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Allium cepa bioassay had been used from decades for the assessment of toxicants and their harmful effects on environment as well as human health.Magnesium oxide (MgO) particles are being utilized in different fields.However,reports on the adverse effects of MgO nanoparticles on the environment and mankind are scarce.Hence,the toxicity of MgO particles is of conc because of their increased utilization.In the current study,A.cepa was used as an indicator to assess the toxicological efficiency of MgO nano-and microparticles (NPs and MPs) at a range of exposure concentrations (12.5,25,50,and 100 μg/mL).The toxicity was evaluated by using various bioassays on A.cepa root tip cells such as comet assay,oxidative stress and their uptake/intalization profile.Results indicated a dose dependent increase in chromosomal aberrations and decrease in mitotic index (MI) when compared to control cells and the effect was more significant for NPs than MPs (at p ＜ 0.05).Comet analysis revealed that the Deoxyribonucleic acid (DNA) damage in terms of percent tail DNA ranged from 6.8-30.1 over 12.5-100 μg/mL concentrations of MgO NPs and was found to be significant at the exposed concentrations.A significant increase in generation of hydrogen peroxide and superoxide radicals was observed in accordance with the lipid peroxidation profile in both MgO NPs and MPs treated plants when compared with control.In conclusion,this investigation revealed that MgO NPs exposure exhibited greater toxicity on A.cepa than MPs.