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 (Lmes/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 C3 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 Lmes/area were positively related to CAM function, and negatively related to C3 function. These results indicate that reduced IAS and Lmes/area are beneficial for CAM function through the reduction of CO2 efflux and the improvement of carbon economy. However, reduced IAS and Lmes/area limit C3 photosynthesis, potentially mediating a bimodal distribution of weak and strong CAM species with high and low IAS and Lmes/area values, respectively.