Ameliorative Effect of Melatonin on Morphological and Biochemical Parameters of Salt‐Tolerant and Sensitive Genotypes of <i>Macrotyloma Uniflorum</i> (L.) Verdc.

Salt stress is a major challenge to the production of legumes as it affects their germination, growth, yield, and general physiology. Further, it also impairs their nitrogen fixation by causing osmotic stress and sodium-induced nutrient imbalances. Treatment with melatonin, a potent antioxidant and growth regulator, leads to the metabolic reprogramming of the plants to improve the stress tolerance capabilities of plants, and therefore, could be useful for improving resilience to salt stress. Horsegram, a nutraceutical legume, shows genotype-dependent variability toward salt stress tolerance; however, how the contrasting genotypes (salt-tolerant and susceptible) respond to melatonin treatment is relatively unclear. Based on our previous findings, two salt-tolerant (DH-22 and DH-29) and two salt-sensitive (DH-11 and DH-12) genotypes of horsegram were selected to investigate the effects of melatonin on salt-stressed seedlings. All four horsegram accessions were subjected to exogenous melatonin treatment at concentrations of 50, 100, and 150 µM under 100 mM NaCl. Results suggested that the melatonin treatment significantly improved growth and biochemical profiles in all horsegram genotypes under salt stress. The tolerant genotypes showed superior root and shoot growth, higher relative water content, chlorophyll, carotenoids, proline, phenolic, and flavonoid compounds, and increased antioxidant enzyme activity. They also exhibited lower levels of oxidative stress markers like malondialdehyde, hydrogen peroxide, and ion leakage compared with sensitive genotypes. The histochemical staining methods involving 3,3'-diaminobenzidine, nitro blue tetrazolium, and trypan blue stains further indicated the melatonin-induced reduction in cell death and reactive oxygen species accumulation in salt-affected horsegram seedlings. Overall, the tolerant genotypes were recorded to respond better to melatonin-mediated stress amelioration than the sensitive genotypes. Further, the present study also underlines the melatonin's potential to improve stress tolerance in legume crops like horsegram to improve salt stress resilience. Future research should concentrate on identifying the molecular processes that explain horsegram's melatonin-mediated salt tolerance. Furthermore, field-based assessments are required to confirm its usefulness for crop development programs.