Potential Effects of Global Atmospheric CO2Enrichment on the Growth and Competitiveness of C3and C4Weed and Crop Plants

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Abstract
Mathematical growth analysis techniques were used to evaluate the effects of CO2 concentrations of 350, 600, and 1000 ppm (v/v) on growth and biomass partitioning in corn (Zea mays L. ‘Dekalb XL 395’), itchgrass (Rottboellia exaltata L. f.), soybean [Glycine max (L.) Merr. ‘Tracy’], and velvetleaf (Abutilon theophrasti Medic). Controlled environment chambers with day/night temperatures of 28/22 C and photosynthetic photon flux densities (PPFD) of 650 μE (microeinsteins) m-2 s-1 were used. Dry matter production in the two C3 species (soybean and velvetleaf) was increased significantly by raising the CO2 concentration above 350 ppm. In corn (a C4 species), dry matter production was least at 1000 ppm CO2 and did not differ between the 350 and 600 ppm treatments. In itchgrass (also C4), dry matter production was greatest at 600 ppm CO2 and did not differ between the 350 and 1000 ppm treatments. Increasing the CO2 concentration increased the rate of dry matter production per unit leaf area (net assimilation rate or NAR) in soybean and velvetleaf but either decreased or did not alter NAR in corn and itchgrass. At 45 days after planting, the weed/crop ratios for total dry matter production for velvetleaf/corn and itchgrass/corn were significantly greater at both 600 and 1000 ppm than at 350 ppm CO2. The weed/crop ratio for itchgrass/soybean was less at 1000 ppm than at 350 or 600 ppm CO2. Compared to the value at 350 ppm, the weed/crop ratio for velvetleaf/soybean was greater at 600 ppm and less at 1000 ppm CO2. We conclude that atmospheric CO2 enrichment probably will make weeds with the C3 photosynthetic pathway more competitive with crops having the C4 pathway. Weeds with the C4 pathway may become less competitive with crops having the C3 pathway.