Origin of the different phytotoxicity and biotransformation of cerium and lanthanum oxide nanoparticles in cucumber

To investigate how the physicochemical properties of nanoparticles (NPs) affect their biological and toxicological effects, we evaluated the phytotoxicity of CeO2 and La2O3 NPs to cucumber (Cucumis sativus) plants and tried to clarify the relation between physicochemical properties of NPs and their behaviors. CeO2 NPs had no phytotoxicity to cucumber at all tested concentrations, while La2O3 NPs showed significant inhibition on root elongation (≥ 2 mg/L), shoot elongation (at 2000 mg/L), root biomass (≥ 2 mg/L), and shoot biomass (≥ 20 mg/L), as well as induced more reactive oxygen species and cell death in roots (2000 mg/L). The different distribution and speciation of Ce and La in plants were determined by synchrotron-based micro X-ray fluorescence microscopy and X-ray absorption spectroscopy. In the aerial parts, all of La was combined with phosphate or carboxylic group, while a fraction of Ce was changed to Ce(III)-carboxyl complexes, implying that La2O3 acted as its ionic form, while CeO2 displayed the behavior of particles or particle-ion mixtures. The higher dissolution of La2O3 than CeO2 NPs might be the reason for their significant difference in phytotoxicity and transporting behaviors in cucumbers. To our knowledge, this is the first detailed study of the relation between the level of dissolution of NPs and their behaviors in plant systems.