ANALYSIS OF THE DIFFERENTIAL RESPONSE OF 5 ANNUALS TO ELEVATED CO2 DURING GROWTH

Old field annual plants - Abutilon theophrasti (C3), Amaranthus retroflexus (C4), Ambrosia artemisiifolia (C3), Chenopodium album (C3) and Setaria faberii (C4) - were grown for their entire life cycle as individuals at CO2 concentration of 350, 500 and 700 μL/L. There was no detectable effect of enhanced CO2 on timing of emergence in any species. Amaranthus relative growth rate (RGR) was always higher at 700 μL/L CO2 than at 350 μL/L. In both Abutilon and Ambrosia, RGR was greater at 700 μL/L than at 350 μL/L during the 1st half of the experiment, but during the 2nd half the reverse was true. Shoot water potential significantly increased (became less negative) with increasing CO2 in Amaranthus and Setaria. Similar but statistically nonsignificant trends were found in Chenopodium and Abutilon. Overall rate of photosynthesis increased with CO2 but there were no significant effects, at the species level of CO2 on photosynthetic rates. Stomatal conductance decreased with increased CO2 at both high and low light levels in C3 species but only at high light levels in C4 species. Amaranthus flowered significantly earlier at 700 μL/L than at 350 μL/L, and Setaria flowered significantly later at 700 μL/L than at either of the other CO2 levels. Of the morphological characters measured at the final harvest only specific leaf area showed a consistent response to CO2, decreasing with increasing CO2. Significant CO2×species interactions were also found for leaf area, leaf biomass, biomass of reproductive parts, and seed biomass indicating species-specific responses for these characters. The response of most characters had a significant species × CO2 interaction, but this was not simply caused by the C3/C4 dichotomy. Reproductive biomass (seed, fruits, flowers) increased with increasing CO2 in Amaranthus and in Chenopodium and Ambrosia, but there was no change in Setaria, and Abutilon showed a peak at 500 μL/L. -from Authors