The Evolution of CAM Photosynthesis in Australian Calandrinia Reveals Lability in C3+CAM Phenotypes and a Possible Constraint to the Evolution of Strong CAM

Synopsis Australian Calandrinia has radiated across the Australian continent during the last 30 Ma, and today inhabits most Australian ecosystems. Given its biogeographic range and reports of facultative Crassulacean acid metabolism (CAM) photosynthesis in multiple species, we hypothesized (1) that CAM would be widespread across Australian Calandrinia and that species, especially those that live in arid regions, would engage in strong CAM, and (2) that Australian Calandrinia would be an important lineage for informing on the CAM evolutionary trajectory. We cultivated 22 Australian Calandrinia species for a drought experiment. Using physiological measurements and delta C-13 values we characterized photosynthetic mode across these species, mapped the resulting character states onto a phylogeny, and characterized the climatic envelopes of species in their native ranges. Most species primarily utilize C-3 photosynthesis, with CAM operating secondarily, often upregulated following drought. Several phylogenetically nested species are C-3, indicating evolutionary losses of CAM. No strong CAM was detected in any of the species. Results highlight the limitations of delta C-13 surveys in detecting C-3+CAM phenotypes, and the evolutionary lability of C-3+CAM phenotypes. We propose a model of CAM evolution that allows for lability and reversibility among C-3+CAM phenotypes and C-3 and suggest that an annual life-cycle may preclude the evolution of strong CAM.