Green foxtail（Setaria viridis） is a notorious weed in corn fields in Heilongjiang Province. To investigate the best method to break the seed dormancy of green foxtail and its physiological response to topramezone, this study selected newly harvested and one-year stored green foxtail seeds as research subjects. The seeds were treated with HCl, Na OH, gibberellic acid（GA）, different water temperatures and polyethylene glycol（PEG） to study the seed dormancy and drought resistance of green foxtail. The results showed that newly harvested seeds exhibited dormancy, and treatments with HCl, NaOH and different water temperatures were unable to break the dormancy. Soaking the seeds in GA could overcome dormancy, but the seeds failed to germinate when exposed to 25% PEG concentration. When topramezone was applied at rates of 22.5 and 45.0 g a.i. · hm-2 at the 3-leaf and 5-leaf stages, respectively, the chlorophyll content reached the lowest value at 28 days after treatment（DAT）. At the 7-leaf stage, the chlorophyll content reached the lowest value at 7 DAT. The activity of 4-hydroxyphenylpy-ruvate dioxygenase（HPPD） enzyme after topramezone application reached the maximum value at 7 DAT for different leaf ages, and the higher the leaf age, the higher the HPPD activity, which was an important factor contributing to the resistance of green foxtail to topramezone.
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