The evolution of crassulacean acid metabolism inAeonium inferred from carbon isotope composition and enzyme activities

Variation in crassulacean acid metabolism (CAM) was investigated in thirtyAeonium species under controlled, well-watered conditions. Carbon isotope ratios and the specific activities of the CAM enzymes phosphoenolpyruvate carboxylase (PEPC) and malic enzyme ME_ are highly correlated and indicate a large variation in CAM. The δ¹³C values range from -13.6 to -27.1‰; PEPC and ME activities vary seven- and five-fold, respectively. Since these observations were made under controlled conditions, the differences are genetically determined and represent adaptations to different habitats. The CAM activity of the species is correlated with the characteristics of their natural habitats. CAM activity is also highly correlated with the growth- and life-forms of the species and accordingly agrees well with the infrageneric classifications which are partly based on these characters. Our results support the hypothesis that the Sempervivoideae have evolved from an EurasiaticSedum, which used C3 photosynthesis or weak CAM, and that evolution withinAeonium was from undifferentiatedSedum-like dwarf shrubs with C3 photosynthesis to woody candelabrum-shaped shrubs exhibiting strong CAM. The monocarpic, rosulate species with intermediate CAM may have evolved via an independent secondary route.