The involvement of indoleacetic acid in paradormancy and sylleptic shoot development of grafted peach trees and hybrid rootstoks.

The physiological mechanisms controlling the induction of lateral branching, which is part of the expression of growth vigour, were investigated in two hybrid rootstocks (GF 677 and Mr.S. 2/5) widely used in peach [Prunus persica (L.) Batsch] cultivation that were grafted to a nectarine scion (cv. 'Big Top'). As expected, field-grown rootstocks showed different degrees of vigour and also induced distinct patterns of growth on the scion. The higher the rootstock vigour, the greater the number and length of lateral shoots developed by the scion. Hence, the growth vigour affected paradormancy and we hypothesized that auxin, which is known to suppress axillary bud development following bud break when transported basipetally along the shoot, might mediate rootstock-induced branching in the top of the tree. The role of indole-3-acetic acid as a hormonal signal in lateral branching was assessed by analysing its concentration in apical and axillary buds collected from growing shoots, both intact and at different times after apex removal. Shoot pruning was used as a means to force axillary buds to overcome paradormancy, assuming that their responses would reflect their intrinsic capacity of resuming growth. The development of lateral buds of both grafted scions and intact rootstocks was positively correlated with the respective auxin concentration and following shoot apex removal the relationships became stronger. Therefore, auxin may be responsible for mediating the expression of growth vigour. The invigorating effect of a rootstock would then be dependent on its ability of inducing high auxin levels in axillary buds. The hormone would act directly within bud tissues to stimulate their growth following bud break.