The occurrence and phylogenetics of Crassulacean acid metabolism in the Portulacaceae

The plant family Portulacaceae is a relatively small family ( similar to 19 genera and similar to 425 species) with a widespread distribution. Species of the Portulacaceae range in size from small herbaceous plants to shrubs that measure up to 4 m in height. Most members of the family have leaves th at range from fleshy to fully succulent and live in very diverse habitats r anging from shady understory plants to plants exposed to full sunlight. The Portulacaceae have members that utilize different photosynthetic pathways ranging from C-3 to C-4 to Crassulacean acid metabolism (CAM) to CAM-cyclin g plants. We are interested in the prevalence of the CAM photosynthetic pat hway in this family. We studied the nocturnal CO2 uptake, diurnal acid fluc tuation, and phosphoenolpyruvate (PEP) carboxylase activity of different me mbers of the Portulacaceae. Our results indicate the highest levels of CAM activity in the genera of Anacampseros, Portulacaria, Ceraria, and Talinops is. Intermediate in their CAM activity were the genera Talinum, Portulaca, and Calandrinia. Low levels of CAM activity were found in the genera Lewisi a, Calyptridium, and Montia. This is the initial report of Talinopsis being a facultative CAM species and the first report of the occurrence of CAM cy cling for the genera of Calyptridium, Lewisia, and Montia. These results in dicate that physiological and anatomical attributes of the CAM pathway are more widespread in the Portulacaceae than was previously thought. The phylo genetic and physiological data in the Portulacaceae indicate a scheme for t he evolution of CAM, possibly starting with changes in leaf anatomy, then p rogressing to increased enzyme activity with increased acid fluctuations, a nd finally ending with the development of the full CAM pathway.