Light and plants : a series of experiments demonstrating light effects on seed germination, plant growth, and plant development

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LIGHT AND PLANTS

A Series of Experiments
Demonstrating Light Effects
On Seed Germination, Plant

Growth, and Plant Development

January 196]

Agricultural Research Service

U.S. DEPARTMENT OF AGRICULTURE

CONTENTS

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Light and seed germination cccccccccccncssccvcccocccssesccusssncsopessaccascesccesss
Light and plant growth. cccccccccccccccccccecccccccccsccsccccccccseccesccossscouscsoccs
Lnght and plant pigments. .ccccccescsenen ss saesniecincsisencpeueseaceaisasancaeeerssaceses
Effectiof duration-of light on! plantstcc-cssescc<senssesssee=snesphoscspeesssenceeas
General cultural hints cece cmcesccenecscsicincisecisnaes ses sisciesselsl=s ss ceessnesateasnaee
General TeterenCeSieccaccssnsensens-sclncevceessenscaesressieusneceeensacseeereeese sas

Demonstrations

A. Light and seed germination

1. Effect of light on germination of seeds planted in soil.......

2. How to test various kinds of seeds to determine their light
FEQCUITEMENE FOYT PETMINALION .cccecccccecccccccccccccccccccccccccccce

3. Effect of duration of imbibition period (soaking) on effec-
tiveness of a given light exposure in promoting germination

Of light-SeNSitive SEEAS ..ccccccccccccccccccccccssccceseccccccccccccece

4. How a light requirement for germination can be induced in
seeds that normally do not require light for germination...

* 5. Photoreversible control of seed germination by red and

far-red lig tesccccceccsceseccacsviscsesiacisscacacssomaaneseceecsseenestenee

B. Light and plant growth

1. Control by light of growth of an internode ..ccccccccccccccccveres
2. Control by light of growth and chlorophyll formation.........
3. Why plants bend toward light (phototropism)....cccccccccccscece
4. Effect of red and far-red light on elongation of stems of

Light- Grown plants .coccccccccccccccccccccesccccescccccccccscccccsccccces

C. Light and plant pigments

1. Effect of light on formation of anthocyanin in seedlings.....
2. Effect of light on tomato skin and fruit COLOT cocccccccecccccccce
3. Localization of response to light by the pigment in tomato

skin SCHOSHSHSHOSNSHSHSHSSSHSSSSHSHSSSHSSSHSHSSSSSSSHSSSHHesesseseesosavesceseeceeseosseoesesesed

4. Effect of light on coloration of appleS.cccccccccccccccccccccccseves

D. Duration of light

1. Photoperiodic control of flowering of short-day plants......
2. Photoperiodic control of flowering of long-day plants........
3. Photoperiodic control of growth and dormancy of woody

PlANCScccccccccccoucevescssescseccusccesisccvacecscesiccacsessel=sicnecssas==snie

4. Photoperiodic control of bulb formation of ONIONS .cccccccceeee

This publication was prepared by the
Crops Research Division
Agricultural Research Service

Page

ONO PR We

24
25

LIGHT AND PLANTS

A Series of Experiments Demonstrating
Light Effects on Seed Germination,
Plant Growth, and Plant Development

By R. J. Downs, H. A. Borthwick, and A. A. Piringer!

INTRODUCTION

Each year scientists in the U. S.
Department of Agriculture receive
many inquiries from students,
teachers, and other interested biolo-
gists for details of simple but dra-
matic experiments todemonstrate the
photomorphogenic effects of light on
plants. To answer these requests for
this specialized information detailed
and systematic experiments and
demonstrations on effects of light on
seed germination, growth, flowering,
and fruiting are outlined herein.
References accompanying each ex-
periment are cited for supplementary
reading and additional details. Certain
of these references will not be readily
available to all interested persons,
but, in general, the cited papers can be
obtained from college and other school
libraries of most metropolitan areas
as well as the personal libraries of
local plant scientists.

LIGHT AND SEED GERMINATION

Seeds of many kinds of plants
germinate poorly or not at all when
planted and covered with soil. Inmany
instances these are seeds that require
light for germination. Some seeds,
such as those of peppergrass (Lepid-

ium virginicum), do not germinate at
all in darkness. Others, suchas seeds
of Grand Rapids lettuce (Lactuca
Sativa), often germinate as much as
30 percent in darkness and some lots
even higher. However, all the seeds
of both peppergrass and Grand Rapids
lettuce germinate following a single
brief exposure to light. A single ex-
posure to light, nevertheless, is "Bis
adequate to promote germination o

all kinds of light-sensitive seeds.
Seeds of the Empress tree (Paulownia
tomentosa), for example, require long
periods of light for several days.

Germination of still other kinds of
seeds, suchas those of henbit (Lamium
amplexicaule), appears to be inhibited
by light.

A favorable temperature is one of
the requirements for germinationand
often a change or alternation of tem-
peratures is more effective than a
constant one in securing maximum
germination. For example, only about
30 percent of peppergrass seeds
imbibed in water and placed on
blotters in Petri dishes at a constant
temperature of 70° F. may germinate
in the light. If the temperature is
alternated, more seeds germinate. If
the seeds are imbibed in a solution

1Plant physiologists, Crops Research Division, Agricultural Research Service, U. S. Department of Agriculture,

Beltsville, Maryland.

containing 0.02-percent potassium ni-
trate and the temperature alternated,
maximum germination is attained.
Such an alternation of temperatures
might be 77°F. for 8 hours per day
and 60° F. for 16 hours per day.

Germination studies are oftenmade
on blotters in Petri dishes inorder to
facilitate handling, planting, and
counting the seeds. If Petri dishes
are not available, plastic sandwich
boxes with lids, and filter paper or
paper towels will perform the same
job. Studies of the effect of light on
germination of seeds implies that
some of the seeds must be kept inthe
dark to act as a check orcontrol. The
term ‘‘dark’* means ‘‘total dark-
ness,’ a complete absence of light.
To provide the darkness required for
the dark controls a proven method is
to make bags of at least two layers of
black sateen cloth. These bags must
be large enough to contain the dishes,
with enough slack at the opening so that
a flap may be folded back to prevent
entrance of light. Analternate method
would be to cover the dishes with two
or more layers of aluminum foil.

Studies on the effect of light on
various plant responses can be made
in greater detail using red and far-
red’? radiant energy. These wave-
lengths are the most effective ones
for regulating many plant responses to
light and they can be obtained by using
colored filters in conjunction with the
proper light source. The fluorescent
lamp emits considerable red but al-
most no far-red light and is, there-
fore, used as a source of red light. A
filter of two layers of red cellophane
removes all visible light except red
and since very little far-redis emitted
by the lamp, the net result is reason-
ably pure red light.

Incanaescent-filament lamps emit
considerable amounts of far-red and

2In Europe this would be referred to as near infrared.

are thus good sources of far red. The
visible light is removed by appro-
priate filters such as a combination
of red and blue cellophane. The red
cellophane absorbs all the visible light
except red and the dark-blue cello-
phane absorbs red. However, neither
color of cellophane absorbs far red,
so the radiation passing through the
filter is, therefore, far-red.

Seeds of trees, shrubs, orna-
mentals, vegetables, grains, and
grasses can be obtained from com-
mercial seed sources thatrangefrom
special seed supply houses to the local
hardware store. Many weed seeds are
light-sensitive and these can be
gathered by the investigator. After
gathering the seeds they should be
stored dry in a refrigerator (about
40° F.) until an appropriate time to
begin the experiments because at
higher temperatures they often under-
go change in their light requirements.
A good supply of seeds should be
gathered toassure an adequate amount
for possible additional experiments.
Any difficulty in identifying the plants
can be resolved with the aid of high
school biology teachers, the botanists
or horticulturists at the State Uni-
versity, State Agricultural Experi-
ment Stations, or agriculture exten-
sion specialists.

Demonstrations A-1 through A-5
tell us the following facts:

® Certain kinds of seeds require light
in order to germinate.

