Crassulacean-Acid Metabolism is a Specific Mode of Photosynthesis

Essentially a loop of CO2 flow via organic acids, mainly malate, is switched before assimilation of CO2 in the Calvin cycle of C3 photosynthesis (see Chapter 2). This allows fixation of CO2 during darkness in the night. The nocturnally fixed CO2 is transiently stored in the form of organic acids in the cell-sap vacuole. It is remobilised again during the day and assimilated in the light (Fig. 6.1). In the evolution of vascular plants CAM has arisen polyphyletically, i.e., independently many times. In all the branches of the evolutionary tree of vascular plants, beginning with the pteridophyta, we find taxa performing CAM (Fig. 6.2). Polyphyletic evolution of CAM not only occurred between higher taxa such as subclasses and families (Fig. 6.2), but also within families and subfamilies and even within genera, e.g., in the bromeliads (Smith, 1989; Crayn et al., 2000, 2004), the Clusiaceae (Holtum et al., 2004; Gustafsson et al., 2007) and the orchids (Silvera et al., 2009), and there were also evolutionary reversions from CAM back to C3 photosynthesis. CAM emerges as a good example of Darwinian evolutionary adaptive radiation. It must have been a rather simple step to evolve performance of CAM. In fact, there are basically no new metabolic requirements for CAM as compared with the phylogenetically older general metabolism and C3 photosynthesis.