Influences of zinc oxide nanoparticles on Allium cepa root cells and the primary cause of phytotoxicity

Zinc oxide nanoparticles (ZnO-NPs) are widely used in consumer products, which have raised concerns about their impact on the human health and environment. In this study, Allium cepa were treated with 5 and 50g/mL ZnO-NPs solutions for 12, 24, and 36h, respectively. The cytotoxic and genotoxic effects of ZnO-NPs in root meristems of Allium cepa cells were characterized by cell membrane integrity, metabolic activity, reactive oxygen species (ROS) accumulation, DNA damage, chromosome aberration, and cell cycle progression. Substantially elevated Zn levels were observed in the cytoplasmic and nuclear fractions, and the accumulation of zinc in the nuclear fraction (up to 9764g/g) was one magnitude greater than that in the cytoplasm (up to 541g/g). The complexation of Zn2+ with diethylene triamine pentacetic acid (DTPA) was performed to explicate the respective contribution of insoluble particles or Zn2+ to ZnO-NPs toxicity. We found that the inhibition of root growth accounted for 24.2% or 36.1% when the plants were exposed to Zn2+ that released from 5 or 50g/mL of ZnO-NPs for 36h, respectively, whereas the exposure to 5 or 50g/mL of insoluble particles resulted in 75.8% or 63.9% of inhibition, respectively. These findings demonstrated that adverse effects exerted not just by Zn2+ released from ZnO-NPs, but also directly from the nanoparticles. These findings contribute to a better understanding of ZnO-NPs cytotoxicity and genotoxicity in plant cells and provide valuable information for further research on the phytotoxic mechanisms of ZnO-NPs. HighlightsSubstantially elevated Zn levels were observed in the cytoplasmic and nuclear fractions of A. cepa roots.ZnO-NPs inhibited plant growth and induced severe cytotoxicity and genotoxicity.The toxicity of ZnO particles is higher than zinc ions released from them.