 
is  relatively  permanent,  notably  in  two  SDP,  Xanthium  and 
Per  ilia. 

The  induced  state  in  Xanthium  is  both  persistent  and  trans- 
missible from  plant  to  plant.  The  transfer  of  a  florigen  from  a  single 
leaf  on  short  days  through  several  grafted  plants  has  already  been 
mentioned,  but  it  is  possible  to  separate  the  final  receptor  from 
the  short-day  donor  in  time  as  well.  If  a  plant  induced  by  short 
days  is  grafted  to  a  receptor  plant  in  long  days,  the  latter  will 
flower.  If  the  first  graft  is  broken  and  the  first  receptor  then  grafted 
to  another  vegetative  plant,  that  plant  will  also  flower  on  long 
days,  and  so  on  (see  Bonner,  1959a).  Thus  the  induced  state,  by 
which  is  meant  here  the  capacity  to  continue  producing  florigen, 
appears  to  be  transferable  from  plant  to  plant  along  with  the 
florigen  itself;  this  might  be  called  "indirect"  induction,  in  con- 
trast to  direct  induction  by  short  days. 

If  all  the  actively  growing  buds  of  a  single-leaved  Xanthium 
plant  are  removed  before  and  for  a  few  days  after  a  single  short-day 
cycle,  the  plant  remains  vegetative.  A  given  leaf  can  produce  the 
flowering  stimulus,  but  not  over  a  long  period  of  time;  the  young 
leaves  and  buds  can  apparently  be  indirectly  induced  by  older 
leaves,  however,  and  can  themselves  either  store  or  continue  to 
produce  the  stimulus  in  quantity.  The  experiments  indicating  this 
interaction  are  too  complex  to  describe  here  (Salisbury,  1955; 
Lincoln,  Raven,  and  Hamner,  1958),  but  suggest  that  in  Xanthium 
the  induced  state  is  not  permanently  localized  but  depends  on  the 
renewed  indirect  induction  of  the  younger  portions  of  the  plant. 

The  situation  obtaining  in  Perilla,  as  reported  by  both  Lona 
(1959)  and  Zeevaart  (1958),  is  quite  different.  A  photoperiodically 
induced  leaf  continues  to  produce  florigen  throughout  its  life.  It 
can  be  grafted  onto  a  plant  on  long  days,  bringing  it  to  flower, 


Permanence  and  Location  of  the  Induced  State    •    85 

then  removed  and  grafted  onto  another  plant,  with  the  same  result; 
this  can  be  repeated  as  long  as  the  leaf  remains  healthy,  which  may 
be  for  several  months  (see  Fig.  5-3).  There  is  no  evidence  that  any 
other  part  of  the  plant  has  a  role  in  the  maintenance  of  the 
induced  state;  detached  leaves  are  easily  induced  by  the  appropriate 
photoperiod,  as  can  be  demonstrated  by  subsequently  grafting  them 
onto  plants  on  long  days.  Experiments  of  this  kind  are  rarely 
successful  with  Xanthium.  The  clearest  difference  between  Perilla 
and  Xanthium  lies  in  the  lack  of  any  indirect  induction  in  the 
former.  When  Perilla  in  long  days  is  brought  to  flower  by  grafting 
an  induced  leaf  to  it,  the  leaves  it  subsequently  produces  remain 
noninduced,  incapable  of  causing  flowering  in  another  plant  on 
long  days. 

On  the  basis  of  these  observations,  the  relationship  between 
florigen  and  the  induced  state  in  Perilla  and  Xanthium.  appears  to 
differ  considerably.  In  the  former,  the  induced  state  is  localized 
in  the  leaf,  produced  only  by  photoperiodic  treatment  and  obviously 
separable  from  the  transmitted  florigen.  In  Xanthium,  indirect  in- 
duction of  the  developing  leaves  goes  on  continually,  either  as  a 
result  of  the  transmission  of  florigen  itself— in  which  case  the  pro- 
duction of  floral  stimulus  in  Xanthium  is  autocatalytic— or  brought 
about  by  a  second  unknown  substance  moving  with  it.  Without 
further  evidence,  the  first  possibility  clearly  requires  the  fewest 
assumptions,  although  it  raises  problems  which  will  be  considered 
later. 

As  the  induced  states  in  Xanthium  and  Perilla  are  maintained 
in  different  ways,  their  permanence  also  differs.  Implicit  in  much 
of  the  Xanthium  literature  is  the  idea  that,  once  induced,  a  plant 
remains  induced  throughout  its  lifetime.  In  a  sense  this  is  not  true, 
since  Lam  and  Leopold  (1960)  showed  that  reversion  can  be  brought 
about  by  constantly  removing  the  flowering  shoots  and  forcing  new 
ones  to  grow  out,  until  finally  vegetative  shoots  appear.  Several 
interpretations  of  these  results  have  been  suggested,  none  preferable 
to  others  on  the  basis  of  available  evidence;  but  it  is  nevertheless 
clear  that  without  such  drastic  treatment,  Xanthium  seldom  or 
never  reverts  even  after  induction  by  a  single  short-day  cycle.  The 
Perilla  plant,  unlike  Xanthium,  reverts  easily  to  the  vegetative 
state  under  long  days,  since  the  induced  older  leaves  die  and  there- 
is  no  indirect  induction  to  reinduce  the  younger.  It  is  thus  some- 


86    •    Floral  Hormones  and  the  Induced  Statf 


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Fig.  5-3.  Experiments  with  grafting  of  single  leaves  in 
Perilla.  (A)  Technique.  Left,  donor  leaf  in  polyethylene  bag. 
Right,  bag  removed;  in  this  case  the  leaf  has  been  trimmed 
to  give  a  standard  surface  area.  (B)  Induction  of  flowering 
in  long  days  by  a  grafted  leaf  previously  exposed  to  36  short 
days.  Photograph  made  41  days  after  grafting.  (Photographs 
from  Zecvaart  [1958],  courtesy  of  Dr.  J.  A.  D.  Zeevaart, 
Agricultural  Institute,  Wageningen.) 


Permanence  and  Location  of  the  Induced  State    •    87 

what  paradoxical  that  the  induced  state  in  Perilla  leaves  themselves 
appears  indestructible.  Attempts  by  Zeevaart  (1958)  to  remove  it 
were  completely  unsuccessful,  for  after  various  treatments  the 
capacity  to  bring  about  flowering  was  retained.  The  treatments 
included  exposure  of  the  detached  leaves  to  continuous  light  of 
low  or  high  intensity,  solutions  of  a  synthetic  auxin  (naphthalene- 
acetic  acid),  high  temperatures  (up  to  5  hours  at  42°  C),  and  the 
respiratory  inhibitors  dinitrophenol  and  sodium  azide.  As  long  as 
a  leaf  survived,  so  did  its  induced  state.  However,  Lam  and  Leopold 
(1961)  have  recently  obtained  results  indicating  that,  under  certain 
circumstances,  the  induced  state  in  a  Perilla  leaf  may  be  gradually 

lost. 

One  of  the  most  curious  properties  of  the  induced  state  in 
Xanthium  is  its  quantitative  nature.  This  is  not  to  be  confused 
with  the  phenomenon  previously  mentioned  (for  example,  in 
Biloxi  soybean)  in  which  eventual  reversion  to  the  vegetative  stale 
is  preceded  by  an  "amount"  of  flowering  proportional  to  the  in- 
ductive treatment.  In  Xantfiium,  too,  the  intensity  of  flowering  is 
quantitatively  related  to  the  inductive  treatment  (for  example, 
Salisbury,  1955),  but  since  intact  plants  do  not  revert,  they  merely 
continue  flower  development  at  a  very  slow  rate  if  the  initial 
induction  treatment  is  minimal.  F.  L.  Naylor  (1941)  compared  the 
development  of  plants  under  repeated  short  days  with  that  of  others 
given  only  a  single  short  day  and  then  placed  in  long-day  con-, 
ditions.  In  the  former,  inflorescences  with  all  flower  parts  complete 
were  evident  after  13  days,  and  the  seeds  were  almost  mature  within 
a  month.  The  second  group  did  not  show  complete  flower  develop- 
ment until  over  two  months  from  the  single  short  day,  but  the  slow 
progress  toward  fruiting  gave  no  sign  of  stopping  before  the  experi- 
ment was  discontinued,  shortly  thereafter.  This  kind  of  observation 
seems  much  more  difficult  to  explain  than  a  mere  reversion  to 
vegetative  growth.  The  latter  could  be  due  to  exhaustion  of  florigen 
or,  as  in  Perilla,  of  the  capacity  to  produce  it,  but  maintenance  of 
a  long-lived  but  low  "steady  state"  of  flowering  cannot  be  visualized 
on  this  basis.  In  a  sense  it  is  analogous  to  the  fractional  induction 
described  in  Chapter  Three,  except  that  in  fractional  induction 
there  is  no  morphological  or  anatomical  change  after  the  first, 
subminimal,  treatment. 

There  is  little  to  be  added  here  to  the  description  of  the  state 


88   •    Floral  Hormones  and  the  Induced  State 

induced  by  vernalization  covered  in  the  preceding  chapter.  Perhaps 
j is  most  remarkable  property,  :ikin  to  the  way  in  which  a  small  Leaf 
area  brings  about  flowering  in  a  large  plant,  is  the  way  in  which  only 
.1  small  portion  (the  meristem)  need  be  vernalized.  Present  evidence, 
however,  does  not  point  to  the  existence  <>i  a  transmissible  stimulus, 

.ind    the    vernalized    State    probably   occurs   only    in    tissues   actually 

derived  from  the  cells  originally  treated.  lake  photoperiodic  in- 
duction,  the  effect  <>l  cold  treatment  is  quantitative  and  "fra<  tional" 
undei  <  ertain  <  onditions. 

THE  BIOCHEMISTRY  ()!•   INDUCTION 

Whit   oi    the  cellular  and   biochemical  changes   involved   in 
induction  and  the  final  (lowering  response?  These  changes  must 

be  understood  il  knowledge  ol  (he  physiology  <>l  flowering  is  to  be 
more  than  supei  Ik  ial,  but  up  to  the  present  time  very  little  evidence 
sufficient  to  answer  the  question  has  been  un<o\eied.  The  subject 
cannot  be  dismissed  so  briefly,  however,  il  only  because  many 
investigators  have  tried  to  remedy  the  situation  and  one  should  be 
awaie   ol    their   attempts. 

As     indicated     in     Chapters    Two    and      Three,     photoperiodic 

induction  is  a  highly  complex  process.  In  SDP,  at  least,  it  is  often 
regarded  as  comprising  several  steps,  or  "partial  processes"-— the 

lust  high  intensity  light  pioeess,  the  dark  process,  the  low  intensity 
light  process  by  which  the  dark  process  can  be  inhibited,  and  the 
second    high  intensity    light    process.    To    these    can    also    be    added 

Horigen  synthesis  (marking  the  attainment  of  the  induced  state), 

followed  by  Horigen  translocation,  and  then  the  changes  in  the 
iiiciistcin  (see,  lor  example.   Bonner,    1959a;   Bonner  and   Liverman, 

1953;  Liverman,  l!).r>r>).   This  analysis  is  more  appropriate  lor  some 

plants  than  lot  others,  and  none  has  been  studied  enough  to 
disclose-  the   iiatuie  ol    ,mv  ol    the   partial    pioc  esses,  except    perhaps 

the  two  involving  high  light-intensity.  These  may  be  photosyntheti< . 

.is  we  have  seen  in  Chapter  Two.  and  thus  supply  both  energy  lot 
the  othci   changes  .ind  c  .u  bohvclrales  with  which  the  Horigen  moves. 

LDP  have  been  less  amenable  t<>  such  an  analysis,  particularly  with 
the  evidence  ol  both  promoting  and  inhibiting  actions  clue  to  the 

leaves  .ind  both  ol  which  may  be-  affected  by  light  and  darkness. 
One   ol    the    lew    consistent   observations   is   that    the   dark    (and    low 


The  Biochemistry  of  Induction    •    89 

light-intensity)  processes  in  most  plants  studied  appear  at  least  to 
have  the  red,  far-red  reversible  system  in  common,  but  its  bio- 
chemical function  is  unknown.  Again,  the  role  of  endogenous 
rhythms  is  uncertain. 

Many  specific  mechanisms  have  been  proposed  for  various 
processes  in  induction,  mostly  involving  transformations  and  inter- 
actions of  hypothetical  substances.  As  Lang  (1952)  has  pointed  out, 
they  are  often  little  more  than  generalized  restatements  of  particu- 
lar data.  Since  expositions  of  these  hypotheses  abound  in  the 
reviews  and  papers  cited,  no  attempt  will  be  made  to  represent 
them  here.  Instead  we  will  briefly  consider  some  of  the  general  areas 
of  investigation  involved. 

One  of  the  earliest  and  still  most  favored  ideas  is  that  auxin 
plays  a  major  part  in  photoperiodic  induction  and  flower  initiation. 
The  possibility  that  induction  might  be  caused  by  a  change  in 
auxin  content  was  tested  by  Chailakhyan  and  Zhdanova  (1938); 
they  concluded  that  this  was  unlikely  since  auxin  content  in  a 
number  of  plants  was  greater  on  long  than  on  short  days,  irre- 
spective of  whether  they  were  LDP  or  SDP.  More  recent  work  of 
the  kind  has  confirmed  their  general  conclusions  (see  Hillman  and 
Galston,  1961;  Doorenbos  and  Wellensiek,  1959),  but  a  major  prob- 
lem is  the  multiplicity  of  auxins  as  well  as  other  growth-promoting 
and  growth-inhibiting  substances  in  plants;  it  is  difficult  to  be  sure 
that  all  the  relevant  compounds  have  been  assayed  in  a  given 
investigation.  Thus  changes  in  one  or  another  identified  or  un- 
identified substance  may  or  may  not  be  correlated  either  with  a 
change  from  one  photoperiod  to  another  or  with  flowering  response, 
but  are  not  easily  interpretable  as  the  cause  of  flowering  (Cooke, 
1954;  Vlitos  and  Meudt,  1954). 

A  study  by  Harada  and  Nitsch  (1959a),  in  which  paper 
chromatography  was  used  to  separate  and  help  identify  various 
compounds,  illustrates  the  complexity  of  the  situation.  They  fol- 
lowed changes  in  the  amounts  of  growth  substances  extractable 
from  an  LDP,  an  SDP,  and  a  vernalizable  plant  at  various  times 
during  or  after  induction.  In  each  plant  there  was  a  number  (3  to 
6,  perhaps  more)  of  active  substances;  the  levels  of  some  changed 
in  such  a  way  as  to  suggest  that  they  might  be  the  cause  of  the 
developmental  changes  rather  than  being  merely  correlated  with 
them.  These  results  are  only  suggestive  at  present,  but  intensive 


90    •    Floral  Hormones  and  the  Induced  State 

pursuit  of  this  kind  of  work  may  eventually  clarify  the  relation 
of  auxins  and  similar  substances  to  flower  initiation. 

Another  approach  is  shown  in  the  work  of  Konishi  (1956).  His 
studies  of  auxin  level  in  several  LDP  (Sileiie,  Rudbeckia,  Spinacia) 
were  based  entirely  on  biological  assays  without  previous  separation 
of  possible  multiple  substances,  but  he  also  considered  enzyme 
systems  that  might  be  involved  in  the  synthesis  and  destruction  of 
the  known  auxin,  indoleacetic  acid.  Increased  activity  of  the  former 
and  reduced  activity  of  the  latter  were  associated  with  the  "bolting" 
—rapid  stem  elongation— characteristic  of  flowering  in  many  LDP; 
evidence  is  lading,  however,  that  these  changes  actually  cause 
bolting  and  flowering. 

Some  indirect  evidence  of  a  role  for  auxin  in  flowering  has 
been  obtained  with  radiations  believed  to  affect  auxin  concentra- 
tion, including  both  ultraviolet  (UV)  and  x-rays.  As  early  as  1887, 
Julius  Sachs  concluded  that  UV  promoted  flowering,  since  both 
Tropaeolum  (nasturtium)  and  Lepidium  flowered  readily  in  sun- 
light filtered  through  water  but  not  through  a  colorless  solution  of 
cjuinine,  which  absorbs  UV.  The  flowering  of  Linum  usitatissimum 
(flax)  and  Statice  bonduelli  is  greatly  hastened  by  exposure  to  a 
minute  or  two  of  intense  UV  each  day,  according  to  von  DenfTer 
and  Schlitt  (1951).  Supporting  von  Denfter's  (1950)  idea  that  auxin 
is  a  major  inhibitor  of  flowering,  they  concluded  that  this  effect  of 
UV  was  due  to  an  inactivation  of  auxin  within  the  plants,  and 
believe  it  explains  the  rapid  flowering  occasionally  encountered  at 
high  altitudes  where  more  UV  readies  the  vegetation.  Many  other 
plants  tested,  however,  did  not  respond  in  this  way.  An  example  of 
the  promotion  of  flowering  by  low  x-ray  doses,  known  to  reduce 
;iuxin  synthesis,  is  reported  by  Leopold  and  Thimann  (1919); 
flowering  in  Wintex  barley  was  increased  by  over  20  percent  after 
three  weekly   treatments  with   25   roentgens. 

Further  indirect  evidence  comes  from  the  eflects  of  gravity. 
Cieotropic  stimulation  is  known  to  cause  a  changed  pattern  of 
auxin  distribution  in  plants,  although  the  mechanism  is  unknown 
(see  Audits,  1959;  Leopold,  1955);  it  can  also  hasten  flowering.  The 
Cabezona  variety  of  pineapple  (Ananas  comosus)  can  be  brought 
to  flower  at  any  time  by  bending  the  stem  into  a  horizontal  position 
and  keeping  it  bent  for  as  few  as  three  days;  assays  confirm  the 
assumption  that  this  treatment  results  in  auxin  redistribution  (van 


The  Biochemistry  of  Induction    •    91 

Overbeek  and  Cruzado,  1948).  In  certain  soybean  varieties  also, 
keeping  the  stem  apex  bent  over  causes  earlier  flowering,  which 
Fisher  (1957)  again  attributes  to  auxin  redistribution,  presumably 
a  lower  level  at  the  older  nodes  resulting  from  an  accumulation  at 
the  apex. 

Hypotheses  on  the  role  of  auxin  in  flowering  have  been  based 
largely  on  the  effects  of  externally  applied  auxins  and  related 
compounds,  to  be  considered  in  the  next  chapter,  rather  than  on 
the  kind  of  work  described  above.  Neither  type  of  evidence  has 
lent  itself  to  any  simple  interpretation.  In  addition  to  hypotheses 
in  which  auxin  simply  inhibits  or  promotes  flowering,  one  of  the 
most  elaborate  schemes  suggested  relates  its  action  directly  to  the 
red,  far-red  reversible  system  (see  Liverman,  1955).  The  evidence 
is  derived  largely  from  work  with  processes  other  than  flowering, 
and  the  "morphogenetic  photocycle,"  as  the  scheme  has  been  called, 
has  not  been  widely  accepted,  at  least  in  its  original  form  (see  Lang, 
1959;  Hillman,   1959c). 

The  gibberellins,  a  class  of  compounds  to  be  discussed  in  the 
next  chapter,  can  cause  flowering  in  many  LDP  when  applied 
externally.  So  far  there  is  little  information  on  whether  the  control 
of  the  level  of  these  substances  by  photoperiod  or  temperature  may 
explain  certain  flowering  responses.  Some  of  the  Harada  and  Nitsch 
(1959)  results  are  suggestive  of  a  change  in  gibberellin  levels  follow- 
ing induction,  but  the  bioassay  used  was  relatively  unspecific.  A- 
more  specific  assay  was  used  by  Lang  (1960),  whose  preliminary 
results  show  a  higher  gibberellin  level  in  induced  than  in  non- 
induced  annual  Hyoscyamus.  That  this  may  be  a  cause  of  flowering 
rather  than  simply  correlated  with  it  is  indicated  by  the  fact  that 
the  increase  shows  up  soon  after  induction  and  is  less  pronounced 
after  flowering  is  well  under  way.  This  sort  of  work  is  now  develop- 
ing rapidly;  and,  as  mentioned  earlier  about  research  on  the 
red,  far-red  pigment,  what  is  reported  here  may  well  be  obsolete 
by  publication. 

The  role  of  respiratory  systems  has  also  been  studied.  Elliott 
and  Leopold  (1952),  for  example,  following  oxygen  uptake  in  leaf 
tissues  of  certain  SDP  and  LDP,  concluded  that  respiration  rate 
increased  in  the  former  and  decreased  in  the  latter  with  photo- 
induction,  whereas  rates  in  two  daylength-indiflerent  plants  were 
dependent  on  the  total  light  given.  Whether  such  correlations  are 


92    ■    Floral  Hormones  and  the  Induced  State 

general,  and  what  their  significance  might  be,  is  unknown.  The  fact 
that  various  well-known  respiratory  poisons,  including  cyanide, 
azide,  and  fluoride,  may  inhibit  the  dark  period  induction  (Naka- 
yama,  1958,  on  Pharbitis  nil)  does  not  afford  any  special  insight 
into  the  processes  involved,  but  indicates  simply  that  normal 
respiration  is  required  to  support  them.  This  is  true  also  of  ver- 
nalization, at  least  on  the  basis  of  the  oxygen-level  and  sugar-feeding 
experiments   mentioned   in    the    previous   chapter. 

