Functional constraints of CAM leaf anatomy: tight cell packing is associated with increased CAM function across a gradient of CAM expression

Increased cell size, increased leaf succulence, reduced intercellular air space (IAS), and reduced surface of mesophyll exposed to IAS (L-mes/area) are traits associated with the Crassulacean acid metabolism (CAM) photosynthetic pathway. An examination was carried out to determine whether these anatomical and structural traits are related to the degree of CAM function in eight CAM species, as measured by CO2 assimilation during the CAM and C-3 phases. Increased cell size and leaf succulence were not closely related to the degree of CAM function, indicating that succulence beyond a certain threshold does not enhance CAM function. Reduced IAS and L-mes/area were positively related to CAM function, and negatively related to C-3 function. These results indicate that reduced IAS and L-mes/area are beneficial for CAM function through the reduction of CO2 efflux and the improvement of carbon economy. However, reduced IAS and L-mes/area limit C-3 photosynthesis, potentially mediating a bimodal distribution of weak and strong CAM species with high and low IAS and L-mes/area values, respectively.