Abstract
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, 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 C3-C4 intermediate species Flaveria pubescens compared with the closely related C3 plant F. cronquistii and the C4 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)CO2 incorporation. Our data suggest that the photorespiratory CO2 pump elevates the intraplastidial CO2 concentration about 3-fold in leaves of the C3-C4 intermediate species F. pubescens relative to the C3 species F. cronquistii.
Highlights
Land plants form three major classes characterized by specific modes of photosynthetic CO2 assimilation
In the mesophyll of C4 plant leaves and in crassulacean acid metabolism (CAM) plants, CO2 is initially fixed by phosphoenolpyruvate carboxylase
Bias becomes even stronger at a varying intercellular distribution of photosynthetic tasks, such as the operation of CO2-concentrating mechanisms
Summary
Land plants form three major classes characterized by specific modes of photosynthetic CO2 assimilation. To determine individual pool sizes Ci and carbon fluxes Ri, the experimental values of the radioactivity of sugar phosphates, metabolites of the glycine and serine branches of the photorespiratory pathway, and of C4-acids were simultaneously fitted to these functions by multi-component non-linear regression analysis.
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