Effects of CO 2 Enrichment on Competition Between a C 4 Weed and a C 3 Crop

The C 4 weed johnsongrass [ Sorghum halepense (L.) Pers. ♯ 3 SORHA] and the C 3 crop soybean [ Glycine max (L.) Merr. ‘Ransom’] were grown separately and in inter- and intraspecific competition at 350 and 675 ppm CO 2 in controlled environment chambers with 29/23 C day/night temperatures and 950 μE·m -2 · s -1 PPFD (photosynthetic photon flux density). In the absence of competition, the higher CO 2 concentration stimulated dry matter accumulation, leaf area expansion, net assimilation rate, and leaf area duration of soybean more than that of johnsongrass. The plant relative yield (PRY) of soybean in competition with johnsongrass increased, and the PRY of johnsongrass in competition with soybean decreased, as the CO 2 concentration was increased from 350 to 675 ppm. Thus, the competitiveness of the C 3 crop with the C 4 weed increased with increasing CO 2 concentration. Relative yield totals were not significantly different from 1.0, indicating that the two species were competing for the same resources. With the increases in global atmospheric CO 2 concentration predicted for the next 50 to 100 yr, the competitiveness of C 3 crops with C 4 weeds could be increased.