STOMATAL CONDUCTANCE AND LEAF-WATER PARAMETERS OF APPLE, PEAR, SWEET CHERRY, AND PLUM IN AN ORCHARD

Stomatal conductance of apple, sweet cherry, and young plum leaves showed a typical midday depression after a maximum in the early morning. In contrast, the stomatal conductance of pear and mature plum leaves changes relatively little throughout the day. Apple and young plum leaves seem to control stomatal conductance by means of a feedforward process. Pear and mature plum leaves show little evidence for stomatal regulation during the light period. Apple, pear, and plum had similar hydraulic conductances between soil and leaves in the early and late season. However, hydraulic conductance in sweet cherry decreased significantly in the late season and caused low leaf water potential and stomatal closure due to feedback regulation during the day. Possible explanations are discussed.