Anatomical aspects of avocado stems and their relation to rooting

Anatomical Aspects of Avocado Stems
and T'ineir Relation to Rooting

By

RICARDO.E. GOMEZ

A DISSERTATION PRESENTED TO THE GRADUATE COUNCIL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF TBI
REQUIREMENTS FOH THE DEGREE OF DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLOIRDA
1971

ACKNOWLEDGMENTS

The author expresses his appreciation to Drs. James Soule and Simon
E. Malo for their guidance, assistance and interest during the planning
and completion of this investigation. Appreciation is also extended to
Drs. R. C. Smith and R. H. Biggs for their constructive criticisms and
suggestions in the interpretation and preparation of the manuscripts, to
Drs. R. A. Conover and A. H. Krezdorn for their support and friendly
guidance.

Eternal thanks are extended to his wife for her moral support and
the typing of this manuscript.

The author also wishes to extend his appreciation to the Agricultural
Research and Education Center Homestead and to the Center for Tropical
Agriculture for providing financial assistance which made this study
possible.

ii

TABLE OF CONTENTS

Page

ACKNOWLEDGMENTS ii

LIST OF TABLES . , iv

LIST OF FIGURES v

ABSTRACT vil

INTRODUCTION 1

LITERATURE REVIEW 2

MATERLALS ANT) METHODS 6

Air Layers 6

Anatomical Studies 6

RESULTS AND DISCUSSION 8

Air Layers 8

Anatomical Studies lU

General Stem Anatomy Ik

Anatomy of Successive Flushes 2k

Anatomy of Cultivars $k

SUMMARY AND CONCLUSIONS 67

BIBLIOGRAFHY 69

BIOGRAPHICAL SKETCH 75

iii

LIST OF TABLES

Table Page

1. Cumulative percentage rooting of air layered avocado

cultivars and seedling trees 12

2. Number of days to maximum rooting of air layered avo-
cado cultivars and seedling trees 13

LIST OF FIGURES

Figure Page

1. Rooting of air layers of all avocado cultivars and

seedling trees 9

2. Rooting of air-layered Mexican seedling trees (M 1

and M 2) , Hickson., and Taylor avocados 10

3- Rooting of air-layered Booth 8, Booth 7, and Pollock

avocados 11

h. Transverse section of second-flush Booth 8 avocado ... 15

5. Tangential section of second-flush Hickson avocado ... 17

6. Transverse section of Booth 7 avocado 19

7. Tangential, section of Hickson avocado 21

8. Typical concentric starch grains of avocado 23

9. Transverse section of etiolated second -flush

Mexicola avocado 25

10. Transverse section of Booth 8 avocado near the

terminal 26

11. Transverse section of Booth 8 avocado near the

terminal 27

12. Transverse section of third-flush Booth 8 avocado .... 28

13. Transverse section of fourth-flush Booth 8 avocado ... 30
Ik. Transverse section of fifth-flush Booth 8 avocado .... 32

15. Transverse section of sixth-flush Booth 8 avocado .... 3^

16. Transverse section of eighth-flush Booth 8 avocado ... 36

17- Transverse section of sixth-flush Taylor avocado 38

18. Tangential section of sixth-flush Booth 8 avocado .... ^-0

LIST OF FIGURES - Continued

Figure Page

19. Tangential section of sixth-flush Booth 8 avocado .... k2

20. Transverse section of first-flush Hickson avocado . kk

21. Transverse section of second-flush Hickson avocado ... k6

22. Transverse section of third-flush Hickson avocado .... k8

23. Transverse section of first-flush Taylor avocado 50

2k. Transverse section of fifth-flush Taylor avocado 52

25. Transverse section of second-flush Pollock avocado ... 55

26. Transverse section of second-flush Booth 7 avocado ... 57

27. Transverse section of second-flush Booth 8 avocado ... 59

28. Transverse section of second-flush Taylor avocado .... 6l

29. Transverse section of second-flush Gainesville seedling
avocado 63

v-

Abstract of Dissertation Presented to the

Graduate Council of the University of Florida in Partial Fulfillment

of the Requirements for the Degree of Doctor of Pnilosophy

ANATOMICAL ASPECTS OF AVOCADO STEMS
AND THEIR RELATION TO ROOTING

By

Ricardo E. Gomez

December, 1971

Chairman: James Soule

Co -Chairman: Simon E. Malo

Major Department: Fruit Crops

Avocado rootstocks of known parentage are desirable for research
and commercial uses. Present stocks are seedlings, which are variable.
This investigation was undertaken to determine whether avocado cultivars
commonly grown in Florida could be propagated as air layers and to inves-
tigate anatomical aspects of stems which influence the rooting of cuttings.

