Exogenous Melatonin Increases Alfalfa (Medicago sativa L.) Tolerance to Salt Stress

Salinity affects the growth and development of plants, including alfalfa (Medicago sativa L.). Melatonin (MT) is an important plant defense component. This study aimed to elucidate the effects of MT on alfalfa under salt stress conditions. Two types of alfalfa with different salt tolerances under normal growth (0 mmol·L-1 NaCl) and salt stress (150 mmol·L-1 NaCl) conditions were treated with 0, 50, 100, 150, and 200 µmol·L-1MT. MT treatment increased plant height, leaf area, and root morphology and repaired chloroplast damage. Osmoregulatory substances such as proline, soluble sugars, and soluble proteins were significantly elevated by NaCl treatment and continued to increase after MT treatment. MT inhibited lipid peroxidation, cellular membrane defects, and reactive oxygen species accumulation induced by NaCl stress. The NaCl stress decreased the K+ concentration and increased the Na+ concentration. However, MT treatment reversed these levels and increased antioxidant enzyme activities and antioxidant nonenzymatic substances to scavenge excess ROS, inhibited the degradation of leaf SPAD values, and promoted photosynthetic gas exchange and chlorophyll fluorescence parameters. The most effective treatment for inhibiting salt stress was 150 µmol·L-1 MT. These results indicate that salt-tolerant and salt-sensitive alfalfa can be sprayed with MT to improve NaCl tolerance.