Stronger expression of crassulacean acid metabolism ( CAM ) requires effective cuticular transpiration barriers but not necessarily strong succulence

Summary Discovering functional and evolutionary interdependencies of hydraulic traits and crassulacean acid metabolism (CAM) is crucial to understand CAM phenotype diversity and convergence. In complex traits such as CAM, the co‐option of associated traits strongly impacts the evolutionary outcome. Here we study Aeonium (Crassulaceae), a diverse Macaronesian genus that exhibits a broad array of CAM expression, focusing on two CAM‐associated traits, minimum conductance ( g min ) and succulence. At the heart of the study, there are two experiments: a comparative cultivation experiment to monitor nocturnal acidification (ΔH + ) under drought and heat treatments and a leaf‐drying curve experiment to quantify g min . Our study group was comprehensively sampled to cover its phylogenetic and ecological diversity. We found a consistently negative correlation of g min and ΔH + , indicating a critical role for the cuticle in the function of CAM. Although ΔH + and succulence were overall not positively correlated, we found evidence that more succulent species remain in the CAM mode when stress is relaxed. We conclude that there is a tight evolutionary link between cuticular transpiration barrier properties and CAM performance. Thereby, the stronger CAM plants express diurnal stomatal closure typical of CAM, the more they may optimise water‐use efficiency through reduced g min .