| To explore the physiological mechanism of polyamine to improve the drought tolerance of melon, 15% polyethylene glycol (PEG-6000) was used to simulate osmotic stress and with ‘Qi Tian No. 2’ cultivated by nutrient solution as the experiment material. The effects of 1 mmol·L-1spermidine (Spd) on the growth, reactive oxygen metabolism，antioxidant enzyme activity, osmotic regulating substances and non\|enzymatic antioxidants of melon seedlings at different drought times were investigated. The results showed that after 4 days of osmotic stress, 1 mmol·L-1 Spd increased the biomass of melon seedlings by 11.34% and the root activity of melon seedlings by 24.20%. The H2O2 content, O2 production rate, MDA content and electrolyte leakage of melon leaves and roots were reduced by 30.72%, 29.99%, 29.34% and 14.62% respectively in leaves, and 29.33%, 25.31%, 28.23% and 20.34%, respectively in roots. The antioxidant enzyme activities of SOD, POD, CAT and APX were increased by 25.54%, 28.70%, 27.72% and 28.41% in leaves, and increased by 23.35%, 27.38%, 25.06% and 20.34% in roots, respectively. Compared with osmotic stress treatment, the contents of proline, soluble sugar and soluble protein in leaves of melon seedlings under Spd treamtment increased by 46.51%, 49.72% and 40.25% after 16 days of osmotic stress. After 16 days of osmotic stress, the content of ASA in melon seedlings increased by 62.50%. After 12 days of osmotic stress, the content of GSH increased by 37.17%, ASA/DHA increased by 74.46%, GSH/GSSG increased by 60.95%. Therefore, under osmotic stress, 1 mmol·L-1 Spd can promote the growth of melon seedlings, stabilize the cell membrane system, enhance the antioxidant capacity of melon seedlings, and effectively alleviated the damage of osmotic stress on melon seedlings. |