Insignificant acute toxicity of TiO2 nanoparticles to willow trees

Manufactured nanoparticles (MNP) are expected to increase in production in near future. In response, their environmental fate and effects are intensively studied. Phytotoxicity of some types of nanoparticles has been observed for annual species in the seed germination and root elongation test. Yet, no results of toxicity tests with trees have been reported. Woody species, dominant in many ecosystems, may be vulnerable in particular due to the large porous wood compartment. This study tests the toxicity of TiO2 nanoparticles on trees with the short-term willow tree transpiration test. TiO2 particles with 25- and 100-nm diameter were suspended in distilled water at concentrations of 0, 1, 10, and 100 mg/L (first test) and 0, 10, 20, and 50 mg/L (second test). Effects on transpiration, growth, and water use efficiency of exposed willow cuttings were monitored. The concentration of nanoparticles was measured by spectrophotometry. None of the measured effect parameters (growth, transpiration, and water use efficiency) showed any significant change during the test. Particles were rapidly lost from solution, probably due to sedimentation as a result of aggregation and also due to adsorption to roots. The loss of nanoparticles from solution was faster for particles with larger diameter and in the presence of trees. Willow trees were not sensitive to short-term exposure to TiO2 nanoparticles. Similar results were obtained for other plant species. Effects of nanoparticles were observed for zinc and zinc oxide particles, but these effects were probably due to heavy metal toxicity and not nanosize specific. In summary, we came to the conclusion that woody species are not in particular vulnerable to nanosized TiO2 particles in the conditions, concentrations, and time periods used in this study. The preliminary results of this study should be confirmed with other types of MNP, other plant species, experiments in soil and experiments combining longer duration, and low exposure concentrations before a final conclusion in this issue can be made.