Initial alteration of scion architecture by dwarfing apple rootstocks may involve shoot-root-shoot signalling by auxin, gibberellin, and cytokinin

Apple rootstock clones of different vigour were examined to determine how they initially modified the architecture of ‘Royal Gala’ scions following grafting of the composite tree. These modifications were compared with those of gibberellins (GA₄₊₇) and/or benzylaminopurine (BAP) applied to the scion, and an inhibitor of auxin transport [1-N-naphthylphthalamic acid (NPA)] applied to the rootstock stem. By the end of the first year of growth after grafting, the dwarfing (‘M.9’) rootstock had significantly decreased the mean total shoot length and node number of the scion compared with invigorating rootstocks (i.e., ‘MM.106’, ‘M.793’, and ‘Royal Gala’). Similarly, NPA applied to the stem of invigorating rootstocks decreased the total shoot growth of the scion, and the architectural changes imposed most closely resembled those of ‘M.9’. Both treatments increased the proportion of primary and secondary shoots (if present) that terminated growth early, which decreased the final length and node number of these shoot types. Also, NPA and ‘M.9’ decreased the number of secondary shoots that formed. For scions on NPA-treated rootstocks or ‘M.9’, BAP re-instated the formation of secondary shoots, while GA₄₊₇ primarily reduced the proportion of primary and secondary shoots that terminated growth early, thereby increasing their final length and node number. An endogenous signalling mechanism is proposed whereby dwarfing rootstocks reduce the basipetal transport of indole-3-acetic acid (IAA) to the root, thereby decreasing the amount of root-produced cytokinin and gibberellin transported to the scion. At the scion, a limited supply of cytokinin may modify the architecture by decreasing branching, whereas a limited supply of gibberellins may primarily reduce the duration of shoot extension growth.