There  has  been  a  series  of  investigations  on  the  fixation  of 
carbon  dioxide  in  darkness,  particularly  by  Kalanchoe,  since  photo- 
period  influences  its  time-course  and  intensity  in  a  manner  sug- 
gestive of  the  effect  on  flowering.  In  addition  other  work  has  shown 
that  exclusion  of  CO,  during  dark  periods  can  reduce  the  induction 
of  several  SDP.  These  results  are  reviewed  by  Kunitake  et  al.  (1957), 
who  concluded  from  their  own  experiments  with  radioactive  tracer 
techniques  that  short-day  induction  of  Kalanchoe  affected  not  the 
proportion  of  COo  fixed  in  various  compounds  but  only  the  total 
amount.  This  conclusion,  together  with  the  fact  that  even  this 
change  occurs  relatively  late  in  induction,  affords  no  support  for 
the  suggestion  of  a  specific  significance  for  dark  C02  fixation  in 
the  inductive  process. 

The  induced  state  in  many  plants  has  some  of  the  character- 
istics of  infection  with  a  virus,  or  some  other  self-replicating  entity. 
This  is  true  both  of  photoperiodically  induced  Xanthium,  in  which 
llorigen  production  appears  to  be  autocatalytic,  and,  in  a  different 
way,  of  vernalization  in  those  plants  in  which  the  vernalized  state 
is  maintained  in  all  cells  descended  from  those  originally  treated. 
Unfortunately  this  stimulating  hypothesis  of  flowering  as  a  virus 
disease  has  as  yet  no  direct  evidence  in  its  favor.  Changes  in  the 
levels  of  both  ribonucleic  and  desoxyribonucleic  acids  during  and 
following  photoinduction  have  been  observed  (Gulich,  1960,  and 
bibliography  therein),  but  all  attempts  to  show  qualitative  dif- 
ferences between  the  nucleic  acids  or  proteins  of  induced  and  non- 
induced  plants  have  been  unsuccessful  (see  Bonner  and  Liverman, 
1953;  Bonner,  1959b).  However,  some  indirect  evidence  has  been 
obtained  by  the  use  of  compounds  believed  to  inhibit  nucleic  acid 
synthesis.  Hess  (1959)  found  that  2-thiouracil  given  during  the 
vernalization  of  Streptocarpus  could  reduce  or  abolish  flower 
initiation    without    affecting    vegetative    growth;    5-fluorouracil    is 


The  Biochemistry  of  Induction     •    93 

reported  to  inhibit  photoperiodic  induction  in  Xanthium  in  a 
manner  possibly  suggestive  of  an  effect  on  the  synthesis  or  effective- 
ness of  the  flowering  hormone  (Salisbury  and  Bonner,  1960).  But 
2-thiouracil  also  causes  a  strong  inhibition  of  induction  in  another 
SDP,  hemp  (Cannabis  sativa);  careful  histological  observations  sug- 
gest that  this  action  and,  by  inference,  those  above  are  due  to  a 
general  effect  on  the  differentiation  capacities  of  the  meristem 
rather  than  to  a  specific  effect  on  flowering  (Heslop-Harrison, 
1960). 

A  question  of  fundamental  importance  concerning  photo- 
periodic induction  was  recently  raised  by  R.  M.  Sachs  on  the  basis 
of  his  and  other  work  with  LSDP  (see  Sachs,  1959).  It  has  been 
widely  assumed  that  the  basic  induction  process  in  both  LDP  and 
SDP  is  alike,  there  being  at  least  two  grounds  for  this  assumption. 
One  is  the  participation  of  the  red,  far-red  system  in  both  types 
and  the  other  is  the  apparent  equivalence  of  florigen  in  both  types, 
at  least  among  many  closely  related  plants.  But  Sachs  points  out 
that  in  the  LSDP  Cestrum  nocturnum  (night-blooming  jasmine) 
long-  and  short-day  induction  appear  to  differ  considerably.  The 
product  of  long-day  induction  ■  is  not  translocated  from  the  treated 
leaves;  short-day  induction  following  long-day  induction,  however, 
gives  rise  to  a  translocatable  flowering  hormone.  Further,  the  se- 
quence of  long-  and  short-day  induction  is  not  reversible  for  any 
plants  requiring  both— in  LSDP  the  former  must  precede  the  latter, 
whereas  in  SLDP  the  reverse  is  true.  Thus  if  one  assumes  that  long- 
day  induction  in  both  LSDP  and  SLDP  (as  well  as  in  simple  LDP) 
controls  the  same  step  in  a  series  of  reactions,  one  then  suspects 
that  the  short-day  induction  step  in  LSDP  is  not  equivalent  to  that 
in  SLDP.  Similarly,  assuming  that  short-day  induction  in  both  types 
(as  well  as  in  SDP)  is  the  same,  then  the  long-day  induction  in  the 
two  types  must  differ.  In  addition  to  indicating  that  short-  and  long- 
day  induction  may  affect  different  processes,  Sachs  suggests  that  "we 
should  be  wary  of  the  assumption  that  LD  induction  affects  the 
same  stage  of  synthesis  of  the  floral  stimulus  in  every  LDP  (the 
same  doubt  exists  with  regard  to  SD  induction  in  all  SDP)."  The 
question  will  be  finally  answered  only  by  a  complete  understanding 
of  the  biochemistry  involved,  which  may  take  many  years.  The  logic 
of  Sachs's  analysis  warns  that  the  answer  will  not  be  simple,  and 
may  also  be  different  for  different  plants. 


94    •    Floral  Hormones  and  the  Induced  State 


CONCLUDING  REMARKS 

An  attempt  at  some  sort  of  evaluation  is  desirable  here,  if  only 
to  avoid  ending  on  a  note  of  complete  confusion.  Some  of  the  views 
to  be  expressed  differ  greatly  from  those  held  by  other  writers,  who 
also  differ  among  themselves;  anyone  seriously  concerned  with 
theoretical  interpretations  should  consult  various  reviews  cited 
earlier. 

The  "all-or-none,"  qualitative  character  of  both  floral  initia- 
tion and  photoperiodic  induction  has  been  widely  stressed  (for 
example,  Lang,  1952).  In  the  writer's  opinion,  it  is  a  questionable 
concept.  Admittedly,  there  are  situations  in  which  one  either  sees 
or  does  not  see  a  floral  primordium,  so  that  the  final  judgment  is 
either  "flowering"  or  "vegetative."  The  same  could  be  said,  how- 
ever, about  the  growth  or  nongrowth  of  a  piece  of  tissue;  at  the 
lower  limit  of  the  technique  used,  one  either  detects  growth  or 
does  not,  yet  there  is  no  general  opinion  that  growth  is  an  all-or- 
none  phenomenon.  Bonner  (1959a),  accepting  the  photoperiodic 
response  as  in  a  sense  quantitative,  nevertheless  goes  on,  "each 
bud  and  each  plant  is  either  reproductive  or  vegetative."  Logically, 
this  is  true  enough.  But  in  developmental,  morphological  terms, 
one  has  only  to  consider  work  like  that  of  Harder  (1948)  on 
Kalanctwe  to  reali/e  that  there  can  be  a  continuum  between  obvi- 
ously vegetative  and  obviously  reproductive  growth. 

One  origin  of  the  all-or-none  view  may  be  an  overemphasis 
on  flower  initiation  (although  such  studies  usually  involve  some 
degree  of  development)  with  too  little  attention  to  the  fact  that 
optimum  flower  development  often  requires  a  continuation  of  the 
inducing  conditions.  A  good  illustration  of  this  common  situation 
was  recently  given  by  Zabka  (1961)  working  with  Amaranth  us 
raudatits.  At  a  certain  age  this  is  a  very  sensitive  SDP;  when  older, 
it  initiates  flowers  even  under  long  days.  Under  any  circumstances, 
however,  inflorescence  development  and  fruiting  are  strongly 
favored  by  short  days,  no  matter  how  initiation  came  about. 

Another  major  support  of  the  all-or-none  view  has  been  the 
fact  that,  in  SDP  lor  example,  flowering  does  not  occur  at  day- 
lengths  above  the  critical  but  does  occur  at  lower  values.  This  thus 
n(  emed   to  represent   a   sharp,   qualitative  cut-oil   in    the  curve  of 


Concluding  Remarks    •    95 

response  versus  daylength,  but  only  on  the  assumption  that  day- 
lengths  above  the  critical  had  no  other  effect  than  to  be  noninduc- 
tive.  Work  mentioned  in  Chapter  Three,  however,  indicates  now 
that  such  daylengths  are  often  positively  antiinductive,  not  merely 
ineffective,  and  that  this  antagonistic  effect  is  quantitatively  related 
to  the  amount  by  which  the  noninductive  daylength  exceeds  the 
critical.  While  no  generalization  is  likely  to  hold  for  all  plants, 
it  is  possible  that  the  processes  involved  in  induction  proceed  con- 
tinuously, and  that  only  the  ratio  of  the  rates  of,  say,  two  or  more 
of  them  differs  under  different  daylengths.  The  critical  daylength 
would  then  be  that  value  at  which  the  ratio  neither  promotes  nor 
inhibits  the  train  of  events  finally  leading  to  flowering. 

Many  of  the  subjects  touched  on  in  the  preceding  chapters, 
including  the  question  of  the  degree  of  difference  between  the 
structure  of  vegetative  and  floral  meristems,  bear  on  this  sort  of 
problem,  but  cannot  be  enlarged  upon  now.  The  relevance  of  such 
theoretical  considerations  to  more  concrete  questions  is  largely 
in  the  suggestion  that  flowering  does  not  represent  a  sudden  change, 
some  sort  of  developmental  "quantum-jump,"  but  is  probably  under 
controls  similar  to  those  affecting  vegetative  growth,  to  the  small 
degree  that  these  are  understood. 

Consider,  for  instance,  the  nature  of  floral  stimuli.  That  some- 
thing moves  between  induced  and  noninduced  parts  of  a  plant,  or 
between  grafted  plants,  cannot  be  doubted.  Movement  of  active 
substances  from  vegetative  to  reproductive  tissue  is  also  highly 
probable.  In  physiological  terms,  then,  both  florigen  and  anti- 
florigen  appear  to  be  valid  concepts,  but  in  the  absence  of  extracted 
samples  one  can  only  speculate  as  to  their  nature  and  whether  they 
are  the  same  in  all  plants.  In  the  light  of  the  considerations  above, 
it  appears  extremely  unlikely  to  the  writer  that  florigens,  whether 
simple  substances  or  as  complex  as  a  virus,  are  likely  to  be  specific 
floral  hormones  in  the  sense  that  they  are  involved  only  in  the 
processes  of  floral  initiation  and  development  but  no  others.  Julius 
Sachs's  concept  of  specific  organ-forming  substances  has  not  stood 
the  test  of  experimentation,  since  most  vegetative  systems  studied 
indicate  that  particular  aspects  of  development  can  be  controlled 
by  the  concentrations  and  interactions  of  substances  that  affect  many 
other  processes  as  well.  A  few  examples  will  be  helpful  here. 

The  use  of  the  auxin  indoleacetic  acid  in  rooting  cuttings  is 


96    •    Floral  Hormones  and  the  Induced  State 

well  known;  in  addition,  much  of  the  rooting  behavior  of  cuttings 
can  be  explained  in  terms  of  their  auxin  content  and  sensitivity. 
Yet  it  is  also  known  that  the  same  compound  plays  a  major  role 
in  other  developmental  processes  having  nothing  to  do  with  root 
initiation,  so  that  it  would  be  grossly  misleading  to  call  it 
"rhizogen"  (root-maker).  That  development  is  controlled  by  the 
balance  of  various  substances  common  to  many  processes  is  strik- 
ingly illustrated  by  the  work  of  Skoog  and  Tsui  (1948)  and  Miller 
and  Skoog  (1953).  Tobacco  stem  segments  grown  in  aseptic  culture 
produce  roots  if  supplied  with  a  particular  level  of  auxin  and 
shoots  if  supplied  with  another  substance,  adenine.  Both  com- 
pounds together  cause  the  production  of  more  or  less  disorganized 
callus  tissue;  but  increasing  the  adenine  again  leads  to  shoot  forma- 
tion, whereas  increasing  the  auxin  leads  to  root  formation.  Thus 
the  balance  of  auxin  and  adenine  controls  the  production  of  roots 
or  shoots  in  this  system.  Adenine,  as  a  component  of  the  nucleic 
acids  and  many  respiratory  co-enzymes,  is  probably  present  in  every 
living  cell;  the  many  roles  of  auxin  have  already  been  mentioned 
(see  Audus,   1959). 

A  simpler  example  of  control  by  an  unspecific  substance  was 
found  by  Wetmore  (1953),  who  studied  the  development  of  young 
fern  apices  in  aseptic  culture.  The  first  few  leaves  produced  by 
ferns,  as  by  many  other  plants,  may  differ  considerably  from  the 
later  ones,  being  characteristically  "juvenile"  in  some  way;  the 
ferns  in  question  (Todea,  Adiantum)  have  juvenile  leaves  with 
few  or  no  divisions,  whereas  the  older  leaves  are  deeply  lobed.  In 
culture,  mere  variation  of  the  sucrose  content  of  the  medium 
suffices  to  bring  about  almost  any  degree  of  "juvenility"  or 
"maturity"  in  leaf  shape,  with  the  lowest  sucrose  level  giving  the 
least  lobed  leaves.  Thus  the  normal  leaf  progression,  regarded  as 
a  fundamental  developmental  property  of  the  meristem  and  one 
of  considerable  evolutionary  significance,  is  susceptible  to  regulation 
by  a  substance  that  presumably  serves  merely  as  a  general  energy 
source.  This  result  may  have  more  than  illustrative  value  here. 
If,  as  Philipson  (1949)  suggests,  the  reproductive  apex  simply 
represents  a  normal  later  stage  in  the  ontogeny  of  the  shoot,  as 
does  the  transition  from  juvenile  to  mature  foliage,  then  perhaps 
a  local  increase  in  carbohydrates  may  play  a  central  role  in 
flowering  itself. 


Concluding  Remarks    •    97 

One  further  study  on  vegetative  growth  should  be  considered 
since  it  bears  comparison  with  the  quantitative  yet  long-lived 
induced  state  which  seems  so  puzzling  in  Xanthium.  The  reader 
whose  sensibilities  were  disturbed  by  "flowering  as  a  virus  disease" 
will  have  to  make  the  best  of  another  similar  analogy,  this  time  to 
the  plant  disease  crown-gall.  In  many  ways  resembling  cancer  in 
animals,  crown-gall  is  brought  about  by  a  bacterium;  following 
infection,  the  tissues  become  tumorous,  growing  rapidly  in  a 
disorganized  fashion,  and  continue  to  do  so  even  when  the  bacteria 
are  no  longer  present.  Pieces  of  such  bacteria-free  tissue  grow 
rapidly  in  culture  on  a  simple  mineral  medium  with  sucrose  and 
a  few  vitamins,  whereas  normal  callus  tissue  from  the  same  plant 
fails  to  grow  under  the  same  conditions.  Braun  (1958)  has  been 
able  to  make  a  whole  series  of  tissue  clones  intermediate  between 
typical  crown-gall  and  typical  normal  tissues  in  their  growth  rate 
on  the  basic  medium.  This  was  done  by  letting  the  bacterial  infec- 
tion proceed  for  different  lengths  of  time  before  a  heat  treatment 
that  stops  it  without  harming  the  tissue.  In  order  to  make  normal 
tissue  grow  as  fast  as  fully  tumorous  crown-gall  tissue  in  culture, 
one  must  add  to  the  basic  medium  6-furfuryl  amino  purine, 
guanylic  and  cytidylic  acids,  asparagine,  glutamine,  inositol,  and 
naphthaleneacetic  acid.  If  the  tissue  has  been  exposed  to  infection 
for  a  short  time,  the  first  compound  may  be  omitted;  if  it  has  been 
exposed  for  a  longer  time,  the  first  four  may  be  omitted,  without 
reducing  the  rate  below  that  of  the  fully  tumorous  tissue. 

Each  strain  of  tissue  maintains  its  particular  nutritional  re- 
quirements in  culture  and  does  not  revert  to  normal.  Braun  con- 
cludes that  "a  series  of  quite  distinct,  but  well-defined,  growth- 
substance-synthesizing  systems  becomes  progressively  activated" 
during  the  crown-gall  induction.  In  short,  a  quantitative  gradation 
exists  as  a  result  of  several  qualitative  changes  in  metabolism. 
Perhaps  photoperiodic  induction  in  some  plants  is  a  process  of  this 
kind,  with  many  intermediate  stages,  and  not  a  unitary  process 
at  all. 

With  such  work  as  background  one  might  envision  florigen 
as  either  a  single  substance,  or  a  combination  of  substances, 
normally  occurring  in  many  plant  cells,  but  frequently  present 
in  insufficient  quantities  or  improper  balance  for  the  meristem 
to  proceed  to  reproductive  development.  If  production  in  another 


98    •    Floral  Hormones  and  the  Induced  State 

part  of  the  plant,  the  leaf,  is  susceptible  to  modification  by  day- 
length,  there  will  be  evidence  of  photoperiodically  induced,  trans- 
locatable  floral  stimuli  or  inhibitors.  When  such  production  is  not 
under  photoperiodic  control,  the  stimuli  or  inhibitors  may  still 
be  demonstrable.  There  is  no  a  priori  reason  to  assume  that  these 
are  the  same  for  all  plants  simply  because  they  appear  to  be  so 
in  certain  closely  related  forms.  (They  do  not  appear  to  be  so  in 
all:  see  Zeevaart,  1958.)  On  the  other  hand,  work  with  the  gib- 
berellins  indicates  that  the  same  compound  can  cause  flowering 
in  many  unrelated  LDP,  although  gibberellins  themselves  cannot 
be  florigen,  as  will  be  indicated  in  the  next  chapter. 

The  fact  that  floral  stimuli  to  the  present  have  proved  non- 
extractable,  and  are  transferable  only  by  grafting,  has  been  used 
as  supporting  evidence  for  the  "virus"  concept  (see  Bonner,  1959b) 
in  spite  of  the  fact  that  many  viruses  are  easily  extracted  and 
transmitted  by  other  means.  It  is  at  least  as  likely  that  the  com- 
pounds involved  are  simply  unstable  under  most  extraction  tech- 
niques. Still  another  possibility  is  precisely  that  florigen  activity 
is  either  due  to  a  particular  balance  of  substances  or,  as  suggested 
by  Went  (1959),  is  the  reflection  "of  rhythmic  concentration 
changes"  of  one  or  more  substances.  In  either  case,  extraction  of 
the  right  combination  would  prove  extremely  difficult,  and  move- 
ment through  a  nonliving  gap  might  disrupt  the  relationships 
involved  even   though  the  substances  themselves  were  stable. 

The  reader  may  well  protest  that  the  intent  of  this  section, 
"to  avoid  ending  on  a  note  of  complete  confusion,"  has  been  badly 
betrayed.  In  answer,  the  entire  point  here  is  that  there  is  no  con- 
fusion, only  ignorance.  There  are  undoubtedly  many  growth- 
regulating  substances  and  systems  of  which  we  know  nothing  as  yet, 
and  which  will  change  present  attitudes  as  much  as  work  with  the 
red,  far-red  system  or  the  gibberellins  is  changing  those  of  the  past 
decades.  Therefore  a  comprehensive  statement  on  the  subject  of 
this  chapter  is  not  only  impossible  but  undesirable,  since  it  would 
have  to  assume  that  all  parts  of  the  puzzle  are  now  in  hand  and 
simply  need  putting  together.  All  of  the  concepts  in  the  literature 
are  valuable  to  the  extent  that  they  are  useful  as  working 
hypotheses,  but  they  should  not  be  mistaken  lor  anything  else. 
What  we  need  is  more  of  the  missing  pieces,  wherever  or  however 
thev  mav  be  found. 


► 
► 
► 


chapter  six  t  chemical  Control 

of  Flowering 


Attempts  to  bring  about  or  prevent  flowering  by  the  applica- 
tion of  chemicals  are  carried  on  for  both  practical  and  theoretical 
reasons.  The  former  are  self-evident,  the  latter  hardly  less  so.  As 
already  indicated,  studies  on  the  mechanism  of  induction  have 
included  work  with  various  metabolic  inhibitors,  which  will  not 
be  considered  further  here.  More  attention  has  been  paid  to  the 
effects  of  naturally  occurring  compounds  and  of  other  substances 
that  modify  plant  growth;  variations  in  the  supply  of  various 
minerals  have  also  been  studied  with  respect  to  flowering. 

