Horseweed has become increasingly difficult to control in the San Joaquin Valley (SJV) of California. Resistance to glyphosate may not fully explain the quasi-invasive behavior of this native species. We contrast glyphosate-resistant (GR) and glyphosate-susceptible (GS) horseweed biotypes for vigor during the vegetative stage and for resistance to ozone (O₃). The SJV is impacted by O₃ air pollution, which could be a factor in competitiveness of GR vs. GS. Both biotypes were exposed during the seedling and vegetative stages of rosette development to a range of O₃ concentrations in greenhouse exposure chambers. Leaf injury was evaluated visually and biomass production and allocation destructively. In O₃-free air, the GR biotype exhibited fewer foliar lesions, more vigorous growth, and 40% greater biomass than the GS biotype. The slope of the response to O₃ was greater in the GR than in the GS biotype, implying greater relative sensitivity to O₃. This was due to greater vigor at low O₃, as the biotypes performed similarly at high O₃. The competitive advantage of the GR biotype may be reduced in polluted environments. There appeared to be no linkage between the evolution of resistances to O₃ and to glyphosate.

Nomenclature: Horseweed, Conyza canadensis (L.) Cronq. ERICA