Melatonin Mitigates Salt-Induced Growth Inhibition Through the Regulation of Carbohydrate and Nitrogen Metabolism in Tomato Seedlings

Soil salinization is a serious threat to agricultural production because it limits the growth and development of crops. Melatonin is a pleiotropic molecule widely used to alleviate abiotic stressors, including salt stress. The objective of the present study was to investigate the function of melatonin in metabolic processes related to the mitigation of salt-induced growth inhibition in tomato seedlings. The hydroponically grown tomato seedlings were foliar sprayed with 100 µM melatonin and salt (NaCl) treatment was applied in tomato roots for 7 days. The experiment was composed of four treatments: (1) control; (2) melatonin (MT, 100 µM); (3) salt (125 mM NaCl); and melatonin + NaCl (MT + salt, 100 µM + 125 mM NaCl). The results revealed that melatonin treatment significantly mitigated salt-induced adverse effects as evidenced by higher biomass production, greater accumulation of K + , and lower content of Na + in salt-stressed tomato leaves and roots, resulting in lower Na + /K + ratio as compared with only salt-stressed seedlings. Melatonin application enhanced photosynthetic efficiency through the elevation of gas exchange and chlorophyll fluorescence parameters values, along with the increment of carbon assimilation enzymes activity. Moreover, sucrose content and sucrose phosphate synthase (SPS) activity were increased in melatonin-received tomato seedlings over to salt-stressed plants. Melatonin also regulates gene expression related to sugar transporters and carbohydrate metabolism in plants exposed to salinity stress. Additionally, plants treated with melatonin showed higher nitrate reductase and nitrate reductase activity by 78% and 62%, respectively, compared with plants under salt stress. Similarly, the nitrogen assimilation capacity (higher nitrate, NO 3 − level) was increased in melatonin treatment plants. Taken together, the present research findings indicate that melatonin attenuates salinity-caused growth inhibition through the coordination action of carbohydrate and nitrogen metabolism and the balancing of Na + /K + homeostasis in tomato seedlings.