Crassulacean acid metabolism: A continuous or discrete trait?
Winter K., Holtum JAM., Smith JAC.
© 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.