Title: Anatomical aspects of avocado stems and their relation to rooting
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Title: Anatomical aspects of avocado stems and their relation to rooting
Physical Description: 75 leaves : ill. ; 28 cm.
Language: English
Creator: Gomez, Ricardo Eduardo, 1938-
Copyright Date: 1971
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Subject: Avocado   ( lcsh )
Fruit Crops thesis Ph. D
Dissertations, Academic -- Fruit Crops -- UF
Genre: bibliography   ( marcgt )
non-fiction   ( marcgt )
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Statement of Responsibility: by Ricardo Eduardo Gomez.
Thesis: Thesis (Ph. D.)--University of Florida, 1971.
Bibliography: Includes bibliographical references (leaves 69-74).
General Note: Typescript.
General Note: Vita.
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Volume ID: VID00001
Source Institution: University of Florida
Holding Location: University of Florida
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Resource Identifier: alephbibnum - 000415155
oclc - 37635724
notis - ACG2391

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Anatomical Aspects of Avocado Stems
and Their Relation to Rooting












By

RICARDO.E. GOMEZ


A DIS'; STATION PRESENTED TO THE GRADUATE COUNCIL
OF THE UNIVERSITY OF FLORIDA IN PAIZTIAL FULFILLMENT OF THE
REQUIREMENTS FOR THE DEGREE nF DOCTOR OF PHILOSOPHY


UNIVERSITY OF FLOIRDA
1971















ACMIOWLEDGKENTS


Tne 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.















TABLE OF CONTENTS


Page

ACKNOWLEDGMENTS ........................................... ii

LIST OF TABLES ............................................ iv

LIST OF FIGURES ........................................... v

ABSTRACT .................................................. vii

INTRODUCTION.................................................. 1

LITERATURE REVIEW ....................................... 2

MATERIALS AND METHODS ..................................... 6

Air Layers .......................................... 6
Anatomical Studies .................................. 6

RESULTS AND DISCUSSION .................................... 8

Air Layers .......................................... 8
Anatomical Studies .................................. 14
General Stem Anatomy ............................ 34
Anatomy of Successive Flushes ................... 24
Anatomy of Cultivars .............................. 54

SUMIMARY AND CONCLUSIOI'S ................................... 67

BIBLIOGfRAtL ..... .... ................................ 69

BIOGRAPHICAL SKETCH ...... .... .................. .......... 75















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

4. 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

14. Transverse section of fifth-flush Booth 8 avocado .... 32

15. Transverse section of sixth-flush Booth 8 avocado .... 34

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 .... 40














LIST OF FIGURES Continued


Figure Page

19. Tangential section of sixth-flush Booth 8 avocado .... 42

20. Transverse section of first-flush Hickson avocado .... 44

21. Transverse section of second-flush Hickson avocado ... 46

22. Transverse section of third-flush Hickson avocado .... 48

23. Transverse section of first-flush Taylor avocado ..... 50

24. 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 .... 61

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









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 2,1, 1970. Cultivars were 'Pollock',

'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 solution. Sections 35 Ai 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 dieneter the fiber-sclereid ring starts

to break down, especially when the phloem rays begin to diverge. Etiolat-

ed stems were found to have less lilnification 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 loot 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 mi-ht be obtained. f.j t, ti of avocado stocks by means of

cuttings and air layerage has been attempted in California (44, 45, 58,

59), Israel (72, 81), and Florida (47, 48, 50, 56) but success thus far

has been limited mainly to cultivars of the Mexican race.

Objectives of the present invest-gation i:ere to determine whether

avocado cultivars con-monly grown in South Florida could be propagated

by air layering and to study anatomical aspects of stems which might

influence the rooting of cutting g 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, 34, 35, 36, 41, 42, 44, 45, 47,

48, 50, 56, 58, 59, 66, 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, 65, 73, 80).

