Exogenous Melatonin Counteracts Salinity and Cadmium Stress via Photosynthetic Machinery and Antioxidant Modulation in Solanum lycopersicum L

Being sessile, plants’ exposure to various environmental stresses during their life cycle is inevitable which can affect their yield and productivity. This study investigates the protective effect of exogenous application of melatonin on 30-day-old tomato plants under NaCl and cadmium stress treatments. Plant growth, photosynthetic pigment, and antioxidant enzymes of plants exposed to NaCl, cadmium (Cd), and NaCl + Cd stress and melatonin treatment on them were analysed. The plants under NaCl- and Cd-induced stress produced an increased amount of ROS and lipid peroxidation, which were significantly reduced upon melatonin application. The maximum quantum energy (FV/FM) and performance index (PIABS) significantly improved in melatonin-treated plants under stress conditions. Redox homeostasis was maintained with a significant increase in superoxide dismutase, catalase, ascorbate peroxidase, and glutathione reductase. This study suggests that exogenous melatonin improves plants’ ability to overcome the combination stress caused by NaCl and Cd by increasing overall photosynthetic capacity and modulating redox balance.