Abstract: Using root-splitting method, the resistance to cucumber damping-off induced by plant growth-promoting rhizobacteria CH1 and the changes in activities of superoxide dismutase ( SOD ) , peroxidase( POD) , catase (CAT) , polyphenol oxidase ( PPO) and phenylalanine ammonialyase ( PAL) in cucumber roots and leaves pretreated with PGPR CH1 and inoculated by Pythium aphanidermatum were studied. The results showed that PGPR CH1 could enhance the surviving seedling of cucumber, control efficacy reached 78.64% after CH1 treatment at 28 d, which indicated that induced resistance was the major factor for CH1 to
control the cucumber damping-off. The activities of SOD, POD, CAT, PPO and PAL in the roots treated by CH1 and the leaves nontreated by CH1 increased higher than those of other treatments to some extent and reached their maximum on 3rd, 3rd, 5th, 5th and 5th in roots, respectively, the activities of these protective enzymes in leaves were accorded with those of enzymes in roots. The experimental results suggested that the increased activities of SOD, POD, CAT, PPO and PAL induced by CH1 against Pythium aphanidermatum are most likely to be associated with the development of ISR in cucumber. Moreover, the resistance induced by CH1 is systemic.

Key words: Cucumber, Cucumis sativus L., Plant growth-promoting rhizobacteria, Protective enzymes, Cucumber dampimg-off, Induced resistance