@ The light requirement is not some-
thing that occurs only under a special
set of experimental conditions, but
occurs when seeds are planted in an
ordinary way in pots of soil.

© A light requirement can be induced
in seeds that normally do not require
light for germination.

@®The photoreaction that allows
germination to proceed is reversible;
red radiant energy drives the reac-
tion in one direction and far-red
drives it in the reverse direction.

®The sensitivity of seeds to agiven
amount of radiant energy changes
with the period of imbibition.

Several questions should come to
mind immediately. How much light is
required to induce germination? What
are the relative amounts of red and
far-red required to drive the reac-
tions? Do all light-sensitive seeds
require the same amount of energy to
trigger germination? What is the
effect of various temperatures on the
light requirements? Are there other
methods of inducing a light require-
ment in seeds that normally are not
light-sensitive? Are there any seeds
that are inhibited from germinating
by light? Experiments can be de-
Signed to answer these and many more
questions relating to the mechanism
by which light controls germination.

LIGHT AND PLANT GROWTH

Vegetative growth of plants is toa
large degree controlled by light.
Plants grown in total darkness have
very long internodes, small leaves,
and are yellow in color because no
chlorophyll is formed. If the dark-
grown plants are exposed to weak light
for a minute or two each day, the
plants have shorter internodes and
normal-size leaves, although they
may still be yellow and without visible
chlorophyll. Daily exposures of the
plants to light of higher intensities
or for a longer duration may not
change the size of the leaves or inter-
nodes of the plants from that obtained
with brief exposures to light of low
intensity, but the plants turn greenas
chlorophyll is formed.

The formative effects of light, but
not chlorophyll formation, result from
the same red, far-red reversible
photoreaction that also _ controls
flowering of photoperiodically-sensi-
tive plants, germination of light-
sensitive seeds, and many other plant
responses. Red is the most efficient
portion of the spectrum in inhibiting
stem elongation and promoting leaf
expansion. A far-red irradiation im-
mediately following the red reverses
the potential effect of the red irradi-
ation and the stems become long.

If light is directed at either light-
grown or dark-grown plants from
one side, the leaves tend to bend and
the leaf petioles twist until the plane
of the leaf blade is perpendicular to
the light. The stems tend to curve in
such a way that the tip of the stem is
directed toward the light source. This
phenomenon is called ‘‘phototropism”"’
and is caused by a different photo-
reaction than the red, far-red one.
Blue light is the most effective kind
of light to promote the phototropic
response.

Demonstrations B-1 through B-4
show several ways in which light
influences plant growth and develop-
ment. These demonstrations tell us
the following facts:

@ Light inhibits stem growth and
promotes leaf expansion.

@ Plants bend toward the light.

@® Chlorophyll formation requires
light and the light must be of
higher intensity than that which
controls stem length.

@ The red, far-red reversible pho
toreaction that controls seed
germination also controls stem
length and leaf size.

Additional experiments can be de-
signed to answer many other questions
relating to the manner by which light
controls plant growth. Examples of

such questions are as follows: Are
bending (phototropism) and growth of
internodes controlled by the same
photoreaction? This question can be
answered by using different regions
of the spectrum (colors of light) and
testing to see if bending and growth
are controlled by the same colors.

Does the duration of darkness fol-
lowing the far-red irradiation of light-
grown plants affect the ultimate length
of the internodes? What is the opti-
mum period of darkness and why is it
optimum?

How concentrated is the pigment
that controls growth? How do we
know it is not chlorophyll? These
questions can be answered by com-
paring the growth responses of albino
and green corn or barley seedlings.

LIGHT AND PLANT PIGMENTS

The autumn coloration of leaves and
stems of woody plants is in part due
to the formation of a red pigment
called anthocyanin. The formation of
anthocyanin is also responsible for the
red color of apple fruits and for the
red to purple color of milo, turnip,
and cabbage seedlings.

A common observation is that
apples often do not turn red uniformly
but that one side of the fruit is green
or at least a lighter shade of redthan
the other side. The reddest side of the
apple is usually facing outward from
the tree. The formation of the red
color (anthocyanin) in apple fruits is,
of course, controlled by light. De-
tailed studies have shown that antho-
cyanin formation in milo, turnip, and
cabbage seedlings and inleaves ofred
maple and other trees is also regu-
lated by light.

Unlike many other light-controlled
plant responses anthocyanin forma-
tion requires high intensity lightfora
relatively long time. However, at the
close of the high-intensity light period

the low intensity, red, far-red photo-
reaction may exert final control on
anthocyanin synthesis. Thus, if the
plant material is irradiated for a few
minutes with far-red at the close of
the high-intensity light period, the
potential anthocyanin synthesis is in-
hibited and very little is formed. If a
brief irradiation with red follows the
far-red, then anthocyanin is formed
in an amount equal to that produced by
the high-intensity light alone.

An example of a low-intensity,
light-controlled coloration is the
yellow color of the skin of the tomato
fruit. Plant breeders recognize dif-
ferences in the color of the skins of
fruits of certain tomato varieties and
have classified the skins as yellow or
clear. The red flesh and a trans-
parent or white skin give the fruit a
translucent pink color, whereas the
yellow skin and red flesh give the
fruit an orange-red appearance. In
many tomato varieties the formation
of this yellow pigment is controlled
by light. Moreover, the same red,
far-red photoreaction that controls
flowering of photoperiodically-sensi-
tive plants, germination of light-seni-
tive seeds, and many other plant re-
sponses also controls the formation
of the yellow pigment in the skins of
tomato fruit.

Demonstrations C-1 through C-4
concern light and its control of plant
coloration. From these demonstra-
tions we know:

@ That light is required for the for-
mation of the red color (antho-
cyanin) of certain seedlings and
apple fruits.

e@ Light is required for the forma-
tion of a yellow pigment in the skin
of tomato fruit.

@ The coloration occurs only inthe
areas that received light--there
is no translocation of the
stimulus.

Additional experiments can be de-
signed to learn more about the light
reaction and about the chemical
processes that result in pigment for-
mation. Questions that one might ask
are: How much energy is requiredto
induce the formation of anthocyanin?
As light energy is increased, doesthe
amount of anthocyanin increase pro-
portionately ? Once the light require-
ment is fulfilled, what is the rate of
anthocyanin formation? What is the
role of temperature? What isthe role
of sugar? Does the red, far-red re-
versible photoreaction operate in the
control of coloration?

EFFECT OF DURATION OF LIGHT
ON PLANTS

Flowering of many kinds of plants
is controlled by the relative length of
the daily light and dark periods. This
Phenomenon is called ‘‘photoperiod-
ism.’’ Some plants, such as certain
varieties of chrysanthemum, poin-
settia, morning-glory, cocklebur, and
lamb’s-quarter, are short-day plants
and flower only when the days are
short and the nights are long. Certain
varieties of spinach, beet, barley, and
tuberous-rooted begonia, are ex-
amples of long-day plants which
flower only when the days are long
and the nights are short. Flowering
of many other kinds of plants is
hastened but not absolutely controlled
by the appropriate daylength. For ex-
ample, scarlet sage, variety America,
flowers quickly on short days but
eventually flowers on long ones. Many
varieties of petunia flower most
rapidly on long days but finally flower
on daylengths as short as 8 hours.

Bulbing and tuber formation are
also controlled by daylength. Tuber-
ous-rooted begonia, which is a long-
day plant for flowering, produces
tubers on short days but not on long
days. Onions, on the other hand,
produce bulbs on long days but not
when the days are short.

Dormancy and thereby preparation
of woody plants for the coming of
winter is another plant response regu-
lated by photoperiod. Even in the
warm greenhouse many woody plants
stop elongation of stems, produce
terminal buds, and ‘‘harden off’* when
the days begin to shorten in the
autumn. However, if artificial light
is used to keep the days long, plants
in the warm greenhouse will continue
growing during the naturally short
days of winter and several years’
‘*field’’ growth is often obtained in
only 1 year.

