Abstract: To understand the effects of 100 μmol·L^-1 exogenous melatonin on photosynthesis and physiology of soybean during podding stage under drought stress, a pot experiment was carried out with soybean cultivar ‘Suinong 26’. The results showed that, compared to normal water supply, drought stress inhibited the growth and decreased the pod number of soybean, and consequently reduced the yield. Drought stress continuously decreased the photosynthesis and chlorophyll fluorescence, with antioxidant enzyme activity and osmotic adjustment substances being increased first and then decreased, membrane lipid peroxidation levels being increased. Exogenous melatonin increased the fresh weight, dry weight, plant height, leaf area and pod number in the podding stage of soybean under drought stress and thus increased yield, alleviated the decline of photosynthesis and chlorophyll fluorescence, increased the activities of antioxidant enzyme (SOD, POD, CAT, APX) and the contents of osmotic adjustment substances (soluble sugar, soluble protein, proline) in leaves, and regulated membrane lipid peroxidation balance (MDA, H₂O₂ and relative conductivity). Meanwhile, melatonin treatment increased the P_n,T_r and G_s of soybean leaves under drought stress by 14.1%-30.0%, 7.3%-19.6%, 12.4%-20.3%, respectively; increased F_v/F_o, F_v/F_m, Φ_PSII and ETR by 3.5%-15.5%, 5.4%-9.4%, 2.6%-5.9%, and 3.0%-18.2%, respectively. Compared with drought stress, the fresh weight of grain, dry weight of grain, grain number, pod number and plant height under melatonin treatment increased by 37.5%-51.4%, 35.2%-150.0%, 23.4%-42.1%, 6.6%-28.7%, 2.6%-9.6% respectively; the number of pods per plant, the number grain of per plants, the grain weight per plant and the 100-grain weight increased by 8.6%, 7.1%, 10.5% and 6.5% respectively. In conclusion, exogenous melatonin can enhance photosynthesis and antioxidant enzyme activities and alleviate growth inhibition under drought stress, which enhances the resistance of soybean to drought and increases its yield.

Key words: total soil nitrogen, soil organic carbon, fine root, litter, vegetation types.