Alleviation of cadmium-induced phytotoxicity and growth improvement by exogenous melatonin pretreatment in mallow (Malva parviflora) plants

The present work was aimed to study the effect of melatonin pretreatments on growth, oxidative stress modulation, cadmium (Cd) accumulation, and tolerance in mallow (Malva parviflora, Malvaceae) plants under the hydroponic system. Application of substances that can modulate the harmful effects of Cd on plant yield and reduce its accumulation in the edible parts is of particular importance. Therefore, the mallow plants pretreated with 15, 50, and 100 µM of melatonin were exposed to 50 µM Cd. Our results showed that melatonin, especially at 15 and 50 µM, led to positive effects on Cd tolerance, including a significant increase in growth, photosynthetic pigments, and soluble protein content. Exogenous melatonin could improve relative water content (RWC) and stomatal conductance in the plants treated with Cd, probably through an increase in proline. Further, lower concentrations of melatonin led to a decrease in Cd translocation to the shoots. Based on the results, melatonin considerably increased catalase (CAT), superoxide dismutase (SOD), and guaiacol peroxidase (GPX) activities as well as the production of phenols. The increased activity of antioxidant enzymes led to a decrease in electrolyte leakage (EL), lipid peroxidation, and H2O2 content in the plants exposed to Cd stress. Under Cd stress, the increased phenols content in melatonin-pretreated plants could be due to the induction of phenylalanine ammonia-lyase (PAL) activity and an increase in shoot soluble carbohydrates. The results showed that the use of melatonin could reduce oxidative stress and improve biomass in the plants exposed to Cd. At least in our experimental conditions, this information appears to be useful for healthy food production.