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This is to certify that the

thesis entitled

INDUCTION OF PRECOCIOUS FLOWERING
IN AMBURANA CEARENSIS

presented by

PAUL M. MUELLER

has been accepted towards fulfillment
of the requirements for

Masters degree in Forestry

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Major profe . I

Date 07 August 1992

 

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cmmnt

 

INDUCTION OF PRECOCIOUS FLOWERING
IN AMBURANA CEARENSIS

BY
Paul M. Mueller

A THESIS
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Forestry

1992

ABSTRACT

INDUCTION OF PRECOCIOUS FLOWERING
IN AMBURANA CEARENSIS

BY
Paul M. Mueller

A greenhouse experiment was conducted to induce precocious
flowering in seedlings of Amburana cearensis, a high value
tropical hardwood native to South America. Eighteen
treatment combinations of chlormequat (CCC) (three levels),
root pruning (two levels) and girdling (three levels) were
applied to 10 month old trees. None of the treatment
combinations induced flowering. Hypotheses are discussed to
explain the lack of flowering: physiological age of the
trees, environmental conditions and treatment levels are
considered. Analysis of family means for ten month height
growth data showed variation among families. No family
differences in branch angle were detected in 12 month old

trees.

ACKNOWLEDGEMENTS

First and foremost, I would like to thank my major
professor, Dr. Douglas 0. Lantagne. The friendly atmosphere
of his office and the advice and suggestions I received
there helped bring this work to completion. I would also
like to thank my other committee members, Dr. Michael A.
Gold and Dr. Amy F. Iezzoni for their help in its
preparation. I offer sincere thanks and appreciation to all
those who answered my questions and helped in the greenhouse
seeding, measuring, and evaluating. Without their help, I
would probably still be at it. Lastly, I would like to
mention my fellow graduate students, friends and the Friday
Afternoon Club - together they helped keep me from going

bonkers during the past two years.

iii

TABLE OF CONTENTS

LIST OF TABLES O O O O O O O O C O O O O O O O O O
1. INTRODUCTION . . . . . . . . . . . . . . . . . .

2. LITERATURE REVIEW . . . . . . . . . .
2.1 INTRODUCTION . . . . . . . . . .
2. 2 AMBURANA CEARENSIS . . . .
2. 3 ONTOLOGICAL DEVELOPMENT OF TREES

2.3.1 Maturation and ageing . . .
2.3.2 Morphological characteristics of
juvenile and mature trees . . . . .
2.3.3 Physiological characteristics of
juvenile and mature trees . . . . .
2. 3. 4 Transition to the mature stage . .
2. 4 GENETICS OF PHASE CHANGE . . . . . . .
2.4.1 Genetic regulation and differential
gene activation . . . . . . . . . .
2.4.2 Inheritance of the juvenile period
2.5 RESPONSE TO DIFFERENT TREATMENTS .
2.5.1 Chlormequat . . . . . . . .
2.5.2 Girdling . . . . . . . . .
2.5.3 Root pruning . . . . . . .

3. MATERIALS AND METHODS . . . . . . . . . . . . .
3.1 FACILITIES DESCRIPTION . . . . . . . . . .

3. 2 EXPERIMENTAL PROCEDURES . . . . . . . .
3.2.1 Soil and container preparation . .

L 2.2 Seeding and seedling care .

.2 2.1 Watering . . . . . .
2. 2 Staking . . . . . .
.2 3 Environment . . . .
.2.4 Pesticide and fertili
application . . . .
3.2.3 Treatment application . . .
3.2.3.1 Root pruning . . . . . . . .

3.2.3.2 Girdling . . . . . . . . . .

3.2.3.3 Chlormequat . . . . . . . .

3.3 EXPERIMENTAL DESIGN . . . . . . . . . . .
3.4 DATA COLLECTION . . . . . . . . . . . .
3.4.1 Flowering . . . . . . . . . . . .
3.4.2 Height . . . . . . . . . . . . .
3.4.3 Branch angle . . . . . . . . . .

iv

Page

vi

0"

common.»

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14

15
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18
20
21

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23
24
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26
27

27
27
28
28
28
29
30
3O
30
3O

3.5 DATA ANALYSIS . . .
3.5.1 Flowering . .
3.5.2 Height . . .
3.5.3 Branch angle

4. RESULTS . . . . . . . .
4.1 FLORAL INITIATIO
4.2 BRANCH ANGLE . . .
4.3 TREE HEIGHT . . .

5. DISCUSSION . . . . . . . .
5.1 FLORAL INITIATION .

5.1.1 Stage of maturation
5.1.2 Treatment levels . . .

5.1.3 Environmental
5.2 BRANCH ANGLE . . . .
5.3 TREE HEIGHT . . . .
5.4 FASCIATION . . . . .

6. SUMMARY . . . . . . . . .

7. LITERATURE CITED . . . . .

conditions

31
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31

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34

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36
36
37
38
39
4O
41

42

45

Table

Table

Table

Table

Table

Table

LIST OF TABLES

Trébol Recuperation Project accession numbers
and geographic origin of seed of Amburana .
GeareHSiSeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeeee

Composition of nutrient solution...... .........

Source, df and EMS for main stem branch

angleCOOOCOO0......00.0.0.0...00.000.000.000...

Analysis of variance for main stem branch

angle...‘IO..00...O...OOOOOOOOOOOOOOO0.00000...

Ranked family means for branch angle...........

Ranked Family means for ten month total height
growth ................................... . .....

vi

25

26

32

33_

34

35

1 INTRODUCTION

For decades, Paraguay has used its forest resources
indiscriminately and unsustainably. Because of this,
Paraguay's most valuable timber species, Amburana cearensis
(trébol), has been depleted to the point of commercial
extinction. Degradation of the population is so severe that
recovery of the species as an integral part of the forest
industry is questionable. Trébol populations are also
severely depleted in other countries of South America such
as Bolivia (F. Bascopé, personal communication 1989) and

Peru (A. Chung, personal communication 1989).

Once the most sought after species by loggers and consumers,
A. cearensis has now disappeared from the Paraguayan market.
In 1990, the Paraguayan Data Conservation Center classified
A. cearensis as a species in danger of extinction. Although
a new Paraguayan law (1991) prohibits cutting of the
species, its population is likely to continue to decline for
three reasons. First, although the intent of the law is
sound, it is unlikely that the Paraguayan National Forest
Service has the resources to enforce it; second, populations

of young, unmerchantable trees will continue to be destroyed

2
through deforestation, and third, large trees left uncut
after removal of the surrounding forest are threatened by

fire and windthrow.

