Sustained growth and increased tolerance to glyphosate observed in a C-3 perennial weed, quackgrass (Elytrigia repens), grown at elevated carbon dioxide

Although the response of crop plants to rising atmospheric carbon dioxide c oncentration ([ CO2]) has been well characterized, little is known concerni ng the long-term growth and/or photosynthetic response of perennial weeds. The growth and photosynthetic characteristics of three cohorts of a perenni al C-3 weedy species, quackgrass (Elytrigia repens (L.) Nevski) were examin ed at similar to 380 mu mol mol(-1) (ambient) and 720 mu mol mol(-1) (eleva ted) [CO2] in temperature-controlled greenhouses during 1998 and early 1999 . Different cohorts were used to assess the sensitivity of growth, photosyn thesis and glyphosate tolerance to elevated [CO2] for different stages in t he life cycle of quackgrass. For the 'old' cohort, planted on Day of Year ( DOY) 187, elevated [CO2] resulted in a consistent stimulation of single lea f photosynthesis, vegetative and whole plant biomass relative to the ambien t [CO2] condition over a 231-d period. Data from the 'intermediate' (DOY 26 8) and 'young' cohorts (DOY 350) indicated that the stimulation of biomass at the elevated [CO2] was time-dependent. To determine if the observed stim ulation of growth at elevated [CO2] altered tolerance to chemical weed cont rol, glyphosate [( N-phosphonomethyl) glycine] was applied to each cohort a nd each [CO2] treatment at rates of 0 (control) and 2.24 kg ai ha(-1) (spra yed). Tolerance was determined by following the growth and slope of each co hort at the growth [CO2] treatment for a 28-d period following glyphosate a pplication. For the young cohort, [CO2] had no affect on glyphosate toleran ce; however, an application rate of 2.24 kg ai ha(-1), reduced but did not eliminate growth for the intermediate and old cohorts grown at elevated [CO 2]. The basis for increased glyphosate tolerance at elevated [CO2] for thes e cohorts was unclear, but was not related to plant size at the time of gly phosate application. Data from this experiment indicate that sustained stim ulation of photosynthesis and growth in perennial weeds could occur as atmo spheric [CO2] increases, with a reduction in chemical control effectiveness and potential increases in weed/crop competition.