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
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Date
2009
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Massey University
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Abstract
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.
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Keywords
Scions, Grafting, Royal Gala apples, Dwarfing