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Plants are essential components of all ecosystems and play a critical role in environmental fate of nanoparticles.However,the toxicological impacts of nanoparticles on plants are not well documented.Titanium dioxide nanoparticles (TiO2-NPs) are produced worldwide in large quantities for a wide range of purposes.In the present study,the uptake of TiO2-NPs by the aquatic plant Spirodela polyrrhiza and the consequent effects on the plant were evaluated.Initially,structural and morphological characteristics of the used TiO2-NPs were determined using XRD,SEM,TEM and BET techniques.As a result,an anatase structure with the average crystalline size of 8 nm was confirmed for the synthesized TiO2-NPs.Subsequently,entrance of TiO2-NPs to plant roots was verified by fluorescence microscopic images.Activity of a number of antioxidant enzymes,as well as,changes in growth parameters and photosynthetic pigment contents as physiological indices were assessed to investigate the effects of TiO2-NPs on S.polyrrhiza.The increasing concentration of TiO2-NPs led to the significant decrease in all of the growth parameters and changes in antioxidant enzyme activities.The activity of superoxide dismutase enhanced significantly by the increasing concentration of TiO2-NPs.Enhancement of superoxide dismutase activity could be explained as promoting antioxidant system to scavenging the reactive oxygen species.In contrast,the activity of peroxidase was notably decreased in the treated plants.Reduced peroxidase activity could be attributed to either direct effect of these particles on the molecular structure of the enzyme or plant defense system damage due to reactive oxygen species.