Extending the Farquhar-von Caemmerer-Berry photosynthesis model to account for various photorespiratory bypasses

The Farquhar-von Caemmerer-Berry model is used as the standard to investigate leaf photosynthetic CO₂- and light-responses in plant physiology. The model was developed for C₃ photosynthesis with the native photorespiratory cycle. However, it is also frequently used to parameterize photosynthesis of plants with synthetic photorespiratory bypasses. Here, we point out that this latter practice is not advised for two reasons. First, unlike the native photorespiratory cycle, synthetic bypasses alter amounts and location of photorespiratory CO₂ release. Second, requirements for reductants and ATP are altered by bypass pathways. We extend the model to account for these changes. Estimates of both V_cmax (Rubisco carboxylation capacity) and J_max (light-saturated linear electron transport rate) differ between original and extended models, and the difference is greater for V_cmax than for J_max. Model analyses show that 17 bypasses reported so far perform contrastingly in terms of photosynthetic efficiency parameters (carboxylation efficiency and quantum yield) and photosynthetic capacity parameter (light-saturated photosynthetic rate). The amount rather than the location of photorespiratory CO₂ release largely determines the benefit of bypasses. Full-decarboxylating bypasses, which have been claimed to act as a CO₂-concentrating mechanism around Rubisco, are least efficient, whereas a carbon-fixing bypass is most promising for improving C₃ photosynthesis.