Cell wall composition strongly influences mesophyll conductance in gymnosperms

Cell wall thickness is widely recognized as one of the main determinants of mesophyll conductance to CO2 (g(m)). However, little is known about the components that regulate effective CO2 diffusivity in the cell wall (i.e. the ratio between actual porosity and tortuosity, the other two biophysical diffusion properties of cell walls). The aim of this study was to assess, at the interspecific level, potential relationships between cell wall composition, cell wall thickness (T-cw) and g(m). Gymnosperms constitute an ideal group to deepen these relationships, as they present, on average, the thickest cell walls within spermatophytes. We characterized the foliar gas exchange, the morphoanatomical traits related with g(m), the leaf fraction constituted by cell walls and three main components of primary cell walls (hemicelluloses, cellulose and pectins) in seven gymnosperm species. We found that, although the relatively low g(m) of gymnosperms was mainly determined by their elevated T-cw, g(m) was also strongly correlated with cell wall composition, which presumably sets the final effective CO2 diffusivity. The data presented here suggest that (i) differences in g(m) are strongly correlated to the pectins to hemicelluloses and cellulose ratio in gymnosperms, and (ii) variations in cell wall composition may modify effective CO2 diffusivity in the cell wall to compensate the negative impact of thickened walls. We speculate that higher relative pectin content allows higher g(m) because pectins increase cell wall hydrophilicity and CO2 molecules cross the wall dissolved in water.