These plant responses are regu-
lated not by the length of the light
period but by the length of the dark
period. Thus, a long-day plant is
really a short-night plant, and a
short-day plant is really a long-night
plant. Therefore, a long-day plant
will flower, dormancy will be pre-
vented, and onions will produce bulbs
when a long dark period is broken into
two short periods bya relatively brief
exposure to light near the middle of
the dark period. Under these same
conditions, short-day plants will re-
main vegetative.

Studies of the responses of green
plants often require plants to be
grown indoors, yet they should have
the same healthy appearance as well-
tended plants grown out-of-doors.
Everyone knows that plants cannot
survive without light of adequate in-
tensity to operate the processes of
photosynthesis. In the field and garden
or in the greenhouse this high-in-
tensity light is obtained from the sun
which often provides an illumination
as high as 10,000 footcandles. In the
average home the light intensity is
usually too low for growth of many
kinds of plants, even on the window
sills. However, plants can be grown
quite successfully with artifical light
in complete absence of sumlight.
Beans, tomato, cereals, and many
ornamentals that grow in open sun-
light make satisfactory growth if the

artificial light intensity is about 1,000
footcandles. Shade-loving plants, such
as African violets, begonias, episcias,
gloxinias, and orchids, will grow well
with intensities as low as 500 foot-
candles.

A practical source of artificial
light for plant growth is the fluores-
cent lamp. These lamps supply the
necessary intensity without excessive
heat and are available in various
lengths, wattages, and colors. They
are usually operated on one- or two-
lamp ballasts, which maintain the
proper current and provide the start-
ing voltage. Prewired lamps and
ballasts of several sizes and types are
available as commercial luminaires
or as channels.

Many kinds of plants can be grown
satisfactorily using only two 40-watt
fluorescent lamps. Since the lamps
themselves are relatively cool, the
plants may be placed quite close to
them without danger of excessive heat
or burning. Table 1 shows the illum-
ination at various distances from two
40-watt cool-white fluorescent lamps
mounted 2 inches apart. If the lamps
are mounted further apart, the illum-
ination at 6 inches or less from the
lamps is markedly decreased.

If the daylength is to be controlled,
plants must be put into complete
darkness at the close of a particular
photoperiod. A dark chamber can be
made of masonite or plywood with
caulked seams, or it could be made of
two or more thicknesses of black
sateen cloth stretched over a wooden
frame. If used carefully, a cardboard
box with all seams and joints sealed
with paper tape could be placed over
the plants to provide darkness.

Experimental procedures can be
facilitated and made more exact if
an electric time switch is available
to turn the lights on and off at any
desired time.

Table 1.--Illumination in footcandles
at various distances from two or four
40-watt standard cool-white fluores-
cent lamps mounted approximately 2
inches froma white -painted reflecting
surface.

; Illumination
Distance

from lamps Two lamps*| Four lamps*

| Used+* | Used*#| New

(in) (fc) (fc) (fc)
1 1,100 1,600 1,800
2 860 1,400 1,600
3 680 1,300 1,400
4 570 1,100 1,300
5 500 940 1,150
6 420 820 1,000
7 360 720 900
8 330 660 830
9 300 600 780

10 280 560 720

11 260 510 660

12 240 480 600

18 130 320 420

24 100 PIO: 3260

* Center-to-center distance be-
tween the lamps was 2 inches.

** These lamps had been used for
approximately 200 hours.

Demonstrations D-1 through D-4
show some of the effects of the rela-
tive lengths of day and night on plant
growth and reproduction. These
demonstrations tell us that:

® Some plants flower on short days
and long nights, whereas others
flower on long days and short
nights.

@ Dormancy of woody plants in the
autumn is brought about by short
days.

@ Daylength controls tuber and bulb
formation as well as flowering
and dormancy.

Additional experiments can be de-
signed to answer and demonstrate
many other aspects of the photo-
periodic control of flowering bulbing,
and dormancy. Forexample, we might
ask, what is the critical daylength for
short-day plants? What isthe longest
day (shortest night) that will include
flowering in short-day plants? What
is the shortest day (longest night) that
will induce flowering of long-day
plants? When a long dark period is
interrupted by a brief interval of light,
what is the minimum energy required
to keep short-day plants vegetative or
to induce flowering of long-day plants ?
When is the most efficient time to
give the interruption during the dark
period? Is the control of flowering
operated through the same red, far-
red reversible photoreaction that con-
trols other plant responses?

GENERAL CULTURAL HINTS

For all demonstrations in which
seeds are germinated in soil, or in
which young seedlings areto be grown,
the soil should be sterilized. Steriliz-
ing the soil destroys harmful insects,
disease-producing organisms, and
weed seeds. Soil may be sterilized by
different methods as follows: (a)
Place small lots of moist soil in a
shallow pan and bake for at least l
hour at a temperature of 215° F,, then
cool but do not use until at least 2
weeks; (b) place soil in an autoclave
or pressure cooker and steam steri-
lize at 15 pounds’ pressure for at
least 1/2 hour, then allow to stand
for a minimum of 2 weeks; (c) sprinkle
1 quart of formaldehyde solution (1
pint 37 percent commercial formalde-
hyde to 3-3/4 gallons water) on 1
square foot by 6 inches of soil placed
in a box or bushel basket, then water
liberally and completely cover with

plastic or heavy cloth for 48 hours,
stirring frequently to hasten escape
of the formaldehyde gas, and allow 2
weeks before use of the soil (CAU-
TION: Do not use for planting as
long as fumes are present, because
formaldehyde gas is an irritating
poison to humans and is toxic tc
plants.)

Plants are usually grown in clay
pots of 3-, 3-1/2-,or 4-inchdiameter
filled with sterilized soil. Before the
soil is put into the pot a piece of
broken pot is placed in the bottom to
cover the hole so that the soil will not
plug it and prevent good drainage.
Clean pots should always be used.

When pots are not available or are
for some reason objectionable, plastic
cups, polyethylene freezer food con-
tainers, or even tin cans may be used.
One or more holes should be punched
in the bottom of these containers and
the holes covered with fiber-glass
matting or plastic window screen be-
fore filling with soil. Good drainage is
imperative for good plant growth.

Studies of the effect of light on plant
growth and flowering require that the
plants be placed indarkness at certain
times. As with seed germination, this
means complete or total darkness.
Because plants require more space
than seeds, light-tight bags are
usually not satisfactory. Instead, a
dark chamber must be constructed in
such a way that there is adequate air
exchange between the inside and out-
side of the chamber to prevent over-
heating. A satisfactory and proven
method is to construct a frame of
wood and cover it with at least two
layers of black sateen cloth. An
entrance or door can be provided by
making an overlapping flap.

GENERAL REFERENCES

Anonymous. Fundamentals of light
and lighting. Bull. LD-2, General
Electric Co., Cleveland, Ohio.

Anonymous. General Electric Fluo-
rescent Lamps, LS-102. Large
Lamp Div., General Electric Co.,
Cleveland, Ohio. 1957.

Borthwick, H. A. Daylength and
flowering. U.S.D.A. Yearbook
1943-47, pp. 273-283. 1947.

Borthwick, H. A. Photoperiodism:
the dark secret; how nights and
lights affect plant growth. Elec-
tricity on the Farm Magazine 26:
11-13. 1953.

Borthwick, H. A. Effect of light on
flowering and production of seed.
_ U.S.D.A. Yearbook 1961.[In press.]

Brown, F. A., Jr. The rhythmic
nature of plants and animals.
Amer. Scientist47: 147-168. 1959.

Butler, W. L., and R. J. Downs. Light
and plant development. Scientific

Hendricks, S. B. Control of growth
and reproduction by light and dark-
ness. Amer. Scientist 44: 229-247.
1956.

Hendricks, S. B. The clocks of life.
Atlantic Monthly 200: 111-115.
1957.

Hicks, GC. B. You can make a plant
do tricks. Popular Mechanics 108:
81-85, 232-236. 1957.

