C(2) photosynthesis generates about 3-fold elevated leaf CO(2) levels in the C(3)–C(4) intermediate species Flaveria pubescens

Formation of a photorespiration-based CO(2)-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 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.