Influence of uranium speciation on its accumulation and translocation in three plant species: Oilseed rape, sunflower and wheat

Chemical speciation greatly influences the accumulation and distribution of metallic pollutants in plants. This study aimed at evaluating plant differential responses to various speciations of a same element. Uranium (U) was chosen as a model, to which wheat, oilseed rape and sunflower were exposed. Using ICP-MS elemental analysis, together with a panel of imaging techniques including scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM) and particle-induced X-ray emission spectroscopy (PIXE), we show that plant behavior in response to U follows three schemes. When exposed to UO22+ free ion, root adsorption and/or accumulation is high, but U transfer to the shoots is limited by precipitation on cell walls. Complexation with carbonate or citrate reduces U content in roots but drastically increases translocation to the shoots, concentrating U in leaves. Inversely, complexation with phosphate considerably reduces U accumulation in all plant tissues, by precipitation and adsorption of U-rich clusters on root epidermal cells. Finally, our results of U chemical extraction from plant tissues suggest the existence of U-binding proteins. Taken together, these data highlight the role of uranium speciation in the transfer of this metallic pollutant into plants and lead to a better understanding of the mechanisms governing its mobilization, accumulation and distribution in plants. These results will be helpful to improve phytoremediation technology of contaminated soils. (C) 2011 Elsevier B.V. All rights reserved.