Facultative crassulacean acid metabolism (CAM) describes the optional use of CAM photosynthesis, typically under conditions of drought stress, in plants that otherwise employ C3 or C4 photosynthesis. In its cleanest form, the upregulation of CAM is fully reversible upon removal of stress. Reversibility distinguishes facultative CAM from ontogenetically programmed unidirectional C3-to-CAM shifts inherent in constitutive CAM plants. Using mainly measurements of 24h CO2 exchange, defining features of facultative CAM are highlighted in five terrestrial species,Clusia pratensis,Calandrinia polyandra,Mesembryanthemum crystallinum,Portulaca oleraceaandTalinum triangulare.For these, we provide detailed chronologies of the shifts between photosynthetic modes and comment on their usefulness as experimental systems. Photosynthetic flexibility is also reviewed in an aquatic CAM plant,Isoetes howellii. Through comparisons of C3 and CAM states in facultative CAM species, many fundamental biochemical principles of the CAM pathway have been uncovered. Facultative CAM species will be of even greater relevance now that new sequencing technologies facilitate the mapping of genomes and tracking of the expression patterns of multiple genes. These technologies and facultative CAM systems, when joined, are expected to contribute in a major way towards our goal of understanding the essence of CAM.
Chronologies of the shifts between photosynthetic modes are provided for a range of facultative CAM species. Combined with new sequencing technologies, facultative systems will contribute towards understanding the principles of CAM.