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Full text of "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 'Pollock 1 , 
'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 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. Rica rd) 
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 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> 5 fj > 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, 'Conference 1 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 t riandra (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 e t 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 



\.«^ 



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 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. 





11 



22 
22 


33 










Nov. 












22 


33 






March, 1970 














1*0 






April 













kk 






June 















25 


Booth 7 


June, I969 





30 


ko 


ho 










Sept. 







50 


50 


50 








Nov. 


















22 




March, 1970 














-: 


60 




April 













20 


40 




June 















33 


Booth 


June, I969 


^ 


63 


rs 












Sept . 




1] 


66 


77 


77 


Tt 


88 




Nov. 








33 


50 


■v. 


83 




March, 1970 














13 


38 




April 













38 


63 




June 















t< 


Hickson 


June, I969 
Sept. 


70 


80 




80 

] : 1 


80 

36 










Nov. 












13 


] j 






March, 1970 
















50 




April 
















^3 




June 















55 


Taylor 


June, I969 





22 


77 












Sept. 




<) 














13 




Nov. 


















38 




March, 1970 

















57 




April 
















63 




June 












6 


50 


Mexican 1 


June, I969 
Sept. 


63 


88 




80 












Nov. 









60 


70 


80 






March, 1970 











25 


100 






April 













60 


80 




June 















90 


Mexican 2 


June, I969 
Sept. 


50 


7 5 



100 












Nov. 












60 


88 


100 




March, 1970 














60 


80 




April 













25 


75 




June 















90 



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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 Anatom y 

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 Lauracea e 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^ ~* 


c N 


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 Anderson 1 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. I Q 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 



28. Gillespie, H. L. 1956. Preliminary investigation of 'residual 

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72 



i+2. Jamaica Agricultural Society. 1932. Vegetative propagation of 
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Clonal propagation of mango (Ma ngifera indica L.) through 
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73 



56. Ochse, J. J. 1950. Avocados and manges propagated by cuttings. 
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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