Crassulacean acid metabolism in tropical dicotyledonous trees of the genus Clusia

The performance of crassulacean acid metabolism (CAM) by dicotyledonous trees of the genus Clusia sampled at three sites in the state of Falcon in northern Venezuela is characterized. Clusia leaves have a somewhat succulent appearance. Unlike leaves of many other CAM plants, which are uniformly built up of very large isodiametric cells, there are distinct layers of palisade and spongy mesophyll, with individual cells being smaller. There is no specialized water storage tissue. delta C-13 values indicate that C. multiflora in the elfin-cloud forest on top of Cerro Santa Ana, at similar to 800m altitude, performs C-3 photosynthesis (delta C-13 -27.1 parts per thousand). However, C. rosea in the tall cloud forest on Cerro Santa Ana (similar to 600 m altitude), and C. rosea and C. alata in the dry forest on Serrania San Luis (similar to 900 m altitude) perform CAM (delta C-13 - 14.1 to - 19.2 parts per thousand). In C. alta and C. rosea there were large day-night changes in the levels of malic and citric acids ranging from 63 to 240mmol 1(-1) for malid acid and from 35 to 112 mmol 1(-1) for citric acid. The sum of the changes in malate and citrate levels accounts for the changes of titratable protons measured. With a day-night change of titratable protons of 768 mmol 1(-1) in one of the analyses, C. rosea showed the highest value yet encountered in a CAM plant. Oscillations of free sugars (fructose, glucose, sucrose) and of starch were also analysed in the CAM performing Clusia species. Carbon skeletons of the precursors involved in nocturnal malate and citrate synthesis largely derive from free sugars and not from polyglucan. Unlike some other CAM plants, there is no clear and quantitative correlation between day-night changes of organic acid levels and cell sap osmolality.