Comparison of foliar silicon and selenium on cadmium absorption, compartmentation, translocation and the antioxidant system in Chinese flowering cabbage

Silicon (Si) and selenium (Se) are beneficial for many higher plants when grown on stress conditions. However, the mechanisms underlying the differential effects between foliar Si and Se in alleviation of plant toxicity exposed to cadmium (Cd) stress are remained unclear. In this study, we investigated the discrepant mechanisms of foliar Si and Se on Cd absorption and compartmentation by roots, its translocation in xylem, and the antioxidant system within Chinese flowering cabbage (Brassica campestris L. ssp. chinensis var. utilis) under low and high Cd stress. Results showed that plant growth was significantly enhanced by foliar additions of Si or/and Se according to an increased plant tissue biomass at high Cd exposure. In addition, the foliar coupled addition of Si and Se showed little effects on the concentrations of Si or Se in plant tissues in comparison with the single addition of foliar Si or Se respectively. The foliar Si alone or combined with Se markedly reduced the Cd concentrations in plant shoots under two Cd treatments. This might be explained by the lower Cd concentrations in symplast and apoplast and the higher Cd concentrations in cell walls of plant roots, and the lower Cd concentrations in xylem sap. However, no great changes in these values were observed under the treatments of foliar Se alone. Moreover, the foliar additions of Si or/and Se all increased the antioxidant enzyme activities of SOD, CAT and APX in plant tissues, especially at high Cd dosage. No significant differences in the increasing degrees of these three antioxidant enzymes were found between the foliar Si and Se treatments. However, only the foliar Se alone or combined with Si markedly promoted the antioxidant enzyme activities of GR and DHAR in plant tissues. Our findings demonstrate that the alleviation of Cd toxicity by foliar Si maybe mainly responsible for inhibition of Cd absorption and its translocation to plant shoots, reinforcing its compartmentation into root cell walls, whilst enhancing the antioxidant enzyme system may be employed by foliar Se.