STOMATAL FUNCTIONING OF INVITRO AND GREENHOUSE APPLE LEAVES IN DARKNESS, MANNITOL, ABA, AND CO2

Leaves from in vitro and greenhouse cultured plants of M. domestica (Borkh.) cv. Mark were subjected to 4 h of darkness; 4 h of 1 M mannitol induced water stress; 1 h of 10-4 M to 10-7 M cis-trans abscisic acid (ABA) treatment; 1 h of 0.12% atmospheric CO2. Stomatal closure was determined by microscopic examination of leaf imprints. In all treatments, less than 5% of the stomata from leaves of in vitro cultured plants were closed. The diameter of open stomata on leaves from in vitro culture remained at 8 .mu.m. In contrast, an average of 96% of the stomata on leaves of greenhouse grown plants were closed after 4 h in darkness; 56% after 4 h of mannitol induced water stress; 90% after 1 h of 10-4 M ABA treatment; 61% after 1 h in an atmosphere of 0.12% CO2. Stomata of in vitro apple leaves did not seem to have a closure mechanism, but acquired one during acclimatization to the greenhouse environment. The lack of stomatal closure in in vitro plants was the main cause of rapid water loss during transfer to low relative humidity.