Does leaf photosynthesis adapt to CO2-enriched environments? An experiment on plants originating from three natural CO2 springs

Atmospheric CO₂ elevation may act as a selective agent, which consequently may alter plant traits in the future. We investigated the adaptation to high CO₂ using transplant experiments with plants originating from natural CO₂ springs and from respective control sites. • We tested three hypotheses for adaptation to high-CO₂ conditions: a higher photosynthetic nitrogen use efficiency (PNUE); a higher photosynthetic water use efficiency (WUE); and a higher capacity for carbohydrate transport from leaves. • Although elevated growth CO ₂ enhanced both PNUE and WUE, there was no genotypic improvement in PNUE. However, some spring plants had a higher WUE, as a result of a significant reduction in stomatal conductance, and also a lower starch concentration. Higher natural variation (assessed by the coefficient of variation) within populations in WUE and starch concentration, compared with PNUE, might be responsible for the observed population differentiation. • These results support the concept that atmospheric CO₂ elevation can act as a selective agent on some plant traits in natural plant communities. Reduced stomatal conductance and reduced starch accumulation are highlighted for possible adaptation to high CO₂.