Exogenous melatonin mitigates drought stress by promoting the living cortical area of roots in cotton

The root cortex plays a critical role in water absorption, influencing root function and metabolic activity. However, the effects of melatonin on root cortical cells and its role in drought resistance remain unclear. This study examines the impact of exogenous melatonin on the root cortex of cotton under drought stress, focusing on its relationship with water uptake and drought resilience. Cotton plants were treated with 8% PEG6000 for drought stress and 100 µM melatonin by foliar application. The results demonstrated that melatonin application significantly increased the living cortical area of roots under drought stress, with notable increases of 26.34% and 59.00% at distances of 1 cm and 13 cm from the root tip, respectively. Melatonin also enhanced root cortical thickness, promoted osmotic regulation, increased respiratory enzyme activity, and improved water and nutrient uptake. Furthermore, melatonin application promoted both root and shoot growth. Notably, the living cortical area positively correlated with osmotic substance accumulation, root respiration, and absorption capacity under drought conditions. In conclusion, exogenous melatonin alleviates drought-induced damage by enhancing the living cortical area, optimizing osmotic regulation, and improving root absorption, thus promoting cotton growth and drought resistance. These findings highlight melatonin as a promising regulator for enhancing drought resistance.