Future atmospheric carbon dioxide may increase tolerance to glyphosate

We tested whether the efficacy of chemical weed control might change as atm ospheric CO2 concentration [CO2] increases by determining if tolerance to a widely used, phloem mobile, postemergence herbicide, glyphosate, was alter ed by a doubling of [CO2]. Tolerance was determined by following the growth of Amaranthus retroflexus L. (redroot. pigweed), a C-4 species, and Chenop odium 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 h a(-1) (0.1 x the commercial rate), and 1.12 kg ai ha(-1) (1.0 x the commerc ial rare) in four separate trials. Irrespective of [CO2], growth of the C-4 species, A. retroflexus, was significantly reduced and was eliminated alto gether at glyphosate application rates of 0.112 and 1.12 kg ai ha(-1), resp ectively. However, in contrast to the ambient [CO2] treatment, an applicati on 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. al bum. Although glyphosate tolerance does increase with plant size at the tim e of application, differences in glyphosate tolerance between CO2 treatment s in C. album cannot be explained by size alone. These data indicate that r ising atmospheric [CO2] could increase glyphosate tolerance in a C-3 weedy species. Changes in herbicide tolerance at elevated [CO2] could limit chemi cal weed control efficacy and increase weed-crop competition.