A  major  motive  of  this  kind  of  work  has  been  the  hope  of 
discovering  compounds,  either  naturally  occurring  or  synthetic, 
with  florigen  activity.  Although  there  have  been  reports  of  success 
from  time  to  time,  none  of  these  has  as  yet  proved  valid.  Either 
the  work  has  been  unrepeatable  or  the  substance  in  question  has 
not  fulfilled  the  criteria  for  florigen.  Drawing  on  the  previous 
chapter,  the  minimal  requirement  for  such  activity  is  the  ability  to 
bring  about  flowering  both  in  LDP^  under  short  days  and  in  SDP 
under  long  days,  as  well  as  in  cold-requiring  but  un vernalized 
plants.  In  addition,  if  the  substance  is  to  be  considered  a  true 
(naturally  occurring)  florigen,  it  should  of  course  be  produced 
only  under  inductive  conditions.  It  is  well  to  keep  these  criteria 
in  mind,  since  the  effects  of  the  first  class  of  compounds  to  be  con- 
sidered are  dramatic  enough  to  be  misleading  in  this  regard. 

99 


100    •    Chemical  Control  of  Flowering 


THE  GIBBERELLINS 


The  single  most  striking  property  of  the  gibberellins,  besides 
the  effects  on  flowering  to  be  discussed,  is  their  ability  to  cause 
greatly  accelerated  growth  in  intact  plants.  This  is  evident  mainly 
in  the  stem,  but  occurs  also  in  other  parts  and  is  especially  obvious 
in  certain  "dwarf"  varieties.  No  other  group  of  compounds,  includ- 
ing the  auxins,  is  known  to  have  such  effects  on  a  wide  variety  of 
intact  plants.  Gibberellins  also  act  on  many  of  the  same  phenomena 
affected  by  red  and  far-red  light.  Such  action  is  not  consistently 
in  one  direction— in  some  cases,  such  as  seed  germination,  gibberel- 
lins appear  to  mimic  the  effect  of  red,  but  in  others  (for  example, 
stem  elongation)  they  act  in  the  same  direction  as  far-red.  It  has 
thus  been  suggested  that  gibberellins  may  be  involved  in  the  action 
of  the  red,  far-red  system,  but  none  of  the  specific  hypotheses  pro- 
posed is  as  yet  sufficiently  grounded  to  be  considered  here. 

Several  gibberellins  have  been  isolated  from  higher  plants,  but 
the  group  was  originally  discovered  as  products  of  a  fungus 
(Gibberella  fujikuroi)  causing  a  rice  disease  characterized  by 
excessive  stem  elongation.  They  are  complex  compounds  that  can 
be  regarded  as  derivatives  of  the  hydrocarbon  fluorene  with  lactone, 
hydroxyl,  and  other  substituents.  The  detailed  structures  of  some 
of  them,  notably  gibberellin  A3  (gibberellic  acid),  are  fairly  well 
established.  Much  of  the  work  to  be  discussed  has  been  done  with 
gibberellic  acid,  but  other  gibberellins  have  been  studied  as  well, 
and  the  general  term  "gibberellin"  will  often  be  used.  Research  on 
the  gibberellins  has  been  pursued  for  several  decades  in  Japan, 
but  became  known  outside  that  country  only  relatively  recently. 
The  first  generally  available  review,  by  Stowe  and  Yamaki  in  1957, 
has  since  been  followed  by  others,  and  all  should  be  consulted  for 
a  thorough  knowledge  of  this  rapidly  developing  topic  (Brian, 
1959;  Phinney  and  West,  1960;  Stowe  and  Yamaki,  1960;  Wittwer 
and  Bukovac,  1958).  For  an  excellent  discussion  of  gibberellin  and 
flowering,  see  Lang  and  Reinhard  (1961). 

The  first  thorough  publication  on  gibberellin  and  flowering 
was  that  of  Lang  (1957),  showing  that  a  few  drops  of  a  dilute  solu- 
tion (chiefly  gibberellic  acid)  given  repeatedly  to  the  growing  point 
or    leaves    brought    about     flowering    of    unvernalized     biennial 


The  Gibberellins    •    101 

Hyoscyamus,  carrot  (Daucus  carota),  and  several  other  biennials, 
all  under  long-day  conditions  (see  Fig.  6-1).  Several  LDP  kept  on 
short  days,  including  annual  Hyoscyamus,  Samolus  pannftorus,  and 
Silene  armeria,  also  flowered  in  response  to  such  treatment.  No 
promotion  of  flowering  occurred  in  the  SDP  Xanthium  and  Biloxi 
soybeans  kept  on  long  days.  These  experiments  were  conducted 
with  gibberellins  of  fungal  origin.  Similar  results  on  both  Samolus 
and  biennial  Hyoscyamus  were  later  obtained  with  extracts  of 
wild-cucumber  (Echinocystis)  seeds,  known  to  be  rich  higher-plant 
sources  of  gibberellins  (Lang  et  ah,  1957).  Evidently,  then,  gibberel- 
lin  can  substitute  for  the  cold  requirement  of  certain  vernalizable 
plants  and  for  the  long-day  requirement  of  certain  LDP,  but  not 
for  the  short-day  requirement  of  SDP.  This  general  conclusion  still 
appears  valid,  but  requires  expansion. 

Vernalization  or  long-day  requirements  have  not  been  suc- 
cessfully replaced  by  gibberellin  in  all  plants  tested.  One  reason 
for  this  may  be  the  known  difference  in  activity,  for  a  given  plant, 
among  the  various  gibberellins  themselves  (see  Phinney  and  West, 
1960)  well  illustrated  by  Fig.  6-2.  Possibly  plants  that  have  not 
responded  so  far  will  do  so  when  other  gibberellins  are  tried.  In 
the  "classical"  experimental  objects  for  vernalization  studies,  the 
winter  cereals,  gibberellic  acid  can  hasten  flowering  in  unvernalized 
seedlings,  but  only  when  applied  at  a  particular  stage;  in  addition 
to  flowering,  which  is  often  abnormal  or  abortive,  other  changes  in 
meristem  development  occur  (Caso  et  al.,  1960;  Koller  et  al.,  1960; 
Purvis,  1960).  Further  lack  of  exact  correspondence  between  gib- 
berellin  effects  and  vernalization  is  found  in  the  work  of  Sarkar 
(1958),  discussed  in  the  next  chapter,  showing  that  optimum 
sensitivity  to  gibberellin  or  to  cold  treatment  need  not  occur  at 
the  same  stage  of  development.  Moore  and  Bonde  (1958)  have 
observed  that  gibberellic  acid  actually  devernalizes  or  prevents 
vernalization  in  a  variety  of  Pisum,  depending  on  whether  it  is 
applied  after  or  before  the  cold  treatment. 

It  is  important  to  realize  that,  at  least  so  far,  all  the  LDP  in 
which  gibberellin  does  replace  long  days  are  those  in  which  flower- 
ing is  associated  with  "bolting"— the  rapid  elongation  of  the  axis 
from  the  almost  stemless  "rosette"  of  leaves  characteristic  of  the 
vegetative  condition.  In  caulescent  LDP,  having  elongated  stems 
even  when  vegetative,  gibberellin  apparently  cannot  bring  about 


102 


Chemical  Control  of  Flowering 


Fig.  6-1.  Substitution  of  gibberellic  acid  (GA)  for  cold  treatment  in  the  flower- 
ing of  the  biennial,  carrot  (Daucus  carota).  Left  to  right:  controls  on  long  days  only; 
long  days  plus  GA,  no  cold  treatment ;  long  days  plus  previous  cold  treatment, 
no  GA.  (Photograph  from  Lang  [1957],  courtesy  of  Dr.  A.  Lang,  California 
Institute  of  Technology.) 


The  Gibberellins 


103 


Fig.  6-2.  Effects  of  various  gibberellins  on  flowering  of  the  LDP  lettuce 
(Lactuca  sativa  var.  Grand  Rapids)  on  short  days.  From  left  to  right:  controls 
(vegetative),  and  gibberellins  Ax  (flowering),  A2,  A3,  and  A4.  Plants  were 
treated  with  a  total  of  4  applications  of  10  microliters  of  10~3  M  solutions  at 
weekly  intervals  starting  when  6  to  8  true  leaves  were  present.  (Photograph 
courtesy  of  Dr.  M.  J.  Bukovac,  Michigan  State  University.) 


flowering.  Examples  of  such  plants  are  Roman  nettle  (Urtica 
pilulifera)  and  enchanter's  nightshade  (Circaea  lutetiana)  (Lona, 
1956).  Since  most  of  the  widely  studied  LDP  are  rosette  plants, 
the  notion  that  gibberellin  promotes  flowering  in  all  LDP  has  been 
current  but  is  probably  untrue.  Not  even  all  rosette  plants  tested 
have  proved  responsive. 


104    •    Chemical  Control  of  Flowering 

Most  of  the  other  situations  in  which  gibberellin  substitutes 
for  long  days  involve  stem  elongation.  It  causes  flowering  in  the 
LSDP  Bryophyllum  crenatum  grown  under  short  days,  thus  satis- 
fying the  long-day  requirement;  this  again  is  a  matter  of  bringing 
about  bolting  (Biinsow  et  al.,  1958).  Another  example  is  its  action 
on  strawberry  plants,  in  which  it  causes  runner  initiation,  petiole 
elongation,  and  flowering  inhibition.  These  effects  are  all  similar 
to  those  of  long  days,  and  the  postulated  flower-inhibiting,  growth- 
promoting  substance  produced  on  long  days  may  be  related  to  gib- 
berellin. (Thompson  and  Guttridge,  1959;  see  also  Chapter  Five 
in  this  volume.) 

The  action  of  gibberellin  on  stem  development  may  well  be 
primary,  with  the  promotion  of  flowering  in  rosette  plants— both 
LDP  and  biennials— an  indirect  result.  Lang  (1957),  for  example, 
noted  that  although  flower  initiation  in  the  rosette  plants  studied 
occurred  with  the  start  of  bolting  under  normal  conditions— long 
days,  or  vernalization  followed  by  long  days— bolting  in  gibberellin- 
treated  plants  generally  preceded  flower  initiation.  In  some  rosette 
plants,  gibberellin  causes  bolting  only,  without  flowering  (Lona, 
1956;  see  Wittwer  and  Bukovac,  1958).  In  many  rosette  plants, 
normal  flowering  occurs  only  if  the  environmental  requirements 
are  partially  satisfied  (see  Brian,  1959;  Chouard,  1960).  Anatomical 
investigations  by  Sachs,  Lang,  and  collaborators  (Sachs  et  al.,  1959, 
1960)  show  that  the  early  effect  of  gibberellin  treatment  on  several 
rosette  plants  is  the  activation  of  the  "subapical  meristem,"  some- 
what below  the  growing  apex.  The  increased  cell  divisions  in  this 
area  are  largely  transverse;  this,  plus  the  subsequent  cell  elongation, 
results  in  rapid  stem  growth.  Gibberellin  can  also  completely 
reverse  the  effects  of  the  complex  growth-regulating  compound 
Amo-1618,  which  causes  a  dwarfed  or  rosette  habit  in  normally 
caulescent  plants  such  as  Chrysanthemum  by  inhibiting  the 
activity  of  trie  subapical  meristem.  While  such  work  bears  no  direct 
relationship  to  flowering,  it  strengthens  the  view  that  gibberellin 
may  indirectly  remove  some  inhibition  on  flowering  through  its 
direct  effect  on  stem  growth. 

Gibberellin  may  either  promote  or  inhibit  later  flower  develop- 
ment in  SDP,  but  is  entirely  unable  to  bring  about  initiation  under 
noninductive  conditions.  In  addition  to  the  work  already  men- 
tioned,  a  striking  example  of  its   ineffectiveness  occurs   with   the 


The  Gibberellins    •    105 

species  Chrysanthemum  morifolium.  In  those  varieties  requiring 
only  cold  treatment  to  flower,  irrespective  of  daylength,  gibberellic 
acid  can  cause  flowering.  In  those  that  are  SDP,  however,  it  does 
not  (Harada  and  Nitsch,  1959b).  In  Kalancho'e,  gibberellin  reduces 
the  flowering  of  plants  kept  on  short  days,  although  it  promotes 
vegetative  growth.  In  spite  of  this,  the  effect  is  not  identical  with 
that  of  long  days  since  it  makes  no  difference  whether  or  not  the 
gibberellin-treated  leaf  lies  between  the  short-day  (induced)  leaf 
and  the  growing  point  (Harder  and  Biinsow,  1956,   1957). 

At  least  two  detailed  studies  on  Xanthium  have  appeared. 
Both  agree  that  gibberellic  acid  cannot  cause  flowering  under  long- 
day  conditions;  it  can,  however,  increase  the  flowering  response  to 
a  limited  number  of  short-day  cycles.  Greulach  and  Haesloop 
(1958)  obtained  such  results  with  intact  plants;  Lincoln  and 
Hamner  (1958),  on  the  other  hand,  found  this  effect  only  in  de- 
budded  plants,  and  concluded  that  the  compound  acted  by 
increasing  the  capacity  of  the  young  leaves  to  store  the  flowering 
stimulus. 

Flowering  in  a  strain  of  the  duckweed  Lemna  perpusilla  may 
take  place  under  any  daylength  or  may  require  short  days,  depend- 
ing upon  factors  to  be  discussed  later.  In  both  situations,  however, 
gibberellin  can  completely  abolish  flowering  at  levels  that  promote 
vegetative  growth,  although  other  associated  morphogenetic  effects 
prevent  this  from  being  considered  a  specific  inhibition  of  flower^ 
ing  (Hillman,   1960). 

In  summary,  the  gibberellins  have  already  contributed  greatly 
to  the  study  of  flowering:  they  are  the  first  compounds  discovered 
with  which  many  kinds  of  plants  can  be  brought  to  flower  almost 
at  will.  Further  understanding  of  the  way  in  which  they  fully  or 
partially  satisfy  requirements  for  long-day  or  cold  treatments,  at 
least  in  rosette  plants,  will  be  of  great  value.  The  closeness  of  their 
relation  to  flowering,  as  compared  with  other  developmental 
processes  such  as  stem  elongation,  is  still  in  doubt,  and  the  results 
with  SDP  indicate  that  no  gibberellin  so  far  tested  can  be  con- 
sidered a  florigen.  However,  there  is  good  preliminary  evidence 
that  native  gibberellin  levels  in  certain  plants  increase  as  a  result 
of  treatments  leading  to  flowering,  and  such  changes  may  be  part 
of  the  normal  mechanism  involved  (Chapter  Five). 


106    •    Chemical  Control  of  Flowering 


AUXINS.  AUXIN  ANTAGONISTS,  AND  OTHER 
GROWTH  REGULATORS 

Since  auxins  were  widely  known  long  before  the  gibberellins, 
there  has  been  more  work  on  their  effects  on  flowering.  In  addition 
to  auxins,  one  must  consider  also  the  effects  of  auxin  antagonists. 
This  broad  term  is  used  here  to  cover  any  substances  believed  to 
act  in  a  manner  opposed  to  that  of  auxin.  Such  action  may  be 
exerted  through  a  molecular  structure  sufficiently  similar  to  that 
of  an  auxin  to  interact  with  the  same  biochemical  site,  yet  not 
sufficiently  similar  to  participate  further  in  whatever  system  auxin 
normally  acts.  Such  an  auxin  antagonist,  competitive  with  auxin 
molecules,  would  be  a  true  "antiauxin."  Other  auxin  antagonists 
may  act  by  interfering  with  native  auxin  synthesis,  by  blocking  the 
transport  of  auxin  from  the  site  of  action,  or  by  interfering  with 
the  effectiveness  of  auxin  in  some  other  way.  Finally,  many  other 
organic  compounds  effective  as  growth  regulators— capable  of  modi- 
fying development  in  various  ways— have  also  been  tested  on 
flowering.  All  of  these  topics  will  be  considered  briefly.  None  of 
the  results  so  far  has  provided  much  clear  information  on  flower- 
ing, since  most  of  the  evidence  suggests  that  the  effects  obtained 
are  extremely  indirect. 

As  noted  in  the  preceding  chapter,  studies  on  the  changes  in 
native  auxin  levels  associated  with  flower  induction  are  incon- 
clusive. In  considering  the  effects  of  applied  auxins,  one  should 
bear  in  mind  that  these  frequently  cause  all  kinds  of  abnormalities 
in  growth,  depending  upon  the  concentrations  (see,  for  example, 
Audus,  1959;  Leopold,  1955).  With  respect  to  auxin  effects  on 
flowering,  comparison  of  earlier  reviews  (for  example,  Lang,  1952; 
Bonner  and  Liverman,  1953)  with  more  recent  ones  such  as 
Leopold's  (1958)  or  the  excellent  critical  article  by  Lang  (1959) 
indicates  a  marked  decline  in  the  certainty  with  which  any  general 
statement  can  be  made. 

There  have  been  indications  that  auxin  treatment  promotes 
flowering  in  LDP  and  inhibits  in  SDP.  The  results  of  some  of  the 
papers  on  this  question  should  illustrate  the  general  uncertainty. 

In  experiments  by  Liverman  and  Lang  (1956)  flower  initiation 
in    annual    Hyoscyamns   and   Silene    was    promoted    by    the   auxin 


Auxins,  Growth  Regulators    ■    107 

indoleacetic  acid  (IAA)  under  conditions  in  which  the  controls 
remained  vegetative.  These,  however,  were  "threshold"  conditions- 
supplementary  light  of  intensities  not  quite  sufficient  to  cause 
flowering  by  itself  was  used  to  extend  the  photoperiod  beyond  its 
critical  value.  No  auxin  promotions  were  observed  under  strict 
short-day  conditions.  Promotion  of  flowering  in  another  LDP, 
Wintex  barley,  has  been  observed  by  Leopold  and  Thimann  (1949). 
This  effect  was  obtained  under  inductive  conditions  and  appears  to 
be  simply  a  promotion  of  later  inflorescence  development.  Note 
that  in  the  same  experiments  (see  Chapter  Five)  x-irradiation, 
which  may  reduce  the  auxin  level,  also  increased  flowering. 

In  the  SDP  Xanthium,  Bonner  and  Thurlow  (1949)  reported 
that  application  of  the  auxins  IAA,  naphthaleneacetic  acid,  or  2,4- 
dichlorophenoxyacetic  acid  (2,4-D)  to  cuttings  or  to  leaves  of  intact 
plants  prevented  the  flowering  response  to  short  days.  This  effect 
was  opposed  by  the  auxin  antagonists  2,4-dichloroanisole  and 
2,3,5-triiodobenzoic  acid  (TIBA).  The  antagonists  themselves,  under 
threshold  conditions— night  interruptions  barely  sufficient  to  keep 
the  controls  vegetative— caused  the  initiation  of  "flowerlike  buds," 
which,  however,  did  not  develop  into  flowers  (Bonner,  1949). 

Auxin  inhibitions  of  flowering  in  Xanthium  have  been  studied 
further  by  Lockhart  and  Hamner  (1954)  who  showed  that  IAA 
increased  both  the  magnitude  and  consistency  of  the  inhibition 
caused  by  a  second  dark  period  following  the  inductive  night 
(Chapter  Two).  Additional  data  on  auxin  inhibition  in  both 
Xanthium  and  Biloxi  soybean  are  provided  in  Hamner  and  Nanda 
(1956).  Salisbury  (1955),  again  with  Xanthium,  found  that  auxin 
inhibited  flowering  only  if  applied  before  translocation  of  the 
"flowering  stimulus"  appeared  to  be  completed— that  is,  before  the 
end  of  the  period  during  which  removal  of  the  induced  leaves 
could  reduce  the  flowering  response.  If  applied  later,  it  promoted 
flower  development,  particularly  under  reduced  light  intensities 
or  in  the  absence  of  actively  growing  buds.  Inhibitions  by  IAA 
applied  before  and  during  the  inductive  dark  period  have  also 
been  reported  in  the  SDP  Pharbitis  (Nakayama,  1958),  although 
earlier  work  showed  promotions  under  similar  conditions  (Naka- 
yama and  Kikuchi,   1956). 

One  of  the  few  plants  in  which  auxins  have  a  major  effect  on 
flowering  is  the  pineapple  {Ananas  comosus).  As  noted  in  Chapter 


108    •    Chemical  Control  of  Flowering 

Five,  one  variety  flowers  in  response  to  geotropic  stimulation,  an 
effect  that  has  been  ascribed  to  a  change  in  native  auxin  distribu- 
tion. In  addition,  a  number  of  varieties  can  be  made  to  flower  by 
applications  of  synthetic  auxins  such  as  naphthaleneacetic  acid 
(NAA)  or  2,4-D.  IAA  appears  to  be  a  native  auxin  in  pineapple, 
and,  paradoxically,  it  has  been  suggested  that  NAA  may  act  in  this 
situation  as  an  auxin  antagonist— an  antiauxin,  in  fact,  competing 
with  the  native  IAA— and  that  flowering  may  result  from  a  lowering 
of  the  effective  level  of  the  native  auxin  (Bonner  and  Liverman, 
1953;  Gowing,  1956).  Whatever  the  explanation,  the  phenomenon 
itself  is  easily  repeatable  and  of  considerable  economic  importance; 
sprays  of  synthetic  auxins  are  used  to  schedule  flowering,  and  hence 
fruiting,  in  commercial  plantations  (see  van  Overbeek,  1952; 
Leopold,   1958). 

