title: Crassulacean acid metabolism: a continuous or discrete trait?
abstract: The key components of crassulacean acid metabolism (CAM) - nocturnal fixation of atmospheric CO2 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 C3 photosynthesis is highly variable. Depending upon species, ontogeny and environment, the contribution of nocturnal CO2 fixation to 24-h carbon gain can range continuously from close to 0% to 100%. Nevertheless, not all possible combinations of light and dark CO2 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 C3 -type isotope signatures. While questions remain concerning the adaptive significance of dark CO2 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 C3 to CAM.
authors.affiliation: Smithsonian Tropical Research Institute, PO Box 0843-03092, Balboa, Ancón, Republic of Panama.
authors.forename: Klaus
authors.initials: K
authors.lastname: Winter
authors.affiliation: Centre for Tropical Biodiversity and Climate Change, James Cook University, Townsville, QLD, 4811, Australia.
authors.forename: Joseph A M
authors.initials: JA
authors.lastname: Holtum
authors.affiliation: Department of Plant Sciences, University of Oxford, South Parks Road, Oxford, OX1 3RB, UK.
authors.forename: J Andrew C
authors.initials: JA
authors.lastname: Smith
mesh_terms: D000222:Adaptation, Physiological; D005075:Biological Evolution; D002244:Carbon / Q000378:metabolism*; D002245:Carbon Dioxide / Q000378:metabolism; D002247:Carbon Isotopes / Q000378:metabolism; D017753:Ecosystem; D018745:Genome, Plant; D008027:Light; D010641:Phenotype; D010788:Photosynthesis; D018526:Plant Transpiration; D010944:Plants / Q000235:genetics / Q000378:metabolism; D014867:Water / Q000378:metabolism*
chemical_list: D002247:Carbon Isotopes; D014867:Water; D002245:Carbon Dioxide; D002244:Carbon
keywords: C3 photosynthesis; CAM photosynthesis; carbon-isotope ratio δ13C; crassulacean acid metabolism (CAM); evolution; phosphoenolpyruvate carboxylase