Does elevated CO2 act as a selective agent? Results of common garden experiments using Plantago asiatica from CO2 spring

Background/Question/Methods

The atmospheric CO₂ concentration is rising markedly and is expected to be approximately 40% higher in 2050 than today. Although the short-term plant responses to elevated CO₂ concentration have been well studied, much less is understood about evolutionary responses to high CO₂ concentration. Natural CO₂ spring, where CO₂ concentration has been consistently high, enables us to study plants exposed to high CO₂ concentration over an evolutionary time scale. We conducted two common-garden experiments using Plantago asiatica seeds acquired from four natural CO₂ springs and the control sites. In open top chambers with elevated growth CO₂ concentration (700ppm) and ambient growth CO₂ concentration (around 400ppm), P. asiatica of each spring and the respective control were cultivated. (i) competition experiment: plants were cultivated in pots; each pot had 4 spring and 4 control plants. Vegetative growth and the productions of reproductive organs were investigated. (ii) comparison of stomata traits: plants from spring and control were cultivated in pots separately to avoid competition, and stomatal density, stomatal index, and stomatal size were measured. The differences in stomata traits were compared between plants from spring and the respective control.

Results/Conclusions

In experiment (i) the results of reproductive production suggest that plants originated from all springs were more competitive against their respective control plants at elevated growth CO₂ concentration than at ambient growth CO₂ concentration, though results of vegetative growth did not show a consistent tendency among springs. In addition, experiment (ii) demonstrated that stomatal size was smaller in plants germinated from seeds acquired in CO₂ spring than in the control, though there were no significant difference in stomatal density and stomatal index. Smaller stomata are adaptive to elevated CO₂ concentration as it enables plants to reduce water loss. These results support that atmospheric CO₂ elevation can act as a selective agent in natural plant populations.