Air layers were put on June 17, September 9> November 11, 1969,
and March 10, April 15, and June 2k, 1970. Cultivars were 'Pollock1,
'Booth 7', 'Booth 8', 'Hickson', and 'Taylor' and 2 Mexican seedlings.
The last had the highest percentage rooting while 'Pollock' and 'Booth 7'
has the lowest percentage. 'Booth 8', 'Hickson' and 'Taylor' were inter-
mediate. Air layers made in June and April required the shortest
period for rooting and the ones in November the longest time.

Six contiguous growth flushes from the terminal end of a branch,
as well as the eighth and tenth, were collected from 'Waldin', 'Pollock',
•Catalina', "Booth 7', 'Booth 8', 'Hickson', 'Lula', 'Taylor*, 'Gainesville'
(parent tree), 'Brogdon', 'Mexicola', and the 2 Mexican seedling trees.

Material was cut into 0 5 cm lengths, killed in FAA and softened in glycerol -
alcohol colution. Sections 35 M thick were cut on a sliding microtome and
treated with phloroglucinol-HCl or IKI. Photomicrographs were made of
selected sections.

General details of stem anatomy corroborated earlier reports. Series
of sections made progressively from the terminal toward the proximal end
revealed that as the stem grows in diameter the fiber-sclereid ring starts
to break down, especially when the phloem rays begin to diverge. Etiolat-
ed stems were found to have less lignification of cells than non- etiolated.
It was also found, that the frequency of the fiber bundles and the sclereid
connection was greatest for West Indian cultivars and least for Mexican
seedling trees. Guatemalan cultivars and hybrid types were intermediate.
The fact that avocados of Mexican origin generally root better than those
of the West Indian race is recognized and has been supported by the air-
layering experiments described above.

The origin of adventitious roots in most plants is in the periphery
of the cambial zone, consequently it is reasonable to presume that if a
barrier of fibers and sclereids is present, the race having the lower
degree of lignification should root best. This has been shown to be true
of the Mexican race as compared to the West Indian cultivars.

INTRODUCTION

Vegetative reproduction by graftage has long been used success-
fully for many tropical fruit crops. Commercial plantings of avocado
(Persea americana Mill.) in many parts of the world utilize plants
grafted on seedling stocks. These stocks are highly variable; therefore,
possible stock-scion interactions can not be readily evaluated. Genet-
ically uniform rootstocks would permit nutritional studies and other
useful investigations from which a larger and more uniform production of
fruits might be obtained. Propagation of avocado stocks by means of
cuttings and air layerage has been attempted in California (hk, kp, 58,
59), Israel (72, 81), and Florida (kj, U8, 50, %) but success thus far
has been limited mainly to cultivars of the Mexican race.

Objectives of the present investigation vere to determine whether
avocado cultivars commonly grown in South Florida could be propagated
by air layering and to study anatomical aspects of stems which might
influence the rooting of cuttings of different cultivars or races.

LITERATURE REVIEW

Avocado, unlike cultivars of some important horticultural crops
such as citrus and mango, does not exhibit polyembryony . Vegetative
reproduction of avocado by means of cuttings and layers has been widely
studied (8, 16, 22, 27, 28, 29, 32, 33, 3^, 35, 36, hi, k2, kk, '45, k7,
48,. 50, 56, 58, 59, &, 67, 72, 81, 82).

The nutritive status of the stock plant greatly influences the
development of roots and shoots (3, 13, 33, 36, 50, 59, 63, 68, 73, 80) ■
Special consideration has been given to the relative amounts of carbo-
hydrates and nitrogen (N). Starring (7^) observed that cuttings taken
from tomato plants which had a high carbohydrate and low N content rooted
better than plants with low carbohydrate and high N. This is true with
other species of plants (3^, 51, 60, 7M • However, Haun and Cornell (37)
noted that cuttings of geranium (Pelargonium hortorum Bailey cv. Ricard)
grown under high N had larger and more numerous roots, but fewer cuttings
rooted when compared to cuttings from low N regimes . Carbohydrate and N
levels can be used to predict the rooting capabilities in some plants
(3^)- Young (82) reported that cuttings taken from avocado trees under
medium and high N regimes retained their leaves for a longer period of
time when those from low N regimes. Rodrigues and Ryan (65) have report-
ed the carbohydrate content in avocado shoots and Cameron and Borst (9)
starch in 6-year old Mexican seedling trees; Bingham (5) and Embleton
etal. (17, 18) the N content of leaves of avocado. High carbohydrate

2

levels may be required to sustain the cutting until they root (34) since
rooting requires several months (47, 48).

