[Chemical forms and subcellular and molecular distribution of Cd in two Cd-accumulation rice genotypes]

A hydroponic experiment combined with subcellular fractionation and gel filtration was carried out to study the chemical forms and subcellular and molecular distribution of Cd in rice roots and leaves, and to compare the differences between the rice varieties 'Guangyuanzhan No. 3' (a low-Cd accumulation genotype) and 'Zhenguiai' (a high-Cd accumulation genotype). The results showed that with increasing concentration of Cd in the medium, the Cd content in all fractions of rice roots and leaves increased significantly, and most of Cd accumulated in F I (cell wall) and F III (soluble substance). Compared with 'Guangyuanzhan No. 3', 'Zhenguiai' had a significantly higher soluble fraction of Cd in its roots and leaves. The greatest amount of Cd in roots and leaves was in the extraction solution of 1 mol x L(-1) NaCl, followed by in 2% HAC, and 0.6 mol x L(-1) HCl. In comparison with 'Guangyuanzhan No. 3', 'Zhenguiai' had a higher proportion of d-H2O and 80% ethanol extractable-Cd, the more mobile form of Cd. In the two rice varieties, the soluble Cd eluted followed the same pattern as the soluble protein in the sample medium, and the majority of soluble Cd was bounded to phytochelatins (PCs) or low molecular weight components. However, the Cd-PCs content in 'Guangyuanzhan No. 3' roots was much lower than that in 'Zhenguiai' roots, which could possibly result in a lower Cd mobility in roots and less Cd translocation from root to aboveground part.