Initial alteration of shoot architecture by dwarfing apple rootstocks involves shoot/root/shoot signalling between auxins, gibberellins and cytokinins

Reduced scion vigour on dwarfing apple rootstocks has been attributed to decreased basipetal transport of indole-3-acetic acid (IAA) to the roots, and lower concentrations of root-produced cytokinins and gibberellins transported in the xylem sap to the scion. The purpose of this study was to understand how rootstocks of different vigour first modified scion architecture and growth, and the probable roles and interactions that auxin, gibberellin and cytokinin may have in these initial growth alterations. For grafted trees of ‘Royal Gala’, an auxin transport inhibitor (± 1-N-Naphthylpthalamic acid (NPA)) was applied to the stem tissue of each rootstock (M.9, MM.106, M.793 and self-rooted ‘Royal Gala’ control) and ± gibberellin (GA4+7) or ± benzylaminopurine (BAP) were applied to the scion. Compared with the ‘Royal Gala’ rootstock (control), M.9 reduced the number of axillary shoots that formed along the primary axis. The final node number and length of both the primary and axillary shoots were also reduced on M.9 because growth slowed or terminated earlier, respectively. Reduced scion branching on M.9 was increased with BAP, however newly initiated axillary shoots generally terminated without additional GA4+7 sprays. Applications of GA4+7 to scions on M.9 and ‘Royal Gala’ resulted in a primary shoot with similar final numbers of neoformed nodes. In contrast, reducing the basipetal transport of IAA to the roots of vigorous rootstocks, by applying NPA, resulted in a major change in scion form that was similar to the dwarfing imposed for scions on M.9 (reduced branching, shoot lengths and node number). The application of BAP to scions on NPA treated rootstocks reinstated branching but, as with M.9, new axillary shoots terminated without GA4+7. For rootstocks treated with NPA, GA4+7 reactivated growth of the primary and axillary shoots, although few additional axillary shoots developed without BAP. Endogenous signalling that may occur between scion and rootstock, together with the analytical work required to confirm the mechanism of vigour control, are discussed.