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 grow th and allocation of soybean and five weeds. individual plants of soyb ean [Glycine max (L.) Merr. 'Braxton'] , johnsongrass [Sorghum halepen se (L.) Pers. quackgrass [Elytrigia repens (L.) Nevski] , redroot pigw eed (Amaranthus retroflexus L.), sicklepod (Cassia obtusifolia L.). an d velvetleaf (Abutilon theophrasti Medic.) were grown in growth chambe rs in all combinations of two temperatures (avg. day/night of 26/19-de grees-C and 30/23-degrees-C) and two CO2 concentrations (350 and 700 p pm) for 35 d. Leaf area and plant biomass were greater at higher tempe ratures, regardless of CO2 level, in all species except quackgrass. Qu ackgrass (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, velvetlea f) 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 a t higher temperatures. These differential responses to elevated CO2 co ncentrations may cause changes in the relative importance of competiti ve pressure from these weeds.