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The effect of ultraviolet-A irradiated or nonirradiated suspensions of agglomerates of titanium dioxide (TiO(2)) or silicon dioxide (SiO(2)) nanoparticles on roots of the onion (Allium cepa) has been studied. The reactive potential of TiO(2) nanoparticles, which have photocatalytic potential, and the nonphotocatalytic SiO(2) nanoparticles with the same size of agglomerates was compared. The authors measured the activity of antioxidant enzymes glutathione reductase, ascorbate peroxidase, guaiacol peroxidase, and catalase as well as lipid peroxidation to assess the oxidative stress in exposed A. cepa roots. A wide range of concentrations of nanoparticles was tested (0.1-1000 µg/mL). The sizes of agglomerates ranged in both cases from 300 nm to 600 nm, and the exposure time was 24 h. Adsorption of SiO(2) nanoparticles on the root surface was minimal but became significant when roots were exposed to TiO(2) agglomerates. No significant biological effects were observed even at high exposure concentrations of SiO(2) and TiO(2) nanoparticles individually. Plants appear to be protected against nanoparticles by the cell wall, which shields the cell membrane from direct contact with the nanoparticles. The authors discuss the need to supplement conventional phytotoxicity and stress end points with measures of plant physiological state when evaluating the safety of nanoparticles.

Extensive production and wide application of TiO2 nanoparticles has stimulated research on its potential biological effects on different groups of organisms but the interaction of TiO2 nanoparticles with higher plants remains poorly understood. We have studied the effect of TiO2 nanoparticles on Allium cepa using a modification of the conventional Allium test with nanoparticles suspended in distilled water as opposed to growth medium. Nanoparticulate TiO2 was found to have low toxic potential and the mitotic index was among the most sensitive measures of the effect of nano-TiO2. We conclude that modified Allium test is suitable to provide comparative data on the biological potential of a variety of nanoparticles and could be used in a tiered approach to nanotoxicity testing. [Copyright &y& Elsevier]