The translocation of metabolites in navel oranges


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The translocation of metabolites in navel oranges
Physical Description:
vi, 64 leaves : ill. ; 28 cm.
Brown, H. Donovan, 1945-
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Subjects / Keywords:
Oranges   ( lcsh )
bibliography   ( marcgt )
non-fiction   ( marcgt )


Thesis (Ph. D.)--University of Florida, 1974.
Bibliography: leaves 56-64.
Statement of Responsibility:
by H. Donovan Brown.
General Note:
General Note:

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University of Florida
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All applicable rights reserved by the source institution and holding location.
Resource Identifier:
oclc - 81961537
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The author extends his appreciation to his committee chairman,

Dr. A. H. Krezdorn, Fruit Crops Department Chairman, for suggesting this

problem and providing encouragement during the course of the research.

The author is further grateful to Dr. Krezdorn for his considerable time

and assistance provided during the preparation of this manuscript.

Appreciation is extended to Dr. R. H. Biggs for his suggestions and

assistance in the conduction of the research and his suggestions in the

preparation of this manuscript. Appreciation is also extended to Drs.

J. Soule and T. E. Humphreys for their constructive criticisms and

suggestions in the preparation of this manuscript and to Dr. W. J.

Wiltbank for his assistance and friendly guidance.

This research was made possible through a graduate assistantship

provided by the Department of Fruit Crops.

Greatest appreciation is extended to the author's wife, Shirley,

for her encouragement, endurance and understanding throughout the comple-

tion of this work.






Fruit Set . . .

Cultivar Selection . .

Cultural Practices and Environment .

Cross Pollination . .

Girdling . . .

Growth Regulators . .

Auxins . . .

Gibberellins . .

Cytokinins . .

Fruit Growth . . .

Fruit Growth Stages . .

Growth Regulators and Fruit Growth .

Auxins . . .

Gibberellins . .

Cytokinins . .

Ethylene . .. .

Control and Movement of Metabolites .























. .







Plant Material .

Treatments .

14CO2 Exposure .

Analyses ..

Leaf Dry Weight .

Fruit Dry Weight

14C Distribution

Fruit .

Leaves .

Stem .

Exposed leaves




. 18

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 Philosophy



H. Donovan Brown

December, 1974

Chairman: Dr. A. H. Krezdorn
Major Department: Fruit Crops

The effects of GA on fruit set and distribution of metabolites in

fruiting shoots of navel orange were studied when GA was applied as

individual flower dips and as whole tree sprays. Individual flowers

dipped in GA resulted in a large increase in fruit set when compared

to the control, but GA sprays applied to entire trees were ineffective.

There was a stronger movement of 14C-metabolites into ovaries and

young fruits throughout the fruit-setting period from both current

season and previous season leaves of the GA-dip treatment than for untreated

trees or those sprayed with GA. In addition, the GA-spray treatment and

the control had a period of reduced transport from old leaves to fruits,

following anthesis, before new leaves began exporting significant quantities

to fruits.

Dry wt of fruits from the GA-dip treatment and the control was much

greater than fruit from the GA-spray treatment.

Fruits were the primary site of importation of metabolites from a

median leaf on the shoot; however there was some basipetal movement,

particularly just prior to expansion of the summer flush.


The strongest fruit-setting treatment maintained a greater movement

of metabolites from the leaves into fruits than other treatments, sub-

stantiating the hypothesis that growth regulators promote fruit set by

enhancing the movement of metabolites into fruits. The failure of whole

tree sprays of GA to enhance fruit set of navel orange appears to be due

to an inability of the fruit to attract metabolites in sufficient quanti-

ties to maintain fruit growth during the fruit setting period.


The navel sweet orange group (Citrus sinensis (L.) Osbeck) consists

of a large number of morphologically similar cultivars recognized by the

presence of a small, rudimentary secondary fruit embedded in the apex of

the fruit -- the navel. Navel cultivars are of world-wide significance

because of their distinctive fine quality, crisp flesh texture and ease

of peeling. Navel oranges are almost entirely seedless because they

rarely produce any functional pollen or fertile ovules. Yields of navel

oranges are characteristically lower than those of seedy cultivars and

strongly parthenocarpic seedless types. Low yields are at times partially

attributable to preharvest fruit drop due to insects and diseases damaging

the navel end of the fruit, but most unfruitfulness results from fruit

drop due to environmental stress (19). Much research has been conducted

to improve yields. Cross-pollination has been ineffective because of the

lack of fertile ovules (28). Girdling has sometimes been effective and is

a common practice in certain areas (94). Results from girdling are, how-

ever, inconsistent. Attempts to increase yields with auxin sprays have

met with very limited success (18). Large increases in yields have

occurred when either navel orange flowers or fruit were dipped into solu-

tions of gibberellic acid (GA) (55), when GA has been applied as a dust to

stigmatic surfaces of flowers and when a solution of GA has been applied

only to the leaves just back of the flowers (53), but it has been ineffec-

tive when sprayed on entire trees.

Crane (22) has suggested that growth regulators, such as GA, promote

fruit set by establishing a high rate of metabolic activity in the ovary

which results in a continuous flow of metabolites from other parts of the

plant and thereby maintaining fruit growth and preventing abscission.

Powell (85), reported a continuous movement of 14C-labelled metabolites

from leaves to young fruit of the seedy Calamondin (C. madurensis Lour.)

when sexually fertilized but not when pollen was excluded. Applications

of GA to the flowers also resulted in a strong movement of metabolites

to the young fruit and excellent parthenocarpic fruiting, thereby bolster-

ing the metabolite hypothesis. Cytokinins and auxins also have a strong

polarizing effect on the movement of metabolites into treated tissues

(44, 39, 34, 38). Thus, the role of growth regulators in fruit set and

development appears to be related to the supply of leaf-metabolites to

the young fruit. The purpose of this work, therefore, was to investigate

the movement of leaf-metabolites in the 'Washington' navel orange follow-

ing the application of GA both as flower dips and as whole-tree sprays in

order to determine why GA-sprays are ineffective on this cultivar.


Fruit Set

Fruits develop from the ovaries of flowers but flower formation alone

does not guarantee a crop. Seed-bearing plants normally require the sexual

processes of pollination and fertilization to stimulate the ovary into

enlarging and remaining on the tree. Ovaries in which no fertilization

of ovules occurs generally drop soon after flowering. The remaining ovaries

soon enlarge perceptibly and often change color. Some describe this some-

what indefinite development as fruit set (61), thereby relating fruit set

to the drop resulting from the lack of fertilization. This definition

leaves much to be desired because many young fruit drop during a period

of a month or more after the drop due to lack of fertilization (32).

