Stomata on the abaxial and adaxial leaf surface contribute differently to leaf gas exchange and photosynthesis in wheat.

Although stomata are typically found in greater numbers on the abaxial surface, wheat flag leaves have greater densities on the adaxial surface. We determine the impact of this less common stomatal patterning on gaseous fluxes using a novel chamber that simultaneously measures both leaf surfaces. Using a combination of differential illuminations and CO₂ concentrations at each leaf surface, we found mesophyll cells associated with the adaxial leaf surface have a higher photosynthetic capacity than the abaxial, supported by a greater stomatal conductance (driven by differences in stomatal density). Blocking vertical gas flux on the abaxial leaf surface demonstrated no compensation by adaxial stomata, suggesting each surface operates independently. Similar stomatal kinetics suggested some co-ordination between the two surfaces, but factors other than light intensity played a role in these responses. Higher photosynthetic capacity on the adaxial surface facilitates greater carbon assimilation, along with higher adaxial g_s that would also support greater evaporative leaf cooling to maintain optimal leaf temperatures for photosynthesis. Furthermore, abaxial gas exchange contributed approximately 50% to leaf photosynthesis, and therefore represents an important contributor to overall leaf gas photosynthesis.