Flowering  in  pineapple  can  be  brought  about  also  by  several 
(<  unpounds  structurally  unrelated  to  auxins,  including  /?-hydroxy- 
ethylhydrazine,  acetylene,  and  ethylene  (see  Leopold,  1958).  Indeed, 
pineapple  is  not  the  only  plant  in  which  ethylene  can  cause 
flowering.  Khudairi  and  Hamner  (1954b)  found  that  ethylene 
chlorohydrin  vapor  caused  flower  initiation  in  Xantliiinn  plants 
under  16-hour  photoperiods.  As  with  the  auxin-antagonist  results 
mentioned  previously,  the  experiments  were  carried  out  under 
threshold  conditions,  with  supplementary  light  of  low  intensities. 

The  mechanism  of  ethylene  action  on  flowering  or  any  other 
plant  process  is  unknown,  but  there  is  some  evidence  that  it  acts 
as  an  auxin  antagonist,  possibly  reducing  the  native  auxin  content. 
If  this  is  so,  then  its  effects  on  both  Xanthium  and  pineapple  are 
in  accord  with  the  hypothesis  that  synthetic  auxins  act  as  anti- 
auxins  for  the  pineapple,  and  the  whole  set  of  observations  can  be 
used  to  support  the  hypothesis  that,  at  least  under  certain  condi- 
tions, flowering  may  occur  after  the  lowering  of  a  superoptimal 
auxin  level.  However,  with  the  bits  of  evidence  discussed  in  Chapter 
Five,  this  hypothesis  remains  highly  speculative. 

Auxin  antagonists  have  provided  another  major  difficulty  in 
analyzing  the  auxin  relationships  to  flower  initiation.  Certain  com- 
pounds believed  to  be  true  antiauxins  (such  as  2.1-dichloro- 
phenoxyisobutyric  acid  or  2,4-6-trichlorophenoxyacetic  acid)  and 
others  that  may  rather  inhibit  auxin  transport  (such  as  2,3,5- 
triiodobenzoic  acid)  promote  flowering  in  annual  Hyoscyamus  under 


Plant  Extracts  of  Various  Kinds    •    109 

threshold    conditions   just    as    do    several    auxins.    No    convincing 
hypothesis  about  such  results  has  yet  been  stated  (see  Lang,  1959). 

Many  growth  regulators  can  speed  or  delay  flowering  some- 
what under  particular  circumstances.  These  effects  are  usually 
minor  and  are  also  associated  with  equal  or  greater  effects  on 
vegetative  growth.  Occasionally,  dramatic  and  at  present  inexpli- 
cable effects  of  particular  compounds  on  particular  plants  are  discov- 
ered, of  which  two  examples  will  be  cited.  For  further  information, 
see  Audus  (1959)  and  Leopold  (1958). 

Furfuryl  alcohol,  a  compound  not  previously  known  to  have 
growth-regulating  activity  for  higher  plants  and  not  obviously 
related  to  known  growth  regulators,  promotes  flowering  and  bolting 
in  the  LDP  Rudbeckia  speciosa  under  short  days  in  the  same  way 
as  does  gibberellin  (Nitsch  and  Harada,  1958).  In  one  of  the  two 
experiments  reported,  some  of  the  control  plants  flowered  as  well, 
so  the  conditions  may  have  been  close  to  threshold.  Effects  on  other 
plants  are  unknown. 

The  compound  N-metatolylphthalamic  acid  is  one  of  a  group 
of  growth  regulators  that  profoundly  affects  flowering  as  well  as 
other  processes  in  a  number  of  plants.  It  is  particularly  effective  in 
increasing  flowering  in  the  tomato  {Ly coper sicon  esculentiim),  a 
daylength-indifferent  plant,  chiefly  by  increasing  the  number  of 
flowers  in  each  cluster.  High  doses  may  even  cause  the  development 
of  a  large  inflorescence  at  the  apex,  causing  further  vegetative 
growth  to  stop.  Such  promotions  of  inflorescence  development 
appear  to  be  due  to  temporary  or  permanent  suppression  of  the 
branch  that  would  otherwise  arise  beneath  an  inflorescence  and 
compete  with  it,  and  are  almost  certainly  not  direct  effects  on 
flower  initiation  (Cordner  and  Hedges,   1959). 

PLANT  EXTRACTS  OF  VARIOUS  KINDS 

Many  naturally  occurring  substances  have  been  tested  for 
possible  flower-promoting  activity,  often  as  extracts  of  uncertain 
composition.  No  such  work,  other  than  that  with  gibberellins,  has 
as  yet  been  conspicuously  successful,  but  it  is  well  to  consider  some 
representative  efforts. 

An  extract  of  the  young  inflorescence  of  a  palm,  Washing- 
tonia  robusta,  apparently  brought  about  flowering  in  Xanthinm 


110    •    Chemical  Control  of  Flowering 

under  long  days  in  experiments  by  Bonner  and  Bonner  (1948). 
Unfortunately  their  attempts  to  repeat  this  work,  with  inflorescence 
extracts  from  the  same  and  other  species  of  palm,  were  completely 
unsuccessful,  so  the  result  remains  unexplained. 

In  1951,  Roberts  also  reported  the  extraction  of  a  substance  that 
induced  flowering  in  Xanthium  under  long  days.  It  appeared  to  be 
of  a  lipide  nature  and  obtainable  only  from  flowering,  not  vegeta- 
tive, individuals  of  a  number  of  species  including  Xanthium  itself. 
Although  attempts  in  several  other  laboratories  have  failed  to  con- 
firm Roberts's  results,  a  long-chain  keto-alcohol  with  activity  as  an 
auxin  antagonist  can  be  prepared  from  certain  plants  by  the  pro- 
cedures used  (see  Struckmeyer  and  Roberts,  1955).  Its  florigenic 
properties,  however,  remain  as  doubtful  as  those  of  the  palm  extract. 
An  extract  with  weak  but  significant  flower-promoting  activity  for 
Xanthium  plants  in  long  days  has  recently  been  prepared  by 
careful  lyophilization  of  Xanthium  inflorescences.  Only  future 
work  will  decide  whether  this  result  will  go  the  way  of  the  others 
cited,  but  the  initial  report  is  very  encouraging  (Lincoln  et  a\., 
1961). 

In  an  extensive  investigation  on  the  development  of  a  straw- 
berry (Fragaria)  variety,  Sironval  (1957)  has  reported  that  unsaponi- 
fiable  lipide  fractions  from  flowering  plants  promote  flowering  of 
those  in  the  vegetative  condition.  In  only  a  few  experiments,  how- 
ever, are  the  untreated  controls  completely  vegetative,  and  often 
the  differences  between  control  and  treated  series  are  discouragingly 
small.  The  active  substances  in  the  extracts  may  include  Vitamin  E, 
which  is  itself  active  in  the  strawberry-plant  test,  and  certain  uni- 
dentified sterols. 

Flowering  in  at  least  one  vernalizable  variety  of  pea  (Pisum 
sativum)  can  be  promoted  by  first  allowing  the  seeds  to  imbibe 
"diffusate"  prepared  from  other  pea  seeds  (Highkin,  1955).  Like 
vernalization,  such  treatment  results  in  flowering  at  a  lower  node 
than  in  the  controls;  in  the  data  published,  the  node  number  to 
the  first  flower  was  about  20  in  the  controls  to  about  18  in  the 
treated,  but  was  highly  significant  statistically.  By  a  "diffusate"  is 
meant  an  extract  prepared  not  by  grinding  seeds  in  water  but 
simply  by  soaking  them,  intact,  under  sterile  conditions  for  varying 
periods  of  time  during  which  active  substances  diffuse  out  into  the 
water.  Such  diffusates  probably  contain  many  metabolically  impor- 


Mineral  Nutrition;  Major  Elements    •    111 

tant  compounds.  In  the  investigation  cited,  the  effect  on  flowering 
was  about  the  same  whether  the  diffusate  was  made  by  soaking 
the  seeds  at  23°  or  at  4°  C;  since  only  the  latter  temperature  would 
vernalize,  the  activity  cannot  be  considered  to  represent  a  vernalin 
(Chapter  Five). 

MINERAL  NUTRITION;  MAJOR  ELEMENTS 

The  question  of  the  relationship  between  mineral  nutrition 
and  flowering  is  embodied  more  in  practical  lore,  and  less  in  experi- 
mental data,  than  almost  any  other  aspect  of  flowering  physiology. 
Because  of  this,  relatively  little  can  be  said  here.  Not  that  such 
lore  is  necessarily  incorrect,  but  it  is  usually  uncertain  and  often 
extremely  local.  One  reason  is  that  distinctions  between  relatively 
specific  effects  on  (lowering  and  those  simply  associated  with  changes 
in  vegetative  growth  are  usually  not  made,  as  indeed  they  do  not 
need  to  be,  for  many  practical  purposes.  Thus  one  frequently  finds 
that  nutritional  conditions  that  simply  favor  optimal  growth  will 
be  recommended  to  increase  flowering  and  fruiting. 

Interestingly  enough,  one  of  the  commonest  examples  of  such 
practical  lore  is  the  opposite  belief,  that  flowering  may  result  from 
conditions  causing  poor  vegetative  growth  or  restraining  growth 
in  some  way.  Although  this  may  be  simply  an  inverse  recognition 
of  the  fact  that  in  many  plants  flowering  and  fruiting  are  associated 
with  and  may  cause  a  reduction  in  vegetative  growth  (see  Leopold 
et  al.,  1959),  there  may  be  more  to  it.  The  clearest  recent  study 
on  this  question  has  nothing  to  do  with  mineral  nutrition,  but 
tends  to  confirm  the  view  that,  at  least  in  certain  plants,  growth 
restraint  can  promote  flowering.  Kojima  and  Maeda  (1958)  studied 
a  variety  of  radish  (Raphanus)  in  which  flowering  is  hastened  by 
vernalization.  In  unvernalized  seedlings,  flowering  and  bolting 
were  promoted  by  several  treatments  that  greatly  impeded  the 
growth  of  the  stem  apex.  The  most  effective  was  to  imbed  the 
upper  part  of  the  seedling  for  several  days  in  gypsum;  another  was 
to  immerse  the  seedlings  in  relatively  concentrated  sugar  solutions, 
which  inhibited  growth  osmotically.  The  mechanism  by  which  a 
growth  restraint  might  promote  flowering  is  unknown,  but  the  data 
seem  clear  and  suggest  that  such  notions  are  better  tested  than 
dismissed. 


112    •    Chemical  Control  of  Flowering 

The  suggestion  that  nitrogen  nutrition  plays  an  important  role 
in  the  control  of  flowering  and  fruiting  in  a  manner  related  to  the 
considerations  above  was  strongly  supported,  although  not  origi- 
nated, by  Kraus  and  Kraybill  in  1918  (see  Kraus,  1925).  They 
concluded  that  fruitfulness  in  the  tomato  plant  depended  on  the 
ratio  of  carbohydrate  to  nitrogen— the  C/N  ratio.  Under  a  given 
light  intensity  (to  supply  the  carbohydrates)  and  at  a  given  tem- 
perature (which  would  govern  the  rate  at  which  they  are  metab- 
olized), the  C/N  ratio  can  obviously  be  controlled  by  controlling 
the  nitrogen  supply.  In  Kraus  and  Kraybill's  experiments,  a 
moderate  ratio  was  favorable  to  flowering  and  fruiting,  whereas 
a  low  ratio  (high  nitrogen)  favored  luxuriant  vegetative  growth 
but  little  reproductive  development.  This  conclusion  in  generalized 
form  was  for  a  while  inflated  out  of  all  proportion  to  the  data 
supporting  it,  which  appear  to  have  been  valid  largely  for  the 
particular  conditions  used.  However,  one  should  note  in  fairness 
that  Kraus  and  Kraybill  were  chiefly  interested  in  later  flower 
development  and  fruiting,  not  in  flower  initiation. 

A  more  recent  study  by  Wittwer  and  Teubner  (1957),  also  on 
tomato,  does  not  support  the  notion  that  high  nitrogen  favors 
vegetative  growth  at  the  expense  of  flowering.  On  the  contrary,  in 
solution  culture  the  highest  nitrogen  level  used  gave  the  best 
flowering  even  under  optimal  temperature  conditions.  With  respect 
to  photoperiodic  plants,  El  Hinnawy  (1956)  found  that  high  nitro- 
gen promoted  earlier  flowering  in  Perilla  and  Kalancho'e  (both 
SDP)  under  inductive  conditions,  slowed  it  in  mustard  (Brassica) 
and  dill,  and  had  no  effect  on  spinach  (all  three  LDP)  under  induc- 
tive conditions.  It  had  no  effect  on  the  photoperiodic  response  as 
such,  and  he  concluded  that  the  effects  of  nitrogen  and  other  major 
element  changes  were  highly  indirect. 

Eguchi  et  al.  (1958)  have  studied  the  responses  of  some  photo- 
periodic, vernalizable,  or  daylength-indifferent  plants  to  levels  of 
nitrogen  and  phosphate  nutrition.  They  concluded  that  in  the  first 
two  types  the  time  of  flowering,  both  chronologically  and  develop- 
mentally,  was  almost  unaffected.  In  the  daylength-indifferent  plants, 
however,  which  included  tomato,  pepper  (Capsicum),  and  eggplant 
(Solanum),  there  was  a  much  greater  effect.  In  a  tomato  variety, 
for  example,  flowering  was  earliest  at  the  highest  levels  of  nitrogen 
and  phosphate  used,  with  the  first  flower  at  node  8  or  9.  Reducing 


Heavy  Metals  and  Flowering    •    113 

either  nitrogen  or  phosphate  to  the  lowest  level  used  delayed 
flowering  to  node  12  or  13  at  the  earliest.  The  authors  proposed 
the  interesting  generalization  that  flowering  in  many  tropical 
daylength-indifferent  plants  is  far  more  dependent  upon  nutrition 
than  it  is  in  photoperiodic  or  vernalizable  plants  in  which  the 
environmental  requirements  have  been  satisfied.  In  this  connection, 
note  that  Gott  et  al.  (1955)  found  that  a  low  nitrogen  level  delayed 
flowering  in  unvernalized  or  partially  vernalized  winter  rye  but 
hardly  affected  vernalized  plants. 

Although  the  literature  on  nutrition  and  flowering  is  more 
extensive  than  that  presented  here,  these  examples  serve  to  indicate 
that,  at  least  at  present,  there  is  no  good  evidence  for  a  close  rela- 
tionship between  a  particular  major  element  and  flower  initiation 
in  most  plants. 

HEAVY  METALS  AND  FLOWERING 

There  is  some  indication  that  iron  nutrition  may  be  more 
critically  involved  in  photoperiodic  induction.  In  a  preliminary 
survey  to  see  whether  any  of  a  large  number  of  different  mineral 
deficiencies  would  reduce  the  capacity  of  Xanthium  to  respond  to 
short-day  treatment,  Smith  et  al.  (1957)  noted  that  iron,  and  possi- 
bly boron  and  magnesium  deficiencies,  had  some  effect.  In  further 
experiments  they  found  that  plants  suffering  from  iron-deficiency 
symptoms  failed  to  flower  or  flowered  abnormally  even  when  trans- 
ferred to  a  high-iron  medium  after  photoinduction.  Such  results 
are  suggestive,  although  the  inhibition  of  vegetative  growth  as  well 
as  the  response  to  short-day  leave  them  somewhat  equivocal.  Any 
special  significance  for  iron  in  flower  initiation  has  been  questioned 
by  Shibata  (1959)  in  a  brief  investigation  on  rice  (Oryza  sativa). 

A  more  clear-cut  result  was  obtained  by  the  writer  (Hillman, 
1961a),  using  a  clone  of  the  duckweed  Lemna  perpusilla  growing 
in  a  well-chelated  medium  (see  below).  The  plants  were  pretreated 
by  growing  them  in  media  with  various  levels  of  iron  for  several 
days,  given  one  (inductive)  long  night,  and  then  all  returned  to  a 
high-iron  medium.  Under  these  conditions,  the  flowering  response 
to  the  single  long  night  was  essentially  abolished  by  pretreatment 
with  a  level  of  iron  not  low  enough  to  affect  vegetative  growth. 
In  other  words,   the  iron  requirement  for  induction  appeared  to 


114    •    Chemical  Control  of  Flowering 

be  higher  than  that  for  vegetative  growth  only.  Whether  this  might 
be  true  also  for  other  micronutrient  elements  in  this  plant,  or 
whether  it  truly  indicates  a  special  role  of  iron  in  photoperiodic 
induction,  is  not  yet  clear.  Yoshimura  (1943)  has  reported  promo- 


Fig.  6-3.  Duckweeds  (Lemna)  as  experimental  organisms  for  the  study  of 
flowering  under  highly  controlled  conditions.  (^1)  An  aseptic  culture  of  L. 
perpusilla.  (B)  A  group  of  L.  gibba,  showing  anthers.  (Photographs  by  Dr.  J.  H. 
Miller  and  Yale  University  Photographic  Services.) 

tion  of  flowering  in  another  duckweed,  Spirodela,  by  molybdenum 
deficiency.  For  a  review  of  other  early  reports  on  duckweed  flower- 
ing, see  Hillman  (1961a). 

The  writer  has  pursued  evidence  of  important  metal  effects 
in  photoperiodism  originating  in  observations  on  the  effects  of 
chelating  agents  on  the  flowering  of  two  species  of  Lemna  (see 
Fig.  6-3).  Chelating  agents  are  compounds  that  form  particularly 
stable  complexes  with  many  metal  ions  and  thus  affect  their  chem- 


Heavy  Metals  and  Flowering    ■    115 

ical  reactivity.  Many  compounds  of  biological  importance  (for 
example,  amino  acids)  are  chelating  agents  in  addition  to  their 
other  properties.  Especially  effective  chelating  agents,  such  as 
ethylenediaminetetraacetic  acid  (EDTA,  "versene"),  bring  about 
considerable  changes  in  plant  metabolism,  probably  by  affecting 
processes  involving  metals. 

When  EDTA  is  added  in  sufficient  quantity  to  a  mineral 
medium  supporting  good  growth,  it  profoundly  modifies  the  photo- 
periodic responses  of  a  clone  of  Lemna  perpusilla  and  a  clone  of 
Lemna  gibba.  Lemna  perpusilla,  previously  daylength-indifferent, 
now  responds  as  a  typical  SDP;  Lemna  gibba,  unable  to  flower 
under  any  photoperiod  on  the  first  medium,  now  flowers  rapidly 
as  an  LDP  in  the  medium  with  EDTA.  The  effects  of  EDTA  on 
vegetative  growth  are  quite  minor  and  not  related  to  photoperiod. 
It  seems  obvious  that  the  major  effect  of  EDTA  here  is  not  directly 
on  flowering  itself  but  on  flowering  through  its  sensitivity  to  photo- 
period, since  in  Lemna  perpusilla  EDTA  permits  a  long-day  inhibi- 
tion of  flowering  whereas  in  Lemna  gibba  it  permits  a  long-day 
promotion.  These  effects  are  related  to  a  report  by  Kandeler  (1955) 
—the  first  in  which  the  control  of  flowering  in  any  duckweed  was 
observed— that  Lemna  gibba  flowered  under  long  photoperiods 
given  with  fluorescent  light  only  in  "aged"  medium,  in  which  the 
plants  had  grown  for  some  time.  It  now  appears  that  EDTA  substi- 
tutes for  this  "aged"  medium  effect  and  vice  versa.  Since,  at  least  in. 
Lemna  perpusilla,  chelating  agents  other  than  EDTA  are  effective, 
the  action  is  not  specific  to  EDTA  alone  and  is  probably  a  conse- 
quence of  chelation  (Hillman,  1959a,  1959b,  1961a,  1961b). 

It  has  recently  appeared  that  in  more  purified  media,  these 
two  plants  show  their  photoperiodic  responses  even  in  the  absence 
of  EDTA.  Under  these  conditions,  very  low  levels  of  cupric  or 
mercuric  ions  promote  Lemna  perpusilla  flowering  in  long  days, 
have  no  effect  in  short  days,  and  inhibit  Lemna  gibba  flowering 
in  long  days.  Thus  these  ions,  by  the  reasoning  above,  appear  to  be 
relatively  specific  inhibitors  of  the  response  to  long  days;  the  action 
of  the  chelating  agents  observed  earlier  probably  represents  preven- 
tion of  the  effects  of  contaminants  (undoubtedly  copper)  in  the 
medium.  Such  results  may  provide  new  tools  for  the  analysis  of 
photoperiodism;  however,  much  further  work  will  be  required  to 
explore  such  a  complex  and  sensitive  experimental  system  (Hillman, 
1961c). 