In 1986, the Trébol Recuperation Project (TRP) was started
in an effort to restore the tree to commercial status, re-
establish its position in the natural forest, and provide an
opportunity for long term economic diversity for area
farmers and ranchers. The TRP concentrated its initial
efforts in collecting and developing information in the
following areas; phenology, nursery production techniques,
spacing for agroforestry and traditional plantation designs,
plantation management, education and gene conservation.
Long-term goals of the TRP included progeny and provenance
testing, establishment of seed production areas, and
breeding. Due to the highly degraded state of A. cearensis
populations in Paraguay, breeding may become one of the more
important of these long term goals if it is to be returned
to a position of economic importance in Paraguay and

elsewhere in its natural range.

Successful breeding of A. cearensis will be impacted in part
by the number of individuals in the breeding population. In
this light, maintaining a highly diverse genetic base of A.
cearensis is of utmost importance. The frequency of

phenotypically superior individuals in the population also

3
influences gain made through breeding. As the frequency of
phenotypically superior individuals decreases, so does the
frequency of alleles that favorably influence the traits of
interest (assuming the trait is a heritable one). As a
result, the probability of finding and capturing those
alleles decreases. Not only is the present overall
population of A. cearensis extremely low in Paraguay, but

phenotypically good individuals are almost non-existent.

Another problem with a trébol breeding program is the long
generation time of the species. Many tropical species, such
as Leucaena leucocephala (NAS, 1977) and Parapiptadenia
rigida (Mueller, personal observation) flower when less than
four years old. The generation time for A. cearensis is not
known, but is thought to be at least 15 years. If the time
from seed to flower can be reduced in A. cearensis, the
efficiency of a breeding program could be increased by
reducing the time between successive generations. Other
benefits of a reduced juvenile period include the ease of
study of floral biology and natural pollination on small
trees, and enhanced seed yields from seed production areas
by inducing flowering in years that the tree does not flower
naturally. Although the benefits of success are
substantial, the induction of precocious flowering is
difficult to achieve and has never been tried on A.

cearensis. There is little published information in the

4
literature on this subject for hardwood forest tree species.
Most work on precocious flowering has been done with the
woody vine Hedera helix L., however Arshad (1980), Johnsson

(1949) and others have conducted studies with Betula spp.

The objective of this study is to stimulate precocious
flowering in A. cearensis using chemical and cultural

treatments.

2 LITERATURE REVIEW

2.1 INTRODUCTION

Forest tree breeding for desired characteristics such as
form, wood quality and growth rate is a time consuming
process. Poor juvenile/mature correlation for important
traits and the generally long time required for forest trees
to flower are two key reasons. The period from germination
to sexual maturity is termed the juvenile stage (Zimmerman,
1973). Some species, such as European beech (Fagus
sylvatica) do not flower until age 30 to 40 years (Metzger,
1988). Amburana cearensis has been observed to require at

least 15 years to flower in plantations.

Transition from the juvenile stage to the adult stage is
influenced by a number of factors, although the exact
mechanisms are not well understood (Zimmerman, Hackett, and
Pharis, 1985; Zimmerman, 1976). Factors include
chronological and physiological age, (Fortanier and Jonkers,
1976), nutritional condition of the plant (Hackett, 1976),
environmental conditions (Wareing and Frydman, 1976), size
(Zimmerman, 1972), genotype (Johnsson, 1949; Poethig, 1990)
and hormone levels (Hackett, 1976). Experiments that deal
with shortening the juvenile stage have produced varied
results. Certain treatments may produce positive results in

one species, but not in another (Zimmerman, 1972).

6
The greatest chance for success in working with trees comes
not only from a thorough understanding of relevant
literature, but also from an understanding of the species in
question (Hanover, 1991 personal communication). Little is
known about the ontogeny of Amburana cearensis.
Phenological studies have been initiated by the Trébol
Recuperation Project in Paraguay, although no conclusions
have been drawn from these observations. The rationale
behind the treatments applied in this experiment comes
mainly from the literature, but also from the experience of

the author in his work with the species.

2.2 AMBURANA CEARENSIS

Amburana cearensis (LEGUMINOSAE), commonly known as trébol
in Paraguay’, is a large sized, deciduous hardwood tree. It
grows to 25 meters in height, and up to 110 cm in diameter
at breast height. It has dark green foliage, with a rounded
crown. Grown with competition, it can produce straight
boles up to 12 meters long. Grown in the open, it produces

a wide, spreading crown and makes an excellent shade tree.

In Paraguay the tree is found principally between the

Aquidaban and Apa Rivers in the North of the Regién

 

' Also known as yvyra piriri guazu (Paraguay), tumi,serioco
and roble (Bolivia), palo trébol, roble del pais and roble
criollo (Argentina), cerejeira, umbruna, imbruna do cheiro,
cumaré and cumaru (Brazil) (Whitmore, et.al., 1990).

7
Oriental, and occasionally in the departments of Chaco,
Presidente Hayes, and Alto Paraguay. It prefers sandy-clay
soils, and is frequently found on calcareous soils (LOpez,

1987).

Leaves of A. cearensis are pinnate and alternate when adult.
Inflorescences are racimes, 2.0 to 7.0 cm long arising from
the leaf axils. Flowers are off-white with one petal.
Fruits range from 5.0 to 7.0 cm long, and contain one winged
seed. Trébol flowers and seeds sporadically, although
exceptions exist. One tree has been reported to flower
yearly, but to varying degrees. Seed production by this
tree ranges from zero to heavy, not necessarily
corresponding to the intensity of flowering (Mueller,

personal observation).

The wood of A. cearensis is moderately heavy, with a
specific gravity of 0.550 Kg/dm3 (Chudnoff, 1984). It is
easily dried and worked, and is prized for quality furniture
production. The tree has no other principal uses, although

it can be burned as fuelwood.

Trébol is the most valuable species in Paraguay, often
bringing 200% of the cost of other commercially important
species. It is also valued in other countries to which it

is native, although specific dollar values are not

8
available. No figures are available on its worth on the

international market.

2.3 QNIOLQGIQAL QEZELQPMENI OF TREES
2.3.1 Maturation and ageing

As a tree grows it undergoes a series of developmental
changes, or stages. Changes associated with the sexual
development of the tree as well as certain morphological and
physiological traits such as thorniness, leaf shape,
phyllotaxy, disease resistance and ability to root are
referred to as maturation (Wareing, 1959; Zimmerman, 1972;
Poethig, 1990). Changes in morphological complexity
(increased branching), vigor and growth rate are considered

a result of ageing (Poethig, 1990; Zimmerman, et al., 1985).

