Plant species-dependent transformation and translocation of ceria nanoparticles†
Uptake of nanoparticles (NPs) by plants is species-dependent; however, its underlying mechanisms are rarely investigated. This study compared the transformation, uptake and translocation of CeO2 NPs (nCeO2) in four different plant species. Corn showed the lowest translocation factor (TF) of Ce, while the differences among the plant species reduced or diminished when the phosphates were removed from the nutrient solution (−P treatment). Using transmission electron microscopy (TEM) and synchrotron-based X-ray absorption near edge spectroscopy (XANES), we found that the removal of phosphate from the nutrient solutions reduces or eliminates the immobilization of Ce3+ on the root surface thus promoting the upward translocation of Ce from roots to shoots, which accounts for the enhanced total Ce detected in all the four plants. Compared with the +P treatment, the Ce(III) contents in the shoots of corn and wheat were enhanced by 27.4 an 4.0 times in the −P treatment, which may account for the reduced biomass of shoots in these two plant species. Our results indicate that different plant species have different abilities of transforming and translocating CeO2 NPs and that the mechanisms differ depending on the chemistry of the surrounding solution (+P/−P). Plant xylems and root exudate compositions are both important factors in determining the transformation of nCeO2 and subsequent translocation of Ce species in plants. Further studies are required to determine the composition of root exudates and to identify the components driving the nCeO2 transformation in different plant species under different culturing conditions.