Abstract:In order to investigate the effects of exogenous spermidine (Spd) on the physiological and molecular mechanism of melon seedlings under chilling stress, we carried out the experiment using the melon varieties ‘Shijimi’ (chillingtolerant) and ‘GL1’ (chillingsensitive) in climate chambers and substrates to investigate the alleviating effects of Spd on melon seedlings for growth, photosynthesis and reactive oxygen metabolism. The expression patterns of four different antioxidant enzyme genes were analyzed using quantitative PCR under chilling stress and recovery. The results showed that: (1) the growth, photosynthesis and chlorophyll contents of melon seedlings were decreased under chilling stress, the antioxidant enzyme activities and the expression patterns of four different antioxidant enzyme genes were increased, and the influence on ‘GL1’ was greater than that on ‘Shijimi’; (2) Spd could increase melonseedling growth, chlorophyll contents, net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr), stomatal limitation (Ls), water use efficiency (WUE), maximum photochemical efficiency (Fv/Fm), light energy capture efficiency of photosystemⅡ (Fv′/Fm′) and the apparent electron rate(ETR),while reduce intercellular carbon dioxide concentration (Ci) and initial fluorescence (Fo) under chilling stress; (3) Spd changed antioxidant enzyme activities and gene expression patterns under chilling stress. The expression of superoxide dismutase (SOD), catalase (CAT), peroxidase (POD) and ascorbate peroxidase (APX) genes increased and antioxidant enzyme activities were improved, and the injury of chilling stress was reduced. (4) Compared with the control, Spd decreased or increased the indexes of ‘GL1’under low temperature stress more than that of ‘Shijimi’. Obviously, Spd was favorable for the melon seedlings to maintain photosynthesis, improve photochemical electron transport efficiency, capture and converse solar energy, the antioxidant enzyme activities and the expression patterns of antioxidant enzyme genes, and thus melon growth were improved, and inhibition of chilling stress on melon was reduced. These findings suggested that ameliorative effect of Spd on melon seedlings under chilling stress was effective, especially for chillingsensitive genotypes.