Abstract: To investigate whether exogenous-H²O²-induced drought tolerance is mediated by the increasing of antioxidant activities, agreenhouse-ecotype cucumber ‘Chunguang 2 ’was pretreated with exogenous hydrogen peroxide (H₂O₂ ) and was subjected to drought stress, which was simulated by 100 g·L ^{- 1} polyethylene glycol 6000. Without pretreatment of exogenous H₂O₂ , drought stress resulted in plants with round chlorop lasts, the indistinct chlorop lastmembranes and thylakoids. Compared to drought treatment itself, the combination of H₂O₂ pretreatment and drought stress did not change the chlorop last ultrastructure obviously, and it not only increased the activities of antioxidants such as superoxidedismutase, catalase, guaiacol peroxidase, ascorbate peroxidase, monodehydroascorbate reductase, dehydrateascorbate reducatase, glutathione reductase,ascorbic acid and the reduced glutathione, but also decreased the contents of malonaldehyde, endogenous H₂O₂ and superoxide radical. We conclude that the app lication of exogenous H₂O₂ increases the antioxidant activities, and therefore decreases the accumulation of reactive oxygen species and alleviates the damage to chlorop lastmembranes under drought stress.

Key words: cucumber, hydrogen peroxide, drought, ultrastructure, antioxidant