Melatonin regulates growth, photosynthesis, and stress responses in pepper (Capsicum annuum L.) under flooding stress.

Flooding stress is a major abiotic constraint that disrupts plant growth, photosynthesis, and cellular homeostasis. The application of biostimulants has emerged as an effective strategy to mitigate damage caused by flooding stress. This study evaluated the effects of exogenous melatonin applied at different concentrations on the physiological, biochemical, and multivariate responses of pepper (Capsicum annuum L.) plants subjected to flooding stress. Treatments included full irrigation (Control), flooding stress without melatonin (Flo-Mel0), and four melatonin doses under flooding conditions (Flo-Mel50, Flo-Mel100, Flo-Mel150, and Flo-Mel200). Flooding stress significantly reduced plant growth, photosynthetic performance, and pigment contents, while markedly altering plant water status and oxidative stress-related parameters. Melatonin application dose-dependently modulated these stress-induced responses by improving growth attributes, photosynthetic traits, and antioxidant defense capacity, while reducing oxidative damage. Multivariate analyses, including principal component analysis and correlation heatmap analysis, revealed a clear separation between growth- and photosynthesis-related traits and those associated with stress and defense responses. Among the tested treatments, the 50 µM melatonin dose produced the most balanced and favorable response across multiple parameters. Collectively, these findings provide integrated physiological and biochemical evidence for the regulatory role of melatonin in enhancing adaptive responses of pepper plants under flooding stress. Moreover, they offer a foundation for future molecular and applied research.