Impact of drought stress on photosynthetic response, leaf water potential, and stem sap flow in two cultivars of bi-leader apple trees (Malus x domestica Borkh.)

Photosynthetic responses and water relation of two apple cultivars, Hongro (medium-maturing) and Fuji (late-maturing), grafted on a single M.9 rootstock, were examined under drought stress conditions. Experimental bi-leader trees were grown in the field and were either well-irrigated or subjected to drought stress for 60 d. The photosynthetic responses and water relation parameters differed between the two cultivars. The net photosynthetic rate (P-n) in Hongro began to decrease at 15 d of drought treatment, while in Fuji this reduction occurred approximately 5 d later. Similarly, the Hongro cultivar in drought treatment showed a higher reduction in midday leaf water potential (Psi(MD)), stem sap flow, and hydraulic conductance (K-s) because of greater reduction of xylem vessel area, vessel density, and vessel diameter as compared to those in Fuji. As the Psi(MD) and stem sap flow decreased in drought stress treatment, the abscisic acid concentration in xylem sap increased; the increase was higher in Hongro than in Fuji. The changes in leaf morphological traits and reduction in total chlorophyll content were lower in Fuji than in Hongro. These results demonstrated that Fuji exhibited greater tolerance to water stress compared with Hongro.