The influence of iron plaque on the absorption, translocation and transformation of mercury in rice (Oryza sativa L.) seedlings exposed to different mercury species

Iron plaque can affect the absorption and accumulation of metal(loid)s in plants. However, it is still unclear whether iron plaque plays different roles in the accumulation of different mercury species in rice plants. The aims of this study are 1) to explore the adsorption of IHg and MeHg onto iron plaque, 2) to investigate the influence of iron plaque on the absorption, translocation of IHg and MeHg in rice plants, 3) to explore whether the process of methylation and demethylation of Hg in vivo occurs in rice plants, and 4) to investigate the effects of iron plaque on the IHg and MeHg transformation in rice. The seedlings were cultivated in an Fe2+ solution for 24 h to induce the iron plaque and then transferred into a nutrient solution containing 500 mu g/L HgCl2 or MeHgCl for 72 h. The Hg content in the iron plaque and rice seedlings was measured by ICP-MS. The chemical forms of Hg in the rice seedlings were determined with HPLC-ICP-MS and XANES. Both IHg and MeHg, particularly MeHg, could be adsorbed by iron plaque. The IHg content of the root and the MeHg content in both the roots and shoots decreased markedly with the increase in iron plaque. The Hg in the root was mainly in the form of RS-Hg-SR with the exposure to HgCl2 and in the form of CH3-Hg-SR and RS-Hg-SR (4:1) with the exposure to MeHgCl. The iron plaque did not change the chemical forms of Hg in the rice plants. Iron plaque greatly decreased the absorption and translocation of both IHg and MeHg in rice seedlings. The demethylation of MeHg occurred in rice plants (in vivo) regardless the formation of iron plaque. This work sheds some light on the understanding of different pathways between IHg and MeHg in rice plants.