► 


chapter  seven  t  ^ge  and  Flowering 


In  the  growth  of  most  plants  from  seed,  an  appreciable  period 
elapses  before  flowers  are  initiated  even  under  conditions  that 
would  cause  rapid  flowering  in  more  mature  individuals.  This  is 
often  expressed  by  saying  that  in  order  to  flower  a  plant  must 
reach  the  stage  of  readiness  or  "ripeness-to-flower,"  the  latter  being 
a  rendering  of  Klebs's  (1918)  term  Bliihreife.  Put  so  abstractly  the 
concept  seems  merely  circular,  but  it  is  not  unique  in  this  regard. 
Dormancy  often  seems  to  be  defined  as  a  state  in  which  growth 
does  not  take  place  under  conditions  favorable  in  all  respects— 
except  for  that  condition  required  to  break  "dormancy."  However, 
this  merely  illustrates  the  limitation  of  abstract  statements  since 
the  questions  involved  in  both  dormancy  and  ripeness-to-flower  are 

quite  real. 

The  relationship  of  age  or  developmental  stage  to  the  ability 
to  flower  is  not  well  understood,  and  differs  vastly  from  species  to 
species.  The  requirement  for  a  considerable  amount  of  vegetative 
growth  is  particularly  marked  in  woody  plants;  many  trees  do  not 
flower  until  at  least  ten  years  of  age,  and  some  "juvenile"  phases 
are  characterized  not  only  by  inability  to  flower  but  also  by  growth 
habits  and  leaf  shapes  differing  from  those  of  the  adult  phase  (see 
Sax,  1958a).  In  herbaceous  species,  similar  events  lasting  a  much 
shorter  time  are  often  observed. 

Since  plants  differ  so  greatly  in  the  speed  with  which  they 
become  ripe-to-flower,  and  probably  in  the  mechanism  involved, 
the  concept  itself  has  little  use  except  to  call  attention  to  a  whole 
range  of  phenomena.  In  spite  of  this,  an  even  more  general  concept, 

116 


Age  and  Flowering  in  Herbaceous  Plants    •    117 

that  of  "phasic  development,"  has  been  associated  with  some  studies. 
It  views  plant  growth  as  a  succession  of  recognizable  phases,  each 
requiring  a  specific  set  of  environmental  conditions  for  its  fulfill- 
ment, and  none  of  which  can  be  bypassed  (see  Murneek  and  Whyte, 
1948).  A  concept  as  unspecific  as  this  is  hardly  susceptible  either  to 
proof  or  disproof  once  it  is  admitted  that  the  characteristics  of 
the  phases  will  not  be  the  same  in  all  plants.  Hence,  it  will  not  be 
considered  further.  Instead,  some  relationships  of  age  and  flowering 
in  some  of  the  familiar  herbaceous  plants  will  be  discussed  first, 
and  will  be  followed  by  a  consideration  of  the  problems  posed  by 
flowering  in  woody  species. 

AGE  AND  FLOWERING  IN  HERBACEOUS  PLANTS 

Certain  plants  produce  a  characteristic  minimum  leaf  number 
before  flower  primordia  are  initiated.  In  the  best-known  examples, 
spring  and  vernalized  winter  rye,  a  minimum  of  seven  leaves 
appear  before  the  inflorescences  no  matter  what  the  conditions 
used,  at  least  in  most  of  the  older  research  with  these  plants.  A 
partial  explanation  is  that  four  leaf  primordia  are  already  present 
in  the  mature  embryo,  and  so  precede  the  inflorescence.  However, 
three  more  are  apparently  differentiated  during  or  after  germina- 
tion. Although  it  is  possible  to  reduce  the  "minimum  leaf  number" 
below  6  by  the  use  of  continuous  light  from  germination,  or  by. 
starting  with  prematurely  harvested  embryos  that  have  differen- 
tiated fewer  leaf  primordia,  apparently  at  least  one  or  two  leaves 
in  addition  to  those  in  the  embryo  still  intervene  before  flower 
initiation  (Gott  et  al.,  1955). 

Holdsworth  (1956)  has  considered  the  concept  of  minimum 
leaf  number  extensively,  and  questions  its  general  usefulness.  The 
number  in  Xanthium— 8— appears  to  be  accounted  for  by  those 
leaves  present  in  the  embryo  plus  those  developing  before  induc- 
tion and  the  translocation  of  the  floral  stimulus  have  taken  place. 
In  certain  other  plants  the  number  is  higher  than  can  be  accounted 
for  in  such  ways.  However,  both  types  of  observation  may  depend 
on  differences  in  the  sensitivity  of  successive  leaves  to  photoperiodic 
induction,  which  will  be  considered  below.  Other  factors  affecting 
minimum  leaf  number  may  be  the  movement  of  flower-inhibiting 
or   promoting   substances    from    the   cotyledons,    as   observed,    for 


118    •    Age  and  Flowering 

example,  in  grafting  experiments  by  Paton  and  Barber  (1955)  and 
Haupt  (1958)  on  early  and  late  (lowering  in  peas  (see  Chapter  Five). 
There  are  also  plants  in  which  the  flower  primordia,  following  a 
certain  number  of  leaves,  are  already  present  in  the  seed  (see 
Naylor,   1958). 

One  should  attempt  to  distinguish  between  minimum  leaf 
number,  as  in  the  case  above,  representing  a  condition  in  which 
a  certain  amount  of  development  takes  place  before  and  during 
the  treatments  leading  to  flowering,  and  ripeness-to-flower  under- 
stood as  a  condition  before  which  a  given  treatment  is  completely 
ineffective  in  promoting  flowering.  In  practice,  such  distinctions 
may  be  difficult  to  make.  If  the  treatment  in  question  is  vernaliza- 
tion, however,  it  is  clear  that  the  difference  between  winter  annuals 
and  biennials  (Chapter  Five)  simply  reflects  the  fact  that  the  latter 
are  not  responsive  until  they  have  attained  a  considerable  size.  In 
this  sense,  some  winter  annuals  are  ripe-to-flower  as  germinating 
seeds.  The  reason  for  the  size  requirement  in  biennials  is  not 
known,  and  has  been  ascribed  to  many  factors,  including  the 
amount  of  food  reserves.  De  Zeeuw  and  Leopold  (1955)  found  that 
the  age  at  which  seedlings  of  Brussels  sprouts,  Brassica  oleracea 
gemmifera,  a  biennial,  could  be  vernalized  was  decreased  if  the 
synthetic  auxin  NAA  was  given  together  with  the  cold  treatment; 
the  effect  was  not  great,  so  that  evidence  that  the  size  requirement 
in  biennials  is  related  to  auxin  content  is  scanty. 

A  series  of  experiments  by  Sarkar  (1958)  on  a  winter-annual 
strain  of  the  crucifer  Arabidopsis  thaliana  illustrates  not  only  the 
complexity  of  possible  relationships  between  development  and 
receptivity  to  cold  treatment,  but  also  the  fact  that  the  cold  treat- 
ment itself  may  have  a  multiple  action,  as  evidenced  by  the  ability 
of  gibberellin  to  replace  it  at  some  stages  but  not  at  others.  The 
strain  of  Arabidopsis  in  question  is  easily  vernalizable  in  the  seed, 
during  germination,  or  in  the  mature  rosette  stage.  Young  rosettes 
are  less  easily  vernalized.  Gibberellic  acid,  however,  is  most  effective 
on  the  young  rosettes,  less  so  on  the  older,  and  totally  ineffective 
on  seeds. 

Many  studies  bearing  on  ripeness-to-flower  deal  with  respon- 
siveness to  photoperiod.  In  certain  plants,  of  course,  previous 
vernalization  is  a  major  factor  aflecting  such  responsiveness  and 
thus  also  ripeness-to-llower  in  this  sense.  Since  this  relationship  was 


Age  and  Flowering  in  Herbaceous  Plants    •    119 

discussed  earlier,  the  discussion  below  will  be  concerned  primarily 
with  other  prerequisites  for  the  photoperiodic  control  of  flowering. 

Klebs  (1918)  originated  this  field  of  inquiry  by  observing  that 
Sempervivum  funkii  did  not  show  a  flowering  response  to  long 
days  until  it  had  been  growing  for  some  time,  and  he  concluded 
that  the  best  conditions  to  bring  about  this  Bliihreife  state  were 
those  involving  a  high  degree  of  carbon  dioxide  assimilation  and 
a  relatively  meager  mineral  nutrition.  This,  as  well  as  other  obser- 
vations by  Klebs,  was  in  part  the  origin  of  investigations  on  the 
C/N  ratio  (Chapter  Six).  It  seems  clear  now  that  for  most  photo- 
periodic plants,  probably  including  Sempervivum,  gross  nutrition 
is  less  important  than  the  morphological  stage  of  development 
attained. 

Certain  plants  do  not  respond  to  an  inductive  photoperiod 
until  they  have  produced  true  leaves,  but  there  are  some  in  which 
the  cotyledons  themselves  are  sensitive.  These  include  the  SDP 
Pharbitis  (Nakayama,  1958)  and  Chenopodium  rubrum,  some 
strains  of  which  may  flower  as  tiny  seedlings  barely  emerged  from 
the  seed  coat  (Cumming,  1959;  see  illustration  facing  page  1).  The 
SDP  Xanthium  and  Perilla,  on  the  other  hand,  are  of  the  former 
type.  The  development  of  at  least  one  true  leaf  is  necessary  before 
Xanthium  can  respond  to  short  days.  Jennings  and  Zuck  (1954), 
testing  the  possibility  that  this  might  be  due  to  insufficient  area  of 
the  expanded  cotyledons,  found  that  an  area  of  true  leaf  consid: 
erably  smaller  than  the  total  cotyledon  area  could  induce  flowering. 

In  Perilla,  the  sensitivity  to  induction  Of  successive  pairs  of 
leaves  increases  from  the  second  to  at  least  the  fifth  pair,  with  the 
first  and  second  being  almost  insensitive.  This  again  does  not  appear 
to  be  a  matter  of  leaf  area  or  even  of  plant  size,  but  represents 
a  developmental  difference  in  the  leaves.  For  example,  if  equal  areas 
(see  Fig.  5-3,  p.  86)  are  cut  from  second  and  fifth  leaves,  grafted 
onto  other  plants  in  long  day,  and  then  induced  with  short-day 
treatments  so  that  they  will  function  as  donors,  the  tissue  from  the 
fifth  leaves  is  by  far  more  effectived  However,  the  fact  that  intact 
older  plants  respond  more  quickly  than  younger  plants  is  also  due 
to  greater  total  leaf  area  (Zeevaart,  1958).  In  the  grass  Lolium 
temulentum,  the  increasing  sensitivity  of  the  entire  plant  to  photo- 
period  is  attributable  entirely  to  the  increasing  sensitivity  of  suc- 
cessively produced  leaves.  When  only  several  lower  leaves  are  left 


120    •    Age  and  Flowering 

on  a  mature  plant,  as  many  long  days  are  required  to  induce  as  are 
required  by  a  much  younger  plant.  However,  a  small  portion  of 
the  area  of  one  later-produced  leaf  is  sufficient  for  induction  by 
one  long  day  (Evans,  1960). 

The  change  in  sensitivity  of  successive  leaves,  as  in  Perilla, 
may  be  a  function  of  meristem  aging.  It  is  also  possible  that  as  the 
meristem  itself  ages,  it  becomes  more  sensitive  to  the  floral  stimulus 
from  other  parts  of  the  plant;  the  general  question  of  meristem 
aging  and  flowering  may  also  be  important  for  flowering  in  woody 
plants  (see  below)  but  little  is  known  about  it. 

At  least  in  Xanthium ,  the  photoperiodic  sensitivity  of  each  leaf 
varies  during  its  development.  Khudairi  and  Hamner  (1954a) 
studied  the  flowering  responses  of  plants  in  which  single  leaves  of 
different  ages  and  at  different  stages  in  expansion  were  present. 
Within  a  wide  range  of  absolute  sizes,  leaves  were  most  sensitive 
when  they  had  expanded  to  about  half  their  final  size,  being  much 
less  so  either  when  very  young  or  when  mature.  Undoubtedly 
similar  relationships  between  individual  leaf  development  and 
photoperiodic  sensitivity  obtain  in  other  plants  as  well. 

It  is  not  always  true  that  photoperiodic  sensitivity  increases 
with  plant  age  or  development.  The  opposite  situation  has  already 
been  noted  in  sunflower  (Chapter  Two).  It  is  an  SDP  when  young 
but  later  becomes  daylength-indifferent  (Dyer  et  al.,  1959);  stated 
otherwise,  long  days  inhibit  flowering  in  the  young  plant  but  not 
in  the  older.  On  the  other  hand,  this  can  still  be  regarded  as  an 
increased  sensitivity  in  the  sense  that  a  shorter  nightlength  is  induc- 
tive in  older  plants.  The  mechanism  is  unknown. 

FLOWERING  IN  WOODY  PLANTS 

It  is  in  the  woody  plants  that  the  problem  of  ripeness-to-flower 
is  most  obvious.  The  two  major  environmental  factors  affecting 
flowering  in  herbaceous  plants— photoperiod  and  temperature— also 
of  course  affect  woody  plants,  and  by  similar  mechanisms;  however, 
the  dominant  factor  here,  that  of  maturity,  appears  to  be  internal. 
The  lack  of  flowering  in  many  trees  until  they  have  attained  a 
given  age  is  of  great  practical  importance  because  it  affects  both 
food  production  and  breeding  programs,  and  also  makes  experi- 
ments slow  and  costly.  Hence  the  effectiveness  of  some  of  the  pro- 


Flowering  in  Woody  Plants    •    121 

cedures  traditionally  used  in  the  hope  of  hastening  flowering  has 
only  recently  been  confirmed  in  controlled  experiments,  and  the 
value  of  some  others  is  still  uncertain. 

Further  problems  are  presented  by  the  fact  that  most  trees  and 
shrubs,  at  least  in  the  temperate  zone,  are  probably  indirect- 
flowering  plants  unlike  most  herbs  studied,  so  that  conditions 
required  for  flower  initiation  may  differ  greatly  from  those  favoring 
flower  development,  and  the  internal  changes  involved  may  differ 
as  well.  As  an  extreme  example,  the  difficulties  faced  by  the  forest 
geneticist  are  evident  in  the  fact  that  not  only  must  most  species 
of  pine  (Piniis)  grow  for  some  five  or  more  years  before  flower 
initiation  is  possible,  but  then  two  and  a  half  years  are  required 
to  obtain  seed.  Flower  primordia  are  formed  in  the  spring  of  one 
year  but  do  not  develop  further  until  the  spring  of  the  next,  when 
pollination  takes  place.  Then  in  the  succeeding  spring  and  summer 
cone  elongation  and  actual  fertilization  finally  occur,  following 
which  the  seeds  mature  in  the  fall  (see  Stanley,  1958).  Clearly,  any 
way  of  reducing  the  age  required  for  flowering  and  speeding  up  the 
reproductive  cycle  itself  would  be  extremely  helpful. 

A  particular  group  of  woody  plants,  the  bamboos  (Tribe 
Bambuseae  of  the  grass  family),  provides  the  most  striking  exam- 
ples of  long-lived  monocarpic  plants  (Chapter  One),  which  flower 
once  and  then  die.  As  summarized  by  Arber  (1934),  there  is  abun- 
dant evidence  that  a  bamboo  will  spend  5  to  50  years,  the  number 
being  characteristic  of  the  species,  in  vigorous  vegetative  growth. 
It  then  flowers,  sets  seed,  and  dies  within  a  short  time.  Usually  all 
plants  of  the  species  within  a  large  area  will  flower  at  the  same 
time,  regardless  of  injury  or  even  of  destruction  of  all  portions 
above  ground  by  cutting  or  fire.  Thus  size  alone  does  not  appear 
to  be  a  factor.  Individuals  transplanted  to,  say,  the  Kew  Botanical 
Gardens  still  flower  the  same  year  as  their  fellows  in  the  tropics, 
making  it  seem  unlikely  that  periodic  environmental  changes  such 
as  droughts  are  the  cause  of  such  behavior— although  this  has  been 
suggested.  Possibly  bamboos  may  provide  instances  of  very  long- 
term  endogenous  rhythms,  but  it  will  take  a  long-lived  plant 
physiologist  or  a  well-endowed  research  institute  to  find  out. 
Certainly  in  no  group  of  plants  is  the  relation  between  age  and 
flowering  more  evident  and  less  understood. 

Most  environmental  factors  affecting  flowering  in  trees  have 


122    •    Age  and  Flowering 

been  studied  relatively  little  because  of  the  obvious  technical 
difficulties.  Increased  soil  fertility  may  be  of  value  (for  experiments 
that  deal  with  this  possibility  using  pine,  see  Hoekstra  and  Mergen, 
1957).  Fraser  (1958)  has  correlated  meteorological  data  with  anatom- 
ical studies  of  spruce  (Picea),  and  concluded  that  earlier  reports 
that  flower  initiation  is  favored  by  high  summer  temperatures  are 
probably  correct.  Reference  to  the  discussions  in  the  papers  cited 
will  indicate  that,  unfortunately,  tree  physiologists  are  generally 
uncertain  about  the  importance  of  any  particular  soil  or  climatic 
factor. 

Photoperiodism  affects  largely  the  vegetative  development  of 
woody  plants  rather  than  flowering,  at  least  according  to  present 
evidence.  The  rate  of  growth,  its  cessation  and  renewal,  branching 
habit,  leaf  shape,  and  resistance  to  cold  are  among  the  characteris- 
tics affected  (see  Wareing,  1956;  Nitsch,  1957).  Such  characteristics 
are  often  of  great  ecological  significance,  and  their  sensitivity  to 
photoperiod  frequently  differs  considerably  within  offspring  of  the 
same  species  gathered  over  a  wide  geographical  area  (see  Vaartaja, 
1959).  In  certain  crop  trees,  such  as  the  SDP  Cofjea  arabico  (coffee), 
flowering  also  is  photoperiodically  controlled  (Piringer  and  Borth- 
wick,  1955),  whereas  the  ornamental  shrub  Cestrum  nocturnum  has 
been  previously  discussed  as  an  LSDP. 

Most  work  with  economically  important  trees,  however,  sug- 
gests a  minor  role  or  none  at  all  for  photoperiodism  in  flower 
initiation.  This  is  almost  certainly  true  for  pines  (Mirov,  1956;  see 
Mirov  and  Stanley,  1959),  for  peaches  (Prunus),  and  probably  for 
apples  (Mains)  (Piringer  and  Downs,  1959).  One  should  note  an 
indication  of  control  by  light  in  the  last-named  tree,  however.  In 
the  paper  cited,  the  variety  used  failed  to  flower  at  all  on  16-hour 
photoperiods  of  which  8  hours  were  under  fluorescent  light,  but 
flowered  well  if  incandescent  light  was  used.  For  such  reasons,  as 
well  as  because  of  the  relatively  few  experiments  done  so  far,  it  is 
impossible  to  guess  whether  or  not  photoperiodically  controlled 
flowering  is  truly  less  common  among  woody  plants  than  it  appears 
to  be  among  herbs.  Certainly,  however,  even  when  photoperiodism 
is  a  direct  factor,  that  of  size  or  maturity  is  still  of  overriding 
interest  both  practically  and  theoretically. 

Because  of  effects  on  vegetative  growth,  photoperiodic  treat- 
ment can  indirectly   hasten   flowering.  A  species  of  birch,   lietula 


Flowering  in  Woody  Plants    •    123 

verrucosa,  normally  requiring  at  least  5  years  from  seed  in  order 
to  flower,  was  used  by  Longman  and  Wareing  (1959)  in  a  study  on 
whether  size  was  the  major  factor  involved  or  whether  a  certain 
number  of  developmental  seasonal  "cycles"  were  necessary  before 
flowering  could  take  place.  Some  seedlings  were  kept  constantly 
under  long  days  or  continuous  light,  in  which  vegetative  growth 
continues  rapidly.  Others  were  allowed  to  make  about  30  centi- 
meters of  growth  under  such  conditions,  given  short  days  to  induce 
dormancy,  and  then  kept  in  the  cold  for  six  weeks,  following  which 
they  were  returned  to  long  days  and  the  cycle  repeated.  There  was 
also  a  control  series  under  natural  conditions.  Fifty  percent  of  the 
trees  in  the  constant  long-day  conditions  flowered  within  the  first 
year,  when  2  to  3  meters  high,  whereas  none  of  the  (smaller)  control 
or  "cycle"  series  flowered  within  two  years.  Hence  in  this  tree  at 
least,  attainment  of  a  certain  size  is  crucial  to  flowering  and  can  be 
speeded  by  constant  long  photoperiods,  although  the  authors  noted 
that  the  plants  so  treated  were  abnormally  spindly. 

