Combined application of melatonin and salicylic acid improves drought tolerance in pepper (Capsicum annuum) through coordinated physiological and biochemical responses

Drought stress is a major environmental constraint limiting growth, water relations, and metabolic stability in pepper (Capsicum annuum). This study evaluated the individual and combined effects of melatonin (MEL) and salicylic acid (SA) on drought tolerance by assessing growth, water status, photosynthetic pigments, oxidative stress markers, osmotic adjustment, and antioxidant defence systems. Plants were grown under non-stressed and drought-stressed conditions and treated with MEL, SA, or their combination. Drought stress significantly reduced fresh weight, relative water content, and photosynthetic pigments, while increasing accumulation of hydrogen peroxide and malondialdehyde, indicating severe oxidative damage. Exogenous application of MEL and SA alleviated these effects, with the combined treatment providing the greatest protection. Under drought, the combined application improved biomass accumulation and tissue hydration, preserved chlorophyll and carotenoid contents, reduced oxidative damage, and increased proline accumulation. It also markedly increased superoxide dismutase, catalase, and peroxidase activities, showing enhanced detoxification of reactive oxygen species. These findings demonstrate that the greater drought tolerance conferred by the combined treatment is associated with coordinated regulation of water balance, redox homeostasis, osmotic adjustment, and photosynthetic stability. The combined application of MEL and SA is a promising strategy for improving drought tolerance in pepper cultivation.