Thesis presented March 15, 2011
Abstract:
Uranium is both a radiological and a chemical toxic, which naturally occurs in the environment as a trace element. Metal accumulation and distribution in plants is modulated by speciation. The aim of this PhD work was thus to assay uranium accumulation,
intra planta repartition and toxicity according to its speciation in solution. Acquired knowledge will be applied in phytoremediation technologies.
We exposed three plant species (sunflower, oilseed rape and wheat) to a panel of hydroponic media containing one or two predominant uranium chemical forms. After exposition in these various contaminated media, we evaluated uranium content in plant organs by ICP-MS. In order to investigate uranium repartition and localization at organ/tissue and cellular scales, we carried out four complementary imaging techniques. The uranium repartition within soluble and membrane fractions in roots and shoot was assayed after fractionation and separation through a chromatography column. In parallel, we used X-ray absorption spectroscopy to determine the molecular-level structure of chemical species formed by uranium in exposure media and plant samples. Finally, we explored toxic effects of uranium on plant growth and metabolism.
Our results revealed three schema of accumulation according to the uranium speciation in the exposure medium: when exposed to UO22+ free ion, root accumulation is high, but uranium transfer to the shoots is limited. Uranium is immobilized by adsorption on root surface and precipitation on root cell walls, associated with phosphorus and calcium. The existence of uranium-binding proteins is also suggested. When complexed with phosphate, root accumulation is considerably reduced and translocation becomes negligible. Uranium is precipitated as described above. Conversely, complexation with carbonate or citrate reduces root accumulation but drastically increases translocation to the shoots. If some uranyl phosphate precipitates are still found in root and shoot, a significant proportion of uranium is associated with carboxyl groups; this fraction could be the uranium form transferred from the roots to shoots. Finally, we noticed among the 3 species differences in accumulation and in translocation.
Keywords:
Uranium, speciation, transfer, phytoremediation