Although  flowering  may  thus  be  hastened  by  speeding  devel- 
opment to  the  requisite  size,  most  of  the  traditional  methods  used 
by  horticulturists  involve  operations  or  mutilations  of  some  kind 
and  bring  about  an  inhibition  of  vegetative  growth.  Of  these 
methods,  one  of  the  most  widely  favored  is  girdling— the  removal 
of  a  ring  of  bark,  including  phloem,  on  an  entire  tree  or  on  a 
branch.  The  immediate  result  is  to  prevent  the  translocation  of 
photosynthate  out  of  the  girdled  top  or  branch,  so  that  materials, 
accumulate  above  the  girdle.  Naturally,  this  can  thus  result  in  the 
death  by  starvation  of  the  root  system  if  it  is  not  permitted  to  heal 
over  within  a  relatively  short  time.  Girdling  is  often  effective  in 
causing  flowering  of  plants  too  young  to  flower  otherwise  in  species, 
as  unrelated  as  Citrus  (Furr  et  al.,  1947),  Pin  us  (Hoekstra  and 
Mergen,  1957),  and  apples  (Sax,  1957,  1958b).  Related  to  girdling 
as  a  means  of  blocking  phloem  translocation  is  the  technique  of 
bark  inversion,  in  which  a  ring  ol,  bark  is  cut  out  and  regrafted 
in  place  upside  down.  Such  procedures  must  be  used  before  the 
period  in  which  flower  initiation  would  normally  be  expected  to 
take  place.  In  apples,  bark  inversion  in  June  will  affect  flowering 
the  following  spring,  even  bringing  it  about  in  2-  or  3-year-old 
seedlings,  whereas  the  same  operation  in  late  summer  is  ineffective 
(Sax,  1957,  1958b). 


124    •    Age  and  Flowering 

With  many  fruit  trees,  grafting  young  scions  onto  dwarfing 
stocks  is  another  method  whereby  both  a  promotion  of  flowering 
and  an  inhibition  of  growth  are  obtained.  The  stocks  are  usually 
varieties  of  the  same  or  a  closely  related  species,  and  may  be  used 
either  as  rootstocks  or  interstocks.  The  latter  method  involves  first 
grafting  the  dwarfing  stock  onto  a  standard  seedling  rootstock  and 
later  grafting  the  variety  to  be  dwarfed  onto  the  developed  dwarfing 
tissue,  so  that  the  latter  is  interposed  between  root  and  crown. 
The  mechanism  by  which  such  procedures  cause  early  flowering 
is  not  known,  but  may  in  some  cases  be  related  to  the  reduction 
of  phloem  transport  out  of  the  scions  and  thus  analogous  to 
girdling.  However,  the  interactions  between  stock  and  scion  in  such 
grafts  are  often  highly  specific,  and  not  all  grafts  that  reduce  growth 
or  transport  promote  flowering.  In  addition,  not  all  grafts  that 
cause  early  flowering  and  dwarfing  appear  to  involve  inhibited 
phloem  transport  (Sax,  1958b). 

Another  traditional  method  of  handling  fruit  trees,  the  espalier 
technique,  in  which  branches  are  bent  horizontally  or  downward 
out  of  their  normal  direction,  suggests  that  orientation  with  respect 
to  gravity  may  affect  flower  initiation.  This  supposition  was  directly 
tested  with  young  plants  of  several  kinds  of  fruit  trees  by  Wareing 
and  Nasr  (1958),  who  found  marked  effects  on  apples.  Nineteen 
young  shoots  held  in  a  horizontal  position  initiated  a  total  of  116 
flower  buds  in  contrast  to  a  control  series  initiating  5.  Smaller  but 
similar  effects  were  observed  in  cherries  (Primus).  Similar  results 
have  also  been  obtained  by  Longman  and  Wareing  (1958)  on  young 
Japanese  larch  (Larix)  trees.  These  are  all,  of  course,  reminiscent 
of  results  with  pineapple  and  soybeans  that  may  involve  a  changed 
auxin  distribution,  and  it  has  also  been  suggested  that  the  flower- 
promoting  effects  of  bark  inversion  may  be  due  to  effects  on  auxin 
distribution,  which  then  affect  phloem  transport  (Sax,  1958b). 

As  repeatedly  noted,  most  of  the  methods  described  above 
have  in  common  either  a  demonstrated  or  possible  effect  of  causing 
the  accumulation  of  photosynthate  near  the  growing  points  affected. 
The  promotion  of  flower  initiation  in  some  trees  by  the  early 
removal  of  fruits  might  also  be  attributed  to  an  increase  in  avail- 
able carbohydrates  (for  experiments  of  this  kind  dealing  with  Citrus, 
see  Furr  and  Armstrong,  1956).  The  general  hypothesis  that  ma- 
turity, and  hence  flowering,  in  many  trees  depends  on  a  high  level 


Flowering  in  Woody  Plants    •    125 

of  carbohydrates  is  by  no  means  unequivocally  supported  by  the 
evidence  at  present,  but  it  is  attractive  in  view  of  Wetmore's  (1953) 
observations,  discussed  in  Chapter  Five,  that  juvenility  and  maturity 
in  fern  leaf  forms,  and  hence  in  the  apex  producing  them,  are 
clearly  correlated  to  sucrose  supply.  On  the  other  hand,  more 
specific  mechanisms  of  a  hormonal  nature  may  be  involved  in  the 
flowering  of  trees. 

In  view  of  the  work  with  herbaceous  plants  leading  to  the 
florigen  hypothesis,  it  is  surprising  how  few  experiments  have  been 
published  on  the  flowering  responses  of  young  scions  after  grafting 
to  mature,  flowering  plants.  Sax  (1958a)  indicates  that  this  tech- 
nique is  common  among  tree  breeders,  but  that  there  is  no  conclu- 
sive evidence  for  its  effectiveness.  Furr  et  al.  (1947)  found  it  com- 
pletely ineffective  in  Citrus.  In  this  connection,  results  of  the 
opposite  kind  of  graft  are  also  of  interest.  Freely  flowering  branches 
from  mature  trees  have  been  grafted  on  young  stocks  in  order  to 
facilitate  seed  collection.  Although  Huber  (1952)  reports  this  tech- 
nique as  successful  in  poplar  (Populas),  there  are  cases  in  which 
mature  scions  on  young  stocks  revert  to  a  nonflowering  condition 
after  several  years  (see  Fraser,  1958).  Whether  this  reflects  an  insuffi- 
cient supply  of  flower-promoting  factors  (florigen,  carbohydrates) 
from  stock  to  scion,  or  the  movement  of  inhibitors,  or  some  other 
relationship,  is  not  known. 

The  entire  problem  of  juvenility  is  obviously  closely  related 
to  the  subject  matter  of  this  chapter.  It  is  particularly  relevant 
with  regard  to  woody  plants,  but  also  probably  important  in  herbs. 
This  problem  has  attracted  relatively  little  attention  in  recent 
years,  but  the  interested  reader  should  consult  Sinnott  (1960)  for 
a  consideration  of  the  literature.  One  striking  if  somewhat  atypical 
example,  related  to  flowering,  is  provided  by  ivy  (Hedera).  The 
young  plant  is  a  vine,  with  lobed  leaves  and  aerial  roots.  After 
10  or  12  years  it  produces  branches  that  grow  upward,  bearing 
entire  leaves  and  no  aerial  roots.  Only  these  branches  are  capable 
of  flowering.  If  they  are  cut  off  and  rooted  they  grow  into  erect 
shrubs  that  may  become  very  large  and  rarely  if  ever  revert  to  the 
juvenile  vine  condition,  although  shoots  produced  from  the  base  of 
old  shrub  (or  arborescent)  forms  may  be  juvenile— a  phenomenon 
observed  also  in  apple  and  other  trees  with  distinct  juvenile  forms 
(see  Sax,  1958a).  Recent  work  by  Robbins  (1957,  1960)  has  shown 


126    •    Age  and  Flowering 

that  reversion  will  occur  after  either  heavy  pruning  or  treatment 
with  gibberellic  acid,  and  also  that  it  is  possible  to  obtain  forms 
intermediate  between  typically  adult  and  typically  juvenile. 
Gibberellic  acid  also  causes  the  production  of  vegetative  inflores- 
cences. However,  the  factors  governing  the  attainment  of  the  adult 
state  in  the  first  place  are  entirely  unknown,  and  further  work 
with  this  sort  of  organism  should  be  valuable  for  an  understanding 
of  both  flowering  and  differentiation  in  general. 


► 
► 


chapter  eight  t  A  Miscellany 


Several  topics  that  have  escaped  the  more  systematic  treatment 
in  preceding  chapters  will  be  considered  briefly  in  this  one.  The 
brevity  does  not  imply  that  these  topics  are  unimportant,  but  is 
a  product  of  space  limitations  and  the  fact  that  this  book,  like  most 
of  the  recent  literature,  is  concerned  with  the  circumstances  bring- 
ing about  flowering  rather  than  with  associated  matters.  In  addition 
to  the  topics  below,  others  connected  with  the  physiology  of  flower- 
ing suggest  themselves,  notably  the  physiology  of  meiosis  and  of 
fertilization.  These  will  be  omitted  entirely  since  an  adequate 
consideration  would  require  a  general  discussion  of  the  physiology 
of  reproduction,  taking  in  material  far  beyond  the  scope  of  this 
survey.  A  few  remarks  on  the  future  of  the  physiology  of  flowering 
conclude  both  chapter  and  book. 

ANTHESIS 

The  culminating  stage  in  flower  development  is  the  opening 
of  the  bud,  anthesis,  with  which  is  often  associated  the  attainment 
of  the  flower's  characteristic  color  and  scent.  Most  of  the  work  on 
anthesis  has  been  concerned  with  the  precise  diurnal  timing  often 
shown  by  this  event.  In  the  literature  on  endogenous  rhythms, 
anthesis  is  considered  as  one  of  the  many  phenomena  under  such 
control.  The  effects  of  light  and  darkness  on  a  number  of  plants 
support  this  view. 

Among  the  earlier  studies,  perhaps  the  most  interesting  are 
two  papers  by  N.  G.  Ball  on  several  plants  whose  flowers  normally 

127 


128    •    A  Miscellany 

open  early  in  the  morning.  For  example,  those  of  the  tropical 
perennial  herb,  Turnera  ulmifolia,  open  about  two  hours  after 
dawn,  then  wither  three  or  four  hours  later.  This  occurs  in  suc- 
cessive groups  of  buds  even  if  the  shoots  are  kept  in  darkness  for 
several  days  so  that  they  are  isolated  from  the  normal  day-night 
changes.  However,  it  is  possible  to  prevent  opening  by  illumination 
during  the  night,  particularly  during  the  second  half  of  the  night, 
and  the  anthesis-inhibiting  effect  of  one  such  illumination  lasts 
for  the  next  three  days.  Air  temperature  and  relative  humidity 
changes  appear  to  have  little  effect  (Ball,   1933). 

Ball  (1936)  found  similar  inhibiting  effects  of  night  illumina- 
tion on  morning  anthesis  in  species  of  Campanula,  Geranium, 
Cist  us,  and  Ipomoea.  He  determined  a  crude  action  spectrum  for 
this  phenomenon,  using  filters,  and  found  that  red  (6500-7000  A) 
was  the  most  effective  and  blue  the  least  effective  color.  With  the 
advantage  of  twenty-five  years,  it  is  easy  to  interpret  these  results 
as  representing  the  disturbance  of  a  circadian  rhythm  originally 
"set"  by  the  light-dark  schedule  through  what  is  presumably  the 
red,  far-red  system.  However,  this  work  was  in  a  sense  before  its 
time,  so  the  (for  then)  unusual  effectiveness  of  red  light  attracted 
little  attention. 

A  paper  by  Arnold  (1959)  on  Oenotliera  (evening  primrose) 
indicates  that  endogenous  rhythms  are  also  involved  here,  though 
relatively  susceptible  to  modification.  If  the  plants  receive  light 
from  6  a.m.  to  6  p.m.  the  flowers  open  at  about  6  p.m.,  as  in  nature; 
with  an  inverse  cycle,  they  open  in  the  morning.  Anthesis  of  a  bud 
that  is  ready  occurs  roughly  12  hours  after  a  dark-to-light  transition, 
which  thus  appears  to  "set"  the  timing  mechanism,  but  the  timer 
is  easily  perturbed  by  the  length  of  the  light  period  itself.  On  a 
schedule  of  18  hours  light-6  hours  darkness  anthesis  is  regularly 
later,  and  on  6  hours  light-18  hours  darkness  regularly  earlier, 
than  would  be  predicted  by  the  12-hour  rule.  However,  it  is  clear 
that  there  is  an  endogenous  component  to  the  timing  since  anthesis 
will  not  follow  any  arbitrary  cycle  of  light  and  darkness.  The  cir- 
cadian periodicity  of  anthesis  cannot  be  made  into  a  12-hour 
periodicity  by  schedules  of  6  hours  light-6  hours  darkness,  nor  into 
a  48-hour  periodicity  by  alternating  24-hour  light  and  dark  periods. 

According  to  Arnold's  investigations  the  light-sensitive  timer  of 
Oenothera  anthesis  must  be  localized  in  the  buds.  In  continuous 


Anthesis    •    129 

darkness,  anthesis  occurs  with  circadian  periodicity  for  several 
days,  but  only  in  those  buds  that  had  developed  largely  under 
normal  day-night  changes.  Buds  that  develop  from  a  young  stage 
in  total  darkness  are  considerably  delayed  in  anthesis,  and  finally 
open  more  or  less  at  random.  In  addition,  light  must  be  given 
directly  to  the  buds  to  reset  or  disturb  the  periodicity  of  anthesis— 
lighting  schedules  given  to  the  leaves  are  ineffective. 

Other  evening-blooming  plants  have  been  studied  recently. 
Anthesis  of  the  giant  tropical  water  lily  Victoria  regia  normally 
occurs  soon  after  sunset  (6  p.m.).  It  can  be  moved  up  as  early  as 
4  p.m.  by  darkening  the  buds  with  black  paper  for  30  minutes,  but 
darkening  earlier  than  this  hour  has  no  effect;  therefore  some  endog- 
enous component,  perhaps  set  by  preceding  illumination  schedules, 
is  involved  in  the  sensitivity  to  darkness.  Illumination  during  the 
night  delays  the  opening  of  buds  during  the  next  days,  but 
eventually  they  open  even  in  continuous  light  (Gessner,  1960). 
The  opening  and  odor  production  of  the  night-blooming  jasmine, 
Cestrum  nocturnum  (an  LSDP  discussed  earlier),  show  a  circadian 
rhythm  in  constant  light  or  darkness.  In  constant  light,  the  period 
length  is  roughly  27  hours  at  17°  C;  higher  temperatures  reduce 
it  by  several  hours,  and  lower  temperatures  increase  it  (Overland, 
1960). 

Daily  timing  of  anthesis  is  probably  regulated  in  the  ways 
indicated  above,  but  much  less  is  known  about  the  control  of 
anthesis  in  those  indirect-flowering  plants  whose  fully  developed 
buds  may  remain  dormant  for  a  considerable  period  and  then  open 
in  the  course  of  a  few  days.  Among  temperate-zone  plants  this  is 
usually  the  result  of  the  breaking  of  dormancy  by  long  cold  exposure 
followed  by  periods  of  favorable  temperatures  for  growth;  as  such, 
it  resembles  the  breaking  of  other  forms  of  dormancy  by  low 
temperature  (see  Chouard,  1960).  Though  this  does  not  explain  it, 
there  is  no  need  for  further  consideration  as  a  separate  topic  here. 
Certain  tropical  plants,  however,  show  the  same  extended  bud 
dormancy,  and  the  same  explanation  cannot  hold  for  these. 

One  of  the  few  examples  studied  with  any  thoroughness  is 
coffee,  Coffea  arabica.  This  is  an  SDP  as  far  as  flower  initiation  is 
concerned  (Chapter  Seven),  but  bud  dormancy  and  anthesis  appear 
to  be  controlled  by  moisture  conditions.  Under  relatively  dry  con- 
ditions, rapid  and  uniform  anthesis  can  be  brought  on  by  heavy 


130    •    A  Miscellany 

rains  or  irrigation— even  by  wetting  the  buds  themselves.  This 
suggests  that  the  seasonal  dormancy  is  simply  due  to  a  water  deficit 
and  disappears  when  water  is  supplied.  But  the  situation  is  probably 
not  this  simple.  Alvim  (1960),  working  in  a  dry  area  where  the 
water  conditions  on  a  plantation  were  completely  controllable  by 
irrigation,  found  that  coffee  plants  irrigated  at  weekly  intervals 
failed  to  reach  anthesis  over  a  long  period  of  time.  Others,  allowed 
to  remain  dry  for  a  shorter  length  of  time  and  then  given  one  good 
irrigation,  responded  with  heavy  anthesis  within  two  weeks.  It 
thus  seems  likely  that  a  period  of  water  deficit  is  required  to  break 
bud  dormancy  in  this  plant,  so  that  anthesis  is  brought  about  by 
a  thorough  wetting  after  a  dry  period.  Alvim  suggests  that  this  may 
be  a  major  form  of  seasonal  control  of  anthesis  in  tropical  plants, 
a  control  in  some  respects  ecologically  analogous  to  that  exerted  in 
temperate-zone  plants  by  low  temperatures  followed  by  warming. 

THE  SEX  EXPRESSION  OF  FLOWERS 

Flower  primordia  in  a  given  species  do  not  always  give  rise  to 
identical  structures,  even  if  development  is  perfectly  normal. 
Although  probably  the  great  majority  of  plants  produce  one  kind 
of  flower,  with  both  functional  stamens  and  functional  pistils— a 
hermaphrodite  or  monoclinous  flower— some  do  not.  Unisexual  (or 
diclinous)  flowers,  either  staminate  or  pistillate,  occur  in  many 
species.  There  are  also  intermediate  conditions  of  various  kinds. 
If  staminate  and  pistillate  flowers  are  borne  on  the  same  individual, 
the  plant  is  said  to  be  monoecious;  if  on  separate  individuals, 
dioecious.  Until  relatively  recently,  these  phenomena  of  "sex  ex- 
pression" have  been  studied  largely  from  the  morphological  and 
genetic  points  of  view,  but  they  are  frequently  modifiable  by 
environmental  and  chemical  means  as  well.  For  a  recent  review  of 
the  genetic  factors,  see  Westergaard  (1958).  A  comprehensive 
review  by  Heslop-Harrison  (1957)  on  the  experimental  modifica- 
tion of  sex  expression  is  the  basis  lor  the  general  statements  not 
otherwise  documented  in  the  discussion  below.  There  is  some 
controversy  over  the  evolutionary  origins  of  sex  expression  in  plants 
and  even  over  the  proper  terms  in  which  to  discuss  it  (see  the 
references  cited  and  also  Heslop-Harrison,   1958). 

Consideration  of  the  effects  of  light  and   temperature  on  sex 


The  Sex  Expression  of  Flowers    •    131 

expression  might  best  begin  with  a  study  by  Nitsch  et  al.  (1952) 
on  a  monoecious  plant,  the  acorn  squash  (a  variety  of  Cucurbita 
pepo).  This  plant  produces  one  flower  primordium  at  each  node, 
and  the  primordia  develop  differently  depending  on  their  position 
in  the  sequence  of  nodes.  The  earliest  give  rise  to  underdeveloped 
staminate  ("male")  flowers;  these  are  followed  by  normal  staminate 
flowers  that  are  followed  in  turn  by  normal  pistillate  ("female") 
flowers;  interspersed  among  the  nodes  bearing  the  latter  are  nodes 
with  inhibited  staminate  flowers.  Still  later,  giant  pistillate  flowers 
occur,  again  interspersed  with  inhibited  staminates;  finally  even 
larger  pistillate  flowers  are  produced  that  are  parthenocarpic,  pro- 
ducing fruits  (but  not  seeds)  without  pollination.  This  trend  of 
"feminization"  occurs  under  all  conditions,  but  the  duration  of 
each  phase  in  terms  of  node  number  is  easily  modified.  High 
temperatures  and  long  days  delay  it,  favoring  the  continued  pro- 
duction of  staminate  flowers,  whereas  low  temperatures  and  short 
days  speed  feminization  greatly.  Either  daylength  or  temperature 
can  be  made  the  dominating  factor  depending  on  the  values  used. 
The  control  exerted  is  striking:  for  example,  female  flowers  can 
be  made  to  appear  as  early  as  the  ninth,  or  as  late  as  the  hundredth 
node. 

It  is  not  clear  whether  the  effects  referred  to  daylength  are 
photoperiodic  in  the  strict  sense.  Supplementary  light  of  1000  foot 
candles  was  used,  and  no  low-intensity  interruptions  attempted. 
One  observation  in  the  paper  suggests  that  lower  intensities  might 
not  be  as  effective.  In  addition,  some  conclusions  on  the  greater 
effectiveness  of  "night"  than  of  "day"  temperatures  are  weakened 
by  the  fact  that  the  former  were  always  given  for  16  hours  daily, 
the  latter  for  only  the  8  hours  of  daily  sunlight  employed,  in  each 
treatment,  irrespective  of  supplementary  light  schedules.  These 
points  do  not  detract  from  the  dramatic  climatic  effects  reported, 
but  the  paper  is  chief  among  those  usually  cited  as  indicating 
control  of  sex  expression  by  "photoperiod"  and  "thermoperiodicity," 
interpretations  that  may  be  overstated. 

