title: Is atmospheric CO2a selective agent on model C3annuals?
abstract: Atmospheric CO2partial pressure (pCO2) was as low as 18 Pa during the Pleistocene and is projected to increase from 36 to 70 Pa CO2before the end of the 21st century. High pCO2often increases the growth and reproduction of C3annuals, whereas low pCO2decreases growth and may reduce or prevent reproduction. Previous predictions regarding the effects of high and low pCO2on C3plants have rarely considered the effects of evolution. Knowledge of the potential for evolution of C3plants in response to CO2is important for predicting the degree to which plants may sequester atmospheric CO2in the future, and for understanding how plants may have functioned in response to low pCO2during the Pleistocene. Therefore, three studies using Arabidopsis thaliana as a model system for C3annuals were conducted: (1) a selection experiment to measure responses to selection for high seed number (a major component of fitness) at Pleistocene (20 Pa) and future (70 Pa) pCO2and to determine changes in development rate and biomass production during selection, (2) a growth experiment to determine if the effects of selection on final biomass were evident prior to reproduction, and (3) a reciprocal transplant experiment to test if pCO2was a selective agent on Arabidopsis. Arabidopsis showed significant positive responses to selection for high seed number at both 20 and 70 Pa CO2during the selection process. Furthermore, plants selected at 20 Pa CO2performed better than plants selected at 70 Pa CO2under low CO2conditions, indicating that low CO2acted as a selective agent on these annuals. However, plants selected at 70 Pa CO2did not have significantly higher seed production than plants selected at 20 Pa CO2when grown at high pCO2. Nevertheless, there was some evidence that high CO2may also be a selective agent because changes in development rate and biomass production during selection occurred in opposite directions at low and high pCO2. Plants selected at high pCO2showed no change or reductions in biomass relative to control plants due to a decrease in the length of the life cycle, as indicated by earlier initiation of flowering and senescence. In contrast, selection at low CO2resulted in an average 35% increase in biomass production, due to an increase in the length of the life cycle that resulted in a longer period for biomass accumulation before senescence. From the Arabidopsis model system we conclude that some C3annuals may have produced greater biomass in response to low pCO2during the Pleistocene relative to what has been predicted from studies exposing a single generation of C3plants to low pCO2. Furthermore, C3annuals may exhibit evolutionary responses to high pCO2in the future that may result in developmental changes, but these are unlikely to increase biomass production. This series of studies shows that CO2may potentially act as a selective agent on C3annuals, producing changes in development rate and carbon accumulation that could not have been predicted from single-generation studies.
authors.affiliation: Department of Botany, Duke University, Durham, NC 27708, USA, , , , , , US.
authors.forename: J K
authors.initials: JK
authors.lastname: Ward
authors.affiliation: Department of Biology, University of Virginia, Charlottesville, VA 22903, USA, , , , , , US.
authors.forename: J
authors.initials: J
authors.lastname: Antonovics
authors.affiliation: Department of Biology, West Virginia University, Morgantown, WV 26506, USA, , , , , , US.
authors.forename: R B
authors.initials: RB
authors.lastname: Thomas
authors.affiliation: Department of Botany, Duke University, Durham, NC 27708, USA, , , , , , US.
authors.forename: B R
authors.initials: BR
authors.lastname: Strain
keywords: Carbon dioxide; Evolution; Key words  Arabidopsis thaliana; Reproduction; Selection