Spermidine application alleviates salinity damage to antioxidant enzyme activity and gene expression in alfalfa

We investigated whether spermidine (Spd) application alleviates salinity-induced damage in alfalfa (Medicago sativa L), and explored defence mechanisms associated with stress-related ion balance, antioxidant metabolism, and gene expression. We examined the response of 30-day-old alfalfa maintained in hydroponic culture tests for 7 days and subjected to one of six treatments: half-strength Hoagland solution (control); 1% NaCl; 10 μM Spd + 1% NaCl; 20 μM Spd + 1% NaCl; 40 μM Spd + 1% NaCl; and 60 μM Spd + 1% NaCl. In salinity-stressed plants, chlorophyll b, chlorophyll a + b, and total protein showed significant decreases, while marked increases were detected in relative electrolyte leakage, H₂O₂ content, glutathione (GSH), superoxide dismutase (SOD), catalase (CAT), peroxidase (POD), glutathione reductase (GR) activity, the Na⁺/K⁺ ratio, and APX1, APX2, GR, and SOD gene expression levels. Chlorophyll a and total protein content markedly increased under exogenous application of 20 μM Spd, while H₂O₂ content, GSH, SOD, CAT, POD, GR activity, the Na⁺/K⁺ ratio, and APX2, GR, and SOD expression levels all decreased. These results indicated that exogenous application of 20 μM spermidine effectively alleviates salinity-induced damage in alfalfa. These findings could benefit alfalfa cultivation and promote the development and utilization of saline-alkali soil.