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To reveal effects of elevated atmospheric CO2 on tolerance to the herbicide glyphosate in the populations of goosegrass (Eleusine indica) known to have resistance to this herbicide, two biotypes of goosegrass, a resistant (R) and a susceptible (S) biotype, were analyzed after exposed to ambient (400 μmol mol-1) and elevated CO2 (800 μmol mol-1) for 25 d. The results showed that elevated CO2 had little effect on growth of both biotypes. At elevated CO2, shoot biomass of R biotype was statistically lower than of S biotype (P<0.5). The dose-response analysis showed that glyphosate tolerance was reduced by 60% in R biotype exposed to elevated Co2 while it was slightly increased in S biotype. Elevated Co2 also had more adverse effects on photosynthetic capacity of R biotype than S biotype. As a result, maximum rate of net photosynthesis (Amax) and carboxylation efficiency (CE) under elevated CO2 in R biotype were significantly lower than in S biotype (P<0.5). These results indicate that elevated atmospheric Co2 can lead to a compromise in glyphosate tolerance of R biotype associated with poor photosynthetic characteristics, and thus will affect weed management involved with herbicide resistance in prospective agricultural systems. © 2015 Friends Science Publishers [ABSTRACT FROM AUTHOR]