EFFECT OF WATER-DEFICIT ON PHOTOSYNTHESIS AND ELECTRON-TRANSPORT IN WHEAT GROWN IN A NATURAL-ENVIRONMENT

O2 evolution and fluorescence were measured simultaneously on leaf discs of durum wheat. Field-grown plants and plants grown in pots in a natural environment were compared with water-stressed plants. In saturating CO2 the biochemical reactions of photosynthesis are not very sensitive to water stress, and electron transport is entirely used for CO2 reduction. The decrease in maximum photosynthetic capacity appears to be related to relative water content and not to leaf water potential. Simultaneous measurements of CO2 uptake and fluorescence of attached leaves at natural CO2 concentrations show that the electron transport rate exceeds the requirement of photosynthetic carbon oxidative and reductive cycles. This suggests that photoreduction Of O2 in natural conditions could occur at higher rates than generally believed, due to the presence of alternative reductions. No evidence of differences between stressed plants and irrigated controls in the way the leaves use photosynthetic electrons was obtained. However, senescence of the leaves probably interferes and masks the effects of the drought.