Submitted to: Australian Journal of Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 15, 2000
Publication Date: N/A
Interpretive Summary: Currently, there is general agreement that rising atmospheric carbon dioxide will increase the growth and yield of a number of crops including wheat, rice and soybeans. However, little is known about the response of weeds, especially perennial weeds, to rising carbon dioxide. In this study we examined the long-term growth and photosynthetic response of quackgrass, ,a perennial weed of forage and pasture crops. Quackgrass, when grown at 21st century carbon dioxide levels (700 ppm), demonstrated significant increases in photosynthesis and growth over a 240-day period compared to quackgrass grown at current levels of carbon dioxide. In addition, control of established stands of quackgrass when grown at the higher carbon dioxide concentration was not achieved when a commercial herbicide, glyphosate ("Roundup"), was applied at the recommended rate. These data indicate that, while crops could grow more with increasing carbon dioxide, weedy growth could also increase. This increase in weedy growth may also not be controlled as effectively by chemical means. Overall, this study suggests that higher carbon dioxide may also mean increases in crop/weed competition, with subsequent effects on crop production.
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 perennial weeds. The growth and photosynthetic characteristics of three cohorts of a perennial weedy species, quackgrass (Elytrigia repens), were examined at 380 umol mol-1 (ambient) and 720 umol mol-1 (elevated) [CO2] during 1998 and early 1999. For the oldest cohort, planted on Day of Year (DOY) 187, elevated [CO2] resulted in a consistent stimulation of single leaf photosynthesis, vegetative and whole plant biomass relative to the ambient [CO2] condition over a 240-day period. To determine if the observed stimulation of growth at elevated [CO2] altered tolerance to chemical weed control, glyphosate ("Roundup") was applied to each cohort at each [CO2] treatment at rates of 0.00 (Control) and 2.24 kg ai ha-1 (Sprayed). Tolerance was determined by following the growth of each cohort at the growth [CO2] treatment for a 28-day period following glyphosate application. For the youngest cohort (DOY 350), [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 elevated DOY 187 cohort. The increase in glyphosate tolerance for this cohort was associated with the larger plant size for the elevated [CO2] grown plants at the time of spraying. Data from this experiment suggest that long-term simulation in perennial weeds could occur with a reduction in chemical control efficacy and increased weed/crop competition as atmospheric [CO2] increases.