This drop is more or less continuous but there is often a distinctly large

drop in late spring or early summer, often called the June drop, that has

been related to competition for water and nutrients (34). Fruit set is

also used to describe the early fruit development period terminated by the

last significant fruit drop (32). This definition is useful because

fruit remaining after the June drop are unlikely to fall prior to maturity

and fruit counts taken after the June drop are often closely related to

yield at maturity.

A relatively large number of fruit cultivars, including those in the

navel orange group, produce fruit parthenocarpically; i.e., without sexual

fertilization and therefore without seeds (34). Thus, fruit set should

not be used to indicate the fruit remaining after the initial drop due

to a lack of fertilization, because in the case of parthenocarpic fruiting,

fertilization does not occur. The term fruit set in this paper refers to

the fruit development period extending from flowering through the June drop.

Cultivar Selection

Navel oranges mutate frequently and this has resulted in a large

number of navel cultivars, many of which are almost indistinguishable

from the original 'Washington' (43). The high rate of mutation has led

numerous researchers and growers to search for higher yields.

Shamel and co-workers in California conducted an extensive but largely

unsuccessful search for higher yielding, better quality strains of navel

oranges (88, 89, 90, 91, 92, 93, 97, 98, 99). Reports from Texas (68)

and Australia (26) indicate that some selections appear to be superior to

old-line cultivars but the problem of low yields persists. Gardner and

Reece (31) in Florida evaluated 28 selections of navel oranges over a 4-

year-period and reported several numbered selections to be higher yielding

than any of the commonly planted old lines. Numerous Florida growers have

introduced mutations such as 'Summerfield', 'Dream' and 'Glen' for which

claims of better yields were made (43) but these claims appear unjustified.

Thus, even though improvement of yield by cultivar selection remains a

possibility and limited success has been attained (31, 79), navel cultivars

in Florida still suffer from low yields.

Cultural Practices and Environment

Colt and Hodgson (19) made an extensive study of abnormal fruit drop

of 'Washington' navel orange. They concluded that the majority of the fruit

abscission early in the season was due to problems involving water relations,

"resulting from the asperity of the climatic complex to which the trees

are subject." They surveyed 'Washington' navel groves in various climatic

areas of California and found that those in the cooler more humid areas

would consistently produce more fruit than those in desert areas. The

authors also found that climatic conditions in the orchard could be modi-

fied by various cultural practices which either reduced soil temperatures

or created more favorable moisture relations. They hypothesized that such

practices could be used to make orchards in the desert regions as pro-

ductive as those in the more climatically favored districts; however, this

has not been demonstrated.

Many environmental factors such as hot dry winds, high temperatures,

low humidity, low rainfall, increase the water deficit of navel orange trees

and cause abscission of young fruit. Site selection based upon soil type,

annual rainfall, mean temperature, relative humidity, and wind protection

has been means suggested to provide a more compatible environment (19, 83).

Effects of rootstock on yield of citrus in general are well established

but the nature of the influence has not been established. Krezdorn and

Phillips (56) working with the 'Orlando' tangelo in Florida reported that

several rootstocks supported high yields of seedless fruit, but trees on

other rootstocks yielded poorly. Later work (10) demonstrated that root-

stocks varied greatly in depth of rooting, suggesting that those penetrating

large volumes of soil might reduce the physiological stress due to inadequate

soil moisture during the fruit-setting period and thereby increase fruiting.

The largest yields were those of trees on the deepest rooted stocks, but

some of the shallowest rooted ones also produced excellent crops. Thus,

even though the reasons some rootstocks resulted in higher yields were not

established, the research demonstrated that a rootstock can in certain cases

bring about satisfactory fruiting of a weakly parthenocarpic citrus cultivar.

There has been no comparable rootstock research with navel oranges in Florida

because certain viruses that have been present in all navel orange budwood

sources cause stunting of trees on certain rootstocks and this has resulted

in the abandonment of rootstock trials. None of the rootstocks used commer-

cially in Florida satisfactorily overcome the navel orange fruiting problem.

Cross Pollination

Self-unfruitfulness results when sexual fertilization is not accom-

plished and parthenocarpy is lacking. The 'Bruce' plum and several peach

cultivars are pollen sterile and require cross pollination (59, 11). The

most common cause, however, of self-unfruitfulness is the result of sexual

self-incompatibility. Cross pollination is necessary for fruit production

in many fruit crops such as almonds (12), sweet cherries (81), many cultivars

of apples (12, 60) and some pears (82). Most citrus cultivars are self-

fruitful. Many interspecific hybrids such as 'Orlando' (57), 'Minneola'

(57), 'Robinson', 'Osceola', and 'Nova' (87) are self-incompatible, how-

ever, and not sufficiently parthenocarpic to produce adequate crops without

cross pollination or some other stimulus to strengthen the parthenocarpy.

Absence of pollen in the flowers of navel oranges removes any possi-

bility of self-pollination and Webber (118) demonstrated that only a slight

increase in seediness was caused by cross pollination. Pomeroy and Aldrich

(84) and El-Tomi (27) reported slightly improved fruit set in some years

by cross-pollinating individual navel flowers. Pomeroy and Aldrich, how-

ever, did not observe increased yield of 'Washington' navel trees planted

adjacent to other citrus cultivars. Workers in South Africa (26) polli-

nated 'Washington' navel flowers with pollen from 20 different citrus

cultivars. There was a wide range of responses in both fruit set and number

of seed per fruit. For example, 'Valencia' sweet orange pollen and 'Morton'

citrange pollen did not increase seed number or fruit set in 'Washington'

navel when compared to the unpollinated control; however, 'Bath' sweet

orange pollen, and pollen from several grapefruit and shaddock cultivars

significantly increased both seed number and fruit set. More importantly,

pollen of 'Hamlin' sweet orange was nearly as effective as the pollen from

the grapefruit and shaddock cultivars, setting 22% of the 'Washington' flowers

pollinated, compared to 3.5% set parthenocarpically. There was an average

of only 0.2 seeds per fruit resulting from the 'Hamlin' pollen, whereas

shaddock pollen produced fruit having an average of 8 seeds per fruit.

Since seedlessness is an important attribute of navel oranges, a pollen

source which increases fruit set but not seed number would make a very

desirable pollinator.


Girdling, a very old horticultural practice, has been used with

varying degrees of success in increasing parthenocarpy of many fruit crops,

including pears (71), grapes (112, 111), persimmon (42), cherry (72),

apples (71), and citrus (3, 33, 51, 58, 80, 95). The physiological response

of fruit set to girdling is not well understood (51, 73, 78), but it is

known to cause an accumulation of carbohydrates (73, 78), growth regulators

(102, 112) and other substances (78, 103, 104, 114) above the girdle.

