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.

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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.