Melatonin-Induced Salinity Tolerance by Ameliorating Osmotic and Oxidative Stress in the Seedlings of Two Tomato (Solanum lycopersicum L.) Cultivars

Melatonin is a crucial biological hormone associated with many physiological and biochemical processes in plants and also enhances resistance against various abiotic stresses. However, the mechanisms underlying the melatonin-assisted mitigation of salt stress in tomato (Solanum lycopersicum L.) plant are still poorly understood. A hydroponic experiment was conducted to investigate the protective role of melatonin in two tomato cultivars (Roma and FM9) under a highly saline growth medium (160 mM NaCl). The one level of melatonin (1.0 µmol L−1) was applied exogenously, sole, or in combination with the salinity stress. NaCl-induced phytotoxicity significantly (P < 0.05) reduced shoot and root dry matter accumulation, chlorophyll contents, relative water contents (RWC), membrane stability index (MSI), and antioxidant enzymatic activities in both cultivars as compared to the control treatment. Moreover, salt treatment alone increased soluble sugar contents (sucrose and fructose), sodium (Na+) uptake, as well as oxidative damage in the leaves of tomato seedlings. However, exogenous supply of melatonin alleviated salt toxicity in tomato seedlings which were more obvious in Roma cultivar as compared to FM 9 cultivar, as demonstrated by a higher increment in the values of growth indicators, RWC, MSI, gaseous exchange attributes, activities of superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and ascorbate peroxidase (APX). In addition, melatonin also alleviated salt-induced oxidative stress by suppressing malondialdehyde (MDA) and hydrogen peroxide (H2O2) contents as well as significantly reduced Na+ uptake at the root surface of tomato plants. It can be concluded that melatonin-induced salt tolerance in tomato is due to enhancement of plant water relations, and improved photosynthetic and antioxidant capacity along with ion homeostasis.