Clusia: Holy Grail and enigma

Abstract

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 photosynthesis 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 photosystem II (ΦPSII) indicating photosynthetic energy use, is larger under photorespiratory than under non-photorespiratory conditions due to the particular energy demand of photorespiration. The heterogeneity of ΦPSII over the leaves as visualized by chlorophyll fluorescence imaging in the C3 mode is larger under non-photorespiratory conditions than under photorespiratory 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 ΦPSII is more dependent on the transitions between CAM phases. Free-running circadian oscillations of photosynthesis 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. ΦPSII and the heterogeneity of ΦPSII oscillate in phase with CO2 uptake and photorespiration only under non-photorespiratory conditions. Under photorespiratory conditions, ΦPSII 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. © The Author [2008]. Published by Oxford University Press [on behalf of the Society for Experimental Biology]. All rights reserved.