Gas exchange Photosynthetic limitations Photosynthetic down-regulation Rubisco speci?c activity Stomatal conductance

The hypothesis that elevated CO2 leads to a faster induction of photosynthesis was tested using 7-8- year-old seedlings of broadleaved Fagus sylvatica and coniferous Picea abies exposed for three growing seasons to ambient (385 mmol mol-1; AC) and elevated CO2 (700 mmol mol-1; EC). Since the photosynthetic induction is primarily determined by light modulations of Rubisco activity and stomata opening, the hypothesis was based on the following presumptions: (1) EC reduces the content of Rubisco in leaves, (2) decreases stomatal conductance, and (3) has no effect on the rates of stomatal opening and Rubisco activation upon illumination. Thus, reduced maxima of Rubisco activity and stomatal conductance should be reached faster in EC compared to AC plants after leaf transition from dark to light.