It is a common practice to girdle navel orange trees to increase

fruit set in California. Adverse effects on tree condition have not been

observed when trees were girdled annually for extended periods of time,

but there have been both positive (86) and negative reports (96) with

respect to reductions in yield of subsequent crops when girdling was


Work in Florida (51) indicates that girdling is inconsistent in

increasing fruit set of navel oranges. Best results have been obtained

on young, vigorous trees, with less success being achieved on older ones.

Growth Regulators

Auxins. The significance of auxins in fruit setting has been recog-

nized for almost 40 years (22). Auxins are present in pollen and is pro-

duced in the style and ovary during pollen tube growth and fertiliza-

tion. The resultant stimulation in growth of the ovary is an established

fact (110). Relatively few citrus cultivars, however, have been induced

to set fruit parthenocarpically by auxin application.

Auxins are most effective on fruits with many ovules, such as egg-

plant, fig, grape, strawberry and tomato (107, 24). Auxins have been

ineffective on stone fruits, such as peach, plum, and cherry (107).

Other fruits such as apple, apricot, avocado and pear may be set by auxin

application, but usually fail to grow to normal size (24).

Work in California on citrus has shown negative results with 2, 4-

dichlorophenoxyacetic acid (2,4-D), napthaleneacetic acid (NAA), indol-

eacetic acid (IAA), indolebutyric acid (IBA), and many other auxins in

attempts to enhance fruit set with these materials (17). In South Africa,

however, 2,4-D has increased yield of navel oranges through an increase

in both numbers of fruit and fruit size (17). The difference in the

reaction of navel oranges to 2,4-D in South Africa and California was explained

as possibly due to the vast differences in environmental conditions

Gibberellins. Many types of fruit such as fig, tomato, eggplant, and

citrus that can be set with auxins can also be set with GA. In addition,

fruit set of many species responds to GA treatment but not to auxins,

(21, 23, 26, 107). For example, some success has been achieved in increas-

ing fruit set of apples with GA (25, 61, 65, 133), even though auxins

have been ineffective. Applications of GA to pears has increased partheno-

carpic fruit development; however, the benefit was only realized on low

bearing cultivars of trees damaged by frost at blossom time (102).

Bukovac and Nakagawa (7) demonstrated a considerable difference in

specificity among gibberellins in research with 'Wealthy' apple. Unpolli-

nated controls abscissed within 4 weeks but the growth rate of fruit

treated with GA4 and GA7 was equal to the pollinated controls. GA5, GA6

and GA8 showed little activity and responses to several other gibberellins

were intermediate.

Application of GA to small 'Bearss' lime fruit, 'Eureka' lemon flowers

and flowering branches of 'Washington' navel orange trees resulted in an

increased number of parthenocarpic fruit (40). Iwasaki et al. (46) applied

GA individually to flowers and young fruit of 'Washington' navel and

improved retention of the fruit. 'Clementine' mandarin has responded favor-

ably to fruit set treatments of GA (80, 101) and substantial increases in

set of 'Orlando' tangelo and other mandarin hybrids have been obtained

with spray applications of GA (53, 54, 55)

Substantial increases in yield resulted when individual navel orange

flowers were dipped in solutions of GA, (55), or applied to young leaves

just behind the fruit (53). Whole tree sprays, however, have given varied

responses from no effect on fruiting or leaf abscission in Florida (53)

to severe leaf drop and death of new vegetative and flower-producing shoots

in California (41).

Cytokinins. Increased fruit set of grapes (114) has been obtained

with 6-benzylamino purine (BA) and 6-benzylamino-9-(2-tetrahydropyranyl)-

9H-purine (PBA). PBA was effective in inducing parthenocarpic fruiting in

fig (23). Cytokinins are reportedly useful in increasing fruit set in

muskmelon (48) and in some apple cultivars (119) although they have been

generally less effective than GA. Kriedemann demonstrated the ability of

cytokinins to increase the transport of photosynthetic assimilates into

young 'Washington' navel fruit; however, no beneficial effects on fruit

set of citrus have been reported(49).

Many interactions and synergisms of cytokinins with other growth

regulators have been demonstrated; however, they generally are less

effective in inducing fruit set than GA.

Fruit Growth

The main blooming period of citrus cultivars is in the spring.

Most cultivars bloom at nearly the same time; however, the developmental

period varies, depending on the cultivar and the climate, from 7 to 13

months (100).

Fruit Growth Stages

Fruit growth is a result of cell division and cell enlargement.

Expansion of intercellular spaces also contributes to the growth of fruit,

especially during the later stages. Cell division usually predominates

during early stages of fruit growth followed by cell enlargement. There

is often an overlapping of the 2 stages and cell division persists until

maturity in some fruits such as avocado (110).

The growth curve of some fruits, such as citrus, apple, pear, tomato,

cucumber and strawberry is sigmoid in shape. Fig, currant, grape, blue-

berry, and many stone fruits, including cherry, olive, apricot, peach and

plum, are characterized by a double sigmoid curve. Growth of the ovary

in the latter type occurs in 2 stages separated by an inactive period of

growth. It is during this period in stone fruits, when the ovary growth

ceases, that the embryo and endosperm develops and lignification of the

endocarp occurs (110, 76, 22, 61).

Morphological development of relatively few citrus cultivars has

been investigated. Bain (4) working with 'Valencia' sweet orange made

the most complete investigation of the development of any citrus cultivar.

Adigun (2) demonstrated the effect of GA, girdling, and cross pollination

on the development of 'Orlando'tangelo fruit.

Bain divided the developmental period of 'Valencia' orange into 3

growth stages: Stage I begins at bloom and lasts about 9 weeks. It is

designated as the period of cell division since the number of cells in all

tissues of developing fruit except the inner cells of the central axis,

increase to form the tissue of the mature fruit during this period. Increase

in fruit size during Stage I is due mainly to growth of the peel.

Stage II lasts for approximately 29 weeks and is the period of maximum

fruit growth, with morphological, anatomical, and physiological changes

being very rapid during this period. There is marked expansion of tissues

accompanied by cell enlargement and differentiation during Stage II. Cell

division continues in the outer peel tissues throughout Stage II The pulp

expands considerably, the juice sacs become larger and the juice content

increases. Stage II is considered as the critical period for fruit growth.

Stage III is designated the maturation period and lasts approximately

28 weeks. Fruit continue to grow during Stage III but at a reduced rate

compared to Stage II. Ripening occurs during this period accompanied

by a change of peel color from yellow to orange.

Bouma (6) made a study of 'Washington' navel orange development in

relation to cultural and fertilizer practices. He found that different

cultural systems evaluated had little effect on fruit morphology but that

nitrogen level had a pronounced effect on peel thickness during Stage II.

He surmised that the stages of development were substantially shorter for

'Washington' than Bain (4) had found for 'Valencia' since the total growing

period was 5-6 months shorter. Bouma found, however, that Stage I in

'Washington' lasted 11 weeks or 2 weeks longer than the same growth stage

of 'Valencia'. Stages II and III were much shorter, lasting only 14 and

12 weeks, respectively.

