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 CO₂ 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 CO₂.12 page(s