Exogenous Melatonin Promotes Nitrogen Use Efficiency in Cotton under Drought and Low Nitrogen Stress

Melatonin (MT) is a key growth-regulating substance in plants that plays a significant role in enhancing crop stress tolerance. This study investigated the effects of foliar application of melatonin on the antioxidant enzyme system, nitrogen absorption and translocation characteristics, and nitrogen metabolism-related enzyme activities in cotton leaves under drought and low-nitrogen stress. Two cotton varieties were used: the low-nitrogen-tolerant cultivar 'Z69' and the low-nitrogen-sensitive cultivar 'X30', in a pot experiment. The results showed that although stress conditions induced proline accumulation and increased antioxidant enzyme activities, they also led to a significant rise in malondialdehyde (MDA) content, indicating aggravated membrane lipid peroxidation. Exogenous MT treatment further enhanced antioxidant capacity, significantly increasing the activities of superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT), as well as proline accumulation, effectively suppressing MDA accumulation and mitigating membrane lipid peroxidation damage. Furthermore, melatonin treatment significantly enhanced the activities of key nitrogen metabolism enzymes, including nitrate reductase (NR), nitrite reductase (NiR), glutamine synthetase (GS), glutamate synthase (GOGAT), glutamate-pyruvate transaminase (GPT), glutamate-oxaloacetate transaminase (GOT), and glutamate dehydrogenase (GDH). The synergistic activation of this multi-enzyme system promoted an increase in plant nitrogen content and nitrogen accumulation, while also improving nitrogen absorption efficiency and nitrogen fertilizer utilization efficiency. Additionally, 'Z69' exhibited stronger stress tolerance compared to 'X30'. This study elucidated the physiological mechanism by which melatonin enhances drought and low-nitrogen tolerance in cotton through the synergistic regulation of the antioxidant system and nitrogen metabolism, providing a theoretical basis for breeding stress-tolerant and high-quality cotton varieties.