Do sulfur addition and rhizoplane iron plaque affect chromium uptake by rice (Oryza sativa L.) seedlings in solution culture?

Heavy metal uptake is confined by other elements, namely iron (Fe) and sulfur (S). There are yet no reports on the contribution of S supply to the attenuation of chromium (Cr) uptake when different species of Cr are employed. The bioaccumulation of Cr in two cultivars of rice seedlings subjected to 1.0 mg L^-1 Cr (III and VI) stress under S deprived or non-deprived conditions were examined in a hydroponic experiment. Sulfur nutrition promoted the root and shoot growth of rice cultivars under Cr stress. For both + S/ - S seedlings, the concentration of both Cr species followed the sequence ACA (ascorbic citrate acetic) extract > root > shoot, with less Cr accumulated in shoots of + S seedlings to that of - S seedlings. The concentrations of Cr and Fe in ACA extracts were found to be significantly correlated. Compared to + S treatment, Cr and Fe contents in iron plaque without S treatment were markedly reduced, especially for Cr (VI). Cr content in roots and shoots was indicated to be at par between cultivars; however, it significantly differed for S and Cr treatments. The Cr translocation between different parts of plaque-harboring seedlings was more pronounced in Cr (VI) treatment relative to Cr (III) treatment. Increased immobilization of Cr in iron plaque of + S seedlings and its subsequent reduction in aerial tissues may likely shed some light on the barrier function of iron plaques in the uptake of both Cr species by rice seedlings.