Melatonin Improves Drought Resistance in Maize Seedlings by Enhancing the Antioxidant System and Regulating Abscisic Acid Metabolism to Maintain Stomatal Opening Under PEG-Induced Drought

Abstract Maize ( Zea mays L.) is highly sensitive to drought stress, resulting in large losses in yield; therefore, strategies aimed at enhancing drought tolerance are essential. Melatonin improves stress tolerance in plants; however, its mechanism in maize seedlings under drought stress remains unknown. Therefore, we investigated the effects of foliar-sprayed melatonin (100 umol L −1 ) on the antioxidant system, photosynthetic gas exchange parameters, stomatal behavior, endogenous melatonin and abscisic acid (ABA)-related gene expression in maize seedling leaves under 20% polyethylene glycol (PEG)-induced drought stress. PEG treatment resulted in oxidative stress and stomatal closure, resulting in chlorophyll degradation and inhibition of photosynthesis; thereby, reducing seedling biomass. Melatonin pretreatment significantly improved the relative water content, photosynthetic gas exchange parameters and stomatal behavior; thereby, maintaining chlorophyll contents and photosynthesis. Melatonin also stimulated the antioxidant system, enhancing the clearance of reactive-oxygen species, preventing severe damage under PEG-induced drought. Pre-treatment also increased endogenous melatonin and inhibited up-regulation of NCED1 , an ABA synthesis-related gene, as well as selectively up-regulating ABA catabolic genes ABA8ox1 and ABA8ox3 , reducing ABA accumulation and inducing stomatal reopening. Overall, these findings suggest that melatonin pre-treatment alleviated the inhibitory effects of drought stress on photosynthesis, enhancing tolerance in maize seedlings.