The biochemical basis for photosynthetic plasticity in tropical trees of the genus Clusia was investigated in three species that were from contrasting habitats and showed marked differences in their capacity for crassulacean acid metabolism (CAM). Physiological, anatomical and biochemical measurements were used to relate changes in the activities/amounts of key enzymes of C₃ and C₄ carboxylation to physiological performance under severe drought stress. On the basis of gas-exchange measurements and day/night patterns of organic acid turnover, the species were categorised as weak CAM-inducible (C.aripoensis Britt.), C₃-CAM intermediate (C. minor L.) and constitutive CAM (C.␣rosea Jacq. 9.). The categories reflect genotypic differences in physiological response to drought stress in terms of net carbon gain; in C. aripoensis net carbon gain was reduced by over 80% in drought-stressed plants whilst carbon gain was relatively unaffected after 10 d without water in C. rosea. In turn, genotypic differences in the capacity for CAM appeared to be directly related to the capacities/amounts of phosphoenolpyruvate carboxylase (PEPCase) and phosphoenolpyruvate carboxykinase (PEPCK) which increased in response to drought in both young and mature leaves. Whilst measured activities of PEPCase and PEPCK in well-watered plants of the C₃-CAM intermediate C. minor were 5–10 times in excess of that required to support the magnitude of organic acid turnover induced by drought, close correlations were observed between malate accumulation/PEPCase capacity and citrate decarboxylation/PEPCK capacity in all the species. Drought stress did not affect the amount of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) protein in any of the species but Rubisco activity was reduced by 35% in the weak CAM-inducible C. aripoensis. Similar amounts of glycine decarboxylase (GDC) protein were present in all three species regardless of the magnitude of CAM expression. Thus, the constitutive CAM species C. rosea did not appear to show reduced activity of this key enzyme of the photorespiratory pathway, which, in turn, may be related to the low internal conductance to CO₂ in this succulent species. Immuno-histochemical techniques showed that PEPCase, PEPCK and Rubisco were present in cells of the palisade and spongy parenchyma in leaves of species performing CAM. However, in leaves from well-watered plants of C. aripoensis which only performed C₃ photosynthesis, PEPCK was localized around latex-producing ducts. Differences in leaf anatomy between the species suggest that the association between mesophyll succulence and the capacity for CAM in these hemi-epiphytic stranglers has been selected for in arid environments.