Application of growth-promoting substances is a common practice in
commercial rooting of cuttings of many species. Initiation of adventitious
roots may be controlled by the level of auxin within the tissue or by a
balance between auxin and other compounds (25). Very high concentrations
of auxin are sometimes needed to enhance rooting in plants (49, "(1, 79)-
Most experiments involving rooting of avocado cuttings have used concentra-
tions varying from 0 to 500 and up to 4,000 parts per million (ppm) (36,
45, 47, 48, 58, 81), or considerably lower than the 10,000 to 30,000 ppm
used for rooting of tea and certain other plants (20, 34).

Cuttings from young avocado seedlings root faster and with a higher
percentage of success than those from more mature plants (22, 33> 45, 8l).
Gillespie (28) obtained sections from a 4-year old Mexican seedling that
had been cut back to 30 cm. He made 3 cuttings from each section and
found that the basal cutting rooted the fastest, and the terminal the
slowest. Contrary to this, Ya'Acob and Kadman (81) and Piatt and Frclich
(58) reported that terminal cuttings rooted better. Leal and Krezdorn
(48) using immature stem tips of 'Gainesville' (a Mexican race seedling)
obtained 90% rooting after 7 months. Eyan et al. (66) observed that
'Hass' avocado cuttings had not rooted after 7 months. T. J. Anderson
of Mulberry, Florida, air layered the top branches (10-15 cm diameter)
of 'Winter Mexican' avocado and obtained rooting after 1 year. Sen
et al. (70) ringed 1, 2, and 3-year old shoots on a 35-year old mango

1
Personal observation by the author.

in June and after kO days detached them. Indolebutyric acid was applied
as a dip (2,000 ppm) and as a powder (5,000 ppm) before planting. The
3-year old wood gave the highest percentage of rooting.

Adventitious roots may arise from pre-existing primordia or be
newly formed in the vicinity of differentiating vascular tissues (1, 2,
4, 10, 11, 12, 15, 20, 30, 31, 62, 73, 78). In young stems, root
primordia are formed from interfascicular parenchyma cells while in
older stems they may be derived from a vascular ray (77)-

Etiolation of shoots from which cuttings and air layers are made
has proved beneficial in many instances (23, 3^, 39, 40, 46, 53> 5fj> 55-»
62, 73) and specifically in avocado (22, 45, 8l). Penfcund (57) report-
ed that stems of Helianthus and Polygonum growing in full sunlight had a
much greater amount of xylem and more and thicker walled fibers and
sclereid cells than those in the shade. Priestley (61) found that
etiolated stems had a well developed endodermis and concluded that an
etiolated stem was somewhat like a root in structure. Bond (6) also
reached a similar conclusion with legumes. The added growth in length
of etiolated stems was the result of cells being longer rather than
being more numerous (7)-

Anatomical structure of the stem has been related to the ability
of stems to form adventitious roots. Beakbane (4) reported that shoots
of difficult-to-root varieties of apples, pears, and other plants are
often characterized by a high degree of sclerification (fibers and sclereids)
in the phloem. For instance, 'Conference1 pear has an almost continuous
cylinder of mature, thick-walled fibers which appears in transverse
section as a ring of lignified tissue encircling the secondary phloem.

Shy-rooting clones of Hevea brasiliensis have also teen found to possess
an almost unbroken cylinder or ring of mature lignified elements. Gardner
(as reported by Beakbane (U)) found that the rooting capacity of stooled
plants diminished as the continuity of the ring increased. Galkin (2k)
was able to determine the rooting ability of apples by the amount of
hard bast fibers in the bark.

The anatomical structure of avocado stems of seedlings of Mexican
or Mexican hybrid parentage has been described by Heismann (38) and
Schroeder (69). Metcalfe and Chalke (52) have reported the general
anatomical characteristics of the family Lauraceae, and Stern (75) has
specifically described the xylem anatomy of Lauraceae.

