Crassulacean acid metabolism: A continuous or discrete trait?

© 2015 New Phytologist Trust. The key components of crassulacean acid metabolism (CAM) - nocturnal fixation of atmospheric CO<inf>2</inf> and its processing via Rubisco in the subsequent light period - are now reasonably well understood in terms of the biochemical reactions defining this water-saving mode of carbon assimilation. Phenotypically, however, the degree to which plants engage in the CAM cycle relative to regular C<inf>3</inf> photosynthesis is highly variable. Depending upon species, ontogeny and environment, the contribution of nocturnal CO<inf>2</inf> fixation to 24-h carbon gain can range continuously from close to 0% to 100%. Nevertheless, not all possible combinations of light and dark CO<inf>2</inf> fixation appear equally common. Large-scale surveys of carbon-isotope ratios typically show a strongly bimodal frequency distribution, with relatively few intermediate values. Recent research has revealed that many species capable of low-level CAM activity are nested within the peak of C<inf>3</inf>-type isotope signatures. While questions remain concerning the adaptive significance of dark CO<inf>2</inf> fixation in such species, plants with low-level CAM should prove valuable models for investigating the discrete changes in genetic architecture and gene expression that have enabled the evolutionary transition from C<inf>3</inf> to CAM.