Developmental changes due to maturation are generally not
easily reversible. Nonetheless, since seedlings grown from
the seed of mature trees are juvenile, reversion must take
place during sexual regeneration (Brink, 1962). Work has
been done to stimulate rejuvenation in a number of species
(Muzik and Cruzado, 1958; Rogler and Dahmas, 1974;
Greenwood, 1987), and has been observed in naturally grown

trees (Brink, 1962).

In contrast, the effects of ageing are generally easily

reversible. For example, a mature twig grafted onto

9
juvenile rootstock will exhibit increased vigor and a
reduction in branching due to reversal of ageing, but will
retain the characteristics of a mature tree, such as mature

leaf form and the ability to flower.

The continued ability to flower signifies that a plant is in
the adult stage (Hackett, 1985). However, a tree may not
flower for a number of years after reaching maturity if all
other requisite conditions such as photoperiod, temperature,
and moisture level are not first met (Zimmerman, 1973).
Zimmerman (1973) terms the time from the end of the juvenile
state to the first appearance of flowers (adult state) the
transitional stage. Poethig (1990) uses the terms juvenile
vegetative, adult vegetative (which would correspond to
Zimmerman's transitional stage) and reproductive stage.
Regardless of terminology used, the major identifying factor
signaling the attainment of the adult (reproductive) stage

is the ability to flower (Borchert, 1976).

It is possible, however, to induce plants to flower while
still in the juvenile stage. Furr, et a1. (1947) have shown
that precocious flowering in three year old citrus is
possible by ringing, and that anomalous flowering can occur
at ages as young as three months. These results however, do
not necessarily signify that the treatment has induced phase

change; removal of the stimulus and subsequent growth of the

lO
plant under normal conditions may not produce flowers for

many years (Furr, et al., 1947).

The progression of a tree through stages of maturation and
ageing are characterized by a number of traits. Marking
the transition of the plant through these stages are
morphological (Poethig, 1990), physiological (Fortanier and
Jonkers, 1976) and genetic (Wareing and Frydman, 1976)

changes in the plant.

2.3.2 Morphological characteristics of juvenile and mature

£23.85:

In some tree species distinct morphological changes take
place as maturity is reached. These changes may include
variation in leaf shape and size, thorniness and phyllotaxy.
The changes that occur vary widely between species, with
some species exhibiting more dramatic changes than others,
while other species show no apparent change in morphology.
Occasionally these changes can be used as indicators of the

maturity of an individual.

The woody vine English Ivy (Hedera helix L.) has a number of
morphological characteristics that can be used to
distinguish mature from juvenile plants. These include
phylotaxy, stem pubescence and plagiotropic versus
oligotrophic growth. Species of the tropical genus

Eucalyptus typically exhibit marked morphological changes

11
between juvenile and adult plants. In most Eucalypts,
juvenile trees exhibit decussate phylotaxis, changing to
spiral as they mature. In addition, four heterophyllous
stages are recognized during the maturation of the genus:
seedling, juvenile, intermediate and adult (Boland, et al.,
1984). Phylotaxis of A. cearensis changes from spiral to
opposite during the seedling stage. How this change is
related to the sexual development of the species is not yet
known. Gleditsia triacanthos (Chase, 1947) as with many
species of the genus Citrus, are characterized in the
juvenile stage by the production of thorns. As the trees

mature, fewer thorns are produced (Furr, et al., 1947).

2.3.3 Physiological characteristics of juvenile and mature
trees

Physiological changes in a tree, such as shifts in hormone
production as it passes from juvenile to adult, are
difficult to quantify. To date, little work has been done
in this area (Zeevart, personal communication, 1991).
Nonetheless, phenomenon that reflect at least some of these
physiological changes are rooting ability, leaf retention

and disease resistance.

Western red cedar's (Thuja plicata) resistance to the leaf
blight fungus Didymascella thujina (Dur.) Maire, has been

shown by Soegaard (1956), to be related to maturity.

12
Juvenile Western red cedar (< four years old) are more

susceptible than older, more mature trees.

In many species of woody plants, cuttings from juvenile
plants, or juvenile parts of mature plants will root easier
than cuttings from mature tissue (Greenwood, 1987). Nolasco
and Schwyzer (1985) showed that basal portions of young
trees of A. cearensis rooted better than terminal portions.
In addition, juvenile tissues in beech (Fagus spp.) and oak
(Quercus spp.) retain leaves during the dormant season while

leaves abscise from mature tissue (Wareing, 1959).

Leaf retention, the ability of cuttings to root, and disease
resistance could possibly be explained by changes in
hormonal balances/concentrations in the plant. However,
research on linking particular hormones to phase change has
been inconclusive (Metzger, 1987) due principally to the
complex nature of the problem. In addition, whether changes
in hormone levels are a result of or the cause for phase
change is unclear. The five known hormones: auxin,
gibberellins, ethylene, cytokinins and abscisic acid, each
have specific and at the same time overlapping roles in the
growth and development of a tree. The role of hormones in
the growth and development of plants has been thoroughly
reviewed in the literature by Davies (1987), Wareing (1982)

and others and therefore will not be covered here.

13
2.3.4 Transitigg tg the mature stage
The majority of the literature focuses on two requisites for
a plant to mature: growth through a predetermined number of
cycles, or attainment of a minimal critical size. The
latter appears to be the currently favored hypothesis
(Hackett, 1985), although no definitive proof has been
presented to account for it. Other factors, such as
distance of the meristem from the roots, the number of cell
divisions gone through by the meristem (Robinson and
Wareing, 1969), hormone balance (Srinivasan and Mullins,
1981) and nutritional status (Sachs, 1977) may also play a

role in triggering phase change.

Relative distance of the meristem from the roots may factor
into the timing of transition. Roots supply hormones such
as cytokinins (Davies, 1987) and possibly gibberellins
(Davies, 1987; Arshad, 1980)) to the upper parts of the
plant. These may inhibit initiation of flower primordia.
As the meristem's distance from the roots increases, a
diffusing factor could play a role in minimizing their

effect as a flower inhibitor.

Conversely, the shoots may direct translocation of certain
hormones to the roots such as gibberellins and abscisic
acid. These substances may act in the root to inhibit

flower formation. Their effectiveness in the root may

14
decrease as the meristem's distance from the root increases.
Arshad (1980) showed that IAA, which is synthesized in the
shoot, may have at least partial control over the basipital

movement of GA through the stem.

