The aims of this study were to investigate the sensitivity of Tradescantia pallida 'Purpurea' to genotoxicity induced by ozone, by means of the micronucleus (MCN) bioassay, to verify whether the intensity of genotoxic responses in inflorescences is modulated by concentrations of ascorbic acid (AA) in their bracts, and/or by air temperature variations during the progress of the bioassay, and to define the time lag necessary after ozone exposure to observe maximal genotoxic effects. Flowering branches were exposed to filtered air (control) and to 60 ppb of ozone (ozone) for 3h in fumigation chambers during spring, autumn, winter and summer. After exposure, they were maintained for 24-120 h under filtered air for recovery. A sub-group of each treatment was taken every 24h, when MCN was scored in inflorescences and the levels of AA were determined in bracts. Ozone caused a significant increase in the frequency of MCN after 24-120 of recovery, compared to measurements in inflorescences from the control treatment, but maximal MCN rate was reached between 72 and 120 h of recovery. The highest percentages of MCN in both fumigation treatments were found during the winter experiment. Ozone exposure did not induce significant changes in the content of AA. However, it was positively influenced by daily amplitude of air temperature during the period of the bioassay. The intensity of genotoxic damage and the time lag necessary to visualize an enhanced number of MCN depended on the levels of ascorbic acid in bracts 24h before MCN scoring and on the daily amplitude of air temperature during the development of the bioassay. Narrower ranges between daily maximum and minimum temperatures (around 4 degrees C) during the days of experiment seemed to promote a more efficient diagnostic of genotoxiciy induced by ozone.