Melatonin-Mediated Modulation of Stomatal Morphology, Osmoregulation and ABA Homeostasis Enhances Drought Tolerance in Okra

Abstract Okra (Abelmoschus esculentus L.) faces serious challenges due to water shortage, which impairs photosynthesis, osmoregulation and hormone-mediated responses. Among the plant growth regulators, melatonin influences the plant’s response to drought by improving osmotic potential, stomatal traits and abscisic acid (ABA) levels. However, its mechanism in drought-stressed okra have not yet been fully explored. In this study, four okra hybrid varieties were exposed to water deficit conditions and treated with 100 μM exogenous melatonin through pretreatment of seeds, foliar application, or a combination of both methods. Several important physiological and biochemical characteristics such as canopy surface temperature and the normalized difference vegetation index (NDVI), which is a non-destructive measure of canopy greenness and photosynthetic vigour, were studied along with leaf water potential and the concentrations of soluble carbohydrates and protein. A detailed analysis of stomatal morphology (stomatal length, width, pore length and pore width) was performed using SEM. ABA levels in okra plants were quantified using liquid chromatography-mass spectrometry (LC-MS/MS). Okra plants under drought stress had higher ABA accumulation (9 ppb) than plants under well-irrigated conditions (3.89 ppb). Melatonin application significantly improved the plant’s tolerance to drought by increasing soluble sugars (79.23 mg/g) and proteins (25.00 mg/g), while also preventing excessive ABA accumulation (6.78 ppb). Under the scanning electron microscope, the melatonin-treated plants altered stomatal morphological traits, resulting in improved structural adaptation to drought stress. The maximum physiological benefit of 31.9% in pod yield was achieved using the dual application method of melatonin, i.e., through seed treatment and foliar application, to optimise productivity in water-limited conditions. The current study establishes that melatonin confers drought tolerance in okra by modulating stomatal traits, cellular osmotic equilibrium and ABA content, which could be a viable approach for improving agricultural yield in arid environments.