Why are sun leaves thicker than shade leaves? Consideration based on analyses of CO2 diffusion in the leaf

Light-saturated rates of photosynthesis on leaf area basis (A) depend not o nly on photosynthetic biochemistry but also on mesophyll structure. Because resistance to CO2 diffusion from the substomatal cavity to the stroma is s ubstantial, it is likely that mesophyll structure affects A through affecti ng diffusion of CO2 in the leaf. To evaluate effects of various aspects of mesophyll structure on photosynthesis, we constructed a one-dimensional mod el of CO2 diffusion in the leaf. When mesophyll thickness of the leaf is ch anged with the Rubisco content per unit leaf area kept constant, the maximu m A occurs at an almost identical mesophyll thickness irrespective of the R ubisco contents per leaf area. On the other hand, with an increase in Rubis co content per leaf area, the mesophyll thickness that realizes a given pho tosynthetic gain per mesophyll thickness (or per leaf cost) increases This probably explains the strong relationship between A and mesephyll thickness . In these simulations, an increase in mesophyll thickness simultaneously m eans an increase in the diffusional resistance in the intercellular spaces (R-ias), an increase in the total surface area of chloroplasts facing the i ntercellular spaces per unit leaf area (S-c), and an increase in constructi on and maintenance cost of the leaf. Leaves can increase S-c and decrease R -ias also by decreasing cell size. Leaves with smaller cells are mechanical ly stronger. However,actual leaves do not have very small cells. This could be because actual leaves exhibiting considerable rates of leaf area expans ion, adequate heat capacitance, high efficiency of N and/or P use, etc, are favoured. Relationships between leaf longevity and mesophyll structure are also discussed.