Future atmospheric carbon dioxide may increase tolerance to glyphosate

We tested whether the efficacy of chemical weed control might change as atmospheric CO 2 concentration [CO 2 ] increases by determining if tolerance to a widely used, phloem mobile, postemergence herbicide, glyphosate, was altered by a doubling of [CO 2 ]. 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 μmol mol −1 ) and twice ambient (720 μmol mol −1 ) [CO 2 ] 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 rate) in four separate trials. Irrespective of [CO 2 ], 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 [CO 2 ] 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 [CO 2 ]-grown C. album. Although glyphosate tolerant does increase with plant size at the time of application, differences in glyphosate tolerance between CO 2 treatments in C. album cannot be explained by size alone. These data indicate that rising atmospheric [CO 2 ] could increase glyphosate tolerance in a C 3 weedy species. Changes in herbicide tolerance at elevated [CO 2 ] could limit chemical weed control efficacy and increase weed–crop competition.