A model of photosynthetic CO2assimilation in C3leaves accounting for respiration and energy recycling by the plastidial oxidative pentose phosphate pathway

Summary - Recently, we reported estimates of anaplerotic carbon flux through the oxidative pentose phosphate pathway (OPPP) in chloroplasts into the Calvin-Benson cycle. These estimates were based on intramolecular hydrogen isotope analysis of sunflower leaf starch. However, the isotope method is believed to underestimate actual flux at low atmospheric CO 2 concentration ( C a ). - Since the OPPP releases CO 2 and reduces NADP + , it can be expected to affect leaf gas exchange under both rubisco- and RuBP-regeneration-limited conditions. Therefore, we expanded Farquhar-von Caemmerer-Berry models to account for OPPP metabolism. Based on model parameterisation with values from the literature, we estimated OPPP-related effects on leaf carbon and energy metabolism in the sunflowers analysed previously. - We found that flux through the plastidial OPPP increases both above and below C a ≈ 450 ppm (the condition the plants were acclimated to). This is qualitatively consistent with our previous isotope-based estimates, yet gas-exchange-based estimates are larger at low C a . - We discuss our results in relation to regulatory properties of the plastidial and cytosolic OPPP, the proposed variability of CO 2 mesophyll conductance, and the contribution of day respiration to the A / C i curve drop at high C a . Furthermore, we critically examine the models and parameterisation and derive recommendations for follow-up studies.