Growth Regulators and Fruit Growth

Auxins. Increases in fruit size is mainly a result of cell enlarge-

ment. Auxins are often cited as having a dominant role in fruit growth

because of their role in cell enlargement (110).

Seeds reportedly produce auxins which move outward toward the carpel

wall and stimulate growth (75). Nitsch (75) was able to control the

development of strawberry by selectively removing achenes from the

receptacle. Where he removed an achene the receptacle did not swell unless

he treated the area with a lanolin paste containing 6-hydroxyethylhydrazine

(BNOA). Gustafson (35) working with tomato showed a concentration gradient

of auxins originating in the seeds and extending through surrounding tissues

with diminution toward the carpel wall.

There have been numerous correlations of seed number and fruit size

(52, 75, 63, 64, 66, 77, 9) and between seeds and fruit shape (1), but

there is usually no close correlation between the total amount of auxin

produced in seeds and fruit growth (110). Moreover, exogenous applications

of auxin have not been as effective as some other growth regulators in

bringing about certain growth responses (110).

Gibberellins. Young seeds are a rich source of gibberellin (110, 107).

The concentration of gibberellin-like substances in various tissues of

apricot fruit correlates closely with growth rates of these tissues through-

out fruit development, but not with general fruit growth (47). In 'Seedless

Tokay' grape, a close correlation between gibberellin-like activity and berry

growth was found during Stages I and II but not in Stage III. The decrease

of gibberellin activity in Stage III was attributed to the early cessation of

seed development (45). Wiltbank and Krezdorn (121) found positive correla-

tions between concentration of gibberellins in the fruit and rate of fruit

growth and also between total gibberellins per fruit and cumulative fruit

growth in 'Washington' navel during the first 9 weeks of fruit development.

Commercial application is made of the ability of GA to enlarge the

fruits of several grape species (122). 'Thompson Seedless', 'Black Corinth'

and 'Delaware' grapes are sprayed commercially with 1 or 2 applications of

GA to reduce fruit set, elongate clusters, increase berry size and increase


GA sprays are used to extend the harvest season of sweet cherries

and produce larger and firmer fruit that are less prone to growth cracks


GA applications have enhanced fruit size of lemons (14,15), grapefruit

(16) and oranges (17, 41), but effects have been variable depending upon

time of application, concentration and other factors.

Cytokinins. Cytokinins have been reported to affect fruit size and

shape in apples (120); however, others working with different varieties

and at other locations have not confirmed this (110). Fruit size was

increased in 'Black Corinth' grape (113) using cytokinins but less effect

was observed with 'Thompson Seedless' grape (114).

Ethylene. Ethylene is now considered one of the main growth regulators

(110). Many plant responses that were formerly believed to result from

auxins are now ascribed to ethylene production (8). Growth of the fig

fruit is more rapid and the time to maturation shortened by the ethylene

produced in the fruit following applications of 2,4,5-trichlorophenoxyacetic

acid (2,4,5-T) (67).

Peach diameter was increased by an application of ethephon, an ethylene

generator, 4 days before harvest. Application of ethylene or ethephon to

grapes during Stage II hastened ripening but benzothiazole-2-oxyacetic

acid (BOA), an auxin, retarded ripening during late Stage I or early Stage

II. Hale et al.(36) suggested that an auxin-ethylene relationship controls

the speed of berry ripening.

Control and Pattern of Movement of Metabolites

There is a preferential movement of metabolites into the phloem

following the buildup of assimilates in the photosynthetic tissue of a

leaf (109). Sugars, primarily sucrose, constitute most of the organic

substances transported in the phloem although nitrogenous substances and

some steroids are also known to be translocated (69, 70, 124, 74, 116,

117, 50, 5, 106). Not only are assimilates selectively moved out of a

photosynthesizing leaf into the conducting tissue, but the movement is

also done against a concentration gradient (123, 108).

A young expanding leaf obtains carbohydrates from other leaves until

it is between 1/3 to 1/2 of its final area, after which time it becomes

self-sustaining and begins exporting assimilates (105, 115, 30, 29).

Kriedemann (50) reported that a lemon leaf does not become a contributing

organ until it is fully mature. The period of maximum export for leaves

of several plants occurs soon after they reach full size, the rate sub-

sequently declining with leaf age (13, 100, 109).

There is a pattern of assimilate distribution that applies to most

plants. Lower leaves supply assimilates to the roots whereas the upper

leaves serve the shoot apex. Assimilates from intermediate leaves may

move in either or both directions, depending on demand (85, 109). These

patterns have been established on small herbacious plants; however, Hale

and Weaver (37) showed much the same pattern with grapes. They demon-

strated fewer assimilates were directed toward the apex as the elongating

vine grew farther from a given source leaf.

Kriedemann (50) reported an apical flush supporting a lemon fruit

directed all photosynthates toward the fruit. There was bidirectional

transport in the subapical flush, but acropetal movement predominated.

Leaves separated from the fruit by 2 flushes had only basipetal move-

ment. Powell (85) observed the same pattern with seedy 'Calamondin'

fruit except that the movement of photosynthates from the apical flush

was not entirely acropetal. A slight basipetal movement occurred.

Changing from a vegetative to a fruiting shoot can rapidly alter

the pattern of assimilate distribution. A shifting pattern in many

cases may be only a change in intensity of assimilate movement in a given

direction (109). Work on peas (62), grapes (37), tomato (109) and citrus

(50, 85) has shown that a developing fruit has a demand for assimilates

from adjacent leaves.

Crane (22) proposed that a high metabolic gradient between the ovary

and adjacent vegetative organs is established by the process of fertili-

zation which diverts assimilates toward the developing fruit, even at the

expense of vegetative growth. Crafts (20) pointed out that as carbohydrates

are transported from their sites of synthesis to those of utilization,

other substances including hormones, growth regulators and viruses are

also carried along and accumulate in sinks to concentrations that are

greater than along the channels of movement.

Crane (22) postulated that the agents which initiate the metabolic

gradient come from the ovary and are probably auxins, gibberellins or

even cytokinins. He assumed these substances were produced by the seeds

or parts of the ovary wall itself.

Later Crane (23) applied auxins, GA, and cytokinins to 'Calimyrna'

fig and obtained fruit similar in general morphology and size to those

produced by pollination. He concluded that fruit growth is controlled

by the ability of growth substances eminating from the seeds to attract

metabolites from other regions of the plant. He suggested that the

hormones were synthesized in the leaves and stored in the fruit.

The role of growth regulators in mobilization of organic materials

has been investigated by exogenous applications of these materials.