2.4 GENETICS OF PHASE CHANGE

Ultimately, all a plant's functions are controlled either
directly or indirectly, wholly or partly by the genetic
makeup of the individual. It is not unlikely that the
prerequisites for the initiation of floral primordia, be it
photoperiod, age or other conditions, trigger transcription
and/or translation of certain genes (Metzger, 1988). The
product of these genes may then be what initiates phase

change, not the prerequisite conditions as we see them.

Studies have been conducted to identify genes that exhibit
control over various aspects of flowering (Reid, 1979;
Johnsson, 1949). Research on the heritability of the
juvenile period has also been conducted (Greene, 1991;
Couranjou, et al., 1988), which can be important in breeding

programs.

The current study deals with mechanical and chemical
treatments to disrupt the physiological processes in
Amburana cearensis to stimulate flowering. Nonetheless,

information in the following section on genetic control and

15
heritability is useful as background in understanding the

problem.

2.4.1 Genetic regulation and differential gene activation

Genetic control of the length of the juvenile stage appears
to be complex and varies between species. In peas the Sn,
Hr and E alleles interact in control of photosensitivity,
age and size of the plant as regards flowering (Reid, 1979;
Kelly and Davies, 1986). The number of days from
germination to flowering in dry bean (Phaseolus vulgaris L.)
is controlled by few genes but is also under control of
photoperiod and temperature (Wallace, et a1. 1991) which may
implicate additional genes. Conversely, Johnsson (1949)
found segregation ratios for early flowering in Betula
verrucosa and B. pubescens that suggested control by single,
dominant genes. Teich and Holst (1969) also found Mendelian
ratios for precocious flowering in scotch pine (Pinus

sylvestris).

Genes connected to phase change in corn mutants have been
investigated by Poethig (1988). He succeeded in identifying
a mutant gene that may control the synthesis or distribution
of a diffusible factor that maintains the juvenile phase.

It may be hypothesized then, that the product of the wild-
type form of this gene triggers transition to the mature, or

reproductive phase.

16
Meeks-Wagner, et al. (1989) showed differential
transcription in tobacco roots between mature and juvenile
plants, indicating that activation of specific genes in the

root may trigger phase change.

What is not clear in any of the examples above is what
regulates the activity of the genes in question. The most
likely answer, however is that a combination of factors is

responsible.

It is known that at least some hormones can have a
regulatory effect over particular genes. Gibberellin (GAg
regulates mRNA synthesis of the a-amylase gene (Jacobsen, et
al., 1982), and auxin has been shown to regulate
transcription of a number of soybean genes (Hagen, 1987).

It is possible therefore, that hormone balance and/or
distribution may ultimately be the catalyst for phase change

through gene regulation.

2.4.2 Inheritance of the juvenile period

Although studies such as the present one may result in
temporary induction of flowering, permanent shifts in
reproductive status are desirable. Selecting and breeding

for shorter juvenile periods can result in earlier flowering

progeny.

17
Variation in the length of the juvenile period exists in a
number of temperate tree species. Where sufficient
variation coupled with adequate heritabilities exist,
breeding for the trait can be worthwhile. Variability has
been shown in birch (Betula spp.) (Johnsson, 1949; Huhtinen,
1976), apple (Visser, 1976), pear (Bell and Zimmerman, 1990;
Visser, 1976; Zielinski, 1963), longleaf pine (Pinus
palustris)(Boyer and Evans, 1967), scotch pine (Teich and
Holst, 1969) and southern magnolia (Magnolia

grandiflora)(Greene, 1991).

Heritability, the ratio of additive genetic variance to
total phenotypic variance (Falconer, 1989), is used in
calculating gain in breeding programs. High heritability
estimates with the same selection intensity will give
greater gain than low estimates. Breeding experiments have
shown heritability of the juvenile period to be generally
high, but variable among species. Greene (1991) estimated
family heritability of early flowering in 36 families of
magnolia to be greater than 0.80. Couranjou, et al. (1988)
found heritability to be from 0.8 to 0.9 in 7 apricot
varieties using parent-offspring regression. In working
with pear and apple, both Visser (1976) and Bell and
Zimmerman (1990) found control of the juvenile period to be
principally through additive gene action as indicated by

high general combining ability and low specific combining

18
ability. Maternal (cytoplasmic) effects have been shown in
cherry (Prunus sp.)(Schmidt, 1976), scotch pine (Teich and
Holst, 1969). In these species, it may therefore be
important to designate individual plants as male or female

parent.

In summary, research in a number of species indicates that
genetic variability exists for length of the juvenile
period, and that improvement is possible. High heritability
estimates for several species, in addition to indications
that principal control is through additive gene action,
suggest that efficient breeding programs can be developed to

shorten the time to flowering.

2.5 RESPONSE TO DIFFERENT TREATMENTS
Three treatments in eighteen different combinations were
applied to A. cearensis seedlings. The following section

discusses these treatments and their major known effects.

2.5.1 Chlormeguat

Chlormequat, also known as CCC, Cycocel or Chlorocholine
chloride, ([2-Chloroethyl]trimethylammonium chloride) is a
growth retardant used in agricultural crops to reduce
lodging (Sponsel, 1988; Nickell, 1979) by stunting
vegetative growth. CCC acts in the plant by inhibiting

activity of ent-kaurene synthetase A, an enzyme in the

19
biosynthetic pathway of gibberellic acid (GA) (Sponsel,
1988). Eht-kaurene synthetase A catalyzes the conversion of
geranylgeranyl pyrophosphate (GGPP) to copalyl pyrophosphate
(CPP), an intermediary in the process. Treatment with CCC
should therefore result in a reduction of endogenous
gibberellins. Hedden (1988) however, states that the exact
site of action of CCC is yet unclear, and that it may also
act on other pathways not related to GA but that inhibit
growth. Arshad (1980) found higher levels of gibberellin-
like compounds in the bark of CCC treated birch, although no
mention was made of growth. Reid and Crozier (1970) found
that small concentrations of CCC (1 ppm) increased levels of
GA in pea plants (Pisium sativum L. cv. Alaska), whereas

higher concentrations (1000 ppm) reduced levels.

In addition to growth retardation, CCC has also been shown
to affect flowering in a number of species. In hollyhock
(Althaea rosea cv. Summer Carnival), CCC was shown to
increase total flowers per plant, and inhibit flower
abortion (Tezuka, et al., 1989). Arshad (1980) found that
application of CCC in combination with stem girdling
increased flowering in seedlings of Betula pendula.
Treatment of Mattiola incana and Althaea rosea with CCC
induced flowering and increased levels of nitrogen,
phosphorus and potassium in the shoots (Hamza and Helaly,

1983).

