Continuous flooding stimulates root iron plaque formation and reduces chromium accumulation in rice (Oryza sativa L.)

Chromium (Cr) contamination in rice poses a serious threat to human health. Therefore, we conducted pot experiments to investigate the influence of water management regimes on the formation of iron plaque on rice roots, and its effect on the accumulation and translocation of Cr in rice grown on contaminated soil. The results showed that water management regimes, including continuous and intermittent flooding, exerted notable effects on soil solution concentrations of Cr(VI) and Cr(III) through changes in redox potential, pH, and dissolved Fe(II) concentrations. In particular, 69.2%-71.8% of Cr(VI) was reduced to Cr(III) under continuous flooding, whereas only 33.3%-38.6% was reduced under intermittent flooding conditions. Additionally, continuous flooding created a rhizosphere environment favorable to the formation of iron plaque. The amount of iron plaque formed increased by 28.2%-47.2% under continuous flooding conditions as compared with that under intermittent flooding conditions. Moreover, compared with intermittent flooding, under continuous flooding, more Cr (18.0%-23.9%) was adsorbed in the iron plaque, thereby sequestering Cr and reducing its mobility. The Cr concentrations in rice root, straw, husk, and grain under continuous flooding conditions were, respectively, 32.0%-36.5%, 32.7%-36.3%, 34.2%-46.9%, and 25.4%-37.7% lower than those under intermittent flooding conditions. Therefore, continuous flooding caused a substantial decrease in the Cr concentrations in rice tissues, as well as an increased distribution of Cr in the iron plaque that acted as a barrier to reduce Cr transfer to the rice roots. These results indicate that continuous flooding irrigation was effective in minimizing the accumulation of Cr in rice plants, as it not only enhanced Cr(VI) reduction in the soil but also improved the blocking capacity of the iron plaque.