Kranze sive Hee ands © herdnz. eel
Gardening indoors under lights.
Viking Press, New York, N. Y.
MOS Ths

Parker, M. W., and H. A. Borthwick.
Influence of light on plant growth.
Ann. Rev. Plant Physiol. 1: 43-58.
1950.

Schultz, Peggy. Growing plants under
artificial light. M. Burrows & Co.,
New York, No Yo 1955.

U. S. Agr. Res. Serv. New light on
plants. U.S. Dept.Agr., Agr.Res. 1:
a= 55) 1953

Wassink, E. C., and J. A. J. Stolwijk.
Effects of light quality on growth.
Ann. Rev. Plant Physiol. 7: 373-
400. 1956.

Weitz, C. E. General Electric Lamp
Bull. LD-1l. General Electric Co.,
Cleveland, Ohio. 1950.

Went, F. W. The role of environ-
ment in plant growth. Amer. Sci-
entist 44: 378-398. 1956.

Withrow, R. B. Photoperiodism and
related phenomena in plants and
animals. Publ. No. 55, Amer.
Assoc. Adv. Sci., Washington, D.C.
1959.

DEMONSTRATIONS

A. Light and Seed Germination

DEMONSTRATION A-1: Effect of light on germination of seeds plantedin soil.

Materials:

Seeds of peppergrass (Lepidium virginicum or L. densiflorum).
Sterilized potting soil.

Six clay pots, or other suitable containers, 3 to 4 inches in diameter.
Small glass squares large enough to completely cover the tops of the
pots.

Glass baking dish or enamel pan large enough to contain all the pots.
Pot labels.

Procedure:

1,

2.

Fill six pots or containers with moist (not wet) sterilized soil to within
2 centimeters of the top of the pots. Smooth the soil surface and tamp
the soil gently but firmly with the bottom of one of the pots.

Prepare six lots of 100 seeds each and distribute one lot of seeds uni-

formly over the surface ofthe soilineach pot. Treat the pots as follows:

(a) Pot 1 - leave the seeds on the surface. Do not cover them with soil.

(b) Pots 2 to 6 - cover the seeds with 1 centimeter of soil. Level the
soil surface and tamp gently.

Do not water the top surface of the soil. Place all the pots in the large
glass dish or enameled pan and sub-irrigate the soil in the pots by
adding water to the dish. Maintain the pots in this dish, being careful to
have them setting in about 1 centimeter of water at all times.
Place all pots in the light.
Keep each pot covered with a glass square, at least until the seedlings
that will develop are well established. The soil will be kept moist by
capillary action. The glass cover will admit light but will prevent
excessive water loss from the soil and maintain a high humidity at
the soil surface. This is important during the critical periods of
germination and early seedling growth.

Write the name of the plant material, the date of planting, the date of

treatment on a label and insert it into the soil at the edge of the pot.

Treatments:

(a) Pot 1 - see 2a.

(b) Pot 2 - see 2b.

(c) Pot 3 - immediately after covering the seeds with soil make a
narrow slit 3 to 4 centimeters deep in the soil across the diameter
of the pot with a knife.

(d) Pots 4, 5, and 6 - repeat the process described for pot 3, 1, 2, and 4
weeks after planting, respectively.

9

Observations:

Record the date of exposure to light, the subsequent date of germination
and the extent of germination. Moist peppergrass seeds exposed to light (as
in pot 1) will germinate in 3 to 4 days after exposure. Seeds covered with l
centimeter of soil will not germinate since they are in the dark (as in pot 2).
Slitting the soil with a knife blade exposes some of the buried seeds to light.
Thus (as in pots 3 to 6) germination of seeds occur in the slit made in the
soil.

The soil may shrink away from the sides of the pot and expose some seeds
to light and seedlings may appear. This can be avoided by planting the
seeds away from the edge of the pots. Slitting the soil at regular intervals
after planting illustrates that germination will occur any time the seeds
are exposed to light. Slitting the soil, in effect, simulates field cultivation.
Thus, cultivation, while destroying plants and seedlings, also brings weed
seeds such as peppergrass to the surface of the soil, where they receive
light, germinate, and produce more weeds.

Supplementary Reading:

Koller, Dov. Germination. Scientific American 200: 75-84. April 1959.
Toole, E. H., S. RB. Hendricks, H. A. Borthwick, and V. K. Toole. Physiol-
ogy of seed gerrnination. Ann. Rev. Plant Physiol. 7: 299-324. 1956.
U.S. Agr. Res. Serv. New Light on Plants. U.S. Dept. Agr., Agr. Res.

LOS 5) pal O5S:.
U.S. Agr. Res. Serv. How Light Controls Plant Development. U.S. Dept.
Agre, Agr. Resi18:19-5.. 1959)

DEMONSTRATION A-2: How to test various kinds of seeds todetermine their
light requirement for germination.

Materials:

1. A minimum of four Petri dishes or plastic sandwich boxes with lids.

2. Ordinary color-fast or white blotters, filter paper, or paper towels.

3. Black sateen cloth bags made of two layers of cloth large enough to
hold the dishes. An alternate method is to wrap the dishes in two layers
of aluminum foil.

4. Seeds of several kinds of weeds. (Although some kinds of seeds are
known to be light-sensitive, many kinds have never been tested. This is
especially true for weed seeds, so they would be the more interesting
group to investigate.)

Procedure:

1. Collect seeds of several kinds of local weeds.In general, seeds will re-
tain their viability fairly well when stored dry ina refrigerator. Some
suggested seeds known to be light-sensitive are peppergrass (Lepidium
virginicum and L. densiflorum), henbit (Lamium amplexicaule), and

10

hedge mustard (Sisymbrium officinale), Other weed seeds worthy of
investigation are Shepard’s purse (Capsella bursapastoris), yellow
rocket (Barbarea vulgaris), tumble-mustard (Sisymbrium altissimum),
chickweed (Stellaria ssp. and Cerastium ssp.), sheep sorrel (Rumex

acetosella), the small-seeded cacti, and, of course, many others.

Four dishes should be used for each kind of seeds tested.

Cut the blotters to fit the dishes and presoak overnight (about 16 hours)

by putting enough tap water into the dishes to flood the blotters.

After the blotters are thoroughly soaked, pour off the excess water and

evenly distribute 100 seeds over the surface ofthe blotters in each dish.

Immediately after the seeds are distributed, place the covers on the

dishes and place them in the black cloth bags.

Allow the seeds to imbibe water in the dark for a period of 16 to 24

hours, then begin treatments.

Treatments:

(a) Dishes 1 and 2 should be kept at about 70° F. during the entire period
of the demonstration. Dishes 3 and 4 should be held at about 60° F.
during the 16- to 24-hour imbibition period, then transferred toa
temperature of 77° F, for the remainder of the demonstration.

(b) Dishes 1 and 3 are placed in the black cloth bags at the time of
planting and left there throughout the demonstration. These will
serve as ‘‘dark controls.”’

(c) Dishes 2 and 4 should be placed in the light for 1 hour each day.
The temperature during the period of irradiation can be between 70°
and 80° F. Light from two 40-watt standard cool-white fluorescent
lamps should be adequate.

(d) When the seeds in dishes 2 and 4 have germinated, the other dishes
are removed from the black cloth bags andthe number of germinated
seeds counted and recorded for each treatment.

Observations:

Record the number of days required for germination, the temperature, light
conditions, and so forth. Count the number of seeds that germinate under
each treatment and record as percent germination. When a light-sensitive
seed is found, demonstrations can be designed to determine how much light
the seeds require, how many times they mustbe exposed to light, and such.
These seeds can also be used in Demonstrations 3, 4, and 5.

Supplementary Reading:

See Demonstration A-l.

1.
2.

DEMONSTRATION A-3: Effect of duration of imbibition (soaking) period on

effectiveness of a given light exposure in promoting
germination of light-sensitive seeds.