20
Although CCC has been shown to induce early flowering in
several species, the exact pathway by which this is
accomplished is not known. Hormone balance and nutritional
status, two factors thought to play a role in phase change
(see section 2.1), are affected by CCC. How CCC affects
these and/or other physiological aspects in trébol is not

known.

2.5.2 Girdling

Girdling, (ringing, scoring) a tree is the act of removing
the phloem in a continuous band around the bole (or branch).
When girdled around the bole, transport of carbohydrates to
the roots is stopped, killing the tree. Girdling upsets the
balance of both carbohydrates and growth regulators (Noel,

1970), and can have a promotory effect as regards flowering.

Lahav, et al. (1986) showed that girdling increased the
number of flowering trees in 9 avocado (Persea americana)
crosses. They concluded that girdling shortened the
juvenile stage, depending on time of girdle. The validity
of this conclusion is doubtful since 47.3% of the control
plants also flowered. Dennis (1967) showed that scoring
increased flowering on those apple progenies that had
already commenced to flower, but had no effect on pear
seedlings. He concluded that scoring did not affect

juvenility per se, but may induce trees in the transition

21

period to flower slightly sooner than normal. Arshad (1980)
found that girdling induced flower initiation in young birch
(Betula pendula) seedlings. He inferred that floral
induction was due to either an accumulation of metabolites
above the girdle, or to interruption of the supply of
photosynthates to the root. Starvation of the root may have
caused a reduction in a flower inhibitor synthesized there

and translocated to the shoots (Arshad, 1980).

2.5.3 Root pruning

Root pruning results in two major changes in a plant.
First, there is a reduction in transport of hormones from
the root to the shoots, principally cytokinins and
gibberellins. Second, due to subsequent increased root
growth there is an increase in the translocation of
photosynthates to the roots (Richards, 1986). It has been
shown that root pruning apple trees can increase flowering
(Geisler and Ferree, 1984). Most of these studies have
dealt with already mature trees and the effect therefore is

not necessarily related to phase change.

Hendry, et al. (1982) have shown that cytokinin levels in
buds of juvenile citrus plants (Citrus sinensis (L.) Osbeck)
were higher than levels in mature plants. Since the
experiment was conducted with grafted material on same age

rootstock, the site of control of either cytokinin

22
production or translocation appears to reside in the shoot.
Regardless, root pruning used in combination with other
treatments that are known to affect juvenile-mature

transition may stimulate the expansion of flower initials.

After a time lag, pruned roots will produce a flush of new
growth. This increase in root growth creates a demand for
photosynthates from the crown. It has been shown by Kramer
and Kozlowski (1979) that an increase in demand for
photosynthates by the root will result in an increase in
photosynthetic rate. Floral initiation is dependent in part
on high levels of photosynthates (Kozlowski, 1971). By
creating a demand in the root, increased photosynthesis in
the shoots may produce an excess of photosynthates which
could be used to set floral initials. Nonetheless, research

to support this hypothesis was not found.

3 MATERIALS AND METHODS

3-1 EACILIII§§_D§§QBIEIIQH

The study was conducted at Michigan State University in East
Lansing, Michigan at the Department of Forestry's Tree
Research Center. It was initiated in mid-June 1991 and
terminated in mid-June 1992. The entire study was conducted
in a greenhouse because A. cearensis is a tropical to sub-

tropical species native to South America.

The greenhouse was of domed fiberglass construction. Heat
was provided by two gas burners at the east end of the house
from September through June. Cooling during summer months
was provided by variable speed exhaust fans and evaporative
cooling pads. Supplemental lighting was used to maintain a
day length of approximately 13 hours. Lights were on during
the entire 13 hour photoperiod. Regular fluorescent lights
of 110 watts were used over the first seven months. These
lights proved to provide insufficient light during the
winter months. Six additional fluorescent fixtures (12
bulbs) were installed on 14 January 1992. All bulbs in all
fixtures were changed to 215 watts for the remainder of the

experiment.

23

24
3.2 EXPERIMENTAL PROCEDURES
Stem girdling, root pruning and chemical treatment in a
3x2x3 factorial were applied to 10 month old container grown

A. cearensis seedlings to induce precocious flowering.

3.2.1 Soii and coptaine; preparation

Seedling containers measured 10x10x58 cm, and were made of
heavy gauge waxed paper. Nine containers were placed in
each of 54 30x30x58 cm plastic cases. The 58 cm plastic
cases were constructed by removing the bottom of one 29 cm
case, and stacking it on a second case. This was done to
accommodate the typically long tap root of the species. The
containers were filled with a prepared potting mix of 75%
peat and spaghnum moss and 25% perlite. To reduce soil
settling after seeding, the soil was compacted by dropping
the cases several times during filling. Cases were then
placed on pallets approximately 10 cm high to facilitate
drainage and air pruning of the roots. Cases were staggered
on the pallets by a space equivalent to one case to allow

for crown development.

3.2.2 Seeding and seedling care

Seed of A. cearensis was obtained from the School of
Forestry at the National University of Asuncién and the
Trébol Recuperation Project in Paraguay. Nine open

pollinated half-sib families were used for the study (Table

25
1). One seed was planted in each container on 10 June 1991,
and covered with approximately 0.5 cm of soil. Each family
was represented once per case. Trébol has a permeable seed
coat so seed scarification was unnecessary.

Table 1. Trébol Recuperation Project accession numbers and
geographic origin of seed of Amburana cearensis.

 

 

Paraguayan

Accession number Location

021.001 Estancia Errante Tranquerita,
Concepcién, Paraguay

021.002 Peguajé-San Fransisco, Concepcién,
Paraguay

021.003 Invernadero La Novia, Arroyito,
Concepcién, Paraguay

021.004 Escuela Agropecuaria de Concepcién,
Concepcién, Paraguay

021.006 Colonia José Félix LOpez (Peuntesino),
Concepcién, Paraguay

021.007 Naranjaty, Concepcién, Paraguay

021.008 Yvy ja'y, Amambay, Paraguay

021.009 Alfonso cué, Concepcién, Paraguay

021.010 Calle 16 - Norte, Arroyito, Concepcién,
Paraguay

 

3.2.2.1 Watering

Containers were watered after seeding to a depth of several
centimeters. Containers were then watered as necessary to
maintain a suitable soil moisture level for germination. On
26, 27, and 28 July 1991 after germination had occurred, all
plants were again watered to ensure that the soil mixture
throughout the container was moist. All water added to this
point was regular tap water. All watering subsequent to

this point was with a nutrient solution (Table 2).

