The present thesis studied the endogenous rhythm of photosynthesis in Clusia minor L. (Clusiaceae). This species is one of the most flexible plants regarding a reversible expression of crassulacean acid metabolism (CAM) in response to a large variety of combinations of environmental parameters. Therefore, it constitutes an interesting model to study the circadian rhythms in C3 and CAM photosynthesis. Well watering or droughting the plants for 3 to 4 days elicited C3 or CAM photosynthesis, respectively. This treatment was used to adapt the plants for dark/light (DL, 12h/12h) and continuous light (LL) experiments, respectively. During LL the plants were well watered. An on-line combination of instruments was used to simultaneously determine gas exchange (net CO2- uptake, JCO2, and stomatal conductance, gH2O) and to calculate internal partial pressure of CO2 (piCO2), photorespiration (JO2), relative quantum use efficiency of photosynthesis (rel. ΦPSII) and its spatial-temporal dynamics over the leaf. The effect of the temperature was studied under DL and LL regimes applying 21°C, 25°C and 30°C in the mini-cuvette system. Photorespiration was rather constant during the light period in the C3-mode leaves, JO2, ranging between 32.1 and 35.7 % of total activity. In the CAM-mode leaves photorespiration depended on the CAM phases. Relative JO2 was still 15.6 % in phase II in the morning and declined from 37.9 % to 17.6 % during phase IV in the afternoon. Anatomically leaves of C. minor are differentiated in palisade and spongy parenchyma with an internal air space of 9.3 % of the total volume and therefore could be structurally homobaric. However, the calculation of heterogeneity of rel. ΦPSII from chlorophyll fluorescence images of leaves under non-photorespiratory and photorespiratory conditions in the C3- and CAM-mode indicates that lateral diffusion of CO2 and O2 is subject to limitations showing that leaves are functionally heterobaric. In LL experiments, the oscillation of photorespiration, demonstrated for the first time for CAM, was coupled to CAM processes that can affect the oxygenase and carboxylase activity of RubisCO. High energy use in light reactions occurred when PEPC activity overlapped RubisCO activitiy in LL for CAM adapted plants. Heterogeneity of quantum use of photosystem II oscillated only under 1 % O2 suggesting that photorespiration compensates variations of energy demand in different parts of the leaf. Heterogeneity is a property of CAM, especially due to desynchronization during transitions between the diurnal CAM-phases. The C3-adapted plants showed C3-photosynthesis in external dark/light rhythms before and after the time in LL. The CAM adapted plants only showed CAM before the time in LL, but had switched to C3-photosynthesis at the end of LL. It happened under temperatures of 21, 25 and 30°C. In the C3-mode leaves still showed a residual CAM activity where the interveinal lamina tissue performed C3-photosynthesis but the major vein chlorenchyma showed features of CAM.