Most  other  investigations  with  temperature,  on  both  monoe- 
cious and  dioecious  plants,  agree  with  the  results  described  in 
showing  low  temperatures  favoring  pistillate  development  and 
high  favoring  staminate.  The  effects  ol  daylength,  whether  strictly 
photoperiodic  or  not,  are  more  complex.  Apparently  the  general- 


132    •    A  Miscellany 

ization  holds  that  pistillate  flowers  represent  a  fuller  intensity  of 
flowering  than  staminate  flowers;  thus,  with  photoperiodic  plants, 
prolonged  short-day  treatment  favors  pistillate  expression  relative 
to  staminate  in  SDP,  whereas  long-day  treatment  does  so  in  LDP. 
For  example,  in  the  LDP  spinach,  normally  dioecious,  short  days 
following  long-day  induction  cause  the  formation  of  some  staminate 
flowers  on  plants  that  would  normally  produce  only  the  pistillate, 
thus  making  the  treated  plants  monoecious  (see  Heslop-Harrison, 

1957). 

The  factors  that  affect  sex  expression  in  plants  with  diclinous 
flowers  may  affect  even  plants  with  hermaphrodite  flowers  in  a 
similar  fashion.  One  particularly  interesting  example,  dealing  with 
the  effect  of  photoperiod,  has  recently  been  studied  by  J.  and  Y. 
Heslop-Harrison  (1958a,b).  The  plant  is  Silene  pendnla,  an  LDP 
in  that  flowering  does  not  occur  with  8  hours  of  daylight  but  is 
brought  about  by  supplementing  this  to  21  hours  with  light  of 
about  300  foot  candles.  Plants  raised  from  germination  on  long 
days  showed  high  male  sterility,  some  50  percent  of  the  anthers 
being  sterile;  in  addition,  pistil  development  was  excessive.  Plants 
that  had  received  some  short-day  exposure  before  being  returned 
to  long  days,  however,  showed  normal  pistil  development  and 
fully  fertile  anthers.  Hence  this  plant,  while  grossly  an  LDP  in 
terms  of  mere  flower  initiation,  is  clearly  an  SLDP  for  normal 
flower  development. 

Chemical  control  of  sex  expression  has  been  studied  in  a 
variety  of  plants.  The  earliest  clear-cut  results  with  auxins  (chiefly 
naphthaleneacetic  acid)  were  obtained  on  monoecious  cucurbits 
such  as  the  cucumber,  Cucttmis  satimis,  in  which  feminization  is 
promoted  (see,  for  example,  Laibach  and  Kribben,  1950).  Subse- 
quent work  on  other  plants  as  well  seems  to  bear  out  the 
generalization  that  high  auxin  levels  favor  pistillate  and  reduce 
staminate  expression.  As  with  other  factors,  such  effects  are  not 
confined  to  plants  with  unisexual  flowers.  The  Silene  work  discussed 
above  also  included  studies  of  the  effects  of  auxin  application; 
these,  like  continual  exposure  to  long  days,  caused  male  sterility 
and  overdevelopment  of  the  pistil. 

Other  growth-regulating  substances  whose  mechanism  of  action 
may  be  related  to  that  of  auxins  also  affect  sex  expression.  Maleic 
hydrazide    and    2,3,5-triiodobenzoic    acid    both    may    cause    male 


The  Sex  Expression  of  Flowers    •    133 

sterility  and  otherwise  suppress  anther  development,  but  often  only 
in  conjunction  with  other  strong  morphogenetic  effects.  A  feminiz- 
ing effect  of  carbon  monoxide  has  been  observed  by  J.  and  Y. 
Heslop-Harrison  (1957)  in  a  monoecious  race  of  Mercurialis.  This 
was  accompanied  by  formative  effects  resembling  those  caused  by 
auxins. 

Three  other  chemical  effects  should  be  mentioned.  High 
nitrogen  levels  generally  promote  pistillate  as  opposed  to  staminate 
expression;  this  has  been  observed  on  monoecious  species  and  on  at 
least  one  hermaphrodite,  the  tomato.  The  question  of  whether  mam- 
malian sex  hormones  may  affect  sex  expression  in  higher  plants  has 
attracted  surprisingly  little  attention.  A  single  major  investigation 
(Love  and  Love,  1945)  with  Melandrium  showed  highly  significant 
effects  in  spite  of  high  toxicity.  Although  similar  work  on  a  few 
other  plants  has  found  nothing  of  interest,  the  problem  may  still 
be  worth  pursuing. 

The  gibberellins  have  so  far  been  little  studied  with  regard  to 
these  phenomena,  but  may  prove  to  be  of  great  importance.  Galun 
(1959)  has  found  that  gibberellic  acid,  unlike  auxin,  causes  a  trend 
toward  "maleness"— prolonged  staminate  and  delayed  pistillate 
expression— in  the  cucumber;  this  effect  is  partially  counteracted 
by  naphthaleneacetic  acid.  Moreover,  certain  cucumber  strains  that 
normally  produce  only  pistillate  flowers  will  produce  staminate 
flowers  as  well  following  gibberellic  acid  treatment.  Besides  its 
theoretical  interest,  this  result  also  holds  promise  for  practical 
breeding  work  (Peterson  and  Anhder,  1960). 

So  far,  the  only  important  hypothesis  on  the  control  of  sex 
expression  is  that  derived  primarily  from  work  with  applied  auxin; 
it  envisages  auxin  level  in  the  plant  as  the  major  controlling  factor. 
Daylength,  temperature,  and  other  factors  are  considered  to  act 
through  their  effects  on  auxin  level.  Probably  the  most  detailed 
statement  is  given  by  Heslop-Harrison  (1957).  In  essence,  optimum 
auxin  levels  for  flowering  are  considered  to  be  lower  than  those  for 
vegetative  growth;  within  the  flowering  range,  the  optimum  for 
staminate  expression  is  lower  than  that  for  pistillate  expression. 
In  a  sense  this  hypothesis  contradicts  the  suggestion,  noted  earlier, 
that  the  pistillate  expression  represents  a  more  intense  flowering 
condition  than  the  staminate.  As  a  working  hypothesis,  however,  it 
has    proved    fruitful.    Experiments    on    the    relationships    between 


134    •    A  Miscellany 

flowering,  sex  expression,  and  leaf  form,  for  example  in  hemp, 
Cannabis  sativa  (J.  and  Y.  Heslop-Harrison,  1958c),  have  provided 
further  evidence  in  its  favor.  Work  of  this  sort  also  has  implications 
for  the  questions  of  juvenility  and  maturity  mentioned  in  the 
preceding  chapter,  but  cannot  be  discussed  in  detail  here.  In  addi- 
tion, further  information  on  the  roles  of  other  growth  substances, 
notably  the  gibberellins,  will  certainly  be  required  before  a  truly 
comprehensive  hypothesis  can  be  framed. 

GENETICS  OF  FLOWERING  RESPONSES 

Flowering  responses  to  photoperiod  and  temperature  are  of 
course  genetically  controlled,  and  from  the  relative  ease  with  which 
"early"  and  "late"  varieties  of  cultivated  plants  are  bred,  one  might 
guess  that  this  control  is  often  quite  simple.  Although  practical 
breeding  work  is  not  done  with  reference  to  narrowly  defined 
physiological  responses,  a  number  of  specific  investigations  confirm 

this  guess. 

The  SDP  characteristic  of  Maryland  Mammoth  tobacco  has 
been  studied  in  crosses  with  Nicotiana  tabacum  var.  Java.  The  ¥1 
generation  is  not  homogeneous,  suggesting  that  the  dominance  of 
Java's  day-neutral  (or,  more  accurately,  weakly  quantitative  LDP) 
characteristic  is  incomplete.  In  the  F2,  however,  the  SDP  character 
occurs  in  approximately  one-fourth  of  the  progeny,  indicating 
dependence  on  a  single  recessive  gene.  The  "mammoth"  (essentially 
SDP)  character  apparently  occurs  frequently  in  various  tobacco 
varieties  as  a  single-gene  mutation,  but  its  expression  is  affected  by 
other  genetic  properties  of  the  variety.  In  the  interspecific  cross  of 
Maryland  Mammoth  with  the  LDP  Nicotiana  sylvestris,  the  LDP 
character  is  completely  dominant  (Lang,  1948).  In  similar  crosses 
between  the  SDP  Coleus  frederici  and  the  quantitative  LDP  Coleus 
blumei  the  Fj  plants  are  entirely  SDP,  indicating  dominance  of 
this  characteristic  (Kribben,  1955). 

The  difference  between  winter  and  spring  varieties  of  Petkus 
rye  appears  to  be  due  to  a  single  gene.  In  the  ¥1  generation  of  a 
(toss,  the  spring  (noncold-requiring)  habit  is  dominant;  the  F., 
generation  segregates  in  a  spring:winter  ratio  of  8:1.  However, 
the  dispersion  in  flowering  time  within  the  spring  and  winter  classes 
of  the  F2  indicates  that  the  situation  may  not  be  quite  as  simple 


Genetics  of  Flowering  Responses    •    135 

as  the  gross  segregation  suggests  (Purvis,  1939).  Sarkar  (1958)  has 
confirmed  and  extended  earlier  work  on  the  cold  requirement  in 
Hyoscyamus  niger.  Here  again,  crosses  between  the  annual  and 
biennial  strains  indicate  a  single-gene  difference  in  this  regard,  but 
there  is  no  dominance.  The  F3  is  intermediate  between  homozygous 
annuals  and  homozygous  biennials.  The  heterozygote  will  eventually 
respond  to  long  days  without  a  previous  cold  treatment,  but  does 
so  more  rapidly  with  it;  a  given  cold  treatment  has  a  greater  effect 
on  the  heterozygote  than  on  the  pure  biennial;  and  the  former 
reaches  a  vernalizable  stage  earlier  in  development  than  the 
latter. 

Not  all  vernalization  requirements  appear  to  depend  on  single 
genes.  Napp-Zinn  (1960)  reports  in  one  paper  of  a  continuing  study 
on  Arabidopsis  thaliana  that  the  difference  between  summer  and 
winter  annual  strains  depends  on  at  least  two  genes.  In  addition, 
the  relation  between  developmental  stage  and  susceptibility  to 
vernalization  is  under  further  genetic  control,  which  has  not  been 
completely  analyzed. 

This  brief  survey  will  be  sufficient  to  suggest  the  nature  of 
such  investigations.  Two  general  observations  are  worth  making  in 
this  connection.  In  the  first  place,  it  seems  evident  even  from  the 
little  that  is  known  that  specific  requirements  for  flowering  are 
not  necessarily  genetically  deep-seated,  but  may  be  easily  acquired 
or  lost.  Hence  conclusions  about  the  distribution— geographical 
or  geological— of  species  and  families  on  the  basis  of  the  present-day 
response  characters  of  certain  members  (for  example,  Allard,  1948), 
although  stimulating,  should  be  entertained  with  the  greatest 
caution.  Second,  and  perhaps  more  important,  there  is  clearly  room 
for  much  more  work  on  the  genetic  control  of  flowering  require- 
ments. Cold  requirements,  at  least,  are  currently  receiving  con- 
siderable attention  (see  Napp-Zinn,  1960)  but  genetic  studies  are 
notably  inconspicuous  or  absent  in  most  of  the  recent  literature  on 
photoperiodism.  The  difficulties  should  not  be  underestimated— 
particularly  those  involved  in  finding  SDP  and  LDP  sufficiently 
closely  related  to  allow  crossing,  a  difficulty  that  in  itself  may  be  of 
great  importance.  However,  with  the  increasingly  precise  knowledge 
that  research  in  flowering  may  be  expected  to  gain  from  investiga- 
tions as  diverse  as  those  on  the  red,  far-red  system  and  with  chemical 
controlling  agents,  a  biochemical  genetics  of  flowering  as  envisaged 


136    •    A  Miscellany 

by  Lang  (1948)  should  be  a  perfectly  attainable  goal,  and  well  worth 
the  effort. 


FLOWERING  AND  DEATH 

In  addition  to  providing  a  melodramatic  heading,  the  relation- 
ship between  these  two  processes  is  sufficiently  intimate  in  some 
plants— the  monocarpic— to  warrant  some  further  mention. 

One  reason  for  death  following  heavy  flowering  might  be 
simply  morphological.  If  all  the  shoot  meristems  are  converted  to 
determinate  structures,  vegetative  growth  cannot  continue— at  least 
without  the  formation  of  adventitious  buds.  Whether  this  complete 
conversion  of  all  meristematic  areas  into  flowers  ever  actually  occurs 
is  of  course  another  question,  but  the  possibility  can  be  envisaged. 

The  usual  explanation  of  death  following  flowering  and  fruit- 
ing is  nutritional— death  is  seen  as  the  result  of  metabolic  patterns 
in  which  the  flowers,  fruits,  and  seeds  in  some  way  compete  so 
successfully  with  the  rest  of  the  plant  for  energy  sources  and  other 
materials  that  death  is  the  eventual  result.  The  evidence  is  largely 
from  observations,  so  often  made,  that  the  life  of  annuals  can  be 
prolonged  by  removing  flowers  and  young  fruits.  However,  it  has 
recently  been  pointed  out  that  there  may  be  other  explanations  for 
such  results,  such  as  the  production  of  inhibitors  at  various  stages 
of  reproductive  development.  For  example,  senescence  in  staminate 
spinach  plants  can  be  put  off  for  a  long  time  by  removing  the 
flowers.  Since  no  fruit  or  seed  could  be  set  by  these  plants  under 
any  circumstances,  and  the  staminate  flowers  themselves  do  not 
appear  to  contain  large  amounts  of  reserves,  the  simple  nutritional 
hypothesis  seems  very  weak  here  (Leopold  et  al.,  1959).  The  article 
cited  contains  additional  experiments  and  references  on  this  topic, 
which  is  largely  unexplored. 

It  has  already  been  mentioned  many  times  that  there  are  close 
relationships  between  flowering  and  vegetative  growth  habit,  de- 
pending upon  the  plant;  it  is  usually  unclear  whether  a  given 
growth  change  is  directly  related  causally  to  flowering  or  whether 
both  express  another  underlying  condition.  The  relationship  in 
monocarpic  plants  thus  represents  another,  and  surely  the  ultimate, 
aspect  of  this  more  general  problem. 


Prospects    •    137 


PROSPECTS 

From  time  to  time  throughout  this  survey  suggestions  for 
future  work  have  been  briefly  made.  In  an  overall  view,  however, 
the  directions  of  research  in  the  physiology  of  flowering  are  hard 
to  predict  with  any  accuracy,  and  harder  still  to  recommend  with 
any  assurance.  The  best  thing  may  be  simply  to  ruminate  a  little 
on  the  subject  before  going  back  to  work. 

One  can  see  that  most  of  the  large  problems  remain.  Indeed, 
one  of  the  major  achievements  of  the  research  of  the  past  few 
decades  was  to  delineate  these  questions  in  the  first  place.  Among 
them  are  the  nature  or  natures  of  the  persistent  states  induced  by 
photoperiodic  or  cold  treatments;  the  nature  of  the  flower- 
controlling  substances  that  move  between  plant  parts  or  between 
grafted  plants;  whether  or  not  endogenous  circadian  rhythms  con- 
stitute the  basic  mechanism  of  photoperiodism;  and  the  relation- 
ships between  juvenility,  maturity,  and  flowering. 

Some  questions  have  been  reduced  to  simpler  forms.  For 
example,  a  question  on  the  role  of  light  and  darkness  in  photo- 
periodism can  be  reshaped,  at  least  in  part,  much  more  sharply: 
What  is  the  biochemical  role  of  the  red,  far-red  pigment?  Some 
developmental  questions— bolting  in  rosette  plants,  for  instance- 
can  now  be  asked,  again  at  least  in  part,  in  terms  of  specific  growth 
substances,  the  gibberellins.  This  increased  concreteness  obviously 
represents  progress;  and  as  long  as  the  answers  to  such  simpler 
questions  are  not  mistaken  for  exhaustive  explanations  of  all  asso- 
ciated phenomena,  they  should  increase  that  progress. 

A  major  goal— perhaps  the  only  goal— of  physiology  can  be 
stated  as  the  understanding  of  growth  and  development  in  terms 
of  simpler  biochemical  systems  and  their  integration.  This  does  not 
mean  that  physiology  is  or  ought  to  be  biochemistry;  in  a  sense, 
the  biochemist's  job  begins  where  the  physiologist's  ends,  although 
in  practice  they  necessarily  overlap  immensely.  One  can  envision 
the  physiologist  as  taking  an  organism  apart  into  relatively  large 
portions— speaking  in  terms  of  processes— that  are  then  susceptible 
to  biochemical  investigation.  Unfortunately,  the  general  recogni- 
tion of  the  close  relationship  between  physiology  and  biochemistry 
has  occasionally  led  to  almost  meaningless  work.  For  example,  an 


138    •    A  Miscellany 

enzyme  or  other  substance  is  assayed  in  tissues  at  two  quite  different 
stages  of  development;  a  difference  is  found,  and  this  biochemical 
difference  is  now  suggested  as  the  cause  of  the  developmental 
difference,  in  spite  of  the  fact  that  it  may  be,  and  probably  is, 
merely  a  correlation.  Such  work  may  be  quite  interesting,  bio- 
chemically speaking,  but  the  physiologist  must  always  keep  in  mind 
the  need  of  a  causal  analysis.  This  at  the  very  least  requires  atten- 
tion to  the  kinetics— relations  in  time— of  any  two  conditions,  one 
of  which  is  believed  to  cause  the  other.  The  physiology  of  flowering 
has  had  and  will  have  its  share  of  both  sorts  of  biochemically 
oriented  investigations,  but  probably  only  the  kind  of  care  with 
which  Lang  (1960)  has  started  to  analyze  the  relations  between 
endogenous  gibberellin  level  and  bolting  in  Hyoscyarnus  will  pro- 
\  ide  real  understanding. 

Assuming,  then,  the  goal  of  taking  organisms  apart  bio- 
(hemically-as  long  as  the  "parts"  so  obtained  fit  together  again, 
physiologically  speaking— what  other  experimental  approaches  are 
available?  A  useful  one  in  the  past  will  continue  to  be  so:  the 
use  of  substances  or  conditions  suspected  of  having  relevant  effects. 
Though  easily  mocked,  in  some  forms,  as  "spray  and  weigh,"  this 
approach  at  least  reduces  the  kinetics  problem;  the  added  substance 
or  changed  condition  surely  precedes  the  effect  in  a  well-controlled 
experiment.  However,  the  problem  still  remains  of  how  directly 
the  two  are  related.  It  is  this  kind  of  approach,  in  the  broadest 
sense,  that  has  led  to  the  basic  discoveries  of  photoperiodism  and 
vernalization,  as  well  as  many  others.  Even  genetic  studies  come 
into  this  general  class. 

Advantages  can  be  gained  here  from  the  use  of  more  convenient 
experimental  materials.  Arabidopsis,  Chenopodium  seedlings,  and 
Lemna  are  all  small  enough  to  be  grown  rapidly  in  aseptic  culture 
under  highly  controlled  conditions,  and  may  thus  partially  replace 
the  unwieldy  Perilla  and  Xanthium  of  classical  investigations. 
However,  the  full  exploitation  of  tissue  culture  techniques  should 
make  the  latter  materials  even  more  useful  than  ever  for  studies  of 
florigen  and  the  induced  state.  For  some  preliminary  thoughts  and 
results  in  this  particular  direction,  see  Chailakhyan  (1961)  and  Fox 
and  Miller  (1959). 

An  approach  related  to  the  two  preceding  has  not  been 
employed    to   any   great   extent.   It   involves   following  changes   in 


Prospects    •    139 

both  meristems  and  other  tissues  with  the  most  sensitive  cyto- 
chemical  and  other  microscopic  techniques.  Ideally,  this  sort  of 
work  could  provide  suggestions  as  to  what  biochemical  changes  to 
investigate  with  grosser  methods.  Even  relatively  traditional  ana- 
tomical studies  can  give  important  information  on  the  action  of 
various  growth  regulators  (for  example,  Sachs  et  al.,  1959,  1960) 
and  it  would  seem  highly  desirable  to  have  such  information  as 
closely  correlated  as  possible  with  that  gained  from  other  ap- 
proaches. Even  some  very  simple-minded  questions  might  have 
valuable  answers:  What  are  the  differences,  if  any,  in  intracellular 
organization  or  content  between  induced  and  noninduced  Perilla 
leaves,  and  how  soon  do  they  arise?  During  the  time  that  florigen 
is  believed  to  be  moving  from  an  induced  leaf  to  a  meristem,  can 
changes  be  observed  along  its  route?  And  so  forth. 

In  short,  the  field  will  undoubtedly  continue  to  progress  as  it 
has  in  the  past— through  critically  tested  guesses,  appropriate  choice 
of  experimental  material,  perseverance,  and  technical  advances.  It 
is  obvious  by  now  that  the  writer  has  no  revolutionary  improve- 
ments in  approach  to  propose,  which  is  hardly  surprising  since 
differentiation  and  development  have  yielded  their  secrets  slowly 
to  better  minds  than  his.  But  the  progressive  understanding  of 
these  problems,  representing  as  they  do  much  of  what  is  contained 
in  that  simple  word,  "life,"  is  surely  an  enterprise  worthy  of 
the  best. 


