Identification of Two Mechanisms of Atrazine Resistance in Setaria faberi and Setaria viridis Biotypes

Two resistant (R) biotypes of Setaria faberi and S. viridis have been detected in maize fields continuously treated with triazine herbicides. The R biotypes of S. faberi and S. viridis were 10.0 and 6.5 times, respectively, more resistant to atrazine than their susceptible (S) biotypes. Both R biotype plastids had less affinity for atrazine than the S biotypes based on chlorophyll fluorescence and Hill reaction assays. As expected, they exhibited cross-resistance to a wide variety of photosystem II inhibitors. The hierarchy of resistance level of these two biotypes was chloro-s-triazines >> methoxy-s-triazines > methyltio-s-triazines >as-triazines. There was no difference in the absorption and translocation of [14C]atrazine in R and S biotypes. Fast fluorescence induction curves showed that atrazine inhibited whole-leaf photosynthesis in S biotypes and they later recovered slight fluorescence activity in both populations. All biotypes produced the same pattern of metabolites, but the R biotypes detoxified the atrazine to conjugate-atrazine at a higher rate than the S biotypes (R > 75% > S). The five wild-type Setaria spp. found in Spain showed differences in detoxification of atrazine. The hierarchy of detoxifying level of atrazine of these Setaria spp. was S. verticilata ~S. adherens >> S. faberi ~S. viridis >>> S. glauca.