Growth and physiological response of root-applied melatonin on cotton (Gossypium hirsutum L.) cultivar ‘Lumian 532’ under drought stress

Drought stress is a major abiotic factor that limits cotton yield stability and severely impairs cotton growth, development, and fiber quality. Exogenous melatonin has significant potential in enhancing crop stress resistance, but its effects and mechanisms when applied to the roots in cotton's response to drought remain unclear. To clarify the regulatory role of melatonin in cotton's drought tolerance, a series of pot and field experiments were conducted using the 'Lumian 532′ variety as the material. Seedlings were subjected to either sufficient watering (soil relative water content 70–75 %) or drought stress (soil relative water content 40–45 %) conditions, with simultaneous treatments of root application of melatonin (50 μM, which was determined as the optimal concentration from preliminary screening of 10–300 μM) and no application of melatonin. A randomized design was adopted for the experiment, with each treatment replicated 3 times. Results showed that root-applied melatonin effectively mitigates the growth inhibitory effects of drought stress on plant growth. Specifically, an optimal concentration of 50 μM melatonin significantly increased plant height, root length, and root surface area by 36.79 %, 35.36 %, and 51.42 %, respectively, compared to untreated drought-stressed plants. Melatonin treatment significantly enhanced the net photosynthetic rate, stomatal conductance, and maximum photochemical efficiency of PSII during drought stress. Additionally, it enhanced the activity of antioxidant enzymes, including superoxide dismutase, peroxidase, and catalase, while promoting the accumulation of ascorbic acid and glutathione. Melatonin significantly reduced H 2 O 2 and O 2 − levels by 19.64–20.23 % and 43.83–68.78 %, respectively. Furthermore, melatonin application significantly increased the levels of gibberellin, indoleacetic acid, and endogenous melatonin in roots, while the content of abscisic acid was significantly reduced by 24.93 % (P < 0.05), thereby alleviating drought-induced hormonal imbalance and enhancing drought. resistance. Field trials revealed that root-applied melatonin significantly increased seed cotton yield by 20.10 % compared to drought treatment, primarily through elevated boll numbers per unit area. These research results indicate that the application of low-dose melatonin to the roots effectively enhances the drought resistance of cotton by synergistically improving photosynthesis, antioxidant capacity, and hormone balance. This provides a theoretical basis and practical reference for incorporating melatonin into agronomic management strategies to stabilize cotton yields under water-limited conditions. • Root-applied melatonin significantly mitigates the inhibitory effects of drought stress on cotton growth and yield. • An optimal concentration of 50 μM melatonin is identified to maximize drought resistance in cotton. • Melatonin boosts cotton drought tolerance via "photosynthetic enhancement-antioxidant synergy-hormone balance" cascade.