Materials:

Eight Petri dishes or plastic sandwich boxes with lids.
Two to four thicknesses of blotters, filter paper, or paper towel cut to
fit the dishes.

11

oe

4.

Eight black sateen cloth bags made of two layers of cloth large enough
to hold the dishes. As an alternate method dishes can be placed between
the folds of a large, double layer of black cloth, or they can be wrapped
with two layers of aluminum foil.

Light-sensitive seeds such as Grand Rapids lettuce or peppergrass
(Lepidium virginicum).

Procedure:

I,

2.

Di

Prepare the dishes as outlined in the procedure in Demonstration A-2,

Use 0.2 percent KNO3 instead of tap water for peppergrass.

All dishes except dish Z2areimmediately placedin darkness (in the black

cloth bags or between folds of a black cloth ‘‘blanket.*") Dish 2 is exposed

to light for a period of 5 minutes, then placed in darkness. Illumination
provided by two 40-watt fluorescent lamps is adequate.

Treatments:

(a) Dish 1 remains in darkness throughout the demonstrationand serves
as the dark control.

(b) Dish 2 is irradiated immediately after distri bultag the seeds, then
placed in darkness.

(c) Dish 3 is irradiated for a period of 5 minutes with light from the
fluorescent lamps after the seeds have imbibed in darkness fora
period of 1 hour; that is, the seeds are exposed to light 1 hour after
soaking.

(d) Dishes 4, 5, 6, 7, and 8 are exposed to 5 minutes of light after 2, 4,
8, 16, and 24 hours of imbibition in darkness.

Following the 5-minute exposure to the fluorescent light the dishes are

returned to the black cloth bags, to the folded black cloth blanket, or to

the aluminum foil.

Four days after the seeds were ‘‘planted’’ the dishes can be removed

from the dark and the number of germinated seeds counted and recorded.

Observations:

Count the number of seeds germinated in each dish and record as percent
germination. These data can be presentedina line graph by plotting percent
germination against the number of hours of imbibition. The results may
show the sensitivity of the seeds to a given dose of light changes during the
period of imbibition.

Supplementary Reading:

See Demonstration A-1l.

DEMONSTRATION A-4: How a light requirement for germination can be in-

duced in seeds that normally do not require light for
germination.

Materials:

1.
2.

Petri dishes or plastic sandwich boxes with lids.
Blotters, filter paper, or paper towels cut to fit the dishes.

12

3. Black sateen cloth bags made of two layers of cloth large enough to
hold at least two dishes.

4, Dark-blue cellophane.

5. Fluorescent lamp (a fluorescent desk lamp will do),

6. Seeds of several kinds of plants including several varieties of lettuce
and tomato.

Procedure:

1. Prepare dishes as previously described in Demonstration A-2,

2. Distribute 100 seeds evenly on the blotters of each dish.

3. Dish 1 is placed in a black cloth bag (darkness).

4, Dish 2 is placed under the fluorescent lamp.

5. Dishes 3 and 4 are completely covered with two layers of dark-blue
cellophane and placed under the fluorescent lamp.

6. The fluorescent lamp is left on continuously.

7. After germination is apparent in dish 2, count and record the number
of germinated seeds in all dishes.

8. Re-cover dish 3 with the blue cellophane and replace both dishes 3 and
4 (without cellophane) under the fluorescent lamp.

9. When germinationis completedindish4, countand record the germinated
seeds in both dishes 3 and 4.

Observations:

The seeds in the dishes under the dark-blue cellophane may not germinate,
whereas those receiving either light or darkness may germinate nearly 100
percent. After a dish has remained under blue cellophane in light for 3 or 4
days it can then be covered with black cloth and the seeds may remain
dormant in the dark. When subsequently given unfiltered light, they promptly
germinate.

Supplementary Reading:

See Demonstration A-1l.

DEMONSTRATION A-5: Photoreversible control of seed germination by red

and far-red light.

Materials:

Three Petri dishes or plastic sandwich boxes with lids.

Ordinary color-fast or white blotters, filter paper, or paper towels.
Red and dark-blue cellophane.

Black sateen cloth bags made of two layers of cloth large enough to hold
each dish,

Light-sensitive seeds such as Grand Rapids lettuce or peppergrass
(Lepidium virginicum).

13

Procedure:

1, Cut two to four layers of blotter to fit each dish and presoak overnight
(about 16 hours) by putting enough tap water into the dishes to flood the
blotters.

2. After the blotters are thoroughly soaked pour off excess water and
evenly distribute 100 seeds over the surface ofthe blotters in each dish.

3. Immediately place the dishes with lids in the black cloth bags.

4, Allow the seeds to imbibe water in the darkness of the black cloth bags
for a period of 16 to 24 hours, then begin treatments.

5. Treatments:

(a) In the dimmest light possible, preferably complete darkness, remove
dishes 1 and 2 from their black cloth bags and wrap each dish with
two layers of red cellophane.

(b) Place both the cellophane-wrapped dishes under the fluorescent
lights for a period of 5 minutes.

(c) Return dish 1 to its black cloth bag without further exposure to light.
If no dark room is available during this transfer, place the dish in
the black cloth bag without removing it from the red-cellophane
wrapping.

(d) Dish 2 is wrapped in blue cellophane so that the seeds are now
covered with two layers of red and two layers of blue cellophane.

(e) Dish 2 is now exposed to light from the incandescent lamps for a
period of 15 minutes.

(f) Place dish 2 in the black cloth, either in complete darkmess or if
necessary still enclosed in the red- and blue-cellophane wrapping.

6. The three dishes of seeds have now received their treatments. Dish 3
has remained in the black cloth bag and serves as a dark control. Dish
1 has been exposed to red radiant energy for 5 minutes and dish 2 has
been exposed to red for 5 minutes and to far-red for 15 minutes.

7. Allow 3 to 4 days to elapse, then remove the dishes from their black
cloth bags and count and record the number of germinated seeds.
Temperatures should be held as close to 70° F. as possible.

Observations:

When counting the number of seeds germinated in each dish, record as
percent germination. These data can be presented in either tabular form
or in a bar graph, using the bars for treatments and the height of the bars
as percent germination. Those seeds that remained in darkness will
probably germinate 0 percent if peppergrass seeds were used, or 5 to 25
percent if seeds of Grand Rapids lettuce were used. Those seeds receiving
red light will probably germinate 90 to 100 percent for both species,
whereas those receiving red followed by far-red might germinate 5 to 10
percent for peppergrass, and 5 to 25 percent for lettuce. Evidence has now
been obtained to show that these seeds require light (red) for germination,
and that the potential germination induced by the exposure to red can be
reversed by a subsequent exposure to far-red radiant energy.

Supplementary Reading:
See Demonstration A-1,

14

B. Light and Plant Growth

DEMONSTRATION B-1: Control by light of the growth of an internode.

Materials:
1. Corn seeds. \
2. Two 4- to 5-inch clay pots or other suitable containers.
3. Sterilized soil.
Procedure:
1, Place a piece of broken pot, fiber-glass mat, or plastic screen over the
drainage hole in the bottom of the pot or container.
2. Fill pot or container 1 with sterilized soil to within 3 centimeters of

the top of the pot, tamp the soil gently, place 3 to 4 corn seeds on the
surface of the soil, and cover them with 2 centimeters of soil. Tamp
firmly.

3, Fill pot 2 with 3 centimeters of sterilized soil, tamp gently, place 3 to 4
corn seeds on the surface of the soil, and fill the pot with sufficient soil
to reach the same level as in pot 1. Tamp firmly.

4. Place both pots ina large glass dish or enameled pan and sub-irrigate
by adding water to the dish or pan.

5. Place both pots in the light at a temperature of about 70° to 80° F.

6. The seedlings of pot 1 will emerge first. Let them grow until the
seedlings of pot 2 emerge and produce a leaf.

7. Knock the soil out of the pots into a bucket of water and remove the
seedlings from the soil, holding the soil and seedling under the surface
of the water until the roots are free of soil.

