Since the first identification of glyphosate resistance in horseweed in California in 2005, the glyphosate-sensitive (GS) biotype has become rare, whereas the glyphosate-resistant (GR) biotype has become dominant in the eastern San Joaquin Valley (SJV). This is an area exposed to regular usage of glyphosate and to high levels of ambient ground-level ozone (O₃). A previous study showed that SJV biotypes of GR are more robust than GS in the absence of ozone. This advantage was reduced, though not eliminated, at elevated O₃. This suggests that the rapid evolution of resistance to glyphosate was not linked to evolution of resistance to O₃. In this study, we contrasted these responses to O₃ in the presence of concurrent glyphosate pressure. The GR and GS biotypes differed in growth and injury, reflecting their known differential sensitivities to glyphosate, but responded similarly to O₃ with no O₃ × biotype interaction. Ozone imposed an unexpected, but ecologically important, impact that enhanced the performance advantage of GR over GS. In the presence of the combination of glyphosate and O₃ the biomass of GR was reduced to low but viable levels, whereas the biomass of GS was reduced to nonviable levels that effectively removed it from the population. These data do not support a genetic linkage between resistance to glyphosate and to O₃, but suggest that air pollution may have accelerated the fixation of glyphosate-resistance alleles in California horseweed populations.

Nomenclature: Glyphosate; horseweed, Conyza canadensis (L.) Cronq. ERICA.