Effects of rare earth oxide nanoparticles on root elongation of plants

The phytotoxicity of four rare earth oxide nanoparticles, nano-CeO₂, nano-La₂O₃, nano-Gd₂O₃ and nano-Yb₂O₃ on seven higher plant species (radish, rape, tomato, lettuce, wheat, cabbage, and cucumber) were investigated in the present study by means of root elongation experiments. Their effects on root growth varied greatly between different nanoparticles and plant species. A suspension of 2000 mg L^-1 nano-CeO₂ had no effect on the root elongation of six plants, except lettuce. On the contrary, 2000 mg L^-1 suspensions of nano-La₂O₃, nano-Gd₂O₃ and nano-Yb₂O₃ severely inhibited the root elongation of all the seven species. Inhibitory effects of nano-La₂O₃, nano-Gd₂O₃, and nano-Yb₂O₃ also differed in the different growth process of plants. For wheat, the inhibition mainly took place during the seed incubation process, while lettuce and rape were inhibited on both seed soaking and incubation process. The fifty percent inhibitory concentrations (IC₅₀) for rape were about 40 mg L^-1 of nano-La₂O₃, 20 mg L^-1 of nano-Gd₂O₃, and 70 mg L^-1 of nano-Yb₂O₃, respectively. In the concentration ranges used in this study, the RE³⁺ ion released from the nanoparticles had negligible effects on the root elongation. These results are helpful in understanding phytotoxicity of rare earth oxide nanoparticles.