Melatonin mediated modulation of physiological, biochemical and gene expression patterns governing salinity tolerance in Capsicum frutescens

Salinity-induced stress severely impairs the growth and physiological performance of Capsicum frutescens L. This study evaluated the role of foliar-applied melatonin in mitigating NaCl-induced stress in salt-tolerant and salt-sensitive genotypes. Under control conditions, plant height was 30.5 cm (tolerant) and 28.4 cm (sensitive), which declined to 21.2 cm and 19.2 cm under NaCl stress. Melatonin application (20 µM) restored height to 24.8 cm and 22.4 cm, respectively. Leaf area decreased from 41.7 to 31.6 cm² (tolerant) and 39.5 to 29.3 cm² (sensitive) under salinity, but improved to 36.4 and 32.8 cm² with melatonin. Fresh weight declined from 15.7 to 9.8 g (tolerant) and 14.2 to 8.4 g (sensitive), while dry weight decreased from 5.78 to 3.54 g and 5.12 to 3.13 g, respectively; melatonin significantly improved biomass accumulation. Salinity reduced SPAD values (52 to 43; 50 to 40) and relative water content (86% to 62%; 81% to 56%) while increasing electrolyte leakage to 39% and 46%. Melatonin enhanced SPAD (46; 42), RWC (70%; 64%), and reduced leakage (29%; 33%). Oxidative damage markers increased under NaCl, as MDA rose to 110 and 125 nmol g-¹ FW and H₂O₂ to 4.2 and 5.5 nmol g-¹ FW in tolerant and sensitive genotypes, respectively; melatonin notably reduced the levels of H₂O₂ and MDA. It was observed that antioxidant activity was reduced due to salinity stress. Accumulation of Na⁺ increased to 35 and 42 mg g-¹ DW, while K⁺ declined, elevating the Na⁺/K⁺ ratio; application of melatonin improved ionic balance. Gene expression results indicated that NaCl exposure upregulated the CaSOS1 and CaSOS2, whereas exogenous application of melatonin reduced their expression and upregulated CaSOD and CaCAT. Conclusively, melatonin improved salinity tolerance through coordinated regulation of growth, antioxidant defense and ionic homeostasis.