Comparative Study of Different Melatonin Application Effects on Antioxidant System in Cadmium-Exposed Chickpea Seedlings

The objective of the current study was to examine the potential ameliorating effect of exogenous melatonin (MT) on cadmium (Cd)-mediated injury in chickpea seedlings and to optimize the MT application approach. To mitigate Cd phytotoxicity, exogenous MT (10 μM) was added to the germination medium of chickpea ( Cicer arietinum L.) seeds contaminated with 200 μM CdCl 2 following two experimental approaches. The first set of seeds was treated simultaneously with Cd and MT at the start of the germination, while the second set was exposed to Cd stress for 3 days before being treated with a combination of Cd and MT. The current results revealed that exogenous MT significantly mitigated the deleterious effect of Cd on seedling growth in terms of root and shoot dry weight. Besides, MT alleviated the Cd-imposed disruption of cellular redox homeostasis. This effect was manifested by reduced cell death and hydrogen peroxide content. Moreover, MT may protect cell membrane integrity by lowering lipoxygenase activity and downregulating its gene expression. Melatonin ameliorated the Cd-induced oxidative stress by promoting key antioxidative enzymes, including ascorbate peroxidase, catalase, and Cu/Zn-superoxide dismutase. This stimulating effect was accompanied by the upregulation of related gene expressions. In contrast, MT downregulated glutathione peroxidase activity and its corresponding gene expression. Seedlings exposed to exogenous MT simultaneously with Cd showed more pronounced beneficial effects than those receiving MT as a late supplementation. Overall, the current results state that exogenous MT, when added simultaneously with the metal, is efficient at enhancing chickpea tolerance to Cd contamination. This favorable effect is mostly mediated by the improvement of antioxidant activities and gene expression.