Effect of rootstock on apple (Malus domestica) tree water relations

The effects of rootstock on mid‐season water relations, under orchard conditions of non‐limiting soil moisture, were determined for bearing ‘Empire’ apple trees (Malus domestica Borkh.) on the clonal rootstocks M9, M26, M7, MM106, and MM104 (most to least dwarfing) in their sixth and seventh growing seasons. Stem water potentials (ψstem) of trees on M9 and M26 were more negative at midday, under warm, sunny conditions, than were the trees on the other three rootstocks. However, change in ψstem per change in stem distance through the canopy (water potential gradient) did not vary among rootstocks at midday. There was no rootstock effect on diurnal variation in transpiration or stomatal conductance. Differences in water storage capacitance, relative to tree size, were determined in a separate study but did not account for the differences observed in ψstem. Calculated hydraulic conductivities of xylem water transport suggest that rootstocks differ in their ability to conduct water to the scion, but hydraulic conductivity of the scion was not affected by rootstock. Root‐stock differences in hydraulic conductivity were not accounted for by differences in tree size.