Melatonin-mediated functional responses to water deficit in Phaseolus vulgaris L.: mechanistic insights across physiological, biochemical, and genomic scales

Agricultural productivity is increasingly constrained by water scarcity, which affects nearly one-quarter of cultivated land and is projected to intensify due to climate change and escalating freshwater demands. Melatonin is widely recognized as a potent biostimulant that plays a crucial role in mitigating various abiotic stresses, particularly drought, across many plant species. This study demonstrates that exogenous application of melatonin (150 μM; foliar spray) confers protection to Phaseolus vulgaris under moderate water deficit (40% field capacity). A randomized complete block design comprising four treatment groups (n = 30 seedlings per group, five replicates) was employed to systematically evaluate morphological, physiological, biochemical, and molecular responses. Melatonin applications at 21 and 28 days after sowing significantly enhanced shoot elongation, leaf area expansion, and photosynthetic efficiency. Biochemically, melatonin markedly increased the activities of key antioxidant enzymes (ascorbate peroxidase (APX), catalase (CAT), and peroxidase (POD)), reduced reactive oxygen species accumulation, elevated proline content by 24%, and decreased electrolyte leakage by 18%, thereby improving osmotic balance and maintaining membrane integrity. Genomic stability was assessed using Inter-Simple Sequence Repeat (ISSR) and Random Amplified Polymorphic DNA (RAPD) markers, revealing that melatonin substantially attenuated drought-induced DNA damage. Marker analysis further demonstrated differential sensitivity, and key statistical indices, including polymorphism information content (PIC), effective multiplex ratio (EMR), and resolving power (RP), exhibited strong linear associations, reinforcing the reliability of molecular diagnostics. Collectively, these results highlight melatonin's multifaceted role in enhancing water-deficit resilience through integrated regulation of physiological homeostasis, oxidative stress mitigation, and genome protection. The findings support melatonin's practical potential as a low-cost, environmentally compatible strategy for improving legume performance in water-deficit environments.