To gain further understanding about the physiological and biochemical effects of Spd,we conducted the effect of spermidine (Spd) on induced resistance to powdery mildew in susceptible Cucumis melon variety ‘066’,‘0544’ and resistance variety ‘Yuntian-930’ seedlings.The activities of superoxide dismutase (SOD),peroxidase (POD),catalase (CAT),polyphenol oxidase (PPO),as well as chlorophyll a,chlorophyll b,total chlorophyll,carotenoid,hydrogen peroxide(H₂O₂) and malondialdehyde (MDA) contents were assayed in melon leaves after treated with Spd and inoculated with powdery mildew.The results show that:(1)the activities of four defense enzymes and the contents of photosynthetic pigments in melon leaves inoculated with powdery mildew were all raised at first,and then decreased.Furthermore,the content of MDA increased observably.(2)Spd mitigated the reduction of defense enzymes and the contents of photosynthetic pigments,and reduced the content of MDA and the disease index,and 1.0 mmol/L Spd had the best effects.(3) Foliar-spraying Spd made the content of H₂O₂ in melon leaves go up whether if inoculated powdery mildew or not.(4) Exogenous Spd had a wide spectrum on improving resistance to powdery mildew and the resistance could conduct upward.One idea is that H₂O₂ may act as a kind of transport signals for the induction resistance to powdery mildew by Spd in melon.In summary,the results indicated that Spd mitigate the powdery mildew stress which cause reduction of defense enzymes activities and photosynthetic pigment contents,increase plants photosynthetic capacity,and decrease disease index observably,then increase melon resistance to powdery mildew.