MATERIAL AND METHODS

Air Layers

Air layers were made at the University of Florida Agricultural
Research and Education Center Homestead, Homestead, Florida. Plants of
West Indian (Wl), Guatemalan (G), and Mexican (M) germplasm were used in
this study. There were 2 plants each of 'Pollock" (Wl) (6<i), 'Booth 7'
(WI x G), 'Booth 8' (Wl x G), 'Hickson' (Wl x g), 'Taylor' (G) and 2
Mexican race seedling trees designated M 1 and M 2. Ten air layers per
variety were applied on June 17, September 9 > and November 11, 19&9, and
March 10, April 15 and June ?.h, 1970- Branches 1 to 2 cm in diameter were
girdled and a strip of bark 2 to 3 cffi wide was removed. Moist sphagnum
moss was placed around the branch at the ringed area and wrapped with
heavy-duty aluminum foil. Experiments simulated commercial conditions.
Individual air layers were examined for the appearance of roots on the
dates when new air layers were applied and on September 30, 1970, and
February 9 , 1971 > 3& to 5l8 days after propagation. Branches were
examined periodically and reringed at the same place if a callus bridge
was found. Percentage rooting was calculated from the number rooted
after subtracting those lost from wind or cultural damage.

Anatomical Studies

Avocados used for microscopic examination were 'Waldin', 'Pollock',
•Catalina' (Wl), "Booth 7', 'Booth 8', 'Hickson', 'Lula', (M x WI),

•Taylor', 'Gainesville' , 'Brogdon' (M x WI), 'Mexicola' (M), and the
2 Mexican seedling trees (M 1 and M 2). Observations were made on 3
other species, Persea scheideana Nees, Phoebe mexicana Meissn., and
Licaria triandra (Sw.) Kostern. Six continuous growth flushes as well
as the eighth and tenth from the terminal end of the branch were collect-
ed from the avocado cultivars and seedlings, while a random sample was
taken from each of the other 3 species. Material was cut into pieces
approximately 0.5 cm in all dimensions. Tissues were killed in formalin-
acetic acid-95^ alcohol solution (FAA; 5,5^5 v:v:v), as described by
Childs et al. (1*0, and softened for at least one month in glycerol-50$
alcohol (1:1, v;v) (21). Sections were cut at 35 p. on a sliding micro-
tome and treated with phloroglucinol-HCl (^3). Some sections were
treated with iodine -potass in:;, iodide (IKI) solution to determine the
presence of starch. Photomicrographs were made of selected sections.
Line drawings were made to aid in the identification of tissues or zones.

1
Material for sections was obtained from parent tree, a Mexican seedling.

RESULTS AND DISCUSSION

Air Layers

Average percentage rooting of all cultivars and seedlings is shown
in Fig. 1. A decrease in rooting is apparent in September and November.
Three distinct groups appear if the data from the cultivars are separated
(Figs. 2 and 3): Mexican seedlings (M 1 and M 2); 'ilickson' and 'Taylor* ;
and 'Booth 7' and 'Pollock'. 'Booth 8* does not fit into any of the
groups but does resemble 'Kicksoa' and 'Taylor' with a time displacement
of about five months. Apparently, the cultivars or seedlings of a race
behave similarly as to rooting. West Indian -Guatemalan hybrids may be-
have like the race of either parent, as 'Booth 7', or unlike either one,
as 'Booth 8'.

The Mexican trees had the highest percentage of rooting throughout
the year, 75 to 100$, 'Booth 7' and 'Pollock' had the lowest percentages,
22 to 60%, and 'Hickson' and 'Taylor' were intermediate, from 13 to 80$
(Table 1). Rooting of 'Booth 8' varied from 38 to 88$. The Guatemalan
group had a marked decrease in rooting in the fall.

An important factor in determining the feasibility of air layering
avocados is the time required for rooting to take place. The time for
initial rooting to take place is shown in Table 1 and the number of
days to maximum rooting, in Table 2.

CO

(9

h

O

a

70

6 0

\.«^

SOh

■J L.

J^_™_J»_„,L

JJASONDJ

1969

M A M

19 70

BATE MADE

Fig. 1. Rooting of air lay<
and seedling trees,

of all avocado cultivars

10

100
90
80
70

e> CO

H SO

O

° 40

fcS SO

20

10

j j ~a s o ra d J f rj a r;i j

1969 1970

DATE MADE

Fig-. 2. Rooting of air-layered Mexican seedling trees (M 1 and
M 2), Hickson (H), and Taylor (T) avocados.

11

100

90

SO

70
O
2 60

O
O

EC
40
30

20

10

G

J A S 0 Fv3 D J F til A M J
1969 1970

DATE MADE

Fig.. 3. Rooting of air-layered Booth 8 (B 8), Booth 7 (B 7),
and Pollock (p) avocados.

12

Table 1, Cumulative percentage rooting of air layered avocado cultivars and
seedling trees

Month made

. >nth

checked

Cultivar

l

969

19

i0

1971

or tree

Sept.