Observations:

Compare and measure the length of the first internode (the distance from
the corn seed to the beginning of the first leaf). Note that the internodes
in both pots 1 and 2 stopped growing when the plant emerged from the soil;
that is, when the seedling received light.

Supplementary Reading:

U.S. Agr. Res. Serv. New Light on Plants. U.S. Dept. Agr., Agr. Res.

wee, 2953 5

DEMONSTRATION B-2: Control by light of growth and chlorophyll formation.

Materials:

l.

A chamber or box that can be made completely dark. If entrance into
the chamber cannot be made without exposing the contents to light,
regardless of how weak the light is, then more than one chamber will

15

(Aye
Sie

4.

have to be made. These chambers can be made of masonite or plywood
with caulked seams and a baffled door, or they can be made of several
layers of black sateen cloth stretched over a wooden frame.

Two 40-watt fluorescent lamps.

Flats, boxes, pots, or plastic freezer cups filled with sterilized soil,
sand, vermiculite, or perlite.

Bean seeds (any kind).

Procedure:

iba

il.

Plant the bean seeds and water. No nutrient solution is required even
when the seeds are planted in sand, vermiculite, or perlite.

The best temperature is 80° to 85° F, Lower temperatures will suffice,
but the rate of germination and growth will be slower.

Place the boxes in the dark chambers immediately after the beans are
planted. If dark red kidney beans are used, planted in sand and kept at
80° to 85° F., they will germinate in 3 to 4 days.

On the fifth day, place box 1 in the light (preferably from fluorescent
lamps) for 5 minutes, then return it to the dark chamber.

Repeat step 4 on the sixth, seventh, and eighth days from planting.
Place box 2 in the light for 20 minutes on the fifth day only.

Place box 3 under the light for 2 hours each day, and box 4 for a period
of at least 8 hours per day.

Remove all boxes from the dark chambers on the ninth or tenth day
from planting. Measure and record the length of each internode and the
length of the leaves.

Slice or mince the leaves and place in a known volume of ethyl alcohol.
Use the same volume of alcohol for each treatment irrespective of the
size of the leaves. A better method is to use 10 milliliters of alcohol
for each gram of leaves.

Calculate the average length of the internodes and the average length
of the leaves for each treatment.

The relative amounts of chlorophyll can be estimated by assigning a
numerical value to each sample based on the visual greenness of the
extract, or the optical density of each sample can be measured.

Observations:

The plants grown in complete darkness (box 5) should have long hypocotyls,
short first internodes, small leaves,andno chlorophyll. Boxes l, 2, 3, and 4
should contain plants that have shorter hypocotyls, longer first internodes,
and perhaps more internodes than the plants of box 5. They should also have
much larger leaves. Plants of box 1 should contain no chlorophyll, and those

of

box 2 none or very little. Plants in boxes 3 and 4, however, should con-

tain a greater amount of chlorophyil, withthose plants of box 4 having more
than those of box 3.

Supplementary Reading:

Downs, R. J. Photoreversibility of leaf and hypocotyl elongation of dark-

grown red kidney bean seedlings. Plant Physiol. 30: 468-473. 1955.
Textbooks of Plant Physiology.

16

DEMONSTRATION B-3: Why plants bend toward light (phototropism).

Materials:

1. A chamber or box that can be made completely dark (see Demonstra-
tion B-2, steps 1 through 4 of Materials, for details).

Procedure:

1. Plant the bean seeds and water. No nutrient solution is required even
when the seeds are planted in sand, vermiculite, or perlite.

2. The best temperature is 80° to 85° F, Lower temperatures will suffice,
but the rate of germination and growth will be slower.

3. After the beans are planted, place one box in the dark chamber and one
in the light, where the plants should receive 8 hours of light per day.

4. When the dark-grown beans are about 6 days old, open the door of the
dark chamber so that the plants receive some light. Better results are
obtained by placing a desk lamp 3 or 4 feet from the open door of the
chamber. In a few hours these plants will bend toward the light entering
the door.

5. When the plants in the light have expanded their first pair of leaves,
place them in the dark chamber. This is conveniently done in the late
afternoon. Again, open the door of the dark chamber and place a desk
lamp 3 to 4 feet from the door.

Observations:

The following morning the leaf blades will have twisted around until they
are perpendicular to the light.

Supplementary Reading:
Textbooks of Plant Physiology.

DEMONSTRATION B-4: Effect of red and far-red light on elongation of stems
of light-grown plants.

Materials:

1. A light-equipped chamber (two 40-watt fluorescent lampsisa minimum).

2. A red chamber (a cardboard box with seams sealed with paper tape).
Cut out the top and most of the bottom of the box. Place two layers of
red cellophane over the opening inthe bottom of the box, using cellophane
tape to hold the cellophane in place.

3. A far-red chamber (a cardboard box prepared in the same manner as
for the red chamber except cover the cut-out opening in the bottom
with two layers of red and two layers of dark-blue cellophane).

4. A dark chamber.

5. Bean plants (preferably Pinto bean).

17

Procedure:

. Plant beans in pots of sterilized soil, water, and place at a temperature
of (80° E.

. After 3 to 4 days the plants will begin to emerge from the soil. At this
time all pots should be placed in the light chamber, where they should
receive light from the fluorescent lamps for 8 to 10 hours each day.
The temperature during this growing period should be 70° to 75° F.,

3. The first pair of leaves should be about half expanded 10 to 12 days
after planting. At this stage of development the plants are ready to
start on treatments.

4. Divide the plants into three equal lots; A, B, and C.

5. Place plants of lot A in the dark at the close of the 8- to 10-hour light
period. Place lots B and C under the red and blue cellophane (the far-
red). Turn off the fluorescent lamps.

6. Place a 100-watt incandescent-filament lamp 3 feet from the red- and
blue-cellophane filter and turn it on for 15 minutes.

7. Plants of lot B are moved in darkness to the dark chamber immediately
after the 15-minute exposure to far-red. Plants of lot C are moved in
darkness and placed under the box with the red-cellophane filter, which
should be placed under the fluorescent lamps.

8. Turn on the fluorescent lamps for 10 minutes, then move plants of lot C
in darkness to the dark chamber. (Great care should be taken to assure
that the plants receive no light of any kind after they are exposed to red
and to far-red.)

9. Return all plants to the fluorescent-light chamber the next morning.

10. The treatments should be given daily until a response is obvious, re-

quiring at least 5 days of treatment.

—

N

Observations:

Record date of planting, date treatments were begun, number of days treat-
ments were given, and the durations of the light period, red exposure, and
far-red exposure. Measure and record daily the length of the second
internode. Data can be plotted as line graphs (length plotted against time).
Three plots should be made: The control (lot A), the far-red treatment
(lot B), and the far-red followed by red (lot C).

Supplementary Reading:

Downs, R. J., S. B. Hendricks, and H. A, Borthwick. Photoreversible
control of elongation of Pinto beans and other plants under normal
conditions of growth. Bot. Gaz. 118: 99-208. 1957.

Wassink, E. C., and J. A. J. Stolwijk. Effects of light quality on growth.
Ann. Rev. Plant Physiol. 7: 373-400. 1956.

C. Light and Plant Pigments

DEMONSTRATION C-1: Effect of light on formation of anthocyaninin seedlings.
Materials:

1. Seeds of turnip (Brassica rapa), variety Purple Top White Globe.
2. Five Petri dishes or plastic sandwich boxes with lids.

18

. Filter paper (Whatman No. 3),

An aqueous solution of 32 parts per million chloramphenicol.

. A solution of 0.01 molal HCl in aqueous 25-percent 1-propanol.
. Black bags made of two layers of black sateen cloth.

Procedure:

1. Place three sheets of filter paper in each Petri dish and moisten with
water and 32 parts per million chloramphenicol to depress chlorophyll
formation.

2. Place 50 seeds of turnip on the filter paper, cover the dishes with the
lids, and place each dish in a black cloth bag, keeping the temperature
at about 77° F.

