Exogenous Melatonin Enhances Pollen Fertility of Drought-Stressed Cotton (Gossypium hirsutum L.) by Improving Antioxidant Metabolism

Exogenous melatonin (MT) can aid crops in mitigating drought stress, yet its impacts on cotton pollen fertility under drought remain understudied. To address this, a study on the effects of exogenous MT on cotton pollen sterility and internal physiological metabolism under drought stress was conducted. Results showed that although antioxidant enzymes catalase (CAT), peroxidase (POD) and superoxide dismutase (SOD) activities increased in drought-stressed anthers, yet this was insufficient to balance reactive oxygen species (ROS) levels, which led to higher hydrogen peroxide (H2O2) and superoxide anion (O2-) accumulation, resulting in oxidative stress, manifested by elevated malondialdehyde (MDA) and reduced pollen viability. Remarkably, the application of MT further bolstered the activities of CAT, POD and SOD in drought-stressed anthers, effectively mitigating the accumulation of O2- and H2O2. This enhancement facilitated the antioxidant defence system under drought conditions. Simultaneously, the activity of ascorbate peroxidase (APX) was reduced, which prevented the reduction of dehydroascorbate (DHA) to ASA, and the ascorbic acid-glutathione (AsA-GSH) cycle was destroyed, leading to the accumulation of GSH. Nevertheless, MT application triggered upregulation of both GhDHAR and GhMDHAR expressions, promoting AsA synthesis and regeneration, respectively, thereby elevating AsA levels. Furthermore, MT revitalised APX activity, accelerating the removal of H2O2 through AsA-mediated reduction, ultimately restoring the balance of the AsA-GSH cycle and enhancing the overall antioxidant capacity of drought-stressed anthers. In summary, exogenous MT can enhance the scavenging ability of ROS in drought-affected anthers by elevating the activities of CAT, POD and SOD and by enhancing APX activity in the AsA-GSH cycle, thereby alleviating drought-induced oxidative stress and improving pollen fertility under water deficit stress.