Leaf Photosynthesis Under Drought Stress

The photosynthetic apparatus is resistant to drought. Net CO2 uptake of a leaf submitted to a mild desiccation decreases because of stomatal closure. As aresult, CO2 concentration in the chloroplast decreases in plants exposed to water shortage. This drop in the chloroplast CO2 concentration causes: (i) a decrease in photochemical yield of open PS II centers and, consequently, an increase of thermal dissipation of the excitons trapped in PS II units; (ii) a decline in the activity of some enzymes, e.g. sucrose phosphate synthase and nitrate reductase; (iii) an increase in the activity ofribulose 1,5-bisphosphate oxygenase. The water status of the plants can be maintained under fluctuating water supply through a regulation of water loss and water uptake, in which abscissic acid plays a major role. The role of photorespiration in protecting the photosynthetic apparatus against high light damage is unclear. Leaf movements (paraheliotropism) and thermal dissipation of excitons trapped in PS II units are probably much more efficient mechanisms than photorespiration to protect photosy stems against photoinhibition. Drought also causes large changes in carbon partitioning at the cellular and plant levels, and profound modifications in the composition of membrane proteins and lipids in the photosynthetic apparatus.