Understanding the phytotoxic effects of CeO2 nanoparticles on the growth and physiology of soybean (Glycine max L. Merrill) in soil media

To investigate the phytotoxic effects of CeO2 nanoparticles (NPs) on crops, soybean plants were cultivated in soil amended with CeO2 NPs at 0, 25, 75 and 225 mg kg-1 to maturity (96 days after sowing, DAS). A similar treatment with 25 mg kg-1 CeCl3 was used as an ionic control for comparison. Plant growth, accumulation and distribution of macro and micronutrients, chemical species of Ce, activities of antioxidant enzymes and levels of malondialdehyde (MDA) in the plants were examined at 21 and 96 DAS. CeO2 NPs can exert negative effects on the growth and development of soybean plants, but only at the highest concentration tested (225 mg kg(-1)) and at the latest timepoint, CeO2 NPs significantly reduced the biomass by 33.8% compared to the untreated control plants. Biochemical assays show that at 96 DAS, the activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) presented a decreasing trend with increased exposure concentrations in both shoots and roots, whereas the accumulation of MDA increased. CeO2 NP treatments modified the accumulation of Ca, Fe, Zn, Mn, Cu, and Mo in roots; Ca, Fe and Zn in stems; and P, Mn and Mo in leaves. Ce3+ ions and CeO2 NPs were found to have very similar but not identical agronomical, physiological and biochemical effects on soybean plants. Synchrotron radiation-based X-ray absorption near edge structure (XANES) analysis shows that in 225 mg kg-1 CeO2 NP-treated plants, the percentages of Ce(iii) in the roots increased from 4.5% (21 DAS) to 11.8% (96 DAS). The results of this study suggest that CeO2 NPs and Ce3+ ions may share a common mechanism of toxicity and Ce(iii) species play an important role in the toxic effects of CeO2 NPs on soybean plants under experimental conditions.