Melatonin alleviates the toxic effects of lead by modulating the antioxidative properties in Plantago ovata seedlings

Our present study focuses on the effect of exogenous application of melatonin on growth, and improvement in oxidative stress in a medicinal herb, isabgol (Plantago ovata). The pretreatment of P. ovata seeds with 25 and 50 μM melatonin under exposure to different concentrations of Pb, i.e., 50, 100, 200 and 500 μM showed variation in dose-dependent responses. Our findings reveal that the melatonin alleviated Pb stress, as reflected in the improvement of plant height and biomass. The photosynthetic pigments and total protein content are also ameliorated in the presence of Pb stress when treated with melatonin. Melatonin, a potent antioxidant, increases the total antioxidant activity, flavonoid contents and DPPH radical scavenging activity while reducing the stress markers such as H₂O₂ content and MDA content. Pb competitively enters into Ca²⁺-gated channels as one of the mechanisms of Pb uptake in plants. Melatonin was found to reduce Pb accumulation while modulating endogenous Ca content in P. ovata. Furthermore, melatonin significantly increased the activities of antioxidant enzymes such as peroxidase (POD), superoxide dismutase (SOD), catalase (CAT), ascorbate peroxidase (APX) and ascorbate oxidase (AO) in Pb-stressed samples. The heatmap and principal component analysis also validated the beneficial effect of melatonin in improving plant growth parameters.