STOMATAL RESPONSES TO WATER DEFICITS AND ABSCISIC-ACID IN LEAVES OF SUNFLOWER PLANTS (HELIANTHUS-ANNUUS L) GROWN UNDER DIFFERENT CONDITIONS

The decrease in diffusive conductance of a leaf exposed to water stress or to exogenous abscisic acid (ABA) was smaller in leaves of sunflower plants (Helianthus annuus L. cv. NK285) that had been grown in a phytotron in humid air than in leaves of sunflowers grown outdoors. Stomata of the phytotron-grown plants were slower to close after detachment of a leaf than those of the outdoor plants. When stomata closed rapidly, as they did in detached leaves and after treatment with ABA, the extent of closure was varied over the leaf's surface, in particular in the case of phytotron-grown plants, and the extent of the heterogeneity was greater in the phytotron-grown plants than in the outdoor plants. When stomata closed gradually, for example, under conditions of limited moisture in the soil, closure occurred uniformly over leaves of plants of both types. The smaller decrease in diffusive conductance of leaves from phytotron-grown plants after treatment with ABA resulted from the presence of patches on the surface in which stomata remained open. The smaller decrease of diffusive conductance in the phytotron-grown plants under conditions of limited moisture in the soil resulted from the uniformly lower responsiveness of stomata on a leaf to the decrease in water potential. When estimates are made of the intercellular concentration of CO2 (Ci) from gas-exchange measurements, heterogeneity in stomatal closure should be monitored when stomata close rapidly, in particular in plants grown in humid air, because heterogeneous stomatal closure can lead to overestimates of Ci.