Maximum CO2diffusion inside leaves is limited by the scaling of cell size and genome size

Summary Maintaining high rates of photosynthesis in leaves requires efficient movement of CO 2 from the atmosphere to the chloroplasts inside the leaf where it is converted into sugar. Throughout the evolution of vascular plants, CO 2 diffusion across the leaf surface was maximized by reducing the sizes of the guard cells that form stomatal pores in the leaf epidermis 1,2 . Once inside the leaf, CO 2 must diffuse through the intercellular airspace and into the mesophyll cells where photosynthesis occurs 3,4 . However, the diffusive interface defined by the mesophyll cells and the airspace and its coordinated evolution with other leaf traits are not well described 5 . Here we show that among vascular plants variation in the total amount of mesophyll surface area per unit mesophyll volume is driven primarily by cell size, the lower limit of which is defined by genome size. The higher surface area enabled by smaller cells allows for more efficient CO 2 diffusion into photosynthetic mesophyll cells. Our results demonstrate that genome downsizing among the flowering plants 6 was critical to restructuring the entire pathway of CO 2 diffusion, facilitating high rates of CO 2 supply to the leaf mesophyll cells despite declining atmospheric CO 2 levels during the Cretaceous.