Melatonin improves antioxidant capacity and ion homeostasis and enhances salt tolerance in maize seedlings

Melatonin (N-acetyl-5-methoxytryptamine) is a biological hormone involved in many important physiological processes in plants. To better understand the protective roles of melatonin in plants against salt stress, we determined the effects of exogenous melatonin (1 µM) on plant growth, photosynthesis, antioxidant enzyme activity and ion homeostasis in maize plants under 100 mM NaCl. Results showed that salt stress decreased plant dry matter accumulation, net photosynthetic rate (Pn) and chlorophyll contents. However, 1 µM melatonin application significantly alleviated this growth inhibition and enhanced Pn by 19 %. Melatonin application also enhanced the activities of antioxidative enzymes of salt-stressed maize leaves, and decreased their electrolyte leakage and MDA content by 25 and 22 %, respectively. In addition, melatonin application significantly increased K+ contents and K+/Na+ ratios in salt-stressed maize shoots by 18 and 52 %, respectively. However, the Na+ content was decreased significantly in melatonin-treated leaves under salinity. The results suggested that the melatonin enhanced maize salt tolerance in maize were most likely due to the improvement of photosynthetic capacity, antioxidative capacity and ion homeostasis in leaves. This study provides, for the first time, the evidence that support the protective roles of exogenous melatonin in maize against salinity.