3. Allow 3 days for the seedlings to germinate and then begin treatments.

4. Place the dishes under the fluorescent lamps for periods of 0, 2, 4, 6,
or 8 hours, then return the dishes to their black cloth bags.

5. The amount of anthocyanin formed can be observed after 16 to 24 hours
after irradiation.

6. Count out a certain number of seedlings (say 25) from each dish and
place each lot of 25 seedlings inthe solution of 0.01 molal HCl in aqueous
25-percent 1-propanol. Place them ina refrigerator (about 40° F.) for
16 to 24 hours.

Observations:

The amount of anthocyanin formed by the various light treatments can be
estimated by assigning a numerical scale that increases with increasing
color, or it can be measured by differences in optical density as measured
with a colorimeter or spectrophotometer. The amount of anthocyanin can
then be plotted graphically by plotting the amount of anthocyanin asa
function of the time (hours of light).

Supplementary Reading:

Siegelman, H. W., and S. B. Hendricks. Photocontrol of anthocyanin
synthesis in turnip and red cabbage seedlings. Plant Physiol. 32: 393-
See tamed OO SY

DEMONSTRATION C-2: Effect of light on tomato skin and fruit color.

Materials:

1, Mature green tomato fruits from a normally red-fruited variety when
ripe, such as the variety Rutgers. Their skin color should be greenish-
white with no visible red, pink, or yellow color. These are readily ob-
tainable from home gardens any time prior to frost and in the larger
cities southern-grown green-mature fruits can be obtained throughout
the winter from local wholesale vegetable distributors.

19

2. A light facility, using incandescent or fluorescent lamps that provide a
light intensity of 20 footcandles or more. Keep the temperature at
MOLT Ey

3. A dark facility that provides total darkness, using light-tight black
sateen cloth bags made of two layers of cloth. Keep the temperature at
TOO ER

4. Scalpel or similar sharp instrument.

5. Small bottles or vials.

6. Acetone or petroleum ether (CAUTION: Flammable solvents.)

Procedure:

1, Divide the green-mature fruits into two uniform lots, A and B.

2. Ripen fruits of lot A for 10 to 14 days with illumination of daily dura-
tion of 1 hour. Longer periods may be given but are not necessary.

3. Ripen fruits of lot B 10 to 14 days in total darkness, taking care not to
remove from darkness until ripe. These should ripen simultaneously
with its lighted counterpart.

4. When the fruits are ripe, remove uniformly shaped and size sections
from a typical fruit from each lot, being careful to keep the sections
separate and properly identified as to treatment. Immerse each section
in boiling water for 1 minute and cool immediately by immersion in
cold water; thus the skin is readily removed from the tomato flesh.
Scrape the adhereing tissue from the skin with the scalpel as carefully
and completely as possible. Place each scraped skin section in one of
the small containers containing acetone or petroleum ether solvent and
leach the skins with several washings (keeping the sections immersed)
over a period of at least several hours.

Observations:

The fruits ripened in the dark will be pink; those ripened in the light will
be orange-red. CAUTION: Fruits will ripenfaster at temperatures higher
than 70° F., but at those higher temperatures the red pigment in the flesh
does not develop well and gives the fruit an off-color appearance.

The skin of fruits ripened in the dark will be colorless; the skin of those
ripened in the light will have a yellow color even after prolonged leaching
with the solvent. The insoluble light-induced yellow pigment left in the
tomato fruit cuticle (skin) has not yet been identified. The presence or
absence of the light-controlled pigment in the skins makes them either
yellow or transparent. When the yellow skin is superimposed over the red,
the fruit has an orange-red appearance, the typical coloration of summer
field-ripened tomato fruits, The combination of red flesh and a transparent
skin produces a fruit that is pink. The pink coloration is characteristic
of fruits commercially available in the North in mid-winter which have
been artificially ripened in darkness by vegetable wholesale distributors.
If fruits ripened in the dark have yellow-tinted skins, the leaching process
was not complete or the fruits were toomature and were already producing
the light-responsive pigment at the time the fruits were placed in the
dark. Light starts to act as soon as the fruits mature.

20

Supplementary Reading:

Piringer, A. A., and P, H. Heinze. Effect of light on the formation of pig-
ment in the tomato fruit cuticle. Plant Physiol. 29: 467-472. 1954.

U.S. Agr. Res. Serv. Light Link in Tomato. U.S. Dept. Agr., Agr. Res.
2 BP eLOS4.

DEMONSTRATION C-3: Localization of response to light by the pigment in
tomato skin.

Materials:

1. Use the same materials as in Demonstration C-2Z.
2. In addition, two small sheets of aluminum foil, enough to completely
cover a tomato fruit.

Procedure:

1. Carefully select three uniform green-mature fruits.

2. Completely and tightly wrap one of the tomato fruits with a sheet of
aluminum foil (dark control).

3. Remove a 5-millimeter-diameter section from the center of another
sheet of aluminum foil. Completely and tightly wrap another tomato
fruit, being careful that the perforation exposes an area of skin on the
side of the fruit.

4, Leave the remaining tomato fruit unwrapped (light control).

5. Place all tomato fruits, both wrapped and unwrapped, in the light and
allow 10 to 14 days for ripening, keeping the temperature at 70° F.

6. When the unwrapped fruit is ripe (soft and red), unwrap all fruits and
note any differences in skin color.

7. Sections of the skin should be removed and processed as in Demonstra-
tion C-2, being careful to include the area exposed to the light through
the perforation in the foil.

Observations:

Skins of the unwrapped fruit should be bright yellow; skins of the com-
pletely wrapped fruits should be colorless; skins from the wrapped fruit
with the small area exposed should be colorless except for the small
exposed area which will be yellow. A novelty can be produced by tightly
wrapping a green-mature fruit with a sheet of aluminum foil having a num-
ber of small perforations.

Supplementary Reading:

Hicks, C. B. You can make a plant dotricks. Popular Mechanics 108:
81-85, 232-236. 1957.

21

DEMONSTRATION C-4: Effect of light on coloration of apples.

Materials:

1, Early harvested (green) Jonathan, Rome Beauty, or Arkansas apples.
Jonathan variety is preferred. Store the apples at 32°F. in bags of 0.38-
millimeter polyethylene plastic.

2. Black cloth bags.

3. Black plastic electrical tape.

4. Aluminum foil.

Procedure:

1, Place one apple in a black cloth bag.

2. Wrap one apple in aluminum foil, then cut holes in the foil and place
the apple under the light of the growth chamber.

3. Using the black plastic electrical tape, put an initial on each of several
apples and place the apples under the light of the growth chamber.

4, Allow 3 to 4 days for the apples to turn red, then remove the foil, tape,
or black bag. Keep the temperature about 75° F,

Observations:

The apple that has been keptinthe dark will still be green. The one covered
with aluminum foil will also be greenexcept where light has entered through
the cut-out holes; here, the apple will be red and may have a polka-dot
appearance of red on green. The apple that was exposed to light except for
areas under the black tape will be red. Under the black tape the apple will
be green and thus show green initials on a red apple.

Supplementary Reading:
U.S. Agr. Res. Serv. How Light Controls Plant Development. U.S. Dept.
Agre, Agr. Res.8:. 5-5, 1g5o.

Siegelman, H. W., and S. B. Hendricks. Photocontrol of anthocyanin
synthesis in apple skin. Plant Physiology 33: 185-190. 1958.

D. Duration of Light

DEMONSTRATION D-1: Photoperiodic control of flowering of short-day
plants.

Materials:

1, A light-equipped chamber (two 40-watt fluorescent lamps isa minimum).
2. A dark chamber.

22

3. Plants of cocklebur, lambsquarter, scarlet sage variety America, or
morning-glory variety Scarlett O’Hara, should be grown on daylengths
of 18 hours or more until large enough to use in the demonstration.
Use morning-glory plants as soon as the cotyledons have expanded.
Plants of cocklebur and lambsquarter are large enough when they
have three leaves above the cotyledons. Photoperiodic treatments of
scarlet sage can be begun as soon as the plants have 4 to 5 pairs of
leaves.

