Melatonin enhances drought tolerance in lemongrass: physiological, biochemical, and antioxidant mechanisms

Drought stress is a critical constraint on plant growth and productivity in arid and semi-arid regions, significantly impairing the physiological traits and antioxidant capacity of medicinal plants. Lemongrass (Cymbopogon citratus), a medicinal and aromatic plant valued for its antioxidant and antimicrobial properties, is highly sensitive to environmental stresses. Melatonin, a multifunctional molecule involved in growth regulation and stress tolerance, shows promise in mitigating drought-induced damage; however, its application in medicinal plants remains underexplored. This study investigated the effects of foliar-applied melatonin (0, 100, and 200 µM) on lemongrass under varying drought stress levels (100%, 70%, and 40% field capacity) in controlled greenhouse conditions. Physiological parameters (chlorophyll content, stomatal conductance, relative water content), growth indices (leaf and root dry weight), compatible solutes (proline), oxidative stress markers (electrolyte leakage, hydrogen peroxide, malondialdehyde), and antioxidant responses (catalase, peroxidase, total phenols, total antioxidant capacity) were assessed. Results revealed that drought stress significantly reduced chlorophyll content (by up to 56.6%), stomatal conductance (by 42.6%), and plant biomass. Conversely, foliar application of melatonin, particularly at 200 µM, effectively mitigated these adverse effects, increasing biomass by 9-11%, improving leaf water status by 14-15%, enhancing proline accumulation, and reducing oxidative stress markers. Furthermore, melatonin enhanced catalase and peroxidase activities as well as phenolic accumulation in a concentration-dependent manner. These findings indicate that melatonin, by enhancing antioxidant defense systems and maintaining physiological homeostasis, offers a promising strategy for improving drought tolerance and supporting the sustainable production of medicinal plants under water-limited conditions.