Auxins have long been known to increase photosynthate movement to treated

areas, and cytokinins have been demonstrated as strong mobilizing agents

in leaves (110). IAA, GA4, GA7 and BA were applied to apple seedlings to

evaluate their ability to mobilize various isotopically labelled sub-

stances to treated areas (38). BA mobilized 3H-GA, acropetally and

basipetally and mobilization was enhanced by the addition of IAA. An

acropetal movement of 14C-kinetin was induced only with a mixture of GA4,

GA and IAA. All of the materials mobilized 14C-sorbitol and 14C-glycine

acropetally, and the movement was greatly enhanced by girdling the stem.

Kriedemann (49) applied cytokinins to citrus fruit and thereby

enhanced the movement of photosynthate into the fruit. Powell (85) applied

GA to 'Calamondin' fruit and observed a pattern of photosynthate distri-

bution that was similar to the strong continuous flow of photosynthates

into seedy fruit of that same cultivar. This pattern differed from that

of the emasculated controls demonstrating that the endogenous mobilizing

force of seeds on photosynthates in 'Calamondin' could be substituted by

an exogenous application of GA.


Plant Material

All plots were located in a 7-year-old 'Washington' navel orchard

on sour orange rootstock in Marion County. Fifteen uniform trees were

randomly divided into 3 groups of 5 trees each. Approximately 75 flower-

ing shoots with a minimum of 5 new leaves each were selected and tagged

on each tree during full bloom. All shoots were thinned to a single

large flower at or just prior to anthesis. Shoots selected were all

located 1 to 2 M above the ground and in the outer canopy.


Treatments consisted of spraying the entire canopy of 1 group of 5

trees with 250 ppm GA and dipping flowers on tagged shoots of the second

group in a solution of 250 ppm GA. The third group of trees received no

treatment and constituted the control.

No statistical experimental design was used because the objective

of the experiment was to study general patterns of movement of metabolites

rather than small treatment differences.

14CO Exposure

The reaction vessel for generating 14CO2 consisted of a 125 ml

erlenmeyer sidearm flask fitted with a piece of Tygon tubing, a rubber

septum for a port on the sidearm and a rubber balloon over the top to

serve as a diaphragm. The 14CO2 was generated by placing 10 mg of Ba 14CO

containing a specific activity of 250 1C in the reaction flask and the

diaphragm was sealed over the top. Just prior to fitting the exposure

chambers over the leaves, I ml of lactic acid was placed in the flask

through the port. Care was taken that the lactic acid went through the

plastic tubing and did not run down the walls of the flask. The lactic
acid was thoroughly mixed with the Ba 1CO3 to ensure complete reaction

and release of the 14CO2. Thirty min were allowed for the reaction to

go to completion.

Plastic bags with zipper-like enclosures were modified for use as

exposure chambers. The interlocking ridges were removed from the center

of the bag for a distance of 5 mm to accommodate the leaf petiole. The

space left by removing the enclosure was filled with modeling clay to

seal the opening around the petiole and black plastic tape was placed on

the lower right side of the bag to make a port for receiving 14CO2. Bags

for each treatment were color coded and numbered consecutively from 1 to


Two leaves, 1 subtending a tagged spring-flush shoot and 1 selected

from near the mid-section of another spring-flush shoot from each tree

in each treatment and the control were exposed to 14CO2 on each of 19

dates (Table 1). Leaves were exposed to 14CO2 during and just prior to

the entire fruit-setting period, starting several days before anthesis

and continuing until the summer flush of growth was fully expanded and

became essentially static.

Table 1. Dates of 14CO2 Application to Navel Orange with Reference to


4/2/72 -5

4/8/72 1

4/10/72 3

4/13/72 6

4/16/72 9

4/20/72 13

4/24/72 17

4/28/72 21

5/2/72 25

5/8/72 31

5/23/72 46

5/30/72 53

6/17/72 68

6/27/72 81

7/1/72 86

7/5/72 90

7/9/72 94

7/12/72 97

7/17/72 102

The exposure chambers were first placed over all leaves that were

to be subjected to 14CO2 on a given date. Then, 2.5 ml of 14CO2 enriched

air containing 5pC specific activity was removed from the reaction flask

through the septum and injected through the wall of the exposure chamber

with a 5 ml disposable syringe. The black plastic tape was peeled back

before injection and replaced afterwards to seal the hole left by the


Injection of 1CO2 into all of the bags required 10 min hence the

starting time for all replicates was approximately the same. Exposure

to 14CO2 began on each date at 8:30 AM and continued until 12:30 PM.

Shoots were harvested after the 4-hr exposure, and the exposed leaf of

each shoot was removed and placed in a coded envelope. Each shoot and

the envelope containing the exposed leaf were placed in a paper bag and

immediately frozen with dry ice. Samples were subsequently stored at

-20C until analyzed. Shoots were harvested in the same order they were

exposed to 14C02.


Frozen plant material from shoots harvested on 19 designated dates

(Table 1) was taken to the laboratory where the shoots were immediately

dissected, placed in coded envelopes, and restored at -20 C. Shoots in

which old leaves were exposed were dissected into 1) ovary (flower or

fruit), 2) exposed leaf, 3) leaves, and 4) stem (Fig. 1). Shoots with

a new median leaf exposed were divided into 1) ovary, 2) exposed leaf,

3) leaves distal to exposed leaf, 4) stem distal to exposed leaf, 5)

leaves basal to exposed leaf, 6) stem basal to exposed leaf, and 7) old

basal leaf from a previous growth flush (Fig. 2).


Fig. 1. Diagramatic sketch of shoot showing position of leaf exposed to
14CO2. Differing numbers indicate subsamples after treatment,
namely, 1) ovary (flower or fruit) 2) exposed leaf 3) leaves
4) stem

Fig. 2. Diagramatic sketch of shoot showing position of leaf exposed to
14CO2. Differing numbers indicate subsamples after treatment,
namely, 1) ovary (flower or fruit) 2) exposed leaf 3) leaves
distal to exposed leaf 4) stem distal to exposed leaf 5) leaves
basal to exposed leaf 6) stem basal to the exposed leaf
7) subtending leaf from a previous flush

Samples were removed from storage and dried in a forced draft oven

for 48 hrs. Fruit larger than 2.5 cm in diameter were dried separately

for 72 hrs.

Each sample was crushed, mixed thoroughly and dry wt determined. A

100-mg aliquot was taken. Fruits heavier than 100 mg were ground in a

Wiley mill through a 40-mesh screen before removing the aliquot.

Several methods of preparing samples for scintillation counting were

attempted including oxidation in a Schononger flask and dissolving the

plant material in various tissue solubilizers. Oxidation in a Schononger

flask was a bulky procedure and was not suited to handling large numbers

of samples.

