Where Does the Transformation of Precipitated Ceria Nanoparticles in Hydroponic Plants Take Place?

Cerium oxide nanoparticles (CeO2 NPs) have been found to be partly biotransformed from Ce(IV) to Ce(III) in plants, yet the transformation process and mechanism are not fully understood. Here, we try to clarify the specific site and necessary conditions for the transformation of precipitated CeO2 NPs in hydroponic cucumber plants. Three different treatment modes were adopted according to whether the NPs were incubated with roots all the time or not. Results showed that exposure modes significantly affect the translocation and transformation of CeO2 NPs. In the normal exposure mode, Ce was present as a Ce(IV) and Ce(III) mixture in the roots and shoots, and the proportion of Ce(III) in the shoots was enhanced obviously with the increase of exposure time. The results of short-time incubation and petiole exposure modes suggested that CeO2 NPs could not be reduced within a short incubation time (3 h) or be further reduced inside the plant tissues. It was deduced that root surfaces are the sites, and the physicochemical interaction between the NPs and root exudates at the nanobio interface is the necessary condition for the transformation of CeO2 NPs in plant systems. These results will contribute to understanding the transformation mechanism of CeO2 and other metal-based NPs and properly evaluate their ecological effects.