Calandrinia polyandra Benth. (Montiaceae), an annual succulent herb endemic to Australia, is an exemplary facultative crassulacean acid metabolism (CAM) plant as demonstrated by continuous whole-plant lifetime CO(2) exchange measurements under controlled conditions in the laboratory. Reduced soil water availability induced a shift from solely daytime CO(2) fixation to dark CO(2) fixation. The shift from C(3) photosynthesis to CAM was reversible either upon rewatering alone, or upon a combination of rewatering and addition of nutrients. These observations highlight the role of edaphic conditions in controlling CAM expression in a plant that has the option of fixing CO(2) either during the day or during the night, providing further evidence that this extreme form of photosynthetic plasticity is primarily controlled by the environment rather than plant ontogeny. The stimulating effect of soil nutrients on CO(2) fixation in the light and its negative effect on dark CO(2) fixation have not been described previously and deserve further attention. In the most widely used CAM model system, the halophytic Mesembryanthemum crystallinum L., CAM is typically induced by high salinity, and some metabolic responses may be CAM-unrelated and related to salt stress per se. C. polyandra could be an excellent complementary system for studying the biochemical and molecular foundations of CAM because drought stress elicits a complete C(3) to CAM transition.
