Melatonin-mediated regulation of growth, physiological, biochemical, and essential oil constituents alleviates drought stress in oregano (Origanum vulgare L.)

Drought stress reduces oregano growth, yield, and yield components, significantly limiting global agricultural productivity. This study aimed to investigate the effects of drought stress and the foliar application of melatonin on the antioxidant properties and secondary metabolites of Origanum vulgare L. sp. vulgare during the 2023 growing season. Drought stress was imposed by withholding irrigation at four levels: control (90–100% readily available water, RAW), 70–80%, 50–60%, and 30–40% RAW. Melatonin was applied foliarly at three concentrations: 0, 100, and 200 mM, during the vegetative growth completion, flowering initiation, and full flowering stages, suggesting its potential as a sustainable tool for enhancing drought tolerance in other crops, with future research directed toward elucidating its molecular mechanisms. Gas chromatography-mass spectrometry (GC/MS) was utilized to identify and analyze the components of the essential oil. The ANOVA results demonstrated that both growth and phytochemical traits were significantly affected by the individual treatments and their interactions. Exposure to different levels of melatonin during drought stress helped mitigate the adverse effects of stress on dry mass and essential oil yield compared to the control group. However, under drought stress, proline content, total phenols, total flavonoids, soluble carbohydrates, antioxidant enzyme activities, and essential oil yield all increased. Interaction results indicated that while drought stress combined with melatonin application (especially at 200 mM) reduced antioxidant activity compared to the control group, certain essential oil components were significantly altered under various experimental treatments. Specifically, after drought stress, only the content of linalool, terpinolene, and caryophyllene increased in plants that had received the 200 mM melatonin foliar application compared to other drought levels. In conclusion, the combined use of melatonin and re-watering after drought stress appears to be a promising and environmentally friendly strategy to enhance drought tolerance, growth, and phytochemical content in oregano plants.