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.
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