Identification of two mechanisms of atrazine resistance in Setaria faberi and Setaria viridis biotypes

Two resistant (R) biotypes of Setal icl faberi and S. viridis have been det ected 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 bio type plastids had less affinity for atrazine than the S biotypes based on c hlorophyll fluorescence and Hill reaction assays. As expected, they exhibit ed cross-resistance to a wide variety of photosystem II inhibitors. The hie rarchy of resistance level of these two biotypes was chloro-s-triazines >> methoxy-s-triazines > methyltio-s-triazines greater than or equal to as-tri azines. There was no difference in the absorption and translocation of [C-1 4]atrazine in R and S biotypes. Fast fluorescence induction curves showed t hat atrazine inhibited whole-leaf photosynthesis in S biotypes and they lat er recovered slight fluorescence activity in both populations. All biotypes produced the same pattern of metabolites, but the R biotypes detoxified th e 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 atraz ine of these Setaria spp. was S. verticilata congruent to S. adherens >> S. faberi congruent to S. viridis >>> S. glauca. (C) 2000 Academic Press.