Citric acid enhances Ce uptake and accumulation in rice seedlings exposed to CeO2 nanoparticles and iron plaque attenuates the enhancement

To assess the role of citric acid, as a typical low-molecular-weight organic acid from root exudates, on cerium (Ce) uptake, accumulation and translocation in rice seedlings (Oryza sativa L.) exposed to two CeO₂ nanoparticles (NPs) (14 nm and 25 nm). A hydroponic experiment was performed under two citric acid levels (0.01 and 0.04 mmol L^-1) combined with iron plaque presence. Citric acid significantly enhanced surface-Ce, root-Ce and shoot-Ce accumulation, irrespective of NPs size and iron plaque presence. The increased surface-Ce was associated with the promoted interactive attraction between NPs and root surface, and the enhanced NPs dissolution. Surface-Ce (containing crystalline and amorphous fractions of iron plaque) accumulation increased with the increase of citric acid concentrations. However, the enhancement influence of 0.01 mmol L^-1 citric acid on root-Ce, shoot-Ce accumulations, rice-Ce distribution and TF_root-shoot was more remarkable than citric acid (0.04 mmol L^-1), which suggested higher food security risk for human health with environment-level citric acid. Iron plaque presence attenuated the enhancement effect of citric acid on rice-Ce accumulation and distribution (containing surface-Ce, root-Ce and shoot-Ce) due to the reduced attractive interaction between NPs and root surface from the effect of Fe²⁺ being dissolved by iron plaque. Above effect of citric acid and iron plaque was more remarkable in 25 nm NP than 14 nm NP.

Iron plaque reduces cerium uptake and translocation in rice seedlings (Oryza sativa L.) exposed to CeO2 nanoparticles with different sizes

This study aims to assess the role of iron plaque (IP) on cerium (Ce) uptake and translocation by rice after CeO₂ nanoparticles (NPs) exposure over a 4 days period. A hydroponic experiment was performed under two IP levels (low and high) combined with two CeO₂ NPs size (14 nm and 25 nm). It was found that CeO₂ NPs as the main form was absorbed by rice due to limited NPs dissolution in hydroponic solution. IP significantly reduced surface-Ce, root-Ce and shoot-Ce accumulation, irrespective of CeO₂ NPs sizes. The reduced uptake of Ce was more obvious in NP₂₅ than NP₁₄. Ce accumulations decreased with increasing IP amounts. In IP treatments, the interactive attraction between NPs and root surface was weakened through the enhancement of hydrodynamic diameters and the reduction of ζ-potential of CeO₂ NPs in solution, as well as the reduction of |ζ| values of rice root, which reduced the Ce bioaccumulation in rice. PCA indicated the negative correlation between surface-Ce (IP-C-Ce and IP-A-Ce) and NPs size, and between shoot-Ce/root-Ce and IP-Fe/tissue-Fe. IP also decreased Ce translocation from root to shoot. A full life study indicated the reduction effect of IP on surface-Ce, root-Ce, shoot-Ce and grain-Ce accumulations. These findings are significant as they imply that the IP formation is a promising approach for preventing Ce accumulation in rice, which would regulate Ce uptake by rice in the following growth stages and decrease the health risk of CeO₂ NPs exposure in agricultural environment.