Crassulacean acid metabolism in tropical dicotyledonous trees of the genusClusia

Summary The performance of crassulacean acid metabolism (CAM) by dicotyledonous trees of the genusClusia 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. ¹³C values indicate thatC. multiflora in the elfin-cloud forest on top of Cerro Santa Ana, at 800 m altitude, performs C₃ photosynthesis (¹³ –27.1). However,C. rosea in the tall cloud forest on Cerro Santa Ana (600m altitude), andC. rosea andC. alata in the dry forest on Serrania San Luis (900 m altitude) perform CAM (¹³C –14.1 to –19.2). InC. alta andC. rosea there were large day-night changes in the levels of malic and citric acids ranging from 63 to 240 mmol 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 performingClusia 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.Dedicated to Professor Dr. Otto L. Lange on the occasion of his 60th birthday.