Comparative responses to silicon and selenium in relation to cadmium uptake, compartmentation in roots, and xylem transport in flowering Chinese cabbage (Brassica campestris L. ssp. chinensis var. utilis) under cadmium stress

•Si and Se both alleviated Cd stress as proved by increasing tissue biomass.•Si and Se showed discrepant strategies in Cd detoxification.•Si reduced the ratios of Cd in symplast and apoplast, and promoted it in cell walls.•Si reduced root-to-shoot Cd transport.•Se showed no effects on uptake, compartmentation, and xylem transport of Cd.

Silicon (Si) and selenium (Se) are generally considered as beneficial elements for the growth of higher plants, especially for those grown in heavy-metal stressed environments. However, the mechanisms underlying the 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 Cd uptake, compartmentation in roots, and xylem transport in flowering Chinese cabbage plants under Cd stress. Addition of Si or/and Se efficaciously alleviated the toxicity of Cd as demonstrated by increasing of tissue (shoots and roots) biomass of plants exposed to high Cd stress, especially for their coupling treatments with high doses. In compare with the Cd-alone treatment, the application of Si alone or in combination with Se greatly decreased plant shoot Cd concentrations as well as its translocation factor (TF), though the Cd concentrations in roots and the total Cd accumulation of whole plants showed increasing trends (especially for the treatments of high Si or coupling with Se), while no marked differences were found in plants exposed to the Se-alone treatment. Additionally, the application of Si alone or in combination with Se considerably reduced the Cd concentrations and its proportions in symplast and apoplast root saps and increased them in cell wall fragments, while little changes were observed for the Se-alone treatment. Furthermore, a greatly decreased tendency was also displayed for the Cd concentrations in xylem saps exposed to the treatments containing Si alone or in combination with Se. Overall, our results reveal that Si-mediated alleviation of Cd toxicity may be due to decreasing Cd concentrations and its proportions in symplasts and apoplasts, enhancing adsorption of Cd on cell walls, and restriction root-to-shoot Cd translocation. However, Se mitigation may involve other mechanisms.