Effects of exogenous nitric oxide on the growth and membrane lipid peroxidation of Medicago sativa under salt stress

Sodium nitroprusside (SNP) agent (0.05, 0.1, 0.2, and 0.3 mmol·L^–1) was used as an exogenous nitric oxide source to study the effects of exogenous NO on the growth of alfalfa (Medicago sativa) seedlings under three different salt stress conditions (24, 72, and 120 mmol·L^–1). The results showed that the fresh weight, dry weight, and chlorophyll contents of alfalfa decreased. Whereas, the relative electrical conductivity, malondialdehyde (MDA), hydrogen peroxide (H₂O₂) content, and peroxidase (POD) activity had a remarkable increase (P < 0.05). Additionally, the ascorbic acid (AsA), superoxide dismutase (SOD), and catalase (CAT) activities were significantly inhibited (P < 0.05) under the salt stress, indicating the toxic effects of salt stress on alfalfa. SNP treatments significantly (P < 0.05) improved the fresh weight, dry weight, chlorophyll, chlorophyll a/b, and AsA content and enhanced the SOD, CAT, and POD activities, but decreased the relative electrical conductivity, MDA, and H₂O₂ contents. This suggests that exogenous NO can enhance antioxidant enzyme activities and decrease lipid peroxidation, thereby improving the resistance of alfalfa to salt stress. The optimum SNP concentration of alfalfa under 24, 72, and 120 mmol·L^–1 salt stress was 0.1, 0.3, and 0.3 mmol·L^–1, respectively.