MODE-OF-ACTION OF ROOTSTOCK/SCION INTERACTIONS IN APPLE AND CHERRY TREES

With apple, the graft unions of dwarfing rootstocks or interstocks with the scions appear to deplete the solutes of the xylem sap and this effect increases with the dwarfing effect of the rootstock or interstock. The depletion is non-selective, affecting all the nutrients and probably also the growth regulators of the sap. Studies with isolated apple shoots in vitro suggest that the cytokinins of the sap have a major role in the control of shoot growth and any depletion of these growth regulators is likely to produce much greater effect on scion development than depletion of the phloridzin or ABA content.

Studies with cherry rootstocks concern effects with cherry Clone 15 (Prunus avium x P. pseudocerasus). This is exceptionally dwarfing as a scion but very invigorating as a rootstock. As an interstock it had no effect unless allowed to produce some very small shoots and then it dwarfed scions by 20–30%. Aspects of the metabolism of the extending shoots of this rootstock, such as possible production of growth inhibitors, merit study in relation to dwarfing in fruit trees.

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