A model of photosynthetic CO2 assimilation and carbon-isotope discrimination in leaves of certain C3?C4 intermediates

A model of leaf, photosynthesis has been developed for C₃–C₄ intermediate species found in the generaPanicum, Moricandia, Parthenium andMollugo where no functional C₄ pathway has been identified. Model assumptions are a functional C₃ cycle in both mesophyll and bundle-sheath cells and that glycine formed in the mesophyll, as a consequence of the oxygenase activity of ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco, EC 4.1.1.39), diffuses to the bundle sheath, where most of the photorespiratory CO₂ is released. The model describes the observed gas-exchange characteristics of these C₃–C₄ intermediates, such as low CO₂-compensation points () at an O₂ pressure of 200 mbar, a curvilinear response of to changing O₂ pressures, and typical responses of CO₂-assimilation rate to intercellular CO₂ pressure. The model predicts that bundle-sheath CO₂ concentration is highest at low mesophyll CO₂ pressures and decreases as mesophyll CO₂ pressure increases. A partitioning of 5–15% of the total leaf Rubisco into the bundle-sheath cells and a bundlesheath conductance similar to that proposed for C₄ species best mimics the gas-exchange results. The model predicts C₃-like carbon-isotope discrimination for photosynthesis at atmospheric levels of CO₂, but at low CO₂ pressures it predicts a higher discrimination than is typically found during C₃ photosynthesis at lower CO₂ pressures.Abbreviations and symbols PEP phosphoenolpyruvate - Rubisco ribulose-1,5-bisphosphate carboxylase-oxygenase (EC 4.1.1.39) - RuBP ribulose-1,5-bisphosphate - p(CO₂) partial pressure of CO₂ - p(O₂) partial pressure of O₂. See also p. 471