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.

稳定同位素通常用于研究CO ₂在光合植物组织中的扩散。用于解释观察到的对C ₃光合作用中较轻的碳同位素的偏好的标准方法涉及Farquhar等人的模型，该模型将碳同位素的辨别力与叶片内的物理和生化过程相关。但是，在许多情况下，该模型对于叶肉电导率对CO ₂扩散的返回不合理的结果（g _m），尤其是在光合作用速率较低时。在这里，我们使用与线粒体呼吸作用的同位素效应相关的修改假设重新推导碳同位素判别模型。特别是，我们将与呼吸有关的碳库与主要同化物库分开对待。我们通过比较同位素组成不同的不同CO ₂原料气测得的g _m值，通过实验测试了该模型，并表明我们的新模型返回的匹配g _m值比以前的模型更合理。我们用我们的结果来讨论叶肉内部的CO ₂扩散特性。