Sustained growth and increased tolerance to glyphosate observed in a C₃ perennial weed, quackgrass (Elytrigia repens), grown at elevated carbon dioxide

Abstract

Although the response of crop plants to rising atmospheric carbon dioxide concentration ([CO₂]) has been well characterized, little is known concerning the long-term growth and/or photosynthetic response of peren-nial weeds. The growth and photosynthetic characteristics of three cohorts of a perennial C₃ weedy species, quack-grass (Elytrigia repens (L.) Nevski) were examined at ~380 µmol mol⁻¹ (ambient) and 720 µmol mol⁻¹ (elevated) [CO₂] in temperature-controlled greenhouses during 1998 and early 1999. Different cohorts were used to assess the sensitivity of growth, photosynthesis and glyphosate tolerance to elevated [CO₂] for different stages in the life cycle of quackgrass. For the ‘old’ cohort, planted on Day of Year (DOY) 187, elevated [CO₂] resulted in a consistent stim-ulation of single leaf photosynthesis, vegetative and whole plant biomass relative to the ambient [CO₂] condition over a 231-d period. Data from the ‘intermediate’ (DOY 268) and ‘young’ cohorts (DOY 350) indicated that the stimula-tion of biomass at the elevated [CO₂] was time-dependent. To determine if the observed stimulation of growth at ele-vated [CO₂] altered tolerance to chemical weed control, glyphosate [(N-phosphonomethyl)glycine] was applied to each cohort and each [CO₂] treatment at rates of 0 (control) and 2.24 kg ai ha⁻¹ (sprayed). Tolerance was determined by following the growth and slope of each cohort at the growth [CO₂] treatment for a 28-d period following glyphosate application. For the young cohort, [CO₂] had no affect on glyphosate tolerance; 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 [CO₂]. The basis for increased glyphosate tolerance at elevated [CO₂] for these cohorts was unclear, but was not related to plant size at the time of glyphosate application. Data from this experiment indicate that sustained stimula-tion of photosynthesis and growth in perennial weeds could occur as atmospheric [CO₂] increases, with a reduction in chemical control effectiveness and potential increases in weed/crop competition.