EFFECTS OF TEMPORARY DROUGHTS ON PHOTOSYNTHESIS OF ALFALFA PLANTS

The effect of temporary droughts on photosynthesis. total conductance to water vapour, intercellular CO2 concentration, CO2 compensation point, light-response curves, photorespiration. dark respiration. chlorophyll content. and ribulose 1.5-bisphosphate (RuBP) carboxylase (EC 4.1.1.39) activity has been examined in nodulated alfalfa plants (Medicago sativa cv. Aragon). Plants were subjected to moderate (S1/RS1) or severe (S2/RS2) cycles of drought (drought recovery). Photosynthetic light-response curves showed decreased light-saturated photosynthetic capacity and decreased apparent quantum yield. Upon rewatering, photosynthesis did not recover whereas conductance did in moderately stressed plants. Calculated electron transport rate also declined in drought-stressed plants, but upon rewatering, moderately stressed plants exhibited a total recovery. Comparison of photosynthetic intercellular CO2 response curves in well-watered and stressed leaves led to the assertion that effects in chloroplast metabolism contribute significantly to photosynthetic inhibition. Although the validity of this entire line of research has been questioned by some recent studies because the occurrence of heterogeneous stomatal closure would affect these curves, in our case, the effect of water stress was investigated in experimental systems where stomata had been removed. Measurements of in vitro RuBP carboxylase activity and protein content showed a strong decline during drought treatments and upon rewatering no recovery was observed. Therefore, our results suggest the major implication of non-stomatal factors in the decline of photosynthesis in alfalfa plants under cyclic drought conditions.