Nov.

March

April

June

Sept.

Feb.

Pollock

June, 1969
Sept.

0

11
0

22
22

33

Nov.

0

0

22

33

March, 1970

0

0

1*0

April

0

kk

June

0

25

Booth 7

June, I969

0

30

ko

ho

Sept.

0

50

50

50

Nov.

0

0

0

0

22

March, 1970

0

0

-:

60

April

0

20

40

June

0

33

Booth 0

June, I969

^

63

rs

Sept .

1]

66

77

77

Tt

88

Nov.

33

50

■v.

83

March, 1970

0

0

13

38

April

0

38

63

June

0

t<

Hickson

June, I969
Sept.

70

80

0

80

] : 1

80

36

Nov.

0

0

13

] j

March, 1970

0

0

50

April

0

0

^3

June

0

55

Taylor

June, I969

0

22

77

Sept.

<)

0

0

0

0

13

Nov.

0

0

0

0

38

March, 1970

0

0

0

57

April

0

0

63

June

6

50

Mexican 1

June, I969
Sept.

63

88

0

80

Nov.

0

60

70

80

March, 1970

0

25

100

April

0

60

80

June

0

90

Mexican 2

June, I969
Sept.

50

7 5
0

100

Nov.

0

0

60

88

100

March, 1970

0

0

60

80

April

0

25

75

June

0

90

13

ff.

r.

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0

a

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VD H

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Ill

The 2 Mexican seedlings and ' Pollock' rooted in the fewest number
of days. Air layers of 'Booth 7' and 'Hickson' were intermediate and
'Taylor' and 'Booth 8* required the longest period for rooting. All
avocados except 'Booth 0' and 'Taylor' took longer to root when the air
layers were made in November. 'Pollock' was inconsistent in the time
required but the others seemed to follow a pattern. 'Pollock' air layers
required a shorter time to root, but only a few rooted. Those from Mexi-
can trees also required a shorter time to root and most of the branches
rooted .

Average number of days to maximum, rooting for all cultivars and
seedlings was 398 for air layers made in ftovember, 300 in March, 280 in
April, 2'i5 in June, and 308 in September.

Anatomical Studies

General Stem Anatomy

Examination of transverse and tangential sections of second-flush
growth (Figs, h and 5), shoved the following features: Isodiametric paren-
chyma cells in the pith, primary xylem composed of lines of vessels increas-
ing in size, secondary xylem with scattered vessels (diffuse porous) occur-
ring singly or 2 or more together and prominent unicellular rays, a more or
less well defined but irregular cambial layer, a definite continuation
of rays, numerous sieve tubes, companion cells, and inclusions in a
broad phloem, clusters of fibers connected by sclereids between the phloem
and cortex (Figs. 6 and 7), a broad, essentially uniform cortex composed
of cells similar to those in the pith, no apparent 'starch sheath'
(although seme cells contained starch grains (Fig. 8), which correspond
to those of the potato ( Solanum tuberosum L.) group as described by

15

A. Line drawing (Ep- epidermis, Co- cortex, PvR- peri-
vascular ring, Ph- phloem, C- cambium, Xy- xylem, P-
pith).

Fig. h. Transverse section of second-flush Booth 8 avocado

16

B. Photomicrograph (x 150)

17

A. Line drawing (Co- cortex, F- fibers, Ph- phloem,
C- cambium, Xy- xylem).

Fig. 5. Tangential section of second-flush Hickson avocado.

18

B. Photomicrograph (x 150)

19

A. Line drawing (Co- cortex, F- fibers, Sc- sclereids,
Ph- phloem) .

Fig. 6. Transverse section of Booth 7 avocado.

B. Photomicrograph (x 171*0

21

A. Line drawing (co- cortex, F- fibers, Sc- sclereids)

Fig. 7. Tangential section of Hickson avocado.

22

B. Photomicrograph (x 150)

23

Fig. 8. Typical concentric starch grains of avocado (x U286),

2k

Esau (19)) and a thick epidermal layer. Etiolated stems of avocado
(Fig. 9) differ from the above in that a well defined collenchyma layer
is present, fiber bundles are discrete with little or no connection of
sclereids, a well defined cambium layer, and a pith which is larger in
diameter than in the non-etiolated stem. This is consistent with those
plants examined by Penfound (57). Differences were apparent among the
cultivars and I gs in the clusters of fibers and the sc.lereid con-
nection. These will be described in a later section. Anatomical details
noted here corroborated earlier reports on Lauraceae and Persea americana
Mill. (38, 52, 69, 75), and vere also similar in the other 3 species of
Lauraceae examined in this investigation.