Special consideration has been given to the relative amounts of carbo-

hydrates and nitrogen (N). Starring (74) 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 (34, 51, 60, 74). However, Haun and Cornell (37)

noted that cuttings of geranium (Pelargonium hortcrum 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

(34). 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

et al. (17, 18) the N content of leaves of avocado. High carbohydrate










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, 71, 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, 81).

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 Platt 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. Ryan 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.l Sen

et al. (70) ringed 1, 2, and 3-year old shoots on a 35-year old mrango


Person obseation by the author.
Personal observation by the author.










in June and after 40 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, 34, 39, 40, 46, 53, 54, 55,

62, 73) and specifically in avocado (22, 45, 81). Penfound (57) report-

ed that stems of Helianthis and Polygonum growing in full sunlight had a

much greater amount of xyiem 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, 'Conference' 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.






5



Shy-rooting clones of Hevea brasiliensis have also been found to possess

an almost unbroken cylinder or ring of mature lignified elements. Gardner

(as reported by Beakbane (4)) found that the rooting capacity of stooled

plants diminished as the continuity of the ring increased. Galkin (24)

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 Heisnann (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 (WI), Guatemalan (G), and Mexican (M) germplasm were used in

this study. There were 2 plants each of 'Pollock" (WI) (64), 'Booth 7'

(WI x G), 'Booth 8' (WI x G), 'Hickson' (WI 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, 1969, and

March 10, April 15 and June 24, 1970. Branches 1 to 2 cm in diameter were

girdled and a strip of bark 2 to 3 cm 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, 36 to 518 days after propagation. Branches were

examined periodically and reringcd 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' (WI), 'Booth 7', 'Booth 8', 'Hickson', 'Lula', (M x WI),










'Taylor', 'Gainesville'1, '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; 5,5,45 v:v:v), as described by

Childs et al. (14), and softened for at least one month in glycerol-50%

alcohol (1:1, v:v) (21). Sections were cut at 35p on a sliding micro-

tome and treated with phloroglucinol-HC1 (43). Some sections were

treated with iodine-potassju., .odide (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 aie separated

(Figs. 2 and 3): Mexican seedlings (M 1 and M 2); 'Hickson' and 'Taylor';

and 'Booth 7' and 'Pollock'. 'Booth 8' does not fit into any of the

groups but does resemble 'Hickson' 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 604, 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 shoi in Table 1 and the number of

days to maximum rooting, in Table 2.



























870


I'~

C 1 0-






.3 A S 0 Ij D F F" A 1.1 .
I 969 1970

DATE MJJADE


Fig. 1. Rooting of air layers of all avocado cultivars
and seedling trees.







































'-U I _,


J 1969
1969


C 0 ) 3J F D i1 A
1970
DATE IViADE


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


1OOr


701-


301-


r Ji J

























F.- 8


100

90

80

70

60

50

40

30

20

10

0


C0 iN D jIF

DrAT E 1W:AD E


1970


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


S7
P


J JA S
1969


I -I-.-~^~~--~I'-P-L---d-~-~----OI ~


' '







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


MNonth checked
Cultivar 19' 19,0 1971
or tree [ic - : I .- .: . .

Pollock June, 1969 0 11 22 33
Sept. O 22
Nov. 0 0 22 33
March, 1970 0 0 40
April O 44
June 0 25

Booth 7 June, 1969 0 30 40 40
Sept. 0 50 50 50
Nov. 0 0 0 0 22
March, 1970 0 0 40 60
April 0 20 40
June 0 33

Booth 8 June, 1969 25 63 75
Sept. 11 66 77 77 77 88
Nov. 14 33 50 66 83
March, 1970 C 0 13 38
April 0 38 63
June 0 66

Hickson June, 1969 70 80 80 80
Sept. 0 10 38
Nov. 0 0 13 13
March, 1970 0 0 30 50
April 0 0 43
June 0 55

Taylor June, 1969 0 22 77
Sept. 0 0 0 0 0 13
Nov. 0 0 0 0 38
March, 1970 0 0 0 57
April 0 0 63
June 0 50

