RESPONSES OF SOYBEAN AND 5 WEEDS TO CO2 ENRICHMENT UNDER 2 TEMPERATURE REGIMES

Rising atmospheric CO2 levels could affect plant growth both directly, through effects on physiology, and indirectly, through the effects of possible CO2-induced temperature increases. In this study we examined the interacting effects of CO2 enrichment and temperature on the growth and allocation of soybean and five weeds. individual plants of soybean [Glycine max (L.) Merr. 'Braxton'] , johnsongrass [Sorghum halepense (L.) Pers. quackgrass [Elytrigia repens (L.) Nevski] , redroot pigweed (Amaranthus retroflexus L.), sicklepod (Cassia obtusifolia L.). and velvetleaf (Abutilon theophrasti Medic.) were grown in growth chambers in all combinations of two temperatures (avg. day/night of 26/19-degrees-C and 30/23-degrees-C) and two CO2 concentrations (350 and 700 ppm) for 35 d. Leaf area and plant biomass were greater at higher temperatures, regardless of CO2 level, in all species except quackgrass. Quackgrass (C3) produced its greatest leaf area and biomass at elevated CO2, whereas johnsongrass (C4) showed little response. Redroot pigweed (C4) and the C3 dicotyledenous species (soybean, sicklepod, velvetleaf) produced their greatest biomass at high CO2, though effects on leaf area were less consistent or absent. In general, when significant CO2 by temperature interactions were found, CO2 responses were smallest at higher temperatures. These differential responses to elevated CO2 concentrations may cause changes in the relative importance of competitive pressure from these weeds.