The light response of mesophyll conductance is controlled by structure across leaf profiles

Mesophyll conductance to CO₂ (g_m) may respond to light either through regulated dynamic mechanisms or due to anatomical and structural factors. At low light, some layers of cells in the leaf cross‐section approach photocompensation and contribute minimally to bulk leaf photosynthesis and little to whole leaf g_m (g_m,leaf). Thus, the bulk g_m,leaf will appear to respond to light despite being based upon cells having an anatomically fixed mesophyll conductance. Such behaviour was observed in species with contrasting leaf structure using the variable J or stable isotope method of measuring g_m,leaf. A species with bifacial structure, Arbutus × ‘Marina’, and an isobilateral species, Triticum durum L., had contrasting responses of g_m,leaf upon varying adaxial or abaxial illumination. Anatomical observations, when coupled with the proposed model of g_m,leaf to photosynthetic photon flux density (PPFD) response, successfully represented the observed gas exchange data. The theoretical and observed evidence that g_m,leaf apparently responds to light has large implications for how g_m,leaf values are interpreted, particularly limitation analyses, and indicates the importance of measuring g_m under full light saturation. Responses of g_m,leaf to the environment should be treated as an emergent property of a distributed 3D structure, and not solely a leaf area‐based phenomenon.