Revisiting carbon isotope discrimination in C₃ plants shows respiration rules when photosynthesis is low

Stable isotopes are commonly used to study the diffusion of CO₂ within photosynthetic plant tissues. The standard method used to interpret the observed preference for the lighter carbon isotope in C₃ photosynthesis involves the model of Farquhar et al., which relates carbon isotope discrimination to physical and biochemical processes within the leaf. However, under many conditions the model returns unreasonable results for mesophyll conductance to CO₂ diffusion (g_m), especially when rates of photosynthesis are low. Here, we re-derive the carbon isotope discrimination model using modified assumptions related to the isotope effect of mitochondrial respiration. In particular, we treat the carbon pool associated with respiration as separate from the pool of primary assimilates. We experimentally test the model by comparing g_m values measured with different CO₂ source gases varying in their isotopic composition, and show that our new model returns matching g_m values that are much more reasonable than those obtained with the previous model. We use our results to discuss CO₂ diffusion properties within the mesophyll.