Environmental Science and Engineering Seminar

CO₂ fixation occurs inside the stroma of chloroplast. Before this event takes place, CO₂ molecules must diffuse first through stomata and then through mesophyll to reach the carboxylation enzyme Rubisco. Convincing evidence exists that mesophyll conductance and stomata conductance are of similar magnitude and thus both strongly affect photosynthesis. However, compared with the extensive research on stomatal conductance, much less attention has been paid to mesophyll conductance. In this presentation, I will present two examples of how mesophyll conductance affects photosynthesis. One example is at the leaf scale while the other is at the global scale. At the leaf scale, I will use A/Ci measurements of a large number of species and model simulations to show that proper consideration of mesophyll conductance is essential for understanding in situ photosynthetic machinery functioning. At the global scale, I will demonstrate that an explicit consideration of mesophyll diffusion is needed to model CO₂ fertilization effect accurately. Without this consideration, the CO2 fertilization effect is underestimated. Since most global carbon cycle models have not explicitly considered mesophyll conductance, the contemporary terrestrial biosphere may be more CO2 limited than previously thought.