Analysis of the Differential Response of Five Annuals to Elevated CO2 during Growth

Abstract

In order to investigate the effects, without competition, of CO 2 on germination, growth, physiological response, and reproduction, we focussed on co—occurring species that are prominent members of an annual community in Illinois. Five species of old field annual plants—Abutilon theophrasti (C 3 ), Amaranthus retroflexus (C 4 ), Ambrosia artemisiifolia (C 3 ), Chenopodium album (C 3 ), and Setaria faberii (C 4 )–were grown for their entire life cycle as individuals at CO 2 concentration of 350 @mL/L, 500 @mL/L, and 700@mL/L. Emergence time, growth rate, shoot water status, photosynthesis, conductance, flowering time, nitrogen content, and biomass and reproductive biomass were measured. There was no detectable effect of enhanced CO 2 on timing of emergence in any of the species. Amaranthus relative growth rate (RGR) was always higher at 700 @mL/L CO 2 than at 350 @mL/L. In both Abutilon and Ambrosia, RGR was greater at 700 @mL/L than at 350 @mL/L during the first half of the experimental period, but during the second half of the period the reverse was true. Shoot water potential significantly increased (became less negative) with increasing CO 2 in Amaranthus and Setaria. Similar but statistically nonsignificant trends were found in Chenopodium and Abutilon. Overall rate of photosynthesis increased with CO 2 but there were no significant effects, at the species level, of CO 2 on photosynthetic rates. Stomatal conductance decreased with increased CO 2 at both high and low light levels in C 3 species but only at high light levels in C 4 species. In all species, intercellular CO 2 increased with external CO 2 . Amaranthus flowered significantly earlier at 700 @mL/L than at 350 @mL/L, and Setaria flowered significantly later at 700 @mL/L than at either of the other CO 2 levels. Both Abutilon and Ambrosia showed a trend towards earlier flowering but this was not statistically significant. Of the morphological characters measured at the final harvest only specific leaf area (SLA) showed a consistent response to CO 2 , decreasing with increasing CO 2 . Significant CO 2 x 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 proportion of nitrogen declined with increasing CO 2 : there was also a significant CO 2 x species interaction caused by the different rates of decline in proportion of nitrogen among the species. The response of most characters had a significant species x CO 2 interaction. However, this was not simply caused by the C 3 /C 4 dichotomy. Reproductive biomass (seed, fruits, and flowers) increased with increasing CO 2 in Amaranthus (C 4 ) and in Chenopodium and Ambrosia (both C 3 ), but there was no change in Setaria (C 4 ), and Abutilon (C 3 ) showed a peak a 500 @mL/L. Species of the same community differed in their response to CO 2 , and these differences may help explain the outcome of competitive interactions among these species above ambient CO 2 levels.