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pathological  plant  growth,  Brookhaven  Symposia  in  Biology,  No.  6,  Upton,  N.Y. 
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related   phenomena    in    plants   and   animals,   Washington,   D.C.:    American 

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in  Xanthium,"  Bot.  Gaz.,  104:409-416. 
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158    •    Bibliography 

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induced  in  Piqueria  trinervia  with  photoperiodic  treatment,"  Contr.  Boyce 

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xiv  -j-  319  pp. 


index  off  plant  names 


Adiantum  96 

Agax'e  (century  plant)  6 

Amaranthus  35,  73,  94 

Anagallis  (pimpernel)  49 

Ananas  (pineapple)  90,  107-108,  124 

Anemone  7 

Anethum  (dill)  18,  37,  46-47,  112 

Apium  (celery)  63 

Apple,  see  Malus 

Arahidopsis  49,  118.  135,  138 

Bambuseae  (bamboos)  6,  121 
Barley,  see  Hordeum 
Bean,  see  Phaseolus 
Beta  (beet)  74,  79 
Be  tula  (birch)  122-123 
Brassica  62,  112,  118 
Brussels  sprouts,  see  Brassica 
Bryokalanchoe  82 
Bryophyllum  14,  82.  104 

Campanula  (bluebell)  7,  62,  128 

Cannabis  (hemp)  134 

Capsicum  (pepper)  1 1 2 

Carrot,  see  Daucus 

Celery,  see  Apium 

Century  plant,  see  Agave 

Cestrum   (night-blooming  jasmine)    14, 

93,  122,  129 
Chenopodium,  facing  1,  18.  25.  27,  52, 

119,  138 
Cherry,  see  Prunus 
Chrysanthemum    37,   59-62,   64,   69-70, 

73,  105 
Circaea  (enchanter's  nightshade)  103 
Cistus  128 
Citrus  123-125 
Clover,  see  Trifolium 
Cocklebur,  see  Xanthium 


Cofjea  (coffee)  122,  129-130 

Cole  us  47,  49,  134 

Corn,  see  7.ea 

Cosmos  4,  73-74 

Cruciferae  (mustard  family)  41 

Cucumis  (cucumber)  132-133 

Cucurbita  (squash)  131 

Dactylis  (orchard  grass)  61 
Datura  (Jimson  weed)  25 
Daucus  (carrot)  101-102 
Dill,  see  Anethum 
Duckweed,  see  Lemna,  Spirodela 

Echinocystis  (wild  cucumber)  101 
Eggplant,  see  Solanum 
Erigeron  71 

Flax,  see  Linum 

Fragaria  (strawberry)    26,   64,   81 
110 


104, 


Geranium  128 

Glycine  (soybean)  8,  12-13,   19.  23,  25, 

ni       or.       At\'     k  l        TO     HO       O"       HI         1  n.  1         in" 


31-32,  49,  51,  72-73,  87,  91, 
124 


101,  107, 


Hedera  (ivy)  125-126 

Helianlhus  (sunflower)  2,  16,  120 

Hemp,  see  Cannabis 

Hippeastrum  66 

Hordeum  (barley)  13,  19,  32-33,  90,  107 

Humulus  (hops)  11 

Hyacinthus  65 

Hyoscyamus  niger  (black  henbane)  an- 
nual 13,  15,  19,  22-23,  26-27,  32,  35. 
37,  41-42.  48-49,  51,  58,  70,  80,  91,  101, 
106.  108,  135;  biennial  58-63,  82,  101, 
135 

159 


160    ■    Index  of  Plant  Names 


Impatiens  51 

fpomoea  (morning  glorv,  sweet  potato) 

14,71-72,  128 
Ivy,  see  Hedera 

Kalanchoe  4-5,  13,  22,  25-27,  47-49,  51- 
52,  71,  74-75,  82-83,  92,  94,  105,  112 

Lactuca  (lettuce)  34-36,  103 
Larix  (larch)  124 

Lemna  (duckweed)  18,  105,  113-115,  138 
Lepidium  90 
Lettuce,  see  Lactuca 
Linum  (flax)  90 
Lolium  18,  119-120 

Lycopersicon  (tomato)  15,  64,  109,  112- 
113 

Madia  47 

Mains  (apple)  122-124 

Maryland  Mammoth  tobacco,  see  Nico- 

tiana 
Matthiola  (stocks)  63 
Melandrium  133 
Mentha  (mint)  7 
Mercurialis  133 
Millet,  see  Setaria 
Morning  glory,  see  Pharbitis,  fpomoea 

Narcissus  7 

Nasturtium,  see  Troparoluiu 
Nettle,  see  Urtica 

Nicotiana    (tobacco)    12-13,   25-26,    70, 
80-82,  96,  134 

Oenothera  128-129 
Oryza  (rice)  113 

Pea,  see  Pisum 

Peach,  see  Prunus 

Pepper,  see  Capsicum 

Perilla  22,  25,  42,  70,   76,   84-87,    112, 

119-120,  138-139 
Pharbitis   (morning  glorv)    18,   27,   38- 

39,  77-78,  92,  107.  119;  see  also  Ipo- 

moea 
Phaseolus  (bean)  44-45,  52 
Picea  (spruce)  122 
Pineapple,  see  Ananas 
Pinus  (pine)  121-123 
Piqueria  (stevia)  77 
Pisum    (pea)    6,    15,   57-58,    81-82,   101, 

110-111,  118 
Plantago  (plantain)  24,  47 
Plum,  see  Prunus 
Populus  (poplar)  125 


Prunus   (cherry,   peach,  plum)   7,    122, 

124 
Pyrus  (pear)  7 

Raphanus  (radish)  111 

Rice,  see  Oryza 

Rudbeckia      (coneflower,      brown-eyed 

Susan)  26.28.71,90,  109 
Rye,  see  Sec  tile 

Salvia  42 

Saxifraga  7 

Secale  (rye)   14,  56-57,  59-62,  113,  117, 

134-135 
Sedum  71 

Sempervivum  (houseleek)  11,  119 
Setaria  (millet)  37-38 
Silene2S,  90,  101,  106,  132 
Solayium  1 12 
Soybean,  see  Glycine 
Spinacia  (spinach)   18,47,  61,  90,    112, 

132, 136 
Spirodela  (duckweed)  114 
Spruce,  see  Picea 
Squash,  see  Cucurbita 
Statice  90 

Stevia,  see  Piqueria 
Stocks,  see  Matthiola 
Strawberry,  see  Fragaria 
Streptocarpus  59,  92 
Sunflower,  see  Helianthus 
Sweet  potato,  see  Ipomoea 

Taraxacum  (dandelion)  7 
Tobacco,  see  Nicotiana 
Todea  96 

Tomato,  see  Lycopersicon 
Trifolium  (clover)  61 
Triticum  (wheat)  14 
Tropaeolum  (nasturtium)  90 
Tulipa  7,  65-66 
Turnera  128 

Urtica  (nettle)  103 

Victoria  129 

Washingtonia  109-110 
Wheat,  see  Triticum 

Xant  hium  (cocklebur)  13,  15,  17-21, 
25-28,  32,  35-39,  43,  50-51,  69-71,  76, 
79-80,  84-85,  87,  93,  97,  101,  105, 
107-110,  113,  117,  119-120,  138 

Zea  (corn)  40 


subject  index 


► 
► 
► 
► 
► 

► 


Acetylene,  108 

Action  spectra,  see  Light-breaks;  Light 
quality 

Age,  and  flowering  in  woody  peren- 
nials, 120-126;  of  leaves,  and  photo- 
periodism,  117,  119-120;  of  plants, 
and  response  to  cold,  118 — and 
photoperiodism,  5,  15-16,  118-120; 
see  also  Juvenility 

Altitude,  90 

Annuals,  6,  54,  136 

Anthesis,  7,  127-130 

Antiauxin,  106-109,  132-133 

Auxin,  definition,  68;  and  induction, 
89-91;  inhibition  of  flowering,  79, 
90-91,  106-108;  promotion  of  flower- 
ing, 90,  106-108;  and  red,  far-red 
system,  91;  and  sex  expression,  132- 
133;  and  vernalization,  118;  see  also 
Antiauxin 

Bark  inversion,  123-124 

Bending,  90-91,  124 

Biennials,   cold   requirements,    54,    58- 

59,  61-62,  118;  definition,  6;  genetics 

of,  135 
Bolting,    caused    by    furfuryl    alcohol, 

109;  definition,  101;  and  gibberellin, 

101-104,  137 
Bulb  plants,  64-66 

Carbohydrate,  and  devernalization,  60- 
61;  -nitrogen  ratio,  112,  119;  promo- 
tion of  flowering,  79;  substitution 
for    high-intensity    light,    21;    trans- 


location, and  florigen  translocation, 
73-77,  79 — and  flowering  in  woody 
perennials,  123-125;  and  vernaliza- 
tion, 57 

Carbon  dioxide,  21-22,  92 

Carbon  monoxide,  133 

Cereals,  winter  and  spring,  devernali- 
zation  of,  59-60;  genetics  of,  134-135; 
and  gibberellin,  101;  minimum  leaf 
number  in,  117;  vernalization  of,  54- 
57 

Chelating  agents,  114-115 

Chlorophyll,  31-32,  41 

Circadian  rhythms,  see  Endogenous 
circadian  rhythms 

Cold  requirements  for  flowering,  of 
biennials,  54,  58-59,  61-62,  118;  in 
bulb  plants,  64-66;  genetics  of,  134— 
135;  of  perennials,  59;  and  plant 
age,  118,  135;  relation  to  photo- 
periodism, 61-62;  satisfaction  of,  by 
diffnsate,  111 — by  gibberellin,  100- 
101— by  short  days,  61-62;  of  winter 
annuals,  54-58;  see  also  Vernalization 

Cold    treatments,    of    bulbs,    64-66;    of 
developed   plants,  54,   58-59,   61-64; 
-     effects  of,  on  dormancy,  62,  129 — on 
seed  germination,  57 — on   vegetative 
growth.    59-60,    64;    of    germinating 
seeds,  54-58,  62;  seasonal  control  by, 
54,  129;  see  also  Vernalization 
Copper,  115 
Cotyledons,  38-39,  119 

Critical  daylength,  definition  and 
qualifications,  13,  15,  20,  22-24;  and 

161 


162    •    Subject  Index 

light  quality.  37-39;  and  red,  far-red 
system,  37-40;  and  temperature.  25, 
43;  see  also  Light  and  dark  periods; 
Photoperiodism 

Critical  nightlength,  see  Critical  day- 
length 

Crown -gall,  97 

Cumulative-flowering  plants,  7 

Dark     periods,    see     Light     and    dark 

periods 
Darkness,  see  Light  and  dark  periods 
Daylength,  see  Critical  daylength 
Daylength-indifferent  plants,  definition, 

14-15;    florigen    production    by,    72; 

genetics  of,  134;  nutrition  and  flower- 
ing of.  112-113 
Dayneutral,    see    Daylength-indifferent 

plants 
Devernalization,    by    gibberellin,     M)l; 

by     high     temperature,     60-61;     of 

perennials,  60-61 
2,4-Dichlorophenox\  acetic  acid  (2,4-D), 

107-108 
Diffusate,  110-111 
Direct-flowering  plants,  7 

Endogenous  circadian  rhythms,  and 
action  of  light-breaks,  47-50;  as  basis 
of  photoperiodism,  42-44.  46-47,  52- 
53;  and  leaf  movements.  44-45,  50; 
and  light  and  dark  period  inter- 
actions, 51-53;  and  red.  far-red  sys- 
tem, 52;  temperature  effects  on,  42- 
46.  52;  in  timing  of  anthesis,  127- 
129 

Ethylene.  108 

Eloral  hormone  or  stimulus,  see  Flori- 
gen 

Florigen,  activity,  criteria  of,  99 — of 
natural  extracts.  109-110;  concept 
examined,  78-82,  95-98;  evidence  for 
existence,  69-72;  production  by  day- 
length-indifferent plants,  72;  relation 
to  induction,  85-87;  transfer  across 
grafts,  69-72,  82,  84-85,  125;  trans- 
location, and  carbohydrate  trans- 
location, 72-77— rate  of.  77-78 

Flower  development  and  initiation,  re- 
lation  to  vegetative  growth,  8-9,  28; 


relations   between,   6-7,  87,  94;   sea- 
sonal occurrence  of,  6-7,  10,  12,  54, 
129-130 
Flower  opening,  see  Anthesis 
Flowering    hormone    or    stimulus,    see 

Florigen 
Furfuryl  alcohol.  109 

Genetics  of  flowering  responses,  134- 
136 

Gibberellin,  devernalization  by,  101; 
effects  on  short-day  plants,  104-105; 
inhibition  of  flowering,  104-105,  126; 
long-day  plants,  91,  101-104;  promo- 
tion of  bolting  and  flowering,  100- 
104,  137;  promotion  of  staminate 
development,  133;  and  red,  far-red 
system,  100;  satisfaction  of  cold  re- 
quirements. 100-102,  104,  118;  and 
vegetative  growth,  100-104 

Girdling,  123-124 

Grafts,  ambiguous  effects  on  flowering, 
79-82,  125;  on  dwarfing  stocks,  124; 
transfer  of  florigen  across,  69-72,  82, 
84-85,  125;  transfer  of  induced  state 
by,  84-85;  transfer  of  vernalin 
across,  82-83 
Gravity,  90-91,  124 

Hormone,     definition.     67;     floral     or 

flowering,  see  Florigen 
^-Hydroxyethvl  hydrazine,  108 

Indirect-flowering  plants,  7 
Induced  state  (Induction),  and  auxin, 
89-91;  by  cold,  vernalization,  56-57, 
87-88;  compared  to  crown-gall 
tumor.  97;  defined,  17;  fractional, 
24-25,  88;  inhibition  by  dark  periods 
in  SDP,  20-22;  and  nucleic  acids. 
92-93.  96;  permanence  of,  83-8S; 
quantitative  nature  of,  87-88,  94-95; 
transfer  across  grafts,  84-85;  and 
vegetative  growth,  28 

Induction,  see  Induced  state 

Inflorescence.  2 

Iron,  113-114 

Juvenility,  and  carbohydrate,  96,  125; 
in  woody  plants,  116.  124-126;  see 
also  Age 


Subject  Index    •    163 


Lamarckism,  55 

LDP,  see  Long-day  plants 

Leaf,  age,  and  photoperiodic  sensitiv- 
ity, 119-120;  blades,  photoperiodic 
perception  by,  17;  movements,  and 
endogenous  circadian  rhythms,  44- 
45,  50;  number,  minimum,  117-118; 
true,  compared  to  cotyledons,  38-39, 
119 

Light  and  dark  periods,  and  endog- 
enous circadian  rhythms,  47-52; 
lengths  of,  10-11,  18-20,  22-24,  51- 
52;  and  red,  far-red  system,  35-39; 
roles  in  photoperiodism,  18-20; 
temperature  interactions  with,  25- 
27,  63-64 

Light -breaks,  in  action  spectrum  stud- 
ies, 30-33;  definition,  19;  and  endog- 
enous circadian  rhythms,  47-50;  and 
red,  far-red  system,  35-39 

Light  intensity,  and  criteria  of  photo- 
periodism, 11.  29,  131;  high,  re- 
quirement for,  20-22;  low,  photo- 
periodic effect  of,  13,  19-20 — and 
red,  far-red  system,  39 

Light  quality,  action  spectra,  30-33, 
52;  and  anthesis,  128;  in  main  light 
periods,  40-42;  and  vegetative 
growth,  34-35,  39-40;  see  also  Red, 
far -red  system 

Long-day  plants  (LDP),  definition,  13; 
and  gibberellin,  91,  100-104;  see  also 
Critical  daylength;  Light;  Photo- 
periodism 

Long-short-day  plants,  13-14,  93 

Lysenkoism,  55 

Mercury,  115 

Meristem,  age  and   flowering,   96,    120, 

124-126;  organization  and  flowering, 

3-5 
Molybdenum,  114 
Monocarpic  plants,  6-7 

Naphthaleneacetic     acid     (NAA),     97, 

107,  108,  118,  132 
Nightlength,  see  Critical  daylength 
Nitrogen,  see  Carbohydrate,   -nitrogen 

ratio;  Nutrition,  major  element 
N-metatolylphthalamic  acid,  109 
Nucleic  acids,  92-93,  96 


Nutrition,  and  (lowering  of  daylength 
indifferent  plants,  112-113;  iron  and 
trace  metal,  113-115;  major  clement, 
111-113.  119,  133 

Perennials,  cold  requirements,  dever- 
nalization,  54,  60-61;  definition,  6; 
woody,  120-126 

Phasic  development,  116-117 

Phloem,  see  Carbohydrate,  transloca- 
tion; Plorigen,  translocation 

Photomorphogenesis,  39 

Photoperiodism,  classes  of  response. 
13-15;  criteria,  definitions  of,  10-11, 
29,  131;  discovery  of,  11-13;  effects 
on  sex  expression,  131-132;  effects  on 
vegetative  growth,  27-28,  122-123; 
and  endogenous  circadian  rhythms, 
42-53;  induction  by,  17-18,  83, 
87;  and  leaf  or  plant  age,  15-16, 
117-120;  and  light  intensity,  11,  20- 
22,  29;  and  light  quality,  30-33,  35- 
39;  and  red,  far-red  system,  35-39; 
relation  to  cold  requirements,  61-62; 
role  of  leaf  in,  17,  84-87,  117-120; 
role  of  light  and  dark  periods  in, 
18-24;  seasonal  control  by,  10-12, 
15;  temperature  effects  on,  25-27;  in 
woody  perennials,  122-123 

Photosvnthate,  see  Carbohydrate 

Photosynthesis,  21-22,  29,  31-32,  41 

Photochrome,  definition,  40;  see  also 
Red,  far-red  system 

Red.  far-red  system,  and  auxin,  91; 
and  critical  daylength,  37-40;  effects 
on,  of  light  and  dark  periods,  35-39 
— of  light-breaks,  35-36 — of  low- 
intensity  light,  39;  and  endogenous 
circadian  rhythms,  52;  and  gibberel- 
lin, 100;  nature  of,  39-40;  and  photo- 
morphogenesis,  vegetative  growth, 
39;  and  seed  germination,  34-35; 
temperature  effects  on,  34,  36-37 

Respiration,  57,  91-92 

Rhythms,  see  Endogenous  circadian 
rhythms 

Ripeness  to  flower,  see  Age;  Juvenility 

Rosette  plants,  see  Bolting 

Scotophile  phase,  46 


164    •    Subject  Index 


SDP,  see  Short-day  plant(s) 

Seasonal  control,  by  cold  treatments, 
54-55,  129;  bv  photoperiodism.  10- 
13,  15;  by  water,  129-130 

Seasonal  flower  initiation  and  develop- 
ment, 7 

Seed  germination,  34-35,  57 

Sex  expression,  and  auxin.  132-133; 
and  gibberellin.  133;  and  photo- 
periodism, 131-132;  and  tempera- 
ture, 131 

Sex  hormones,  animal.  133 

Short-day  plants  (SDP)  definition,  13; 
inhibition  by  long  days  in.  25;  see 
also  Critical  davlength 

Short-long-day  plants,  13-14.  r>l-62 


Temperature,  effects  of,  on  bulb 
plants,  64-66 — on  critical  daylength, 
26.  43 — on  endogenous  circadian 
rhythms,  42-46,  52 — on  photoperiod- 
ism, 25-27 — on  red,  far-red  system, 
34,  36-37 — on  sex  expression,  131; 
high,  devernalization  by,  60-61; 
interactions  with  light  and  dark 
periods,  25-27,  63-64;  see  also  Cold 
requirements;  Cold  treatments;  Ver- 
nalization 

Thermoperiodism,  25-27,  63-64.  131 

Trees,  see  Perennials,  woody 

2,3,5-Triiodobenzoic  acid  (TIBA),  107- 
109, 132-133 


Tropical  plants,  lack  of  cold  require- 
ments in  bulbs,  65-66;  lack  of 
knowledge  about,  G;  water  effects  on 
seasonal  anthesis,  129-130 

Ultraviolet  radiation,  90 
United  States  Department  of  Agricul- 
ture, 11-13,32-39 

Vegetative  growth,  effects  on,  of  cold, 
vernalization,  57,  59-60,  64 — of  light 
quality,  39 — of  photoperiodism,  27- 
28,  122-123;  and  gibberellin  (stem 
elongation),  100-105;  relation  to 
flower  development  and  initiation. 
8-9,  28;  restraint  of,  111;  see  also 
Age;  Bolting;  Meristem 

Vernalin,  82-83 

Vernalization,  and  auxin,  118;  and 
carbohydrates,  57,  60-61;  definition 
and  qualifications,  55,  58,  62-64; 
induction  by,  56-57,  87-88;  and 
political  ideology.  55;  see  also  Cereals, 
winter  and  spring;  Cold  require- 
ments; Cold  treatments;  Devernali/a- 
tion;  Vernalin 

Viruses,  78,  92 

Vitamin  E,  110 

Water,  129-130 

Woody  plants,  see  Perennials 

X-rays,  90