Functional leaf anatomy of plants with crassulacean acid metabolism

Crassulacean acid metabolism (CAM) has evolved independently on dozens of occasions and is now found in over 7% of plant species. In this study, the leaf structure of a phylogenetically diverse assemblage of 18 CAM plants was compared with six C₃ plants and four C₄ plants to assess whether consistent anatomical patterns that may reflect functional constraints are present. CAM plants exhibited increased cell size and increased leaf and mesophyll thickness relative to C₃ and C₄ species. CAM species also exhibited reduced intercellular air space (IAS) and reduced length of mesophyll surface exposed to IAS per unit area (L_mes / area). The low volume of IAS and low exposure of mesophyll surface to IAS likely increases internal resistance to CO₂ in CAM tissues. While this diffusional barrier may limit uptake of CO₂ during Phases II and IV, carbon economy could be enhanced through the reduced loss of internal CO₂ during all four phases of CAM.