SPECIAL ISSUE REVIEW PAPER Clusia: Holy Grail and enigma

Clusia is the only genus with bona fide dicotyledonous trees performing Crassulacean acid metabolism (CAM). Clusia minor L. is extraordinarily flexible, being C3/CAM intermediate and expressing the photosynthetic modes C3, CAM, CAM-cycling, and CAM-idling. C3 photosyn-thesis and CAM can be observed simultaneously in two opposite leaves on a node and possibly even within the same leaf in the interveinal lamina and the major vein tissue, respectively. The relative activity of photosys-tem II (FPSII) indicating photosynthetic energy use, is larger under photorespiratory than under non-photorespiratory conditions due to the particular en-ergy demand of photorespiration. The heterogeneity of FPSII over the leaves as visualized by chlorophyll fluorescence imaging in the C3 mode is larger under non-photorespiratory conditions than under photores-piratory conditions. These observations indicate that photorespiration, presumably by its particular energy demand, synchronizes photosynthetic activity over the leaves. In the CAM mode, the heterogeneity of FPSII is more dependent on the transitions between CAM phases. Free-running circadian oscillations of photo-synthesis are strongly dampened in both the C3 and the CAM mode. Photorespiration is under circadian clock control in both the C3 and the CAM mode. FPSII and the heterogeneity of FPSII oscillate in phase with CO2 uptake and photorespiration only under non-photorespiratory conditions. Under photorespiratory conditions, FPSII does not oscillate and there is no heterogeneity, again indicating the stabilizing function of photorespiration. Plants acclimatized to perform CAM switch to C3 photosynthesis during free-running oscillations while subjected to constant illumination