C2photosynthesis generates about 3-fold elevated leaf CO2levels in the C3–C4intermediate species Flaveria pubescens

Formation of a photorespiration-based CO 2 -concentrating mechanism in C 3 –C 4 intermediate plants is seen as a pre-requisite 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 CO 2 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 14 CO 2 incorporation. Our data suggest that the photorespiratory CO 2 pump elevates the intraplastidial CO 2 concentration about 3-fold in leaves of the C 3 –C 4 intermediate species F. pubescens relative to the C 3 species F. cronquistii.