Future atmospheric carbon dioxide may increase tolerance to glyphosate

We tested whether the efficacy of chemical weed control might change as atmospheric CO2 concentration [CO2] increases by determining if tolerance to a widely used, phloem mobile, postemergence herbicide, glyphosate, was altered by a doubling of [CO2]. Tolerance was determined by following the growth of Amaranthus retroflexus L. (redroot. pigweed), a C-4 species, and Chenopodium album L. (common lambsquarters), a C-3 species, grown at near ambient (360 mu mol mol(-1)) and twice ambient (720 mu mol mol(-1)) [CO2] for 14 d following glyphosate application at rates of 0.00 (control), 0.112 kg ai ha(-1) (0.1 x the commercial rate), and 1.12 kg ai ha(-1) (1.0 x the commercial rare) in four separate trials. Irrespective of [CO2], growth of the C-4 species, A. retroflexus, was significantly reduced and was eliminated altogether at glyphosate application rates of 0.112 and 1.12 kg ai ha(-1), respectively. However, in contrast to the ambient [CO2] treatment, an application rate of 0.112 kg ai ha(-1) had no effect on growth, and a 1.12 kg ai ha(-1) rate reduced but did not eliminate growth in elevated [CO2]-grown C. album. Although glyphosate tolerance does increase with plant size at the time of application, differences in glyphosate tolerance between CO2 treatments in C. album cannot be explained by size alone. These data indicate that rising atmospheric [CO2] could increase glyphosate tolerance in a C-3 weedy species. Changes in herbicide tolerance at elevated [CO2] could limit chemical weed control efficacy and increase weed-crop competition.