Abstract: Spraying melatonin is one of the effective ways to alleviate the damage of soybean under stress. Clarifying the regulation effect of spraying melatonin on the growth of soybean seedlings under soda saline-alkali stress can provide theoretical basis and technical support for alleviating soda saline-alkali stress of soybean seedlings. Using a sand culture experiment method, we set up four treatments, including conventional treatment + spraying water (CK), conventional + spraying melatonin treatment (CKM), soda saline-alkali stress treatment + spraying water (S), soda saline-alkali stress + spraying melatonin treatment (SM). The response of different physiological indices in seedling stage of soybean cultivar Henong 71 to soda saline-alkali stress was investigated. The results showed that compared with conventional treatments (CK and CKM), the growth of soybean seedlings under soda saline-alkali stress (S and SM) was inhibited. Compared with S treatment, SM treatment significantly increased the initial fluorescence (F₀) and maximum fluorescence yield (F_m) of PSII reaction center, increased the non-photochemical quenching coefficient (NPQ), enhanced the excess light energy dissipation of crop photosynthetic mechanism, improved the photoprotection ability of PSII reaction center, and significantly increased net photosynthetic rate (P_n). The activity of antioxidant enzymes (SOD, POD, and APX) and scavenging ability of ROS in leaves, and the contents of sugars and soluble proteins in roots and stems were significantly increased. The MDA production rate in leaves and roots was significantly decreased. Root dry matter accumulation, total surface area, total volume and average root diameter were significantly increased. The intracellular hormone levels in leaves were maintained, and the contents of IAA, CTK, ETH and MT in leaves were increased by 15.9%, 31.9%, 8.7%, and 55% (P<0.05), respectively. In conclusion, spraying melatonin can alleviate the adverse effects of salinealkali stress on the physiological indices of soybean seedlings and improve their saline-alkali tolerance.

Key words: soda saline-alkali stress, melatonin, soybean, photosynthetic fluorescence characteristics, endogenous hormone.