EFFECTS OF PATCHY STOMATAL CLOSURE ON GAS-EXCHANGE MEASUREMENTS FOLLOWING ABSCISIC-ACID TREATMENT

Gas exchange data and images of leaf fluorescence were collected concurrently as stomata responded to abscisic acid (ABA) application, When 10(-5) kmol m(-3) ABA was applied to the transpiration stream in a short pulse, stomatal conductance (g(s)), photosynthesis (A) and intercellular CO2 concentration (c(i)) decreased rapidly after a short lag period and became approximately constant after 2h, There was an apparent reduction in the A versus c(i) relationship as stomata closed, but the data returned to the A versus c(i) curve while stomatal conductance was constant or slowly rising during the second hour after ABA treatment, Larger amounts of ABA administered during the pulse caused larger deviations from the A versus c(i) relationship, When 10(-7) kmol m(-3) ABA was applied continuously through the transpiration stream, g(s), A and c(i) decreased, but there was no substantial deviation from the A versus c(i) curve, Fluorescence images were patchy as stomata closed for all experiments, but became slowly more uniform during the time that gas exchange was returning to the A versus c(i) curve, The distribution of conductance among patches was not bimodal, and larger deviations from the A versus c(i) curve had greater ranges of pixel values and more pixel values representing low values of c(i) during stomatal closure than did experiments showing small or no deviation, Estimates of A and g(s) from fluorescence images compared favourably with measured values in most cases, suggesting that the patchy distributions of fluorescence were caused by patchy distributions of stomatal conductance and that apparent reductions in the A versus c(i) relationship were the result of these patchy stomatal distributions and not direct effects of ABA on mesophyll functioning, The data show that stomatal patches can be temporary and that patchiness may not be reflected in gas exchange data if the range of stomatal conductances is not large, These observations may explain some of the discrepancies among previous studies concerning the effect of ABA on the A versus c(i) relationship.