C2 photosynthesis generates about 3-fold elevated leaf CO2 levels in the C3-C4 intermediate species Flaveria pubescens

Formation of a photorespiration-based CO2-concentrating mechanism in C-3-C-4 intermediate plants is seen as a prerequisite for the evolution of C-4 photosynthesis, but it is not known how efficient this mechanism is. Here, using in vivo Rubisco carboxylation-to-oxygenation ratios as a proxy to assess relative intraplastidial CO2 levels is suggested. Such ratios were determined for the C-3-C-4 intermediate species Flaveria pubescens compared with the closely related C-3 plant F. cronquistii and the C-4 plant F. trinervia. To this end, a model was developed to describe the major carbon fluxes and metabolite pools involved in photosynthetic-photorespiratory carbon metabolism and used quantitatively to evaluate the labelling kinetics during short-term (CO2)-C-14 incorporation. Our data suggest that the photorespiratory CO2 pump elevates the intraplastidial CO2 concentration about 3-fold in leaves of the C-3-C-4 intermediate species F. pubescens relative to the C-3 species F. cronquistii.