THE ROLE OF CYTOKININS IN THE REGULATION OF APICAL DOMINANCE IN Pisum sativum BY ...

224 Pages·2013·3.07 MB·English

THE ROLE OF CYTOKININS IN THE REGULATION OF
APICAL DOMINANCE IN Pisum sativum
BY
RICHARD ANTHONY KING
Submitted in fulfilment of the requirements for the degree of
Doctor of Philosophy
in the
Department of Botany, Faculty of Science
University of Natal, Pietermaritzburg
December 1988
PREFACE
I hereby declare that this thesis, submitted for the degree of Doctor
of Philosophy in the Faculty of Science, University of Natal, Pieter
maritzburg, except where the work of others is acknowledged, i's the
result of my own investigation.
Richard Anthony King
December 1988
0)
ACKNOWLEDGEMENTS
My sincere thanks to Professor J. Van Staden for his enthusiasm and
guidance throughout the course of this project and the preparation of
this manuscript.
Thankyou to my father and late mother who provided generous assistance
in many forms and were always encouraging and concerned with my
endeavours.
Special thanks to Neville Quinn for efficient proof-reading, and to
Jeanette Stead for her friendship and support.
Financial assistance in the form of a bursary from the Council for Scien
tific and Industrial Research and a graduate assistant bursary from
the University of Natal is gratefully acknowledged.
(ii)
ABSTRACT
Intact or decapitated Pisum sativum seedlings were used to investigate the correla
tive inhibition of lateral buds by the shoot apex - a developmental process known
as apical dominance. Apical dominance is considered to be regulated by the relative
ratios of growth regulators, especially auxin and cytokinin, and resource availability
in the plant. This study considered the role of cytokinins in the regulation of
correlative inhibition, and was closely linked to theoretical models of auxin and
nutrient gradients in the whole plant.
Firstly, the response of all lateral buds on the plant to decapitation of the shoot
apex, and the subsequent growth of these buds, was documented. The pattern
of lateral bud branching following decapitation, noted to change with increasing
age of the plant, was found to be consistent with the view that apically synthesized
auxin, moving basipetally, exerts an inhibitory influence on lateral bud growth.
Removal of selected lateral buds on decapitated plants which left various combina
tions of buds to compete indicated that correlation between lateral buds on the
same plant was likely an important factor controlling the patterns of lateral bud
branching.
Secondly, a quantitative study of the biological responses which result from the
application of a cytokinin to a lateral bud were performed. The different abilities
of ten cytokinins tested to release lateral buds from dominance paralleled their
activity in a soybean callus bioassay, and were likely a result of the intrinsic activi
ties of the cytokinins due to their structure and their subsequent rr.etabolism in
the plant following application. A consistent feature of these experiments was
the low biological activity of isopentenyladenine in relation to the high activity
of zeatin. Further investigation of the role of isopentenyladenine and zeatin in
apical dominance control indicated that lateral buds differed in their sensitivity
to these two cytokinins. On decapitated plants, lateral buds were more able to
(iii)
utilize applied isopentenyladenine, and high zeatin concentrations were no longer
supraoptimal for growth.
Finally, the metabolism of [3H] isopentenyladenine in the intact plant or in isolated
explants was investigated in an attempt to relate the biosynthesis, transport,
interconversion and degradation of cytokinins to the developmental process of
apical dominance. Comparison of the uptake and metabolism of [3H] isopentenyl
adenine by isolated organs of Pisum sativum indicated that the roots, leaves and
stems, but not isolated lateral buds had the capacity to metabolise ~HJ isopen
tenyladenine to zeatin metabolites. This metabolite activity was very notable
in the stem, where it occurred as a gradient and was related to the age of the
plant. Auxin was considered to be the factor controlling this distribution of cyto
kinin metabolic activity in the stem since parallel gradients had been noted in
the patterns of lateral bud branching and the response of buds to cytokinin applica
tion. Indeed, it was shown that indole-3-acetic acid affected the metabolism
of [3H] isopentenyladenine in isolated stem sections.
These results are discussed in relation to the control of lateral bud growth via
the auxin mediated distribution and metabolism of root produced cytokinins in
the shoot system, necessary for the release of apical dominance.
