Exogenous silicon alleviates cadmium toxicity in rice seedlings in relation to Cd distribution and ultrastructure changes

Abstract

Silicon (Si) has inhibitory effects on cadmium (Cd) toxicity in rice, but the effects of Si-containing materials on the chemical properties of hydroponic media have been ignored. The increased pH value and introduction of excess cations when utilizing alkaline reagents may disguise the true function of the Si. Thus, the present study investigated Si-alleviated effect on Cd toxicity in rice seedlings by eliminating the aforementioned interferences. The hydroponic cultivation of rice seedlings was carried out to demonstrate the inhibiting effects of exogenous Si on Cd accumulation and distribution. The chlorophyll and carotenoid contents in leaf cells under Cd stress were also measured. Transmission electron microscopy was used to investigate ultrastructural changes in rice mesophyll cells, chloroplasts, and mitochondria under different Cd-stressed and Si-amended conditions. The alkalinity of the Si source was neutralized with nitric acid, and discrepancies in Na+ levels was eliminated by the addition of sodium nitrate to different treatments. Toxicity symptoms were observed in rice seedlings under Cd (5 mg L−1) treatment after 35 days, and the biomass, and chlorophyll and carotenoid contents, were lowered. The levels of these indices increased after adding 120 mg L−1 Si. The bioconcentration and translocation factor values indicated that Si reduced Cd accumulation in rice plants and also decreased the ratio of Cd translocation from roots to shoots, which effectively alleviated the Cd toxicity. The positive effects of Si were mainly reflected in the recovery of grana lamellae and membrane structures in chloroplasts, and restored membrane morphology and increased numbers of cristae in mitochondria. Exogenous Si reduced Cd uptake in rice seedlings, especially in shoots, and inhibited Cd transport into aboveground parts. In addition, Si restored the functions of chloroplasts owing to a decrease in lipid peroxidation. The application of Si indicated that Cd toxicity is alleviated partly because of the improvement in chloroplast biosynthesis and the diminution of Cd bioavailability.