Tissue solubilizers all resulted in highly colored solutions due to

chlorophyll pigments contained in the plant tissues. This caused severe

quenching of 14C-counts when counted in a liquid scintillation spectro-

photometer. Attempts to use bleaching agents to remove the color were

unsuccessful due to quenching from the bleach. Quenching curves were

constructed, but the amount of quenching was so large that reliable data

could not be obtained.

Automated oxidation assemblies were available, but their cost was

prohibitive, so an oxidation train (Fig. 3) was constructed.

The apparatus consisted, in part, of a brass tube which was connected

to an 02 source by Tygon tubing on 1 end and contained a nichrome sample

basket on the other. The 02 tube and sample basket were screwed into

another tube to create a closed system for oxidation. The tube was heated

and an 02 flow of 1 L/min was passed through the system allowing for

complete oxidation of the sample and sweeping the oxidation products

through the system. Heat was supplied by a Bunsen burner placed under



1 2- -_ -

Fig. 3. Diagram of apparatus used to oxidize plant material and trap the
resulting 14C02. It consisted of: 1) oxidation tube 2) sample
basket 3) oxygen tube 4) water trap 5) vial containing
phenethylamine, a C02 trapping agent 6) Bunsen burner
phenethylamine, a CO2 trapping agent 6) Bunsen burner

the oxidation tube immediately behind the location of the sample basket

inside. The location was critical. Placing heat directly under the basket

heated the sample slowly and caused unoxidized material to be driven off

briefly before ignition of the sample. Placing heat too far behind the

sample delayed ignition, but by locating heat just behind the sample, the

walls of the tube became very hot and anything driven off before ignition

passed through this area and was oxidized. Glass tubing was bent to

create a water trap and the tip was drawn out in order to scrub the CO2

from the oxidation products as they passed through the phenethylamine,

contained in a scintillation vial.

A sample was wrapped tightly in cigarette paper and placed in the

sample basket for oxidation. The flow of 02 was started and the 02 tube

and basket were screwed into the oxidation tube. A scintillation vial

containing 15 ml of scintillation solution was placed under the bubbling

tube to trap the oxidation products and the Bunsen burner was ignited.

Ignition of the sample was viewed through the end of the 02 tube which

was connected to Tygon tubing. Oxidation lasted approximately 15 sec and

the system was flushed with 02 for 40 sec after oxidation was complete.

The scintillation vial containing the oxidation products was removed

before the 02 flow was stopped which kept the inside of the tube free of

any scintillation solution to facilitate cleaning.

Samples were oxidized systematically by running replicates and clean-

ing the system after each set of 5 replicates. Cleaning was accomplished

by blowing out the train with air, washing the water trap with ethyl

alcohol and replacing the glass wool filter.


Samples for each 14C exposure date were arranged in a Packard Tri

Carb model 3380 liquid scintillation spectrophotometer with replicates

of each treatment consecutive. The 1C preset channel was used and an

automatic external standardization ratio was determined for each sample,

which provided a comparison of the relative quenching between samples.

Each sample was counted for 5 min.


Leaf Dry Weight

Dry wts of leaves increased until 25 days after anthesis but there

were no differences due to treatments (Fig. 4). Dry wts of leaves from

GA-sprayed trees were greater than for the other 2 treatments, from 46

days after anthesis until the termination of the experiment. Leaves

from shoots dipped in GA were intermediate in dry wt between the control

and the GA-sprayed treatment.

Fruit Dry Weight

Mean dry wts of fruit from all treatments were similar at anthesis

and for several days thereafter (Table 2). Thirteen days after anthesis,

however, dry wts of GA-dip fruit had become noticeably greater than those

for the GA-spray treatment and the control, and their difference was main-

tained until 86 days after anthesis. Dry wts of fruit from the control

trees were much greater than for the GA-spray treatment 31 days after

anthesis. The GA-dip fruit did not vary greatly from the control from

86 days after anthesis until the conclusion of the experiment, but the

dry wt of fruit from both the control and the GA-dip treatments remained

greater than for fruit from the GA-spray treatment until the termination

of the experiment.

Dry wts of fruit from the control were initially less than those of

the GA-dip treatment, but appeared to catch up toward the end of the

experiment. This was probably due to the manner of determining dry wt

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from whole trees sprayed with GA and untreated controls.

Days after Anthesis




















Dry Wt. (mg)
























































of the fruit. Fruit harvested for 14C-analysis on each date were used

for dry wt determinations. Initially, fruit of all degrees of ability to

survive were in the lot sampled, but as the experiment progressed, only

the larger more competitive fruit of the control survived, leaving only

the largest fruit for experimental use.


Fruit. The importation of 14C-photosynthates into the fruit from

the median leaf on the shoot increased up to the 17th day after anthesis

in both the control and the GA-spray treatments (Fig. 5), as indicated
by increases in concentration of 1C in the fruit. The concentration of

14C in the fruit decreased gradually the next 14 days and became almost

negligible by 68 days after anthesis. Fruit from the GA-dip treatment

had an increase in 14C concentration which peaked at the same time as

the other treatments, but remained high for 14 days before declining.

The concentration of 14C in the GA-dip treatments also became negligible

by 68 days after anthesis.

Concentrations of 14C derived from a median leaf in the fruit from

the control and GA-spray treatments were similar throughout the experiment,

but concentrations of 14C in fruit from GA-dip treatments were generally

higher than for other treatments for 58 days after anthesis. Concentra-

tions of 14C in fruit from all treatments were similar and extremely low

during the last 34 days of the experiment, caused primarily by a dilution

effect due to substantial increases in fruit size.

Concentrations of 14C-photosynthate exported from a leaf of a

previous growth flush into fruits of all treatments were very strong at

anthesis and decreased each date until 68 days after anthesis after which


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14C-photosynthate concentration was minimal (Fig. 6). The pattern of 14-

labelled photosynthate movement into fruits of the control and the GA-

spray treatments were very similar on each date but movement of labelled

photosynthate in the GA-dip treatment became substantially stronger by 6

days after anthesis. The movement of 1C-photosynthate into the GA-dip

fruit remained strong until movement from the newly formed leaves became

strong, as Powell (85) found in 'Calamondin.'

Seasonal patterns of movement of 14C-labelled photosynthate to the

fruit differed from those portrayed on a dry wt basis, because the dilution

effect due to increase in fruit size was taken into consideration. The

total importation of 14C from an old basal leaf to the fruit had 3 main

peaks during the course of the experiment: a) at anthesis, b) at 53 days

after anthesis, and c) at 97 days after anthesis (Fig. 7). In general,

the strongest movement was sustained by the GA-dip treatment except for a

period of 26 days, from 68 to 94 days after anthesis, when the total move-

ment was stronger into the control and GA-sprayed fruit than into GA-dip


Total movement of labelled photosynthate from the new median leaf

to the fruit followed the same general pattern as from the leaf of a previous

growth flush (Fig. 8). There was generally a greater difference between

the concentration of 1C-labelled metabolites in fruit from the GA-dip

and the other treatments when the 14C was derived from a new leaf than

when the source leaf was from a previous growth flush. Also there was

not a corresponding period in which the GA-dip fruit accumulated less

total 14C photosynthates than the other treatments as was found from the

"old" source leaf.