Anatomy of Successive Flushes

The gi- xpansion of an avocado stem is shown in transverse
sections of the first, third through sixth, and eighth flush of 'Booth 8'
(Figs. 10, 11, 12, 13, 14, 15, 16). Sections cut about 1 mm from the
terminal showed that epidermal hairs (Fig. 10) were abundant, fibers were
not lignified (Fig. 11) and little cellular organization occurred.
Figures 10 and 11 are serial photomicrographs of a transverse section.
Sections of older stem tissues (Figs. 12-16) showed that the progressive
expansion of the stem was accompanied by a separation of the fiber bundles
and a decrease in the width of the layer of sclereids connecting them.
Divergent phloem rays appear in the fourth-flush (Fig. 13). They become
more prominent as the stem increases in diameter. There is an almost
complete break down of the sclerenchy^a ring at the eighth flush (Fig. l6) .
A transverse section of the sixth-flush of 'Taylor' avocado (Fig. 17)
shows a divergent ray, parenchyma type ray cells and a few lignified

25

; v *\

Fig. 9- Transverse section of etiolated second- flush
Mexicola avocado.

26

Fig. 10. Transverse section of Booth 3 avocado near the
terminal (x ^30) •

27

Fig. 11. Transverse section of Booth 8 avocado near the
terminal (x 1+30) .

23

A. Line drawing (Co- cortex, PvR- perivascular ring,
Ph- phloem, C- cambium, Xy- xylem, P- pith).

Fig. 12. Transverse section of third-flush Booth 8 avocado.

29

B. Photomicrograph (x 150)

30

A. Line drawing (Ep- epidermis, Co- cortex, PvR- peri-
vascular ring, Ph- phloem, C- cambium, Xy- xylem).

Fig. 13- Transverse section of fourth-flush Booth 8 avocado.

31

B. Photomicrograph (x 150)

32

A. Line drawing (Ep- epidermis, Co- cortex, PvR- peri-
vascular ring, Ph- phloem, C- cambium, Xy- xylem).

Fig. Ik . Transverse section of fifth-flush Booth 8 avocado.

33

B. Photomicrograph (x 150)

3^

A. Line drawing (Co- cortex, PvR- perivascular ring,
Ph- phloem, PhR- phloem ray, C- cambium, Xy- xylem),

Fig. 15. Transverse section of sixth-flush Booth 8 avocado.

35

Photomicrograph (x 150)

36

A. Line drawing (Co- cortex, PvR- perivascular ring,
Ph- phloem, PhR- phloem ray).

Fig. l6. Transverse section of eighth-flush Booth 8 avocado.

37

B. Photomicrograph (x 150)

38

A. Line drawing (Co- cortex, F- fibers, Sc- sclereids,
PhR- phloem ray, Ph- phloem).

Fig. 17. Transverse section of sixth-flush Taylor avocado.

39

B. Photomicrograph (x ^30)

uo

A. Line drawing (Co- cortex, F- fibers, Sc- sclereids),

Fig. 18. Tangential section of sixth-flush Booth 8 avocado.

Ul

B. Photomicrograph (x 150)

k2

A. Line drawing (F- fibers, Sc- sclereids)

Fig. 19. Tangential section of sixth-flush Booth 8 avocado.

^3

B. Photomicrograph (x *+30)

kk

A. Line drawing (Co- cortex, PvR- perivascular ring,
Ph- phloem) .

Fig. 20. Transverse section of first-flush Hickson avocado.

h$

******

B. Photomicrograph (x ^30)

4b

A. Line Drawing (Co- cortex, PvR- perivascular ring,
Ph- phloem) .

Fig. 21. Transverse section of second-flush Hickson avocado.

hi

B. Photomicrograph (x H30)

kQ

A. Line drawing (Co- cortex, PVR- perivascular ring,
Ph- phloem) .

Fig. 22. Transverse section of third-flush Hickson avocado.

h9

B. Photomicrograph (x ^30)

50

A. Line drawing (Ep- epidermis, Co- cortex, PvR peri-
vascular ring, Ph- phloem, C- cambium, Xy- xylem,
P- pith).

Fig. 23. Transverse section of first-flush Taylor avocado.

51

B. Photomicrograph (x 150)

52

A. Line drawing (Ep- epidermis, Co- cortex, PvR- peri-
vascular ring, PhR- phloem ray, Ph- phloem, C- cambium,
Xy- xylem, XyR- xylem ray).

