Effects of elevated CO₂ on the response of glyphosate resistant and susceptible Palmer amaranth (Amaranthus palmeri S. Wats.) to varying rates of glyphosate

Repeated application of herbicides has led to herbicide-resistant weed populations. Elevated CO2 often increases weed growth, exacerbating problems including impacts on crop yield and weed control. How rising CO2 affects herbicide-resistant weeds remains largely unknown. Palmer amaranth (Amaranthus palmeri) populations resistant to or susceptible to glyphosate were exposed to ambient or elevated (ambient +200 mu mol mol(-1)) concentrations of CO2 in open-top chambers. Two weeks after CO2 exposure, each population was sprayed with glyphosate at 0, 0.5x, 1.0x, or 1.5x the label rate. Afterwards, CO2 exposure continued for an additional two weeks when visual herbicide injury and leaf gas exchange were assessed. Plants were harvested for determination of treatment effects on growth. Elevated CO2 had little effect on most variables except for decreasing stomatal conductance (similar to 25.5%) and increasing water use efficiency (similar to 49.2%). Glyphosate resistant plants showed decreased height (9.8%) and damage (similar to 56.4%) and increased root dry weight (12.1%), photosynthesis (similar to 41.2%), stomatal conductance (similar to 63.3%), and water use efficiency (similar to 32.0%). In general, increasing rates of glyphosate decreased growth and gas exchange variables, which was frequently only seen in the susceptible ecotype. Data suggest that rising atmospheric CO2 is unlikely to affect Palmer amaranth, including control of glyphosate-resistant populations.