Using climate-controlled glasshouses, the growth of grain sorghum was evaluated with and without the presence of common cocklebur at current and projected future atmospheric concentrations of carbon dioxide [CO₂]. Single-leaf photosynthetic rates declined for both species in competition; however, elevated CO₂ reduced the percentage decline in common cocklebur and increased it in sorghum by 35 d after sowing (DAS) relative to ambient CO₂. In monoculture, elevated CO₂ significantly stimulated leaf photosynthetic rate, leaf area, and aboveground dry weight of common cocklebur more than that of sorghum. However, the stimulation of aboveground biomass or leaf area for monocultures of sorghum and common cocklebur at elevated CO₂ did not adequately predict the CO₂ response of aboveground biomass or leaf area for sorghum and common cocklebur grown in competitive mixtures. Overall, by 41 DAS, plant relative yield (PRY), in terms of aboveground biomass and leaf area, increased significantly for common cocklebur and decreased significantly for sorghum in competitive mixtures at elevated CO₂. Data from this study indicate that vegetative growth, competition, and potential yield of economically important C₄ crops could be reduced by co-occurring C₃ weeds as atmospheric carbon dioxide increases.

Nomenclature: Common cocklebur, Xanthium strumarium L. XANST; sorghum, Sorghum bicolor L. Moench.