Differential Effects of Melatonin on Nitrogen Metabolism and Growth in Capsicum chinense Jacq

Melatonin has been recognized as a regulator of plant growth and stress responses; however, its role in nitrogen metabolism, particularly in Capsicum chinense Jacq., remains poorly understood. In this study, we evaluated the effect of melatonin priming alone (chemopriming, CP) or in combination with substrate reinforcement (soil drench, SD) (CP + SD), at different doses, on physiological, biochemical, and molecular parameters in habanero pepper plants growing in non-stressed conditions. Growth traits, nitrogen-related metabolites, total carbohydrates, the expression of key genes involved in nitrogen metabolism, and glutamate dehydrogenase (GDH) activity were analyzed. The results showed that priming increased biomass mainly at 25 and 50 µM, whereas the combined treatment promoted improvements from early doses (5 µM). Additionally, the evaluated genes exhibited tissue-specific expression patterns. Most genes were downregulated in leaves, whereas proline and total carbohydrate levels correlated with fresh and dry weight. Our results provide new insights into the regulation of nitrogen metabolism according to the application method and dosage, establishing an application strategy to optimize nitrogen metabolism and growth in habanero peppers. The mechanism of melatonin in regulating the balance of C and N metabolites and its impact on the response of chili plants to growth and environmental signals should be addressed later.