Weedy adaptation in Setaria spp. III. Variation in herbicide resistance in Setaria spp

Setaria is a grass genus of about 125 species that includes both food crops and a number of important agricultural weeds. Setaria viridis, S. faberii, S. glauca, and S. geniculata are major agricultural weeds worldwide and in North America. There is currently an inadequate knowledge of inter- and intraspecific herbicide-resistance variation in these foxtail species despite the importance of this knowledge to understanding evolution of herbicide resistance and improving weed management. Previous isozyme analyses of these species indicate that significant variation in genetic diversity exists among foxtail populations. It is unknown whether this genetic diversity is correlated with variability in important adaptive traits such as herbicide resistance. Studies were conducted to determine if inter- and intraspecific differences in atrazine and metolachlor resistance exist in foxtail species. Three assays were utilized to make these determinations: whole plant dose response, in vivo leaf chlorophyll fluorescence, and glutathione S-transferase (GST) conjugation assays. Significant variations in atrazine and metolachlor resistance were revealed within and among foxtail species. Green and giant foxtail were more resistant to atrazine than was yellow foxtail. Although green and giant foxtail again had a similar level of resistance, yellow foxtail was the most resistant species to metolachlor. These results indicated that the resistance mechanisms (quantitative or qualitative) to these two herbicides may be different in yellow, green, and giant foxtail. Intraspecific differences in atrazine resistance were found within both green foxtail populations and with yellow foxtail populations. Intraspecific metolachlor resistance differences were detected among green foxtail populations, but not in other foxtail species. No evidence for population shifts to more resistant foxtail variants with prolonged atrazine exposure was found in several detailed studies. When populations from several farms with a long history of atrazine use were compared, no differences in atrazine resistance were detected among populations from treated areas and adjacent untreated areas. Chlorophyll fluorescence assays indicated a similar pattern of atrazine resistant among foxtail populations, although it was less sensitive in detecting differences than the whole plant assay. No differences in GST-mediated atrazine or metolachlor conjugation were detected within or between foxtail species. These results may indicate that GST-mediated glutathione-herbicide conjugation may not be the primary detoxification mechanism for these herbicides in these foxtail species. Alternatively, these results may be a reflection of mitigating factors, such as differences at the target site, or in fitness related to growth rate at critical growth stages, among the populations. Several foxtail species had significant inter- and intraspecific differences in GST-mediated 1-chloro-2,4-dinitrobenzene conjugation activity. In some instances these responses were similar to those observed in the whole plant responses to metolachlor, although the significance of these similarities was not clear.