Formation of a photorespiration-based CO2-concentrating mechanism in C3-C4 intermediate plants is seen as a prerequisite for the evolution of C4 photosynthesis, but it is not known how efficient this mechanism is. Here, usingin vivoRubisco carboxylation-to-oxygenation ratios as a proxy to assess relative intraplastidial CO2 levels is suggested. Such ratios were determined for the C3-C4 intermediate speciesFlaveria pubescenscompared with the closely related C3 plantF. cronquistiiand the C4 plantF. 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 14CO2 incorporation. Our data suggest that the photorespiratory CO2 pump elevates the intraplastidial CO2 concentration about 3-fold in leaves of the C3-C4 intermediate speciesF. pubescensrelative to the C3 speciesF. cronquistii.
Photorespiration raises cellular CO2 levels about 3-fold in leaves of C3-C4 intermediate Flaveria species. This was shown by using 14C-based fluxomics to determine the Rubisco in vivo carboxylation-to-oxygenation ratios.