Exploring Xylem Anatomical Adaptations Associated with Crassulacean Acid Metabolism and Hydraulic Capacitance in Clusia Leaves: Lessons for CAM Bioengineering

• Background and Aims Many succulent species are characterised by the presence of crassulacean acid metabolism (CAM) and/or elevated bulk hydraulic capacitance (C FT ). Both CAM and elevated C FT substantially reduce the rate at which water moves through transpiring leaves. However, little is known about how these physiological adaptations are coordinated with leaf vascular architecture and xylem anatomy. • Methods The genus Clusia contains species spanning the entire C 3 -CAM continuum, and also is known to have > 5-fold interspecific variation in C FT . We used this highly diverse genus to explore how interspecific variation in vein density and xylem vessel dimensions are coordinated with CAM and C FT . • Key Results We found that constitutive CAM phenotypes were associated with lower vein length per leaf area (VLA) and vein termini density (VTD), compared to C 3 or facultative CAM species. However, when vein densities were standardised by leaf depth, this value was higher in CAM than C 3 species, which is likely an adaptation to overcome apoplastic hydraulic resistance in deep chlorenchyma tissue. In contrast, C FT did not correlate with any xylem anatomical trait measured, suggesting CAM has a greater impact on leaf hydraulic physiology than C FT . • Conclusions Our findings strongly suggest that CAM photosynthesis is coordinated with leaf vein densities. The link between CAM and vascular anatomy will be important to consider when attempting to bioengineer CAM into C 3 crops.