R script used to generate the theoretical conductance values based on cell size and leaf porosity. from Maximum CO2 diffusion inside leaves is limited by the scaling of cell size and genome size

Maintaining high rates of photosynthesis in leaves requires efficient movement of CO2 from the atmosphere to the mesophyll cells inside the leaf where CO2 is converted into sugar. CO2 diffusion inside the leaf depends directly on the structure of the mesophyll cells and their surrounding airspace, which have been difficult to characterize because of their inherently three-dimensional organization. Yet, faster CO2 diffusion inside the leaf was likely critical in elevating rates of photosynthesis that occurred among angiosperm lineages. Here we characterize the three-dimensional surface area of the leaf mesophyll across vascular plants. We show that genome size determines the sizes and packing densities of cells in all leaf tissues and that smaller cells enable more mesophyll surface area to be packed into the leaf volume, facilitating higher CO2 diffusion. Measurements and modelling revealed that the spongy mesophyll layer better facilitates gaseous phase diffusion while the palisade mesophyll layer better facilitates liquid-phase diffusion. Our results demonstrate that genome downsizing among the angiosperms was critical to restructuring the entire pathway of CO2 diffusion into and through the leaf, maintaining high rates of CO2 supply to the leaf mesophyll despite declining atmospheric CO2 levels during the Cretaceous.

维持叶片的高光合速率，需要二氧化碳（CO₂）高效地从大气扩散至叶片内部的叶肉细胞（mesophyll cells），而CO₂正是在这类细胞中被转化为糖类物质。叶片内部的CO₂扩散过程直接取决于叶肉细胞及其周边气隙的结构，由于这类结构本身具备三维组织特性，此前很难对其进行精准表征。然而，叶片内部更快的CO₂扩散，或许是被子植物（angiosperm）类群光合速率提升的关键驱动因素之一。本研究针对维管植物（vascular plants）的叶肉三维表面积开展了系统性表征。研究表明，基因组大小（genome size）决定了所有叶片组织中细胞的尺寸与填充密度；更小的细胞可使更多叶肉表面积被容纳于叶片体积内，从而提升CO₂扩散效率。通过测量与建模分析发现，海绵叶肉（spongy mesophyll）层更利于气相扩散，而栅栏叶肉（palisade mesophyll）层则更利于液相扩散。本研究结果证实，被子植物的基因组小型化事件，对于重构CO₂进入并穿过叶片的整条扩散通路至关重要；这使得即便在白垩纪（Cretaceous）大气CO₂浓度持续下降的背景下，叶片仍能维持向叶肉细胞的高CO₂供应速率。