Is atmospheric CO₂ a selective agent on model C₃ annuals?

Atmospheric CO₂ partial pressure (pCO₂) was as low as 18 Pa during the Pleistocene and is projected to increase from 36 to 70 Pa CO₂ before the end of the 21st century. High pCO₂ often increases the growth and reproduction of C₃ annuals, whereas low pCO₂ decreases growth and may reduce or prevent reproduction. Previous predictions regarding the effects of high and low pCO₂ on C₃ plants have rarely considered the effects of evolution. Knowledge of the potential for evolution of C₃ plants in response to CO₂ is important for predicting the degree to which plants may sequester atmospheric CO₂ in the future, and for understanding how plants may have functioned in response to low pCO₂ during the Pleistocene. Therefore, three studies using Arabidopsis thaliana as a model system for C₃ annuals 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) pCO₂ and 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 pCO₂ was a selective agent on Arabidopsis. Arabidopsis showed significant positive responses to selection for high seed number at both 20 and 70 Pa CO₂ during the selection process. Furthermore, plants selected at 20 Pa CO₂ performed better than plants selected at 70 Pa CO₂ under low CO₂ conditions, indicating that low CO₂ acted as a selective agent on these annuals. However, plants selected at 70 Pa CO₂ did not have significantly higher seed production than plants selected at 20 Pa CO₂ when grown at high pCO₂. Nevertheless, there was some evidence that high CO₂ may 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 pCO₂ showed 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 CO₂ resulted 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 C₃ annuals may have produced greater biomass in response to low pCO₂ during the Pleistocene relative to what has been predicted from studies exposing a single generation of C₃ plants to low pCO₂. Furthermore, C₃ annuals may exhibit evolutionary responses to high pCO₂ in the future that may result in developmental changes, but these are unlikely to increase biomass production. This series of studies shows that CO₂ may potentially act as a selective agent on C₃ annuals, producing changes in development rate and carbon accumulation that could not have been predicted from single-generation studies.