Melatonin-Mediated Alleviation of Soil Salinity Stress by Modulation of Redox Reactions and Phytochemical Status in Guar (Cyamopsis tetragonoloba L.)

Salinity is one of the major environmental problems threatening crops production, especially in semiarid and arid areas of the world. Melatonin, a growth regulator, has recently been receiving significant attention for its critical role in improving plant salt tolerance. In this experiment, a 2-year field trial was conducted to examine the influence of four exogenous melatonin application rates (0, 100, 200, and 400 µmol L−1) on various physiological, biochemical, and yield characteristics of guar plants grown under four different NaCl salinity levels (0, 5, 10, and 15 dS m−1) conditions. According to the results, severe salinity stress caused reductions in leaf relative water content (RWC), total chlorophyll, carbohydrates, and phenol contents (by 9.8, 13.6, 42.1, and 25.3%), and stimulated proline level, superoxide dismutase (SOD), catalase (CAT), peroxidase (POX), ascorbate peroxidase (APX), and glutathione reductase (GR) activities (by 54.8, 60.6, 25.1, 51.9, 51.2, and 60.5%) in guar leaves, respectively. In addition, photosynthesis (Pn) and transpiration (Tr) values showed a significant decline with increasing salinity levels up to 45.9 and 77.2%, respectively. However, exogenous melatonin application, particularly at 200 µmol L−1, relieved physiological impacts of salt stress through increased proline (6–80%), phenol (8–27%), and chlorophyll (7–16%) contents and improved hydraulic functions in guar plants. Furthermore, melatonin-applied plants showed the greatest improvement in yield characteristics including biomass (14–106%), root length (8–13%), seeds per plants (8–36%), and seed gum (11–23%), and maintained higher viscosity and contents of protein and fat in the guar gum relative to untreated plants across four salinity levels. These findings suggested that melatonin improves plant resistance to salinity stress in guar and hence could be utilized to sustain crop yields in various regions affected by soil salinity.