Mexican 1 June, 1969 63 88
Sept. 0 80
Nov. 0 60 70 80
March, 1970 0 25 100
April 0 60 80
June 0 90

Mexican 2 June, 1969 50 75
Sept. 0 100
Nov. 0 0 60 88 100
March, 1970 0 0 60 80
April 0 25 75
June 0 90










Table 2. Number of days to maximum rooting of air layered avocado cultivars and
seedling trees



Cultivars Seedling trees
Date made Average
Pollock Booth 7 Booth 8 Hickson Taylor Mexican 1 Mexican 2


1969

June 17 301 301 265 301 265 146 146 245

Sept. 9 181 287 518 287 518 181 181 303

Nov. 11 323 455 455 323 455 323 455 398


1970


March 10 204 3336 336 336 336 204 336 300

April 15 167 299 299 299 299 299 299 280


Average 235 336 377 309 375 231 283











The 2 Kexican seedlings and 'Pollock' rooted in the fewest nu:xber

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 8' 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 tiv.e to root, but only a few rooted. Those fro:' Mexi-

can trees also required a shorter time to root and rest of the branches

rooted.

Average number of days to naximum.roosin.; for all cultivars ond

seedlings was 398 for air layers made in Lovenber, 300 irt MIarch, 280 in

April, 2!5 in June, and 308 in September.


Anatomi'ca.l Studies


General Stem Anatomy

Examination of transverse and tangential sections of second-flush

growth (Figs. 4 and 5), shoved the following features: Isodianetric 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 sin ly or 2 or rore together and prominent unicellular rays, a more or

less well defined but irregular cu :bial la er, a definite continuation

of rays, numerous sieve tubes, companion cells, and inclusions in a

broad phlcem, clusters of fibers corrected by sclereids between the phloem

and cortex (Figs. 6 and 7), a broo e, essentially unilor:i cortex opposed

of cells similar to those in tha pith, no apparent 'starch sheath'

(&a. .... ome cells contained :,trch -rains (Fi:. 8), wnich correspond

to those of the potato (Solanue Laberosi .) group as described by






















Co


PYR





C





Xy




P


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



Fig. 4. Transvorse section of second-flush Booth 8 avocado


















































B. Photomicrograph (x 150).



























































A. Line drawing (Co- cortex,
C- cambium, Xy- xylem).


F- fibers, Ph- phloem,


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



















































B. Photomicrograph (x 150).
























































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


Fig. 6. Transverse section of Booth 7 avocado.




















































B. Photomicrograph (x 1714).
























































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


Fig. 7. Tangential section of Hickson avocado.




















































B. Photomicrograph (x 150).





















A 3


IP

'!!!!h! 4,


:*
a
=


Fig. 8. Typical concentric starch grains of avocado (x 4286).











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 ca bium 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 wtere apparent among the

cultivars and seedlines in the clusters of fibers .nd the sclercid con-

nection. These will be described in a later section. Anatomical details

noted here corroborated earlier reports on Lauraceie and rersca americana

Mill. (38, 52, 69, 75), and were also similar in the other 3 species of

Lauraceae examined in this investigation.


Anatomy of Succcs: ve Flushes

The gradual expansion cf an .voc:,do stem is -hon in tranoverCe

sections of the first, third throuIgh sixrh, and eighth, flush of 'Booth 81

(Figs. 10, 11, 12, 13, 14, 15, 16). Sctions cut, about 1 mr from the

terminal showed that epidermal hairs (Fi 10) were abun sCt, fibers were

not lignified (Fig. 11) and little celi.ulr organization occurred.

Figures 10 and 11 are serial photoricrographs of a transverse section.

Sections of older stem tissues (Figs. 12-16) showed that the progressive

expansion of the stern was acccnpanied by a separation of the fiber bundles

and a decrease in the width of the layer of sclereids connecting them.

Divergent phloea, rays appear in the fourth-flush (Fig. 13). They become

more prominent a; the ster increases in diameter. There is an almost

complete break down of the sclerenchy, a rind; at the eighth flush (Fig. 16).