(iv)
CONTENTS
Page
0)
PREFACE
ACKNOWLEDGEMENTS (ii)
ABSTRACT (iii)
CONTENTS (v)
LIST OF FIGURES (v iii)
LIST OF TABLES (xiii)
CHAPTER I: LITERATURE REVIEW 1
1.1 Introduction 1
1.2 The release of apical dominance 2
1.3 Early research: A historical perspective 6
1.4 Plant nutrition and apical dominance 9
1.5 Growth regulators and apical dominance 18
1.5.1 Auxin and cytokinin transport in relation
to apical dominance 19
1.5.1.1 Auxin transport 19
1.5.1.2 Cytokinin transport 25
1.5.2 The role of growth regulators in apical
dominance 31
1.5.2.1 Auxin 31
1.5.2.2 Cytokinins 36
1.5.2.3 Gibberellins 44
1.5.2.4 Ethylene 47
1.5.2.5 Abscisic acid 49
1.6 Environmental factors affecting apical dominance 52
1.6.1 Light 52
1.6.2 Water 54
1.6.3 Nutrients 55
1.6.4 Gravity
55
1.6.5 Other environment factors 56
1.7 A hypothesis of apical dominance control
56
Page
CHAPTER II: THE GROWTH AND DEVELOPMENT OF LATERAL
BUDS 59
2.1 Introduction 59
2.2 The response of lateral buds to decapitation: An
introduction to the experimental system 59
2.2.1 Materials and methods 59
2.2.2 Results 62
2.3 Correlative effects between lateral buds on the
same plant following decapitation 65
2.3.1 Materials and methods 65
2.3.2 Results 65
2.4 Discussion 71
CHAPTER III: THE APPLICATION OF CYTOKININS TO LATERAL
BUDS 83
3.1 Introduction 83
3.2 An activity profile of the ability of cytokinins to
release lateral buds from apical dominance 84
3.2.1 Materials and methods 84
3.2.2 Results 88
3.3 Application of isopentenY,ladenine and zeatin to
la teral buds 90
3.3.1 Materials and methods 90
3.3.2 Results 91
3.4 Discussion
97
CHAPTER IV: CYTOKININ METABOLISM IN RELATION TO
APICAL DOMINANCE
114
4.1 Introduction
114
4.2 General materials and methods
115
(vi)
Page
J
4.3 Application of [3H isopentenyladenine to bud 2
on the whole plant 119
4.3.1 Materials and methods 119
4.3.2 Results and discussion 119
J
4.4 The metabolism of [3H isopentenyladenine
by isolated organs 126
4.4.1 Materials and methods 126
4.4.2 Results 126
4.4.3 Discussion 135
4.5 The metabolism of [3HJ isopentenyladenine
by stem segments 145
4.5.1 Materials and methods 145
4.5.2 Results 146
4.5.3 Discussion 149
4.6 The effect of auxin on the metabolism of [3HJ
isopentenyladenine in stems 152
4.6.1 Materials and methods 152
4.6.2 Results 153
4.6.3 Discussion 153
CONCLUSION 159
REFERENCES 163
( vii)
LIST OF FIGURES
Page
1 Diagrammatical representation of two early and diverse views
on apical dominance. 7
2 Diagrammatical representation of nutrient relations in the plant
considered to influence the expression of apical dominance. 10
3 Diagrammatical representation of two hypothetical mechanisms
by which apically synthesized auxin might regulate apical domi
nance via control of nutrient distribution in the plant. 14
4 Diagrammatical representation of the correlative inhibition of one
shoot by another shoot on the same plant. 17
5 The auxin relationships in Pisum sativum as determined by SCOTT
and BRIGGS (1960). 22
6 The distribution of extractable auxin in each of 20 millimetre
Pisum sativum epicotyl sections at various times following ex
cision of the shoot apex (SCOTT and BRIGGS, 1960). 24
7 Diagrammatical representation of the translocation of cytokinins
applied to the roots of intact or decapitated plants. 28
8 A hypothetical time sequence of cytokinin redistribution and
events induced by decapitation or cytokinin application to the
lateral buds of a plant.
31
(viii)
Page
9 Diagrammatical representation of a hypothetical mechanism of
lateral bud inhibition mediated by auxin. 37
10 Diagrammatical representation of a hypothetical interaction be
tween auxin and cytokinin controlling lateral bud inhibition. 43
11 A Pisum sativum plant bearing four nodes with trifid bracts at
the lower two nodes and true leaves at the upper two nodes. 61
12 The extension of lateral bud 1, 2, 3 and 4 following decapita
tion of the shoot apex of four-node Pisum sativum plants
after 14 days. 63
13 The extension of lateral buds following decapitation of the
shoot apex of four, six, eight and ten-node Pisum sativum
plants after 14 days.
64
14 The extension of three cotyledonary buds on a decapitated four
node Pisum sativum plant 14 days after the excision of all lateral
buds.
67
15 A comparison of the extension of lateral bud 1, 2, 3 or 4 on the
separate four-node Pisum sativum plants 14 days following de
capitation of all but one lateral bud.
68
16 The extension of two lateral buds on a decapitated four-node
Pisum sativum plant 14 days after the excision of two lateral
buds.
69
17 The extension of three lateral buds on a decapitated four
node Pisum sativum plant 14 days after the excision of one
la teral bud.
70