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Shoot-root allometry and growth of nashi and tomato : the effects of budding, gibberellins and cytokinins : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Horticultural Science at Massey University
Growth of the root and the shoot systems of plants is generally, positively correlated, although the mechanism(s) controlling such relationships is not well understood. A series of experiments were carried out on young nashi trees (Pyrus serotina) and tomatoes (Lycopersicon esculentum Mill.) to explore this homeostatic phenomenon. Two nashi cultivars, Hosui and Nijiseiki, were budded on to each of three clonal rootstocks, which differed in vigour (scion-budded trees). Buds from each rootstock was also budded on their own roots (rootstock trees). Growth, measured by individual organ and total plant dry weight, leaf attributes (leaf area, leaf number and leaf size) and root attributes (root length, root number and root volume) over two years after budding, indicated that scion-budded trees were markedly smaller than rootstock trees, irrespective of rootstock vigour. The imbalance of shoot-root ratio occurred following pruning after bud take; this remained in rootstock trees for one year but persisted for two years in scion budded trees. Vigour of all rootstocks appeared to diminish with time and final tree size was not well related to initial rootstock vigour. Neither rootstock nor scion morphological characteristics appeared to be changed by the partner, although presence of the cultivar bud on rootstocks delayed commencement of root activity in early spring. These results indicate that two-year-old (scion budded growth) nashi trees are not appropriate material for studying allometric relationship. Plant growth regulators, gibberellins and cytokinins, were applied to 6- and 5-week-old tomato seedlings, respectively, in three separate aeroponic experiments. Gibberellic acid was sprayed twice to the shoot (at 2.9 X10-5 M), while root application was achieved by incorporating GA3 into the nutrient solution (cone. 5.8 X10-5 and 2.9 X10-4 M). Compared to the control, stem elongation, stem dry weight and stem weight ratio (SWR) was increased while root attributes (dry weight and root weight ratio (RWR)), leaf attributes (leaf area, leaf area ratio and leaf dry weight), and consequently total plant dry weight were reduced in GA3 treated plants. Gibberellic acid promoted apical dominance. Shoot applied GA3 was quantitatively more effective than root application, suggesting that the organ in which physiologically active GA(s) originate may be an important component of plant response to environments. In addition, GA3 effects were additive as indicated by the increasing difference with time in SWR and shoot-root ratio. The increased SWR and reduced leaf weight ratio (LWR) were responsible for an increase in the allometric value between stem and root dry weight (kS), and a reduction in the allometric value between leaf and root dry weight (kL), respectively. However, allometric value between shoot and root dry weight (kT) was unaltered by GA3. These results suggest no feedback mechanism of de novo GA synthesis occurred, and indicate that GA has no role in regulation of shoot-root allometry. A synthetic cytokinin, benzylaminopurine (BA), was applied to roots at 2.2 X10-8, 2.2 X10-7 and 2.2 X10-6 M. The control gave an intermediate response in all parameters measured, compared to the enhanced response at 2.2 X10-8 M BA and the inhibitory response at other BA concentrations. This suggested that BA supplemented, and had a similar effect to, endogenous cytokinins. Benzylaminopurine initially or transiently stimulated shoot and leaf primordia and thus released buds from apical dominance, leading to an increase in leaf attributes (leaf number, leaf area, leaf dry weight and leaf weight ratio (LWR)), increased shoot-root ratio and reduced RWR. Benzylaminopurine had no effect on stem attributes (stem elongation, stem dry weight and SWR). There were, however, no changes induced in kL and kT. It is suggested that cytokinins participate in the homeostatic mechanism regulating plant growth allometry. A model in which both gibberellins and cytokinins integrate to affect plant growth via allometric relationships is proposed. The usefulness of allometric studies to detect and analyse dynamic changes of organs and plant productivity in response to environment, as well as explain mechanisms regulating shoot-root equilibrium is strongly endorsed by this study.