Thicker leaves, thicker cell walls, low mesophyll conductance

Photosynthesis is the key driver of grain yield in agricultural crop species, so reducing any limitations to photosynthetic processes in these plants could improve yield and contribute to future food security. Diffusion of CO₂ from the air to the site of carboxylation is regarded as one of the most important limiting factors for photosynthesis in C₃ species. After entering the leaf through the stomata, CO₂ molecules need to diffuse through the mesophyll cell to reach the chloroplasts. This pathway of diffusion includes anatomical and biochemical components and is referred to as mesophyll conductance. Of the many anatomical features that relate to mesophyll conductance, the thickness of cell walls is particularly important, accounting for almost half of the total mesophyll resistance to diffusion. Studies have shown that leaves with thicker cell wall typically have higher leaf mass per area (LMA). There is however no consensus on whether mesophyll conductance is affected by differences in leaf structure and anatomy or whether this varies within species.

In their new study published in AoBP, Ye et al. report that high LMA rice plants invest more leaf mass to cell walls and possess a low mesophyll conductance, limiting photosynthesis. Leaf thickness rather than leaf density is the main driving factor for the difference in LMA. With increased leaf thickness, the surface area of mesophyll cells (and the chloroplasts inside them) exposed to the intercellular airspaces is increased, however thickness of cell walls is also increased. Cell wall compounds were found to account for the majority of the leaf dry mass in rice leaves. The authors found significant differences between rice genotypes in their study, including one genotype with thick cell walls and a high mesophyll conductance, and suggest that future studies explore this intraspecific variation in more detail.