Intact or decapitated Pisum sativum seedlings were used to investigate the correlative
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 combinations
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 activities
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
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] isopentenyladenine
by isolated organs of Pisum sativum indicated that the roots, leaves and
stems, but not isolated lateral buds had the capacity to metabolise [³H] isopentenyladenine
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 cytokinin
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 application.
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. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1988.
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:ukzn/oai:http://researchspace.ukzn.ac.za:10413/10523 |
| Date | January 1988 |
| Creators | King, Richard Anthony. |
| Contributors | Van Staden, Johannes. |
| Source Sets | South African National ETD Portal |
| Detected Language | English |
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