4. Sterilized soil.

Procedure:

1. When the plants are large enough to use, divide them into lots A and B.

2. Both lots of plants should receive 8 to 10 hours of light in the light
chamber each day.

3. Place lot A in darkness at the close of each daily light period. Turn off
fluorescent lamps. Place lot B 3 to 4 feet from a 40-watt incandescent-
filament lamp, which is now turned on.

4. If an electric time switch is available, give lot B a total light period of
18 to 20 hours (8 to 10 hours fluorescent light and 8 to 10 additional
hours incandescent light).

5. If an electric time switch is not available, leave the incandescent lamp
on throughout the night.

6. Return both lots A and B to the light chamber each morning.

7. Continue these daily treatments until flower buds are obvious. If the
treatments are discontinued now, the flowers will usually continue to
develop. During this period of development both lots of plants should
receive photoperiods of 18 hours or more.

Observations:

Record date of planting, date treatments began, length of the light and dark
periods, and include the number of short days required to induce flower
formation.

Supplementary Reading:

Doorenbos, J., and S. J. Wellensiek. Photoperiodic control of floral in-
duction. Ann. Rev. Plant Physiol. 10: 147-184. 1959.

Lang, A. Physiology of flowering. Ann. Rev. Plant Physiol. 3: 265-306.
E952.

Liverman, J. L. The physiology of flowering. Ann. Rev. Plant Physiol.
G-el77-210. 1955,

Naylor, A. W. The control of flowering. Scientific American 186: 49-56.
1952.

Parker, M. W., and H. A. Borthwick. Influence of light on plant growth.
Ann. Rev. Plant Physiol. 1: 43-58. 1950.

Salisbury, F. B. The flowering process. Scientific American 198: 109-
Wis 1 958%

23

DEMONSTRATION D-2: Photoperiodic control of flowering of long-day plants.

Materials:

1. A light-equipped chamber (a minimum of two 40-watt fluorescent lamps).
2. A dark chamber.
3. Plants of tuberous -rooted begonia, petunia, or barley.

Procedure:

1, Divide the plants into lots Aand Bas soon as they emerge from the soil.

2. Both lots should receive 8 tol0hours of light daily in the light chamber.

3. Place lot A in darkness at the close of each 8- to 10-hour light period.
Turn off fluorescent lamps and expose plants of lot B to light froma
40-watt incandescent-filament lamp.

4, If an electric time switch is available, allow a total light period of 16
to 18 hours, 8 to 10 hours from the fluorescent lamps and another 8
hours from the incandescent lamp.

5. If an electric time switch is not available, leave the incandescent lamp
on throughout the night.

6. Return both lots to the fluorescent-lighted chamber each morning.

7. These treatments should be given daily until flower buds are obvious.
If the treatments are stopped at this point, the flower buds will usually
continue to develop. During this period of development all plants should
be grown on short days of 8 to 10 hours’ duration.

Observations:

Record date of planting, date demonstration began, length of the light and
dark periods, and also record how many long days were required to induce
formation of flowers. If the tuberous-rooted begonia is used as the experi-
mental plants, make observations on the extent of tuber formation as well
as flowering.

Supplementary Reading:

U.S. Agr. Res. Serv. Prescription for Better Plant Form. Dept. of Agr.,
Agra Res. 8: 1427 1959.
See Demonstration D-1.

DEMONSTRATION D-3: Photoperiodic control of growth and dormancy of
woody plants.

Materials:

1. A light-equipped chamber.

2. A dark chamber.

3. Seedlings or rooted cuttings of some woody plant material such as
deciduous trees (caftalpa and red maple), evergreen trees (spruce,
loblolly pine, slash pine, or Virginia pine), and shrubs (hollies and
Weigela).

24

Procedure:

1. Divide rooted cuttings or seedlings into lots A and B and place both lots
in the light chamber for 8 to 10 hours daily.

2. Place lot A in darkness at the close of each 8- to 10-hour light period.

3. Turn off the fluorescent lamps and place lotB 3 to 4 feet from a 40-watt
incandescent-filament lamp, which is now turned on.

4. If an electric time switch is available, the total daily light period for
plants of lot B should be 16 to 18 hours, 8 to 10 hours of fluorescent
light plus 8 more hours of incandescent light.

5. If an electric time switch is not available, leave the incandescent light
on throughout the night.

6. Return both lots A and B to the light chamber each morning.

7. These daily treatments should be continued until the plants on 8- to 10-
hour day (those of lot A) take on the aspects of dormancy and there is a
marked difference in the size of plants of lots A and B. This should
require at least 30 days.

Observations:

Record date treatments began, length of the light and dark periods, and
also record the number of short days required to induce dormancy or to
stop growth of the main axis. Measurements can be made at daily intervals
and the length of the main axis of plants from both lots can be plotted
against time in days.

Supplementary Reading:

Borthwick, H. A. Light effects on tree growthand seed germination. Ohio
Jeuroci., 5f:351-s04. 1957. :

Downs, R. J., and H. A. Borthwick. Effects of photoperiod on growth of
trees. Bot. Gaz. 117: 310-326. 1956.

Thimann, K. V. The physiology of forest trees. Ronald Press, New York,
N.Y., pp. 529-583. 1958.

Wareing, P. F. Photoperiodism in woody plants. Ann. Rev. Plant Physiol.
7: 191-214, 1956.

DEMONSTRATION D-4: Photoperiodic control of bulb formation of onions,

Materials:

1, Onion seeds. Plants of southern varieties White Bermuda, Crystal Wax,
Eclipse, Excel, and Granex hybrid will bulb on 12-hour days. Plants of
northern varieties Australian Brown, Sweet Spanish, Elite hybrid, and
Yellow Globe (Early Yellow Globe, Yellow Globe Danvers, Downing
Yellow, and Globe) will bulb on 15-hour days.

2. Sterilized soil.

25

3. Four wooden boxes 8 to 10 inches wide, 10 inches deep, and 12 inches
long with drainage holes.

4. A light-equipped chamber.

5. A dark chamber.

Procedure:

1, Fill boxes with sterilized soil and level the soil surface.

2. Make two shallow furrows lengthwise of the box 1/4-inch deep and 4
inches apart.

3. Plant the seeds thinly in the furrows, cover the seeds with soil, and water
carefully. Label each box with the name of the variety, the date of
planting, and the daylength treatment.

4, When seedlings are well established, thin the plants to 2 inches apart
in the row.

5. Germinate the seeds and grow the plants at room temperature (70° to
S0.2E)e

6. Place two boxes on short days and the remaining two boxes on long
days immediately after planting the seeds.

7. Additional varieties and intermediate daylengths can be used to broaden
the experiment.

Observations:

Note any differences in the top growth or plant habit at regular intervals
during the course of the demonstration. Differences should become apparent
in about 60 days. Carefully remove a few plants at random from a box on
each daylength and note any differences in bulbing. Do this at regular
intervals to determine the time of bulbing and the treatment on which it
occurred. When bulbing is definitely apparent, the experiment may be
terminated. The plants in the remaining box on each daylength may be
harvested and the extent of bulbing on each daylength noted and recorded.

Supplementary Reading:

Boswell, V. R., and H. A. Jones. Climate and vegetable crops: Onions.
In CLIMATE AND MAN, U.S. Dept. of Agr., Yearbook of Agriculture,
1941. Government Printing Office, Washington, D.C., pp. 388-389.

Jones, H. A. Onion improvement: Varietal adaptation. In BETTER
PLANTS AND ANIMALS, II. U.S. Dept. Agr., Yearbook of Agriculture,
1957. Government Printing Office, Washington, D.C., pp. 325-326.

Magruder, R., and H. A. Allard. Bulb formation in some American and
European varieties of onions as affected by length of day. Jour. Agr.
Ress 54:07 19-152. roi.

Growth Through Agricultural Progress

26