Chroloplast CuZn-superoxide dismutase and catalase are earliest targets of osmotic stress in lettuce plants. ,Chroloplast CuZn-superoxide dismutase and catalase are earliest targets of osmotic stress in lettuce plants.

Abstract

Early biochemical events of osmotic stress occurring in lettuce (Lactucta sativa L.) leaves were investigated in order to elucidate the mechanisms of drought-induced damages in plants. Osmotic stress was given to 3-4 week-old water-cultured lettuce plants by transferring them into sorbitol solutions for limiting water uptake in roots. A mild stress (0.25 M sorbitol, 100 µmol/m2/s white light), which did not wilt leaves up to 4 h, enhanced the level of the methylviologen-and-light-mediated monodehydroascorbate radical in leaves in 1 h, as determined by in vivo ESR. At the same time, superoxide dismutase (SOD) activity in leaves was decreased by 40%. The activity decrease was mostly attributed to the inactivation of chloroplastic (Cp) CuZnSOD, as judged by electrophoresis/activity staining. Cytosolic CuZnSOD was much less affected by the same treatment. Sorbitol did not inhibit SOD activity in vitro. Inactivation of Cp-SOD in leaves required light. At the concentration higher than 0.5 M, catalase also was inactivated in 1 h. Thylakoid-bound ascorbate peroxidase, MDA reductase and Cp-electron transport chain (H2O to NADP+) were not affected by a 1 M sorbitol treatment up to 4 h. These results indicate that Cp-CuZnSOD is the specific and primarily inactivated target and catalase is the secondly sensitive one. Inactivation of these enzymes would lead to oxidative damages in droughted leaves.