Carbon isotope composition of plant photosynthetic tissues reflects a Crassulacean Acid Metabolism (CAM) continuum in the majority of CAM lineages

The stable carbon isotope composition of plant tissues, commonly expressed as δ13C, holds a wealth information about photosynthetic pathway, water relations and stress physiology. Crassulacean acid metabolism (CAM) is derived form photosynthesis that allows plants to fix at higher water-use efficiency compared the ancestral C3 photosynthesis. While central carbon-fixing enzyme plants, Rubisco, strongly discriminates against heavy 13C isotope, CAM characterized by dual use Rubisco much less discriminating PEP carboxylase enzymes, causing δ13C values tissues be generally negative than those found in plants. Past studies variation lineages have repeatedly bimodal distribution with very few samples representative range around -20‰ intermediate between C3- CAM-like values. Although facultative long been known extend well into below -20‰, this value often tentatively used threshold for character coding distinguish from species evolution. Compiling 6623 reported literature CAM/C3 vascular presenting new data 581 accessions mainly succulent Mesembryanthemoideae (Aizoaceae) Aeonieae (Crassulaceae), we here investigate diverse patterns different families sub-familial taxa demonstrate not universally present all lineages. Moreover, show means mixture modelling full dataset well-sampled Bromeliaceae best described combination three rather two Gaussian distributions one intermediary cluster more evident clusters In view these results furthermore emerging unimodal mean close conclude continuum cautions usage macroevolutionary studies. Finally, observed diversity monophyletic urges lineage-specific reconstructions unifying model