ANALYSIS OF INHIBITION OF PHOTOSYNTHESIS DUE TO WATER-STRESS IN THE C-3 SPECIES HORDEUM-VULGARE AND VICIA-FABA - ELECTRON-TRANSPORT, CO2 FIXATION AND CARBOXYLATION CAPACITY

A C-3 monocot, Hordeum vulgare and C-3 dicot, Vicia faba, were studied to evaluate the mechanism of inhibition of photosynthesis due to wate r stress. The net rate of CO2 fixation (A) and transpiration (E) were measured by gas exchange, while the true rate of O-2 evolution (J(o2)) was calculated from chlorophyll fluorescence analysis through the str ess cycle (10 to 11 days). With the development of water stress, the d ecrease in A was more pronounced than the decrease in J(o2) resulting in an increased ratio of Photosystem II activity per CO2 fixed which i s indicative of an increase in photorespiration due to a decrease in s upply of CO2 to Rubisco. Analyses of changes in the J(o2)/A ratios ver sus that of CO2 limited photosynthesis in well watered plants, and RuB P pool/RuBP binding sites on Rubisco and RuBP activity, indicate a dec reased supply of CO2 to Rubisco under both mild and severe stress is p rimarily responsible for the decrease in CO2 fixation. In the early st ages of stress, the decrease in C-i (intercellular CO2) due to stomata l closure can account for the decrease in photosynthesis. Under more s evere stress, CO2 supply to Rubisco, calculated from analysis of elect ron flow and CO2 exchange, continued to decrease. However, C-i, calcul ated from analysis of transpiration and CO2 exchange, either remained constant or increased which may be due to either a decrease in mesophy ll conductance or an overestimation of C-i by this method due to patch iness in conductance of CO2, to the intercellular space. When plants w ere rewatered after photosynthesis had dropped to 10-30% of the origin al rate, both species showed near full recovery within two to four day s.