Exogenous Application of Melatonin Induces Tolerance to Salt Stress by Improving the Photosynthetic Efficiency and Antioxidant Defense System of Maize Seedling

Melatonin is a ubiquitous signaling plant hormone that plays a crucial role in regulating the growth and development of plants under stress conditions. Since a few species have been investigated to unveil the effect of exogenous melatonin on salt stress, in the current research study, we investigated the effects of melatonin by measuring the photosynthetic characteristics and antioxidant defense system of maize seedling under salt stress (200 mM NaCl), along with different concentrations of melatonin (MT 1 —30, MT 2 –60, and MT 3 –90 µM) treatments. Salt stress reduced the plant growth characteristics and photosynthetic efficiency by increasing the ROS accumulation and reducing the antioxidant enzyme activities of maize seedling. However, pretreatment with melatonin on roots alleviated NaCl-induced decrease in photosynthetic rate and oxidative stress in a dose-dependent manner. Our results revealed that exogenous application of melatonin at an optimum concentration (60 µM) under salt stress conditions significantly increased the growth of plants, chlorophyll content, photosynthetic efficiency, antioxidant enzymes activities, i.e., superoxide dismutase (SOD), peroxide (POD), catalase (CAT) and ascorbate peroxidase (APX), and reduced the reactive oxygen species (ROS). This study unraveled the crucial role of melatonin in salt stress mitigation and thus can be implicated in the management of salinity in maize seedling.