Foliar treatment with melatonin modulates photosynthetic and antioxidant responses in Silybum marianum L. under salt stress

Abstract

Salinity is a significant global environmental problem that alters the physiology and morphology of plants. Melatonin modulates several physiological activities in plants under various environmental stresses. However, the contribution of melatonin to stress tolerance in response to salinity remains yet unclear. Therefore, the present study was conducted to investigate the potential impact of melatonin on various physiological reactions and metabolic processes in milk thistle (Silybum marianum) exposed to salinity stress. Salinity causes a reduction in photosynthesis and an increase in free radical production, ultimately resulting in growth inhibition. To combat the adverse effects of salinity stress, milk thistle produced increased levels of antioxidant compounds compared with control group. However, this increase in antioxidant activity could not help the plant in tolerating salinity stress. Conversely, foliar treatment of 100 µM melatonin improved photosynthesis (Pn) 64 %, relative water contents RWC (30.8 %), photochemical efficiency 0.44, antioxidant activities, and seedling characteristics in salinity-exposed plants compared with those in the negative control. Moreover, treatment with 100 µM melatonin resulted in better salinity tolerance by enhancing physiological changes such as stomatal conductance, transpiration, CO2 concentration, photosynthesis, germination, and generation of reactive oxygen species and reducing the hydrogen peroxide concentration in milk thistle plants under salinity stress.