The effect of exogenous spermidine(Spd) on inducing resistance of Cucumis melo L.to powdery mildew was conducted with the susceptible cultivars ‘0544’,and the photosynthetic parameters,activities of antioxidant enzymes,contents of endogenous polyamines and the genes expression of polyamine synthesis and metabolism were tested in melon seedlings after pretreated with Spd then inoculated with powdery mildew.The results showed that:(1)1.0 mmol·L^-1 exogenous Spd could significantly reduce the disease index in melon seedlings resistance to powdery mildew and relieve the symptoms;(2)the genes related to polyamine syntheses and metabolism(SAMDC,ADC,ODC,Spd,Spm,PAO) were significantly induced by the exogenous Spd treatment during the seedling stage;(3)the contents of putrescine (Put) and spermine (Spd) were induced more significantly by the exogenous Spd while the spermine (Spm) content accumulated more by both the exogenous Spd treatment and powdery mildew inoculation;(4)the defense enzymes of melon seedlings such as polyphenol oxidase(PPO),peroxidase (POD) and superoxide dismutase(SOD) activities were induced after powdery mildew inoculation,with both the exogenous Spd treatment and powdery mildew inoculation.Meanwhile,the H₂O₂ contents increased as well after the exogenous Spd treatment;(5)during the period of observation(120 h),the photosynthetic parameters were less affected after the powdery mildew inoculation.However,the photosynthetic rate(P_n),stomatal conductance(G_s) and transpiration rate (T_r) increased significantly by both the exogenous Spd treatment and powdery mildew inoculation.This study suggested that pretreatment melon seedlings with the exogenous Spd before powdery mildew inoculation could obviously improve the resistance of melon to powdery mildew,which might via rapid accumulation of PAs and H₂O₂ contents metabolized form polymine,and then induced the activities of defense enzymes.