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, ArbutusxMarina', 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.