A transverse section of the sixth-flush of 'Taylor' avocado (Fig. 17)

shows a divergent ray, parcnchyma type ray cells and a few liCnified































I-'


ad
h Q


& '


A dc

a~~ -i.


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





































Fig. 10. Transverse section of Booth 8 avocado near the
terminal (x 430).


Aw
























































Fig. 11. Transverse section of Booth 8 avocado near the
terminal (x 430).


















Co







Ph










Xy






P


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.






































B. Photomicrograph (x 150).


al.ri;CJ
























































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.























































B. Photonicrograph (x 150).


~PL~i~B i

























































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


Fig. 14. Transverse section of fifth-flush Booth 8 avocado.




















































B. Photcmicrograph (x 150).






















































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.





















































B. Photomicrograph (x 150).

























































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



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




























































B. Photomicrograph (x 150).


`f' ~L~rlY~



























































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



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



















































B. Photomicrograph (x 430).























































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




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










































r30-


B. Photomicrograph (x 150).
























































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


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



















































B. Photomicrograph (x 430).



























































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



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




























































B. Photomicrograph (x 430).


siqmlwl-jo-jpq
























































A. Line Drawing
Ph- phloem).


(Co- cortex, PvR- perivascular ring,


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


CO













Ph

















































B. Photomicrograph (x 130).























































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




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


Co







Pvp





Ph



























SN Ail
Pt ~. \ :t


B. Photomicrograph (x 430).















































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.


Ep


Co


V P.

Ph



Xy
I-p























































B. Photomicrograph (x 150).


























































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



Fig. 24. Transverse section of fifth-flush Taylor avocado.









~Th
I


B. Photomicrograph (x 150).









cells across the broad end of the ray. It may be noted in tangential

sections of the sixth-flush of 'Booth 8' (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 'Hickhon' (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 24).


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, 28, and 29, respectively. These cultivars ant 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 'eristematic characteristics and give rise to

root initials. Etiolated stems resemble the apical portion of the


























































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).

























Ep

Co

PvR









Xy












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.



















































B. Photomicrograph (x 150).

























































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.




















































B. Photomicrograph (x 150).




















































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.


Ep Co Ep


PvR






Ph



-y









































in Lt 'iI


B. Photomicrograph (x 150).

























































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


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


Co








Ph





Xy























































B. Photomicrograph (x 150).










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 (34). It may also explain

the success in rooting immature 'Gainesville' avocado cuttings, reported

by Leal and Krezdorn (48) 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 (34). In 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, 45, 47, 48, 81).

This could be due to the cultivars or seedlings used in the tests.

Kadman and Ya'Acob (45) 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 explain the results obtained with the

air layers reported previously. This is in accord with Beakbane's (4)

conclusion that shoots of shy-rooting plants are often characterized by

a high degree of sclerification in the phloem and with Galkin (24), 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 (4). 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', 'Iickson', 'Lula', 'Taylor', 'Gainesville' seedling,

'Brogdon', 'Mexicola', and the 2 Mexican seedling trees. It was found,

when the sawe 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 24, 1970. Cultivars tested were 'Pollock',

'Booth 7', 'Booth 8', 'Hickson', and 'Taylor', and 2 Mexican seedling

trees. The last had the highest percentage rooting while 'Pollock' and

'Booth "' had the lowest percentage. 'Hickson' and 'Taylor' were intermediate.






68



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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73



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BIOGRAPHICAL SKETCH


Ricardo E. Gomez was born July 13, 1938 at Havana, Cuba. In June

1956 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 Educdtion Center, Homestead and the Center for Tropical Agri-

culture from 1968 to 1971. He was awarded the degree Doctor of Philosophy

in December 1971. He received the T. J. Anderson 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

Siga 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.










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.





aes 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.





Rbert H. Biggs
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.





Al r d H, Kredo
Pr essor and Chai an f 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.




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


Dean, Graduate School




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