Modulation of redox state, antioxidant systems and photosynthetic capacity induction of osmotic stress tolerance in Crocus sativus L. through exogenous melatonin

The potential of melatonin as a plant growth regulator biomolecule for mitigating the negative effects of environmental abiotic stresses has gained significant attention. This study aimed to investigate the influence of melatonin on various morpho-physiological and biochemical characteristics of saffron plants under osmotic stress conditions. Osmotic stress was simulated using polyethylene glycol (PEG)-6000 at a concentration of 30% (w/v). Melatonin was applied through foliar spray and root treatment at a concentration of 100 µM. The results revealed that osmotic stress caused an increase in oxidative injury indices, such as ROS (55.62%), MDA (50.43%), H2O2 (3.05%) and EL (16.40%), while decreasing growth parameters including FW (25.65%), DW (38.57%), SH (21.77%), RWC (36.3%), and photosynthetic traits such as chl a (55.43%), chl b (42.56%) and car (23.75%) content, Pn (64.92%), Fv/Fm ratio (1.275%), C (35.92%) and Ci (16.56%) concentration. In response to water stress, the concentration of compatible osmolytes and enzymatic and non-enzymatic antioxidants were increased. Both foliar and root applications of melatonin resulted in a reduction in oxidative injury indices by enhancing the levels of enzymatic and non-enzymatic antioxidant compounds. Overall, the findings demonstrated that exogenous melatonin application had a positive impact on physiological indicators, including photosynthetic parameters and the antioxidant system, thereby enhancing plant resistance during osmotic stress. Additionally, the results indicated that foliar application of melatonin was more effective than root application in increasing plant resistance to osmotic stress.