The physiological basis of vigour control by apple rootstocks - an unresolved paradigm : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Physiology at Massey University, Palmerston North, New Zealand

For millennia, scions have been grafted onto dwarfing apple rootstocks to reduce final tree size. However, it is unclear how scion architecture is first modified by the dwarfing apple rootstock, the time from grafting when this occurs and the endogenous hormonal signalling mechanisms that may cause the initial modifications in growth that then define the future architecture of the scion. In this study, the dwarfing (M.9) rootstock significantly decreased the mean total shoot length and node number of ‘Royal Gala’ apple scions by the end of the first year of growth from grafting when compared with rootstock(s) of greater vigour (MM.106, M.793 and a ‘Royal Gala’ rootstock control). Similarly, the auxin transport inhibitor 1-N-naphthylphthalamic acid (NPA) applied to the stem of vigorous rootstocks significantly decreased mean total shoot length and node number of the scion, and the architectural changes imposed were generally similar to those imposed by M.9. For example, both treatments decreased the mean length and node number of the primary shoot, reduced the formation of secondary axes on the primary shoot and caused a greater proportion of primary and secondary shoots (if present) to terminate growth early. Decreased formation of secondary axes imposed by both treatments was reversed by applying the cytokinin benzylaminopurine (BAP) repeatedly to the scion, whilst applications of gibberellins (GA4+7) reduced the proportion of primary and secondary shoots that terminated growth early, therefore increasing the final mean length and node number of these shoot types. Both M.9 and NPA also significantly decreased the final mean dry mass and length of the root system. Given these general similarities, it is proposed that the basipetal IAA signal is of central importance in rootstock-induced scion dwarfing, and that a shoot/root/shoot signalling mechanism may exist whereby the stem tissue of the M.9 rootstock decreases the basipetal transport of IAA to the root during summer, thereby decreasing root growth and the amount of rootproduced cytokinin and gibberellin transported to scion. Reduced amounts of cytokinin transported to the scion may decrease branching, whilst reduced amounts of gibberellins may decrease the duration for which a large proportion of primary and secondary shoots grow. Analysis of endogenous hormones for newly grafted composite ‘Royal Gala’ apple trees on rootstocks of different vigour provided some additional support for these ideas. It is recommended that future studies elucidate what unique properties of the M.9 bark act to restrict IAA transport, whilst it is concluded that gene(s) regulating rootstock-induced scion dwarfing are likely to control processes within the rootstock that modify the metabolism of IAA, its basipetal transport and the subsequent synthesis of root-produced vigour-inducing hormones including cytokinins and gibberellins