Dual Melatonin Enhances Coordination Between Carbon and Nitrogen Assimilation in Soybean

Soybean production is currently insufficient to meet global demand, highlighting the need for strategies to enhance growth. Melatonin (MT) has emerged as a promising solution due to its growth-promoting properties. This study investigated the effects of a dual MT treatment—combining seed soaking and foliar spraying—on soybean carbon and nitrogen metabolism using metabolomics analysis. The results demonstrated that MT treatment significantly upregulated the TCA cycle, providing energy and precursors for amino acid and carbohydrate synthesis. Key amino acid pathways, including histidine and phenylalanine metabolism, were enhanced, with histidine metabolism stimulating purine synthesis to improve biological nitrogen fixation and phenylalanine metabolism promoting secondary metabolite production to support growth. Additionally, carbohydrate pathways such as starch and sucrose metabolism and glycolysis/gluconeogenesis were positively regulated, ensuring energy supply and carbon homeostasis. Overall, dual MT treatment enhanced soybean metabolic capacity by promoting amino acid and carbohydrate metabolism, stimulating purine and secondary metabolite production, and maintaining carbon and nitrogen balance. These findings underscore melatonin’s regulatory role in soybean growth and provide insights for improving crop productivity.