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There was a short interval just before the expansion of the summer

flush in which fruit from all treatments had a period of reduced impor-

tation of photosynthates. This agrees with other work on citrus and

shows a competition between newly developing leaves and fruit for current

photosynthates (85).
Leaves. The movement of 1C-metabolites from a median leaf on

the shoot was primarily acropetal regardless of treatment or date (Fig. 9).

Movement from a median leaf into leaves distal to it became successively

stronger for all treatments from anthesis until 9 days later and then

decreased for a period of 14 days. Then, the general trend was for the

movement of labelled metabolites in all treatments to become stronger

again, with a peak about 68 days after anthesis. Movement of labelled

metabolites for the GA-dip and GA-spray treatment then decreased until a

level similar to the acropetal movement prior to anthesis was reached.

Movement of labelled photosynthates from the median leaf of the control

increased substantially 97 days after anthesis then decreased on the final

date of monitoring to a level twice the GA-dip treatment.

The basipetal movement of 14C-photosynthates from a median leaf to

other leaves of the same flush was fairly uniform throughout the experi-

ment for all treatments (Fig. 10). There was a slight increase in photo-

synthate movement in the GA-dip treatment over other treatments from 13

days to 46 days after anthesis.

Photosynthate movement from a median leaf into a leaf of a

previous growth flush was fairly constant until 90 days after anthesis,

at which time the movement from the GA-dip treatment was many times greater

than the control or GA-spray treatment (Fig. 11). This time period

corresponded with the expansion of the summer growth flush.



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The pattern of movement of 14C-labelled photosynthates from the

leaf subtending the current season shoot into the leaves of that shoot

was similar to the pattern of 14C-photosynthate movement to the fruit

(Fig. 12). The movement was strong until shortly after anthesis and

gradually decreased to 13 days after anthesis. All treatments remained

constant until 46 days after anthesis, at which time there was an upward

trend similar to the acropetal movement from the median leaf.

Beginning 3 days after anthesis and lasting 6 days, 14C-photo-

synthate movement from the previous flush leaf into the leaves of the

new flush was much stronger for the GA-spray treatment than for the


The 1C-photosynthate movement from the previous flush leaf into

the new leaves became substantially greater in the GA-dip treatments than

in either the control or the GA-spray treatments, toward the end of the

experimental period when the summer leaf flush began emerging.

Stem. Movement of 1C in the stem apical to the median leaf that

was exposed to the 14CO2 was essentially the same as that from the median

leaf to the fruit for all treatments (Fig. 13). The basal portion of the

stem retained substantial concentrations of 14C for all treatments from

13 days until 46 days after anthesis (Fig. 14). The stem of the new flush

retained 14C derived from an old leaf at the base of the flush similar

to the importation of 14C by the fruit from the same source leaf (Fig. 15).

Exposed Leaves. The general trend of 14C retention by the new

leaves was that which might be anticipated (Fig. 16). There was a strong

retention of 14C by all treatments at anthesis and as the leaves matured

they exported a larger portion of the labelled photosynthates.



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There did not appear to be any substantial differences due to

treatments; however, leaves of the sprayed trees retained more of the

labelled photosynthate during the first 46 days than the others and the

GA-dip treatment retained less, which agrees with the patterns of export

to the fruit from the same source leaves.

Retention of 14C in the previous flush source leaves followed

a bell-shaped curve, which peaked during the period of maximum import

into the fruit from the new-flush source leaves (Fig. 17).

There appeared to be greater retention of labelled material in

the previous flush source leaves from the GA-spray treatment during

the beginning of the experiment, but little difference was observed

after the 1st 25 days.

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Conditions desired for this study were established: i.e., an increase

in fruit set with flowers dipped in GA, no increase in fruit set from

whole-tree sprays and low fruit set on control trees. Fruit counts were

not taken but it was evident from the number of tagged fruit remaining

on each treatment at the end of the experiment that the fruit set on the

GA-dip treatment was much greater than the GA-spray treatment or the

control. The GA-spray treatment did not cause any increase in fruit set.

These results confirm previous research in Florida. In this study, trees

treated with whole-tree sprays shed some of their leaves whereas no loss

of leaves was observed in previous studies on navel orange when concen-

trations up to 500 ppm GA were used (53). On the other hand, GA applied

to navel oranges in California has resulted in leaf drop and twig dieback

even at concentrations below 250 ppm (41). Such differences in response

have not been explained, but differences in climate or specific weather

conditions at the time of spraying may contribute to the response by

affecting absorption or the amount of material deposited on the leaf.

More 14C-metabolites moved into the ovaries and young fruit though-

out the fruit-setting period from both current season and previous season

leaves following GA-dip treatment than was found with untreated trees or

those sprayed with GA. In addition, there was a short period from 6 days

to 13 days after anthesis, in which fruit that had been dipped in GA received

much more 14C-photosynthates than fruit from the GA-spray treatment or the


The period immediately following anthesis appeared to be a critical

time in fruit set. It was a transitional time in which the primary source

of photosynthates received by fruit shifted from the old leaves to the

current flush. Newly developing leaves and fruit initially imported

photosynthates from mature leaves subtending the new flush. New leaves

gradually ceased to import as they matured and they began to export photo-

synthates to the developing fruit when they were fully expanded. The

older mature leaves gradually became a less significant source of photo-

synthates for the fruit. Fruits which had been dipped in GA received a

substantial supply of photosynthates without interruption; whereas fruit

from the GA spray treatment and the control had a period of approximately

7 days in which the total amount received from both mature leaves and new

leaves was quite low. This time coincides with a normal wave of heavy

fruit drop. There was a heavy fruit drop from the control of this experi-

ment during that period, but with the GA-spray treatment, the other weak

fruit-setting treatment, fruit drop was lighted However, fruit from the

GA-spray dropped later in the experiment. Abscission was delayed, but

there was not a strong movement of photosynthates into fruit from surround-

ing leaves and the small fruit gradually dropped as evidenced by very few

fruit remaining on trees sprayed with GA at the termination of the experi-


These data could explain greater fruit set in the GA-dip treatment

because they follow the hypothesis of Crane (22) that growth regulators

promote fruit set by establishing a high rate of metabolic activity in the

ovary resulting in a continuous flow of metabolites from other parts of

the plant into the fruit. Cytokinins applied to newly developing navel

oranges have been demonstrated to enhance importation of 14C-labelled

photosynthates from nearby leaves (49). Furthermore, fruit from 'Calamondin'

flowers which had been emasculated and then treated with GA, received a

continuous flow of 14C-labelled photosynthates from nearby leaves as was

also the case with seedy fruit of the same cultivar, but not when flowers

were emasculated (85).

