Exogenous melatonin increases alfalfa (Medicago sativa L.) tolerance to salt stress

In contrast to acute stresses like drought, chilling, or pest infestation, salt stress causes progressive and long-term damage to plants. NaCl is the most important salt in nature and an important factor causing salt stress. Exogenous hormone regulation is one of the most influential and environmentally friendly methods recognized in recent years, Melatonin (MT) is an important plant defense component, and it has low cost and large income compared with traditional saline-alkali land improvement. To explore the effects of exogenous MT on the salt tolerance of alfalfa seedlings and the physiological response mechanism, in order to provide new ideas for salt-resistant cultivation of alfalfa, which is very important for sustainable agriculture. 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.