Comparative responses to silicon and selenium in relation to antioxidant enzyme system and the glutathione-ascorbate cycle in flowering Chinese cabbage (Brassica campestris L. ssp. chinensis var. utilis) under cadmium stress

Silicon (Si) and selenium (Se) are generally considered as contributing elements for plant resistance to abiotic stresses, especially for those in heavy-metal stressed environments. However, the mechanisms underlying the different roles of Si and Se in mitigating cadmium (Cd) stress in flowering Chinese cabbage (Brassica campestris L. ssp. chinensis var. utilis) are still poorly understood. Here, we investigated the comparative responses to Si and Se in relation to antioxidant enzyme system and the glutathione-ascorbate cycle in flowering Chinese cabbage plants under Cd stress. Addition of Si and/or Se at equivalent concentrations alleviated Cd toxicity as demonstrated by increasing plant tissue (shoots and roots) biomass and reducing plant tissue (leaves and roots) concentrations of malondialdehyde (MDA) and hydrogen peroxide (H2O2) in plants exposed to high Cd stress. Additionally, in comparison with the high Cd-alone treatment, the application of Si and/or Se significantly increased the activities of antioxidant enzymes such as superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX), while no differences in the size of these effects between Si and Se were observed at equal concentrations, suggesting that these three antioxidant enzymes were not the key factors involved in differences of Cd detoxification between Si and Se. Furthermore, the single addition of Se or in combined with Si markedly stimulated the efficiency of the GSH-AsA cycle by increasing the concentrations of glutathione (GSH) and ascorbate (AsA) as well as the activities of glutathione reductase (GR) and dehydroascorbate reductase (DHAR) in plant tissues (leaves and roots), especially at high dose, while little changes were observed in the Si-alone treatment, indicating that Se has the greater ability of increasing the efficiency of GSH-AsA cycle rather than of Si exposed to Cd stress. Overall, our results reveal that Se-mediated alleviation of Cd toxicity is due to increasing antioxidant enzyme activities and the GSH-AsA cycle efficiency. However, Si mitigation may involve other mechanisms apart from increasing antioxidant enzyme activities.