Fig. 2k. Transverse section of fifth-flush Taylor avocado.

53

B. Photomicrograph (x 150)

5k

cells across the broad end of the ray. It may "be noted in tangential
sections of the sixth-flush of 'Booth 6' (Figs. 18 and 19) that the lig-
nified cells connecting the fibers are not as compact or as continuous
as those in Fig. 7. The separation of fiber bundles and decrease in
thickness and continuity of the ring is clearly noted in transverse
sections of contiguous flushes of 'Hickson* (Figs. 20, 21 and 22). The
separation of bundles and discontinuity of the fiber ring is even more
apparent in non-contiguous growth flushes of 'Taylor' (Figs. 23 and 2k).

Anatomy of Cultivars

Transverse sections of the second-flush of 'Pollock', 'Booth 7',
'Booth 8', 'Taylor' and 'Gainesville' avocado are shown in Figs. 25, 26
27, 23, and 29, respectively. These cultivars ani seedling ('Gainesville')
were chosen as representative of those examined since all follow more or
less closely the same structural pattern. It was evident from these sec-
tions that the fiber bundles were larger, closer together, and definitely
interconnected by more sclerenchyma cells in the West Indian cultivar
(Fig. 25) than those of the other races or hybrids. 'Gainesville' (Fig. 29)
appeared to have the most loosely organized ring. 'Taylor' (Fig. 28) was
intermediate. 'Booth 7' (Fig. 26) was similar to the West Indian type,
while 'Booth 8' (Fig. 27) resembled the Guatemalan parent rather than
the West Indian.

The discontinuity of the perivascular sclerenchyma ring, divergence
of the rays and separation of the fiber bundles found in sections examined
in the present study were consistent with Esau's (19) model for the
thickening of a dicotyledonous stem. The parenchyma type ray cells may
be capable of reverting to 'meristematic characteristics and give rise to
root initials. Etiolated stems resemble the apical portion of the

55

A. Line drawing (Ep- epidermis, Co- cortex, PvR- peri-
vascular ring, Ph- phloem, C- cambium, Xy- xylem).

Fig. 25. Transverse section of second-flush Pollock avocado.

56

B. Photomicrograph (x 150)

57

A. Line drawing (Ep- epidermis, Co- cortex, PvR- peri-
vascular ring, Ph- phloem, C- cambium, Xy- xylem) .

Fig. 26. Transverse section of second-flush Booth 7 avocado.

58

B. Photomicrograph (x 150)

59

A. Line drawing (Ep- epidermis, Co- cortex, PvR- peri-
vascular ring, Ph- phloem, C- cambium, Xy- xylem) .

Fig. 27. Transverse section of second-flush Booth 8 avocado.

6o

B. Photomicrograph (x 150) .

61

Co

/S. — "^

PvfU

jf

C Ph

Xy

^

A. Line drawing (Ep- epidermis, Co- cortex, PvR- peri-
vascular ring, Ph- phloem, C- cambium, Xy- xylem).

Fig. 28. Transverse section of second-flush Taylor avocado.

62

B. Photomicrograph (x 150)

63

Ac- — s£~~~^^.

Co

PvR

%J^^

Ph

x^ ~*

cN

N^. ■ — -" — "

^- -\

Xy

~-

A. Line drawing (Co- cortex, PvR- perivascular ring,
Ph- phloem, C- cambium, Xy- xylem).

Fig. 29. Transverse section of second-flush Gainesville
avocado .

6U

B. Photomicrograph (x 150)

65

terminals in the lack of sclereids between the fiber bundles. The above
may explain in part why some plants which are difficult to root by cut-
ting are successfully rooted as air layers (3^). I-t mav also explain
the success in rooting immature 'Gainesville* avocado cuttings, reported
by Leal and Krezdorn (hQ) as well as that of Anderson1 in rooting 'Winter
Mexican' (G x M) 10 to 15 cm in diameter by air layerage.

Many investigators have had success with stimulating rooting with
the use of auxins (3*0. IQ the case of avocado little success has been
obtained with auxin in stimulating rooting. Some have shown promotion,
others have not obtained a promotion of rooting (35, ^5, ^7, ^8, Si).
This could be due to the cultivars or seedlings used in the tests.

