Sensitivity of various apple rootstocks to water stress

The apple industry needs to better understand tree responses and resilience to irrigation deficits. Rootstock choices are critical to achieving high yield and fruit quality, but little is known on differential sensitivities to water stress. The study objective was to assess the influence of rootstocks of varying vigour on water use under well-watered and water deficit regimes, and to assess fruit quality after storage.

We used field-grown full-bearing ‘Rosy Glow’ (years 1, 3) and non-bearing potted ‘Rosy Glow’ (year 2) trees, on five different rootstocks. Well-watered and water deficit (no irrigation) treatments were imposed in two cycles (years 1, 2) or one cycle (year 3) in summer. We quantified: soil water content, soil water potential, leaf gas exchange, transpiration, hydraulic resistance, root growth, water productivity, basal crop coefficient (K_cb), Crop Water Stress Index (CWSI), post-storage quality.

Transpiration was positively correlated with rootstock vigour-inducing capacity, where trees on the dwarfing rootstock (M.9 RN.29) used the least water, even when calculated for high density planting. Transpiration of field-grown trees was not reduced by water deficit, but differential reductions were found between rootstocks in pot-grown trees. M.793 and M.9 RN.29 use different resilience strategies. Trees on M.9 RN.29, G.222 and MM.109/M.9 (more dwarfing) had higher hydraulic resistance than trees on M.793 and M.7 (more vigorous). Rootstocks MM.109/M.9 and M.9 RN.29 gave the highest crop water productivity.

More dwarfing rootstocks confer production and water use benefits without increasing risks to water deficits (at least in this soil type). Resilience is conferred by divergent strategies in different rootstocks. More dwarfing rootstocks require adherence to recommendations for optimum maturity at harvest to reduce the risks of quality defects developing in storage, and may not store well after a season with drought stress.

More dwarfing rootstocks are not more sensitive to drought stress in the field. Nevertheless, care should be taken in harvesting and storing the fruit after a season with drought stress.