Phenotypic and genetic differences in a perennial herb across a natural gradient of CO₂ concentration

The atmospheric CO₂ concentration [CO₂] has been increasing markedly since the industrial revolution and is predicted to reach 500-1,000 μmol mol^-1 by the end of this century. Although the short-term and acclimatory responses to elevated [CO₂] have been well studied, much less is understood about evolutionary responses to high [CO₂]. We studied phenotypic and genetic differences in Plantago asiatica populations around a natural CO₂ spring, where [CO₂] has been consistently high over an evolutionary time scale. Our common-garden experiment revealed that plants transferred from habitats with higher [CO₂] had higher relative growth rates, greater leaf to root ratios, lower photosynthetic rates, and lower stomatal conductance. The habitat-dependent differences were partly heritable because a similar trend of leaf to root ratio was found among their offsprings. Genetic analyses indicated that selfing or biparental inbreeding might promote local adaptation in areas with high [CO₂] despite substantial gene flow across the [CO₂] gradient. These results indicate that phenotypic and genetic differences have occurred between high and normal [CO₂] populations.