Kadman and Ya'Acob (k^>) concluded in a review of experiments on
avocado propagation that Mexican avocado generally roots better from
cuttings than does Guatemalan, while West Indian roots the poorest. This
statement would still be true if the sclercnchyma ring were to act as a
barrier to root emergence and would explai.n the results obtained with the
air layers reported previously. This is in accord with Beakbane's (k)
conclusion that shoots of shy-rooting plants are often characterized by
a high degree of sclerification in the phloem and with Galkin (2U), who
was able to predict rooting ability by measuring the amount of bast
fibers. The difference in rooting ability of mature and juvenile types
of material may result from the degree of sclerification in the primary
phloem being much less in very young material (k) . Many years ago,
Gardner (26)' suggested that anatomical differences existed. Stoutemyer

1
Personal observation by the author.

66

(76) found that tissue from mature and juvenile wood of apples were
nearly identical histologically with the exception that the mature phase
contained more pericyclic fibers than the juvenile.

SUMMARY AND CONCLUSIONS

General details of stem anatomy corroborated those observed by
earlier investigators. Series of sections made in progression down the
stem from the terminal revealed that as the stem grows in diameter the
fiber-sclereid ring starts to break down, especially when the phloem rays
begin to diverge. This was true of 'Waldin', 'Pollock', 'Catalina',
•Booth 7', 'Booth 8', 'Hickson', 'Lula', 'Taylor', 'Gainesville' seedling,
'Brogdon', 'Mexicola', and the 2 Mexican seedling trees. It was found,
when the same flush of the cultivars or seedlings was examined, that the
frequency of the fiber bundles and the chickness of the sclereid connec-
tion was greatest, for the West Indian and least for the Mexican. The
Guatemalan and hybrids were intermediate. Etiolated shoots have been
shown to have a smaller degree of lignification than non-etiolated stems.
The origin of adventitious roots in many plants is in the periphery of
the cambial zone, therefore, if the sclerenchyma ring acts as a barrier,
the race having the lower degree of lignification should root best.
Mexican avocados were found to have less lignification than the West Indian.

Air layers were put on June 17, September 9, November 11, 1969 and
March 10, April 15, and June 2k, 1970. Cultivars tested were 'Pollock',
•Booth 7', 'Booth 8', 'Hickson', and 'Taylor', and 2 Mexican seedling
trees. The lost had the highest percentage rooting while 'Pollock' and
'Booth ">[% had the lowest percentage. 'Hickson' and 'Taylor' were intermediate.

<H

Air layers made in June and April required the shortest period for
rooting. The ones made in November took the longest time to root.

The experiments with air layering further exemplifies the differ-
ences in rooting ability between races and would still be true if the
fiber-sclereid ring acted as a barrier. Mexican avocados were found
to have a lower degree of lignification and rooted best while West
Indian had the most continuous lignified ring and rooted the poorest.

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■n

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73

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7*

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BIOGRAPHICAL SKETCH

Ricardo E. Gomez was born July 13, 1938 at Havana, Cuba. In June
19 56 he was graduated from Lafayette School in Havana. In April 1966 he
received the degree Bachelor of Science in Agriculture with a major in
Soils from the University of Florida and received the 1965-66 Kroger
Award for high scholarship. In the same year he enrolled in the Graduate
School of the University of Florida. In August 1968 he received the
degree Master of Science in Agriculture with a major in Soils from the
University of Florida. He was a graduate student in the Department of
Fruit Crops and held a graduate assistantship provided by the Agricultural
Research and Education Center, Homestead and the Center for Tropical Agri-
culture from I968 to 19717 He was awarded the degree Doctor of Philosophy
in December 1971. He received the T. J. Andersen Memorial Award for 1971
for work in tropical fruits .

He is a member of American Society for Horticultural Science, Ameri-
can Society for Horticultural Science, Tropical Region, Alpha Zeta, Gamma
Sigma Delta, and Phi Sigma honorary fraternities.

He is married to the former Maria Martha Callejas of Chinandega,
Nicaragua. He is the father of four children, three boys and a girl.

75

I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.

v/niCd vVv^vL*

kmes Soule, Chairman
ofessor of Fruit Crops

I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.

Simon E. Malo, Co-Chairman
Associate Horticulturist

I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.

Robert H. Biggs ff
Professor of Fruit Crops

I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.

I certify that I have read this study and that in my opinion it
conforms to acceptable standards of scholarly presentation and is fully
adequate, in scope and quality, as a dissertation for the degree of
Doctor of Philosophy.

Richard C. Smith
Associate Professor of Botany

This dissertation was submitted to the Dean of the College of
Agriculture and to the Graduate Council, and was accepted as partial
fulfillment of the requirements for the degree of Doctor of Philosophy.

December, 1971

ulture

Dean, Graduate School

UNIVERSITY OF FLORIDA

3 1262 08552 5763