Crassulacean acid metabolism (CAM) is a strategy tomaintain photosynthesis under conditions of closed sto-mata. Owing to its release from carboxylic acids formedduring the night, CO2in CAM plants can be converted intosugar by the Calvin cycle despite the gas exchange isprevented (Cushman and Bohnert1999). Photosynthesis ofthe CAM type is, however, not uniform over the day, butundergoes diurnal changes, which can be assigned to fourdistinct phases with respect to the predominant enzymeactive in carboxylation (Osmond1978; Winter and Smith1996; Borland and Taybi2004; Winter and Holtum2007).While at night fixation of CO2by PEPC leads to accu-mulation of carboxylic acids (phase I), at the onset of theday (phase II) stomata temporarily open and a transitionfrom carboxylation by PEPC to carboxylation mediated byRubisco occurs. Decarboxylation of carboxylic acids inconcert with the fixation of CO2by Rubisco during mosttime of the day (phase III) is followed by a transition to C3photosynthesis towards the end of the day when resourcesof accumulated CO2are depleted (phase IV). Mesophyllcells of CAM plants are subjected to considerable fluctu-ations of the internal CO2level. During phases I and II,they have a low level of CO2and high acidification, inphase III CO2concentration is increased and accompaniedby an increased level of oxygen, and then the level of CO2is decreased again in phase IV.