Exogenous melatonin modulates growth, antioxidant defense, and secondary metabolism of Matthiola incana under drought stress.

Drought stress is a major environmental constraint that adversely affects plant growth, physiological performance, and the accumulation of bioactive compounds. The use of biostimulants such as melatonin (MT) has emerged as a promising strategy to enhance plant tolerance to abiotic stress. In this study, the effects of exogenous MT on growth, physiological traits, antioxidant defense, and secondary metabolism of Matthiola incana under different irrigation regimes (100, 70, 40, and 25% field capacity, FC) were investigated. The experiment was conducted as a factorial design with four MT concentrations (0, 100, 200, and 400 mg L⁻¹). Drought stress significantly reduced growth parameters, chlorophyll content, and relative water content (RWC), while increasing oxidative stress indicators. However, MT application mitigated these adverse effects by enhancing photosynthetic efficiency (Fv/Fm), improving water status, and stimulating antioxidant enzyme activities, including superoxide dismutase and catalase. Under optimal irrigation, 200 mg L⁻¹ MT significantly enhanced plant height (up to 29.6%), leaf number (22.1%), plant fresh weight (43.3%), flowering (17.2%), and chlorophyll content (16.5%). Under drought stress, MT markedly mitigated growth inhibition. At 40% FC, plant height increased by up to 29.5% with 400 mg L⁻¹, while leaf production rose by 21.4%. Principal component analysis further confirmed the integrative role of MT in coordinating physiological and biochemical responses. Overall, the results demonstrate that exogenous MT enhances drought tolerance in M. incana through improved antioxidant defense, maintenance of photosynthetic performance, and regulation of osmotic balance. These findings highlight the potential application of MT as an effective biostimulant for improving plant performance under water-limited conditions.