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

Garbutt, K., Williams, W. E., Bazzaz, F. A.
Ecology 1990 v.71 no.3 pp. 1185-1194
Abutilon theophrasti, Amaranthus retroflexus, Ambrosia artemisiifolia, C3 plants, C4 plants, Chenopodium album, Setaria faberi, annuals, biomass, carbon dioxide, carbon dioxide enrichment, flowering, flowers, fruits, germination, leaf area, leaves, nitrogen, nitrogen content, photosynthesis, physiological response, reproduction, stomatal conductance, water potential, Illinois
In order to investigate the effects, without competition, of CO₂ 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 (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 @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 CO₂ on timing of emergence in any of the species. Amaranthus relative growth rate (RGR) was always higher at 700 @mL/L CO₂ 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 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. In all species, intercellular CO₂ increased with external CO₂. 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 CO₂ 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 CO₂, decreasing with increasing CO₂. Significant CO₂ 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 CO₂: there was also a significant CO₂ 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 CO₂ interaction. However, this was not simply caused by the C₃/C₄ dichotomy. Reproductive biomass (seed, fruits, and flowers) increased with increasing CO₂ in Amaranthus (C₄) and in Chenopodium and Ambrosia (both C₃), but there was no change in Setaria (C₄), and Abutilon (C₃) showed a peak a 500 @mL/L. Species of the same community differed in their response to CO₂, and these differences may help explain the outcome of competitive interactions among these species above ambient CO₂ levels.