Comparison of growth potentials of epicormic shoots of nectarine trees grown on size-controlling and vigorous rootstocks

The vegetative performance of four nectarine tree scion/rootstock combinations, with varying growth capacities, were analysed in the Spring after the canopy:root ratio was dramatically reduced by severe pruning in the dormant season. We anticipated that severe pruning and reduction of the shoot:root ratio would mitigate known root water supply limitations related to the various rootstocks and determine if other factors associated with the rootstocks would control the rates of epicormic shoot growth.The trees used in this field experiment were 3 years-old, with ‘May Fire’ nectarine grafted onto four different rootstocks: ‘Nemaguard’ (a vigorous seed-propagated control, P. persica P. davidiana hybrid) and three size-controlling rootstocks, ‘K146-43’, ‘P30-135’ (P. salicina P. persica hybrids) and ‘K-119-50’ (P. salicina P. dulcis hybrid). At the beginning of the trial, there were clear differences in plant dimensions, confirming the higher vigour induced by ‘Nemaguard’ compared to the other rootstocks. Reducing the canopy:root ratio was able to annul potential differences in weekly shoot-extension growth rates and cumulative extension growth of individual shoots during the Spring flush of re-growth after pruning.This supported the idea that there were no rootstock-specific signals (hormonal or physical) that caused large differences in extension growth rates of individual epicormic shoots on trees. However, there were clear differences in total epicormic shoot regrowth per tree that were associated with initial tree size at the time of severe pruning, indicating the importance of trunk and root storage in influencing the amount of growth stimulation after dormant pruning.