26

Table 2. Composition of nutrient solution.

 

 

Concentration
Nutrient (ppm)
Nitrogen 200
Potassium 100
Phosphorus 107
Magnesium 1.1
Iron .75
Manganese .42
Boron .15
Zinc .12
Copper .07
Molybdenum .07

 

Frequency of watering after these dates was approximately
once per week from 28 July 1991 through 12 August 1991. In
early August it was determined that tap roots were not
developing as expected, and overwatering was the suspected
cause. In an effort to stimulate tap root development no
watering occurred from 12 August to 22 October 1991. After
22 October 1991 through to the end of the experiment the

plants were watered every three weeks.

3.2.2.2 Staking

In late July trees were staked with bamboo poles to maintain
the plants in an upright position. Low light levels and
lack of wind stimulation were suspected causes of spindly
tree growth. Trees continued to be staked as necessary over

the study period.

27
3.2.2.3 Engirpppppp
Air temperatures were monitored daily during the initial
stages of germination and growth, and then weekly to monitor
maximum and minimum temperatures. Mean air temperature
during the germination period was approximately 34.2 degrees
celsius (C). Overall summer temperatures during 1991 ranged
from 27.2°C to 38.9°C. Winter greenhouse temperatures ranged
from 7.8°C to 30.0°C. Target temperatures were approximately
32.2°C. The heaters were unable to adequately heat the

greenhouse during the winter months.

3.2.2.4 Pesticide and ertilizer a lication

Fungicides (captan and metalaxyl) to control damping off and
other soil borne fungi were applied approximately two weeks
after seeding. Nicotine gas bombs were used three times to
control insects, and two miticides, dienochlor and
fluvalinate were used twice to control mites. Osmocote
brand slow release 18-6-12 fertilizer (15 grams/tree) was
also applied two weeks after seeding, and again on 30

January 1992 at 6 grams per tree.

3.2.3 Treatment application

Treatments were applied on 14 and 15 April 1992. The trees

were approximately 10 months old and averaged 88.5 cm tall.

28
3.2.3.1 Bppp_ppppipg
Two levels of root pruning were applied; hand pruned at the
case interface (approximately 50% of the root length), and
air pruned out the bottom of the containers. Hand pruned
roots were cut with a sharp machete, slicing through the
containers and roots at the case interface. Cases were

lifted apart to assure that all roots had been cut.

3.2.3.2 Girdling

Three levels of stem girdling were applied with grafting
knives: 0, 50 and 90 percent of the circumference. Girdles
were one centimeter wide, starting at a height of 10 cm from
the soil surface. Any branches that originated below 10 cm
on the stem were also girdled at the same level (0, 50, 90%)
as the main stem. Percentages of circumference were
ocularly estimated. In most cases the bark slipped off
easily. In cases where the bark was not easily removed, the
girdled area was scraped until no inner bark remained.
Callus tissue was prevented from bridging the girdle during
the experiment. Sprouts originating below the girdle were

removed.

3.2.3.3 Chlormeguat
Three levels of Chlormequat (CCC) were applied: 0, 750 and

1500 ppm. CCC was dissolved in tap water and applied as a

soil drench in two 0.5 liter applications one week apart.

29
Two applications were used to assure that the solution did
not penetrate past the case interface. Solution penetrating
past the case interface in root pruned treatments would not

have been available for uptake.

CCC solutions were prepared separately for each replication
and applied to each tree individually. Tap water was added
to those treatments that received 0 ppm CCC. Before

treatment initiation trees were not watered for six weeks to

increase water stress and encourage uptake of the solution.

3.3 EXPERIMENTAL DESIGN

The experiment was established as a randomized complete
block factorial (3x2x3) design with four replications. A
total of 18 treatments were tested.‘ Each treatment
contained nine trees. Treatments were randomly assigned to
each case of nine trees. Families were randomly assigned to
containers within each case. Initially, each treatment
contained one representative of each family. Due to poor
germination, only enough plants for three replications were
available. Seedlings from the fourth replication were used
to replant empty pots in replications one, two and three.
Because germination was not equal across all families, final
representation of families was not equal across
replications. Inequality of families across replications

did not affect tests of significance for flowering, branch

3O
angle or height since these analyses were conducted as
though bulked seed had been used. Family identity was
maintained throughout the experiment to facilitate the
identification of individuals showing better than average or

unusual growth.

3.4 DATA COLLECTION
3.4.1 Flowering

At the conclusion of the experiment, each tree was
individually checked for flowers, or flower buds arising
from leaf axils. Treatment values were the sum of the

racimes on the individual plants within each treatment.

3.4.2 Height

Total height of each plant, from soil level to the terminal
meristem, was measured monthly. Total height was recorded

to the nearest tenth of a centimeter.

3.4.3 Branch angle

Main stem branch angle was measured on four randomly
selected trees of each family in each replication. Inside
branch angle was measured in degrees from the main stem
using clear plastic protractors. The first three branches

were measured on each of the trees selected.

31

3.5 DATA ANALXSIS
3.5.1 Eiowepipg

Flowering data was to be analyzed as a completely randomized

block design with three replications of 18 treatments.

3.5.2 Height

Statistical analysis of family means for ten month height
growth was not possible because staking may have influenced
total heights among families. Family differences in height
growth detected by statistical analysis would not
necessarily reflect true differences. Simple means were
used as an indicator of possible differences in family

height growth.

3.5.3 Branch angle

The angle at which a branch arises from the main stem can
influence the quality of the lumber produced. Branch angles
on 12 month old seedlings were measured to provide an

estimate of juvenile variation among and within families.

Branch angle data were analyzed as a completely randomized
block design with sub-samples (Table 3). There were three
replications of eight treatments (families)(Table 1).
Family 021-008 was not used in the analysis because
approximately 33% of the trees measured had no branches.

Data were checked for normality using the Kolmogorov-Smirnov

32

Table 3. Source, df and EMS for main stem branch angle.

 

 

Source df EMS

Total 95

Rep. 2 03+To§+TFof
Family 7 afi+Taf,+TRo§
Family x rep. 14 03+To§
Trees w/in plot 72 03

 

two sample test (Steel and Torrie, 1980). Variance
components and small sense individual tree and family
heritabilities (hz) were also estimated. Individual tree h2
was estimated using the following formula from Zobel and

Talbert (1984)

 

 

 

2_ at
hr 02 02
_w If +02:
tr t
(Zobel and Talbert, 1984).
where: of = 4o} 0% = family x replication
0% = a; + 0% + 0% variance
of = family variance 0: = within plot variance

t = number of trees r = number of replications
per plot

4 RESULTS

4.1 ELQRAL INITiATION

No flowers or flower buds were observed during the
experiment. Possible reasons for the failure of the plants
to respond are varied, and will be discussed in section 5.
Information not related to flowering was also gathered on

the species, and is presented below.

