Influence of iron plaque on the uptake and accumulation of chromium by rice (Oryza sativa L.) seedlings: Insights from hydroponic and soil cultivation

The effects of iron plaque formation on chromium (Cr) uptake and accumulation by rice seedlings (Oryza sativa L.) were assessed using hydroponic and soil experiments, where each 3 levels of Fe supplementation were added to Hoagland solution (0, 30, and 100 mg Fe2+ L−1) and a typical paddy soil (0, 1, and 2 g Fe2+ kg−1). For each treatment, rice seedlings were exposed to different levels of Cr as chromate at 0, 0.5, 2, 5, 10, and 20 mg L−1 in solution or 300 mg kg−1 in soil. Low levels of Cr supply (0.5, 2, and 5 mg L−1) promoted root biomass, while high levels (10 and 20 mg L−1) decreased root and shoot biomass and undermined the density and integrity of iron plaque. Iron supply significantly increased the proportion of Cr in iron plaque, but decreased that in rice plants. The results of hydroponic experiment showed that iron plaque formed with Fe supply at 100 mg L−1 markedly reduced Cr accumulation in shoots of rice seedlings when exposure to 10 and 20 mg L−1 Cr. The soil culture experiment also demonstrated that exogenous Fe addition significantly decreased Cr concentration in leaf and stem of rice seedlings. These results suggested that iron plaque with appropriate amount was effective to reduce the uptake and accumulation of Cr in rice plants, which have strong implication for taking measures to regulate Cr accumulation in rice grains.