A physiological investigation of rootstock effects on the water use of irrigated ‘Rosy Glow’ apple trees under water deficit conditions

Water resources for irrigation are constrained in many apple production regions, and water-saving technologies are required. This study investigated the effect of apple rootstocks on water use (canopy transpiration, E c ) of irrigated apple trees under well-watered (WW) and water-deficit (WD – irrigation was fully suspended during two cycles). A field trial was conducted in South Africa using full-bearing ‘Rosy Glow’ apple trees grafted on different rootstocks: M.793 (semi-vigorous), MM.109/M.9 and G.222 (semi-dwarfing), and M.9 RN.29 (dwarfing). WW and WD treatments were imposed in two late-summer cycles. Under WW conditions, total E c correlated with rootstock vigour, being approx. 995 mm (M.793), 725 mm (G.222), 715 mm (MM.109/M.9) and 375 mm (M.9 RN.29). During WD, M.793 showed the greatest reduction in transpiration (31%), followed by M.9 RN.29 (17%), MM.109/M.9 (4%) and G.222 (1%). The recovery capacity for E c was high in M.793 and M.9 RN.29, accompanied by enhanced post-stress root development (shallow roots in M.793 and deeper roots in M.9 RN.29). In the second season, a pot trial with young non-bearing trees on five rootstocks supported the key findings of the field trial. In the pot trial, transpiration per unit leaf area decreased under WD in all rootstocks, with G.202, M.7, and G.222 showing the largest decreases. Conversely, M.793 and M.9 RN.29 displayed greater resilience, showing quicker recovery and transpiration rates during recovery that surpassed those measured in the pre-deficit period. Rootstock selection significantly influences water use and resilience to water deficit, with implications for orchard choices and management in water-limited regions