4.2 BRANCH ANGLE

Analysis of variance for family differences in main stem
branch angle showed significant differences at the five
percent level of confidence for family x replication

interactions (Table 4).

Table 4. Analysis of variance for main stem branch angle.

 

 

 

sum of mean
Source df sguares sguare. F valueiEMS
Total 92
Rep. 2 94.41 63+To§+TFaf
Family 7 550.43 78.63 0.74 afi+Ta§+TRo§
Family x rep. 14 1479.64 105.69 2.19‘ 63+Tofi
Trees w/in plot 69 3333.32 48.31 03

 

* Significant at a = 0.05

33

34
Table 5 contains ranked family means for branch angle across

replications.

Table 5. Ranked family means for branch angle.

 

Branch angle

 

Eamilv (degrees)
021-002 62.7 i 5.9
021-007 59.1 i 6.5
021-006 58.4 i 7.2
021-003 57.5 i 8.2
021-001 57.3 i 6.6
021-009 56.7 i 7.1
021-004 55.0 i 8.2
021-010 54.4 110.1

 

Individual tree heritability estimate was -0.15. Family
heritability estimate was -0.41. Negative heritabilities
are a result of the failure of statistics to accurately
estimate the amount of variation present as the variability
between families approaches zero. Since negative
heritabilities are theoretically impossible, both these
estimates can be assumed to be zero and will not be

discussed further.

4.3 TREE HEIGHT

A comparison of family means at 10 months of age showed that
genetic variation in early height growth may be present.
Family 021-008 was 146% of the population mean, while the
second ranked family, 021-007, was 104% of the population
mean. Family means ranged from 77% to 146% of the

population total (Table 6).

35

Table 6. Ranked Family means for ten month total height

 

 

growth.

Number of Mean height % of
Eamily individuais (cm) pop. meap
021.008 58 164.9 143.9 146
021.007 54 117.6 $36.2 104
021.001 77 113.1 137.0 100
021.002 26 105.5 133.2 94
021.009 56 103.7 129.0 92
021.006 74 102.2 126.8 91
021.004 59 100.2 132.6 89
021.003 39 93.4 126.5 83
021.010 20 86.4 123.5 77

 

5 DISCUSSION

5-1 ELQBAL_INITIAIIQN

It would be difficult, if not impossible, to determine why
the trees did not respond as expected to the treatments.
Several hypotheses, however, can be drawn to explain the
results based on different aspects of the experiment. It
should be noted that each of these aspects are discussed
individually. Nonetheless, they are all related and the
observed results are likely a consequence of many

interactions between them.

5.1.1 Stage of maturation

The appearance of flowers is typically used as an indicator
of maturity. No observable indicators, however, have yet
been identified to distinguish the end of the juvenile stage
or the end of the transitional stage. Supposedly,
transitional trees are "ready" to flower, but only lack the
necessary external stimuli to do so. Since it is currently
impossible to distinguish between the two phases the problem
becomes apparent: studies that have induced "juvenile" trees
to flower may have actually induced trees in the
transitional stage to flower. Successful treatments in one
species may fail to produce results in another of the same
chronological age due to the developmental differences

between the species.

36

37
Although the treatments used in this study induced flowering
in Betula pendula seedlings (Arshad, 1980), they did not in
similarly aged seedlings of A. cearensis. It may be
hypothesized that B. pendula seedlings were actually
transitional as regards physiological development, and A.
cearensis was juvenile, thereby rendering the treatments

ineffective.

5.1.2 Treatment levels

Physiological differences between species give rise to
variation in developmental processes. Artificial stimuli
are therefore likely to produce different reactions in
different species. The treatment levels applied in this
study were based on previous studies and a review of related
literature. Since work had never been carried out with A.
cearensis, knowledge of appropriate treatment levels were

based on work with other species.

One assumption made was that A. cearensis was similar enough
to these other species that positive results would be
produced by application of similar treatments. It may be
concluded that this assumption did not hold, as evidenced by
the lack of flowering. It should not be assumed however,

that the treatments per se were not appropriate.

38
Higher resistance of A. cearensis to the action of CCC may
have necessitated increased levels of the chemical to
stimulate flowering. Similarly, more intense root pruning

or stem girdling may also have been required.

5.1.3 Environmental conditions

Although attempts were made to simulate environmental
conditions similar to those of A. cearensis' native habitat,
actual conditions fell short of optimum. Large changes in
temperature from day to night were common over the colder
months, often dropping to temperatures (IJNC) that occur
infrequently in its natural range. Also, light intensities
within the greenhouse, even after the replacement of the
original bulbs with higher intensity bulbs, was
significantly less than direct sunlight due to the opaque
plexiglass roof. These conditions may have sufficiently
stressed the trees to upset normal physiological processes

which thereby inhibited floral initiation.

Photoperiod may play a role in flowering in A. cearensis.
Photoperiod was maintained at approximately 13 hours during
the experiment. It is not known at what point in A.
cearensis' phenological cycle floral initials are formed.
It is possible, however, that the photoperiod at that time
in Paraguay is shorter than that in central Michigan when

the treatments were applied. If A. cearensis is photoperiod

39
sensitive as regards flowering, too long (or short) a
photoperiod at the time of treatment could be enough to

inhibit flower formation.

5.2 PPANCH ANGLE

Analysis of variance for branch angle showed no
statistically significant differences among families, and a
significant family by replication interaction at the 5%
level. Although the family x replication interaction was
statistically significant, it is doubtful that there is any
practical importance in it. Although family rankings change
substantially over replications, no environmental
differences between replications could be identified to
account for these changes. Since among family variance can
be assumed to be near zero, it follows that a small change
in value for a family mean could result in a significant
change in rank relative to other families. This change in
rank would be reflected in a significant interaction. The
change in value over replications could have been a result
of microenvironment differences, sampling errors,

measurement errors or any combination of these factors.

Two points should be mentioned about this analysis that are
of interest. First, it is possible that the environment in
which the trees were grown affected branch angles. A test

of the same families in a native environment may produce

 

4O
quite different results. Second, correlation of traits
between juvenile and mature trees is often very poor. In
other words, what is true in a juvenile tree will not

necessarily be the case in a mature tree.

