Endogenous melatonin content confers drought stress tolerance in pepper

Pepper plants are susceptible to drought stress, which occurs when there is a prolonged period of water shortage or inadequate soil moisture. Drought stress can lead to a range of physiological and biochemical changes in plants, which can ultimately result in reduced growth, yield, and overall crop quality. Numerous studies have shown that exogenous application of melatonin can improve drought tolerance in pepper plants by reducing oxidative damage, maintaining better water status, and enhancing photosynthetic efficiency. However, there are few studies that answer the question whether and how the level of endogenous melatonin affects drought stress tolerance of pepper plants. In this paper, the potential relationship between endogenous melatonin content and drought stress tolerance of sixty genotypes of long-fruited pepper (Capsicum annuum L.) subjected to drought stress for twelve days was investigated. Physical (leaf area), physiological (electrical conductivity (EC), visual damage index and water potential) and biochemical parameters (pre-stress and post-stress melatonin, hydrogen peroxide (H2O2), TBARS, proline, chlorophyll, carotenoid levels, as well as catalase-CAT, peroxidase-POX, and superoxide dismutase-SOD activities) were analyzed in the collected leaf samples. Correlation analysis was performed between pre-stress melatonin levels and all other parameters and when significant correlation was found, polynomial regression was carried out to determine the relationship between these components. The correlation analysis showed that pre-stress melatonin contents were in positive relation to the activity of antioxidant enzymes, post-stress melatonin and chlorophyll contents, while in negative relation to H2O2, TBARS, carotenoids, EC, visual injury index, and water potential parameters. The contents of endogenous melatonin in leaves of 60 genotypes ranged from 4.9 to 73.9 μg kg−1 FW. Low melatonin genotypes showed lower water potential, proline and chlorophyll contents and antioxidant enzyme activities (SOD, CAT, and POX), but higher EC, H2O2 and TBARS, suggesting they are much more susceptible to stress. Additionally, weighted scores of the genotypes considered to be susceptible were significantly lower than those of the tolerant ones. All these results suggest that there is a significant relationship between endogenous pre-stress melatonin content and drought stress tolerance. Thus, the possible use of endogenous melatonin content as a physiological marker in selecting drought stress-tolerant varieties of pepper was demonstrated.