EFFECTS OF WATER-STRESS ON WATER RELATION PARAMETERS AND STOMATAL CONDUCTANCE OF FUJI APPLE-TREES

Effects of water stress on water relation parameters and stomatal conductance of 'Fuji' apple trees were studied by cultivating one-year-old trees/M26 in plastic gets. No osmotic adjustment was observed even 18-25 days after water stress was initiated, when predawn leaf water potential decreased to -1.27 MPa and some leaves turned yellow and were shed. Stomata, however, reacted sensitively to water stress treatments so that stomatal conductance in severely stressed trees declined to 8-10 % of well-watered controls. This reduced stomatal conductance in the water stress treatments, resulting in less transpiration, minimized treatment differences in leaf water potential, and turgor was maintained even under severe water stress conditions. Stomata began partly to close even at a relatively high water status (or closing stomata caused the leaves to maintain positive turgor), strongly suggesting that roots sense the availability of water in drying soil and produce chemical signals such as abscisic acid, which regulate stomatal behaviour accordingly, whatever the water status of leaves. 'Fuji' apple trees seem not to tolerate water stress by active osmotic adjustment but to avoid it by sensitive stomatal closing.