The Occurrence and Phylogenetics of Crassulacean Acid Metabolism in the Portulacaceae

Abstract

The plant family Portulacaceae is a relatively small family (∼19 genera and ∼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 that range from fleshy to fully succulent and live in very diverse habitats ranging from shady understory plants to plants exposed to full sunlight. The Portulacaceae have members that utilize different photosynthetic pathways ranging from C₃ to C₄ to Crassulacean acid metabolism (CAM) to CAM‐cycling plants. We are interested in the prevalence of the CAM photosynthetic pathway in this family. We studied the nocturnal CO₂ uptake, diurnal acid fluctuation, and phosphoenolpyruvate (PEP) carboxylase activity of different members of the Portulacaceae. Our results indicate the highest levels of CAM activity in the genera of Anacampseros, Portulacaria, Ceraria, and Talinopsis. Intermediate in their CAM activity were the genera Talinum, Portulaca, and Calandrinia. Low levels of CAM activity were found in the genera Lewisia, Calyptridium, and Montia. This is the initial report of Talinopsis being a facultative CAM species and the first report of the occurrence of CAM cycling for the genera of Calyptridium, Lewisia, and Montia. These results indicate that physiological and anatomical attributes of the CAM pathway are more widespread in the Portulacaceae than was previously thought. The phylogenetic and physiological data in the Portulacaceae indicate a scheme for the evolution of CAM, possibly starting with changes in leaf anatomy, then progressing to increased enzyme activity with increased acid fluctuations, and finally ending with the development of the full CAM pathway.