Abstract: This study was conducted to clarify the mechanism underlying the alleviation saline-alkali stress of exogenous prohexadione-calcium (Pro-Ca) of soybean seedlings. The soybean variety ‘Hefeng 50’ was used as experimental material to examine the effects of foliar spraying Pro-Ca of 100 mg·L^-1on the growth, photosynthetic characteristics, antioxidant metabolism, AsA-GSH cycle and osmotic adjustment of soybean seedlings under 110 mmol·L^-1  saline-alkali stress. Results showed that saline-alkali stress significantly inhibited the growth of soybean seedlings, diminished the net photosynthetic rate (P_n) and the contents of chlorophyll, soluble protein, soluble sugar, sucrose, and starch, and increased catalase (CAT) activity and the accumulation of proline, reactive oxygen species (ROS), malondialdehyde (MDA), and electrolyte leakage. Compared with the saline-alkali treatment, exogenous application of Pro-Ca improved aboveground and root growth of soybean seedlings, increased the contents of chlorophyll a, chlorophyll b, carotenoid, and total chlorophyll, maintain higher P_n, and promote the accumulation of sucrose, fructose, and starch. Furthermore, the application of Pro-Ca drastically increased the activities of six antioxidant enzymes (SOD, POD, CAT, GR, MDHAR, and DHAR), the levels of two nonenzymatic antioxidants (AsA and GSH), and the content of proline in leaves. Meanwhile, the electrolyte leakage, O₂^-_· production rate, and contents of MDA and H₂O₂ significantly decreased. In conclusion, the exogenous application of Pro-Ca can improve the tolerance of soybean seedlings to saline-alkali stress by regulating antioxidant enzymes and AsA-GSH-cycle, increasing osmotic solute accumulation and reducing ROS damage.

Key words: prohexadione-calcium, soybean, saline-alkali stress, photosynthetic characteristics, AsA-GSH cycle.