Regardless of these two drawbacks, the analysis is also
important for two reasons. First, the angle at which
branches arise from the main stem is of principal importance
only during the initial years when the harvestable stem is
being formed. Plantation grown A. cearensis must be pruned
to produce quality boles. Branches that arise at a near 90°
angle are much easier to prune, leave smaller scars, and
smaller knots in the final product. If the branch angle
changes after the merchantable portion of the stem is
formed, it is of little practical importance. Second, this
study could shed some light on the effect of environment on
branch angle in A. cearensis if similar studies are carried

out in its natural range.

5.3 TREE HEIGHT

As mentioned in a previous section, statistical analysis was
not possible with ten month height data. Comparison of
means, however, showed that variance among families may be
present for early height growth. Although this was an
extremely small trial in terms of number of families, one

family, 021-008, performed noticeably better than the rest.

41
Rapid early growth can prove to be a survival advantage.
Weeds are a common problem in forest plantations.
Individuals that out-compete weeds for canopy space early on
may have a better chance for survival. In addition, this
initial advantage could take years off the rotation age by
minimizing the establishment time required for the

plantation.

5.4 EASCIAILON

During the course of the experiment three trees exhibited
fasciated stems. Similar fasciations on A. cearensis had
been previously observed by the author in Paraguay. The
fasciation appeared as a widening and flattening of the
terminal meristem. Appearance of the fasciation temporarily
stopped growth of the affected meristem. After an
undetermined length of time, height growth resumed and the

abnormality in the stem was covered over by diameter growth.

Transmission electron micrographs of an affected meristem
taken by the Michigan State University Plant & Pest
Diagnostic Center showed what could possibly be mycoplasma-
like-organisms in some cells. Positive identification of
the cellular inclusions could not be made due to lack of

sufficient affected material.

6 SUMMARY

Induction of precocious flowering is a complex matter, and
it is doubtful if continuing research in this area will be
fruitful until more basic knowledge is gathered on the
species. Detailed phenological studies combined with
environmental data will aid in defining conditions necessary
for flowering, as well as natural flowering and fruiting

cycles.

Early flowering trees, whether naturally or through
artificial stimulation, would be useful in a breeding
program of A. cearensis for reasons already discussed. It
is this author's opinion that a more productive use of TRP
resources at this time is germplasm collection. Conserving
a wide array of genotypes from the Paraguayan population
will be critical if future breeding efforts with the species

are to be successful.

Data collected on early height growth indicated that genetic
variation likely exists among half-sib families in A.
cearensis. Properly conducted progeny and/or provenance
trials in Paraguay or elsewhere in its natural range would
aid in determining the extent of variability for this trait.
Increased early growth can decrease both the time needed for

establishment and the time to harvest. Practically

42

speaking, this will reduce establishment costs by requiring
less maintenance in the early life of the plantation, and
increase the rate of return on investment for the plantation

(all other things being equal).

No genetic variation was found among the eight families
tested for branch angle. This is not to say however that
variation does not exist for the trait within A. cearensis.
The present study looked at only eight families. To
accurately asses the amount of variability, or lack of
variability present in the species, many more families
should be tested. To determine if a breeding program to
increase branch angle is appropriate, additional information
is needed. More plantation data is required to determine
the relative importance of branch angle in high quality

timber production of A. cerensis.

The fasciation noted in three of the trees is interesting,
and could be of major importance in future work with the
species. Past experience of the author in Paraguay has
shown that the fasciation can affect stem quality, although
in most cases it does not result in serious damage. The
abnormality is typically covered over with new wood as the
tree's diameter increases. In all cases however, regardless
of the extent to which the stem is deformed, growth is

temporarily stopped. This pause in growth may seriously

43

44

retard a plantations progress.

Observations in Paraguay indicate that the appearance of the

fasciation may be cyclic in nature (Zarza, personal
communication 1992.). If this is true, then it would have
impact on a planting program principally during the high
points of the cycle, and would be of minor consequence at
other times. The importance of identifying the cause and
possible ways to deal with it depends in part then, on the
frequency of the cycles. The more frequent the cycle, the

more important it is to initiate research on the subject.

The results of this study are important in that they serve
as a starting point for further research on A. cearensis.
Identifying superior performing families for growth rate,
branch angle or other traits will be a step towards
ultimately providing superior planting stock to those

interested in planting A. cearensis.

7 LITERATURE CITED

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in relation to flowering in seedlings of Betula pendula
Roth. Ph.D. Thesis, University College of Wales,
Aberystwyth. 106pp.

Bell, R. L. and R. H. Zimmerman. 1990. Combining ability
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Boland, D. J., M. I. H. Brooker, G. M. Chippendale, N. Hall,
B. P. M. Hyland, R. D. Johnston, D. A. Kleinig and J. D.
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Borchert, R. 1976. The concept of juvenility in woody
plants. Acta Horticulturae. 56:21-36.

Boyer, W D. and S. R. Evans. 1967. Early flowering in
Longleaf pine. Journal of Forestry. 65:806.

Brink, R. A. 1962. Phase change in higher plants and somatic
cell heredity. The Quarterly Review of Biology. 37(1):1-22.

Chase, S. B. 1947. Propagation of thornless honeylocust.
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Chudnoff, M. 1984. Tropical timbers of the world. United
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Couranjou, J., J. M. Duffillol and A. Pomar. 1988. Survey of
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Davies P. J. 1987. The plant hormones: Their nature,
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development. Kluwer Academic Publishers. 681 p.

Dennis, F. G. Jr. 1967. Growth and flowering responses of
apple and pear seedlings to growth retardants and scoring.
American Society for Horticultural Science. 93:53-61.

Falconer, D. S. 1989. Introduction to Quantitative Genetics.
3rd ed. John Wiley & Sons, Inc. New York. 438 p.

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46

Fortanier, E. J. and H. Jonkers. 1976. Juvenility and
maturity of plants as influenced by their ontogenetical and
physiological ageing. Acta Horticulturae. 56:37-44.

Furr J. R., W. C. Cooper and P. C. Reece. 1947. An
investigation of flower formation in adult and juvenile
citrus trees. Am. J. Bot. 34:1-8.

Geisler, D. and D. C. Ferree. 1984. Response of plants to
root pruning. Horticultural Review. 6:155-188.

Greene, T. A. 1991. Family differences in growth and
flowering in young southern magnolia. HortScience.
26(3):302-304.

Greenwood, M. S. 1987. Rejuvenation of forest trees. Plant
Growth Regulation. 6:1-12.

Hackett, W. P. 1976. Control of phase change in woody
plants. Acta Horticulturae. 56:143-154.

Hackett, W. P. 1985. Juvenility, maturation and rejuvenation
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