INDUCIBILITY OF CRASSULACEAN ACID METABOLISM (CAM) IN CLUSIA SPECIES - PHYSIOLOGICAL BIOCHEMICAL CHARACTERIZATION AND INTERCELLULAR LOCALIZATION OF CARBOXYLATION AND DECARBOXYLATION PROCESSES IN 3 SPECIES WHICH EXHIBIT DIFFERENT DEGREES OF CAM/

The biochemical basis for photosynthetic plasticity in tropical trees of the genus Clusia was investigated in three species that were from c ontrasting habitats and showed marked differences in their capacity fo r crassulacean acid metabolism (CAM). Physiological, anatomical and bi ochemical measurements were used to relate changes in the activities/a mounts of key enzymes of C-3 and C-4 carboxylation to physiological pe rformance under severe drought stress. On the basis of gas-exchange me asurements and day/night patterns of organic acid turnover, the specie s were categorised as weak CAM-inducible (C. aripoensis Britt.), C-3-C AM intermediate (C. minor L.) and constitutive CAM (C, rosea Jacq. 9.) . The categories reflect genotypic differences in physiological respon se 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 phosphoenolpyruva te carboxylase (PEPCase) and phosphoenolpyruvate carboxykinase (PEPCK) which increased in response to drought in both young and mature leave s. Whilst measured activities of PEPCase and PEPCK in well-watered pla nts of the C-3-CAM intermediate C. minor were 5-10 times in excess of that required to support the magnitude of organic acid turnover induce d by drought, close correlations were observed between malate accumula tion/PEPCase capacity and citrate decarboxylation/PEPCK capacity in al l the species. Drought stress did not affect the amount of ribulose 1, 5-bisphosphate carboxylase/oxygenase (Rubisco) protein in any of the s pecies but Rubisco activity was reduced by 35% in the weak CAM-inducib le C. aripoensis. Similar amounts of glycine decarboxylase (GDC) prote in were present in all three species regardless of the magnitude of CA M expression. Thus, the constitutive CAM species C. rosea did not appe ar to show reduced activity of this key enzyme of the photorespiratory pathway, which, in turn, may be related to the low internal conductan ce to CO2 in this succulent species. Immuno-histochemical techniques s howed that PEPCase, PEPCK and Rubisco were present in cells of the pal isade and spongy parenchyma in leaves of species performing CAM. Howev er, in leaves from well-watered plants of C. aripoensis which only per formed C-3 photosynthesis, PEPCK was localized around latex-producing ducts. Differences in leaf anatomy between the species suggest that th e association between mesophyll succulence and the capacity for CAM in these hemi-epiphytic stranglers has been selected for in arid environ ments.