This does not, however, explain why there was a stronger movement

into ovaries and young fruit of the flower dip treatment than when entire

trees were sprayed with the same concentration of GA. One might speculate

that the leaves become more competitive than the fruit for substrates due

to the spray application of GA on the leaves; however dry wt data of the

leaves do not substantiate this hypothesis. GA in the leaves may have,

however, caused a higher rate of respiration, creating a higher demand for

substrates but not causing the accumulation of more dry material than non-

sprayed leaves. Data showing translocation of 14C-photosynthates from

old leaves into new leaves subtending the fruit support this hypothesis.

There was more photosynthate moving from the old leaves into the new

leaves in the GA-spray treatment than in the GA-dip treatment or the

control during the period following anthesis, which appears to be a

critical time in fruit set. This time coincides with the period in which

translocation of current photosynthates to fruit of the GA-dip treatment

was greater than the movement of photosynthates to the GA-spray treat-

ment or the control. Therefore, the GA-spray may have caused an increase

in demand for substrates by the new leaves making them more competitive

for substrates than fruit of that treatment while fruit that had been

dipped in GA became more competitive than the leaves of the treatment which

had not received any GA. This hypothesis could be clarified by an experi-

ment designed to determine the respiration rate of navel orange leaves in

which these treatments had been imposed.

Another possibility is that spraying an entire tree may have created

a multitude of sites in which metabolic gradients were established, but

none had a particular competitive advantage so that the total effect was

similar to the control. Translocation patterns in fruiting shoots of the

GA-spray treatment and the control were very similar throughout the experi-

ment which would support this premise. In addition, the fruit wt data

demonstrate that the control fruits remaining on the tree at the end of

the fruit-setting period were just as large as fruits from the GA-dip

treatment, whereas fruits from the GA-spray treatment were much smaller.

One might assume that the control fruits that set did so because their

ability to compete for substrates was greater than those that did not set.

The few fruits that did survive were apparently as competitive for sub-

strates as fruit that had been dipped in GA. If the premise that a

multitude of sites are activated by spraying the tree with GA be true,

then the competitive advantage of the few fruits on the tree that would

have set without treatment becomes diluted by the many competing sites

and the resulting fruits would be smaller, which was the case in this

experiment. This premise might be determined by dipping varying quan-

tities of flowers on a tree with GA, including 1 treatment in which all

flowers on the tree would be dipped in GA. Another means would be to

compare translocation patterns of the treatments imposed on this experi-

ment in a cultivar, such as 'Orlando', in which fruit-set is increased

by GA-sprays to the translocation patterns of the same treatments on

navel orange. Crane's hypothesis should then be re-evaluated if the trans-

location patterns in the GA-spray treatment of 'Orlando' did not differ

from the control, as in this experiment.

There is a chance that flowers are not sufficiently wet by spraying

navel trees with GA. This could be determined by spraying an entire

tree with GA then dipping selected flowers in the same concentration of

GA and tagging other flowers without dipping to determine if there would

be a difference in fruit set due to treatment.

Many questions relating to the cause of failure of GA sprays to

improve fruit set of navel oranges remain unanswered; however, it is clear

from this study that there was a much stronger flow of metabolites into

fruit from the GA dip treatment, the most effective fruit setting treat-

ment, than into fruit of the other treatments. There appears to be a

critical time in fruit set, following anthesis, in which the principal

source of substrates switches from older leaves to the new leaves sub-

tending the fruit. These data suggest that a smooth transition must take

place causing no interruption in the flow of substrates to the fruit for

fruit set to take place.


1. Large increases in yields have occurred when either navel orange

flowers or fruit were dipped into solutions of gibberellic acid, but GA

has been ineffective when sprayed on entire trees. The role of growth

regulators appears to be related to the supply of leaf-metabolites to

the young fruit, hence this work was to investigate the movement of leaf

metabolites in the 'Washington' navel orange following application of GA

both as flower dips and as whole-tree sprays.

2. More fruit remained at the conclusion of the fruit-setting period,

through the June crop, on the GA-dip plots than on the GA-spray or control


3. Movement of 14C-photosynthate into fruits from the GA-dip treatment

was greater than the GA-spray treatment or the control.

4. There was a stronger movement of 14C-metabolites into the ovaries

and young fruits throughout the fruit-setting period from both current

season and previous season leaves of the GA-dip treatment than for untreated

trees or those sprayed with GA. In addition, the GA-spray treatment and

the control had a period of reduced transport from old leaves to fruits,

following anthesis, before new leaves began exporting significant quan-

tities to fruits.

5. Dry wt of fruits from the GA-dip treatment and the control was much

greater than fruit from the GA-spray treatment.

6. There was bidirectional movement of labelled photosynthate from the

median leaf on the shoot; however, the greatest quantities were transported

acropetally, fruits being the primary site of importation.

7. The strongest fruit-setting treatment maintained a greater movement

of metabolites from the leaves into fruits than other treatments, sub-

stantiating the hypothesis that growth regulators promote fruit set by

enhancing the movement of metabolites into fruits. The failure of whole-

tree sprays of GA to enhance fruit set of navel orange appears to be

due to an inability of the fruit to attract metabolites in sufficient

quantities to maintain fruit growth during the fruit setting period.


1. Abbott, D. L. 1959. Growth substances as fruit thinning agents
for apples: progress report. The effects of seed removal on
the growth of apple fruitlets. In Ann. Rept. Agr. Hort. Res.
Sta., Long Ashton, Bristol, England, 1958, pp. 52-56.

2. Adigun, O. 0. 1972. The influence of seed, Gibberellic Acid and
Girdling on the development of 'Orlando' Tangelo fruit. Doctoral
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H. Donovan Brown was born in Lakeland, Florida on March 21, 1945.

He received his secondary education in Florida and graduated from Lake-

land Senior High School in 1963. He received his Bachelor of Science in

Agriculture degree in 1968 and his Master of Science in Agriculture in

1970 from the University of Florida.

He is a member of the Florida State Horticultural Society, American

Society for Horticultural Science, American Horticultural Society and

Alpha Zeta, Gamma Sigma Delta and Sigma Xi societies.

He is married to the former Shirely Wear and has two sons, Scott

and Todd. The family resides in Lakeland, Florida.

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.

A. H. Krezdorn/ Caran
Professor of ru rps

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.

R. H. Biggs
Professor of Horticulture

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.

T. E. Humphreys
Professor of Botany

This dissertation was submitted to the Graduate Faculty 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

December, 1974

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