A model of photosynthetic CO2 assimilation in C3 leaves accounting for respiration and energy recycling by the plastidial oxidative pentose phosphate pathway

Abstract

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 the actual flux at low atmospheric CO2 concentration (Ca). Since the OPPP releases CO2 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 Ca ≈ 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 Ca. We discuss our results in relation to regulatory properties of the plastidial and cytosolic OPPP, the proposed variability of CO2 mesophyll conductance, and the contribution of day respiration to the A/Ci curve drop at high Ca. Furthermore, we critically examine the models and parameterisation and derive recommendations for follow‐up studies.