IS CRASSULACEAN ACID METABOLISM ACTIVITY IN SYMPATRIC SPECIES OF HEMI-EPIPHYTIC STRANGLERS SUCH AS CLUSIA RELATED TO CARBON CYCLING AS A PHOTOPROTECTIVE PROCESS

A comparison of carbon isotope discrimination characteristics, crassul acean acid metabolism (CAM) activity and gas exchange together with co ncurrent analysis of photosystem II (PSII) chlorophyll fluorescence wa s conducted on leaves of sympatric species of Clusia from the restinga of Barra de Marica, Brazil. The carbon isotope discrimination (Delta) and leaf-sap titratable acidity for leaves collected in the field ind icated that the carbon metabolism of one species, C. lanceolata, was p redominantly C3-like, and a second, C. fluminensis, constitutive CAM. When well-watered under glasshouse conditions C. lanceolata displayed a gas exchange pattern expected of a C3 plant, where values of instant aneous discrimination (Delta) rose from 13.5 parts per thousand shortl y after dawn to 21.9 parts per thousand at midday, suggesting that all CO2 uptake was mediated solely by ribulose 1,5-bisphosphate carboxyla se (RUBISCO). C. fluminensis showed a gas exchange pattern which clear ly exhibited all four phases of CAM. Delta values during phase II rang ed from -0.4 parts per thousand at dawn to 5.9 parts per thousand some 3 h later, indicating that C4 carboxylation dominated CO2 uptake duri ng the morning with an increasing contribution by RUBISCO, suggested b y the 5 parts per thousand shift in Delta at this time. The dominance of phosphoenol-pyruvate carboxylase (PEPc) activity was also found dur ing phase IV, and extended throughout the dark period (phase I) in C. fluminensis, such that values of Delta measured were negative (-5.0 to -0.4 parts per thousand). This is the first time that negative Delta values have been reported, close to those predicted theoretically for PEPc activity. The daytime uptake of CO2, mediated by PEPc could lead to futile cycling through RUBISCO. In C. fluminensis organic acids wer e subjected to carbon turnover between PEPc and RUBISCO during phase I I of CAM, serving perhaps to dissipate ATP and reductant at a time whe n excess photons are absorbed. Under low levels of photosynthetically active radiation (PAR) the two species displayed similar chlorophyll f luorescence characteristics, although for the CAM C. fluminensis a low er rate of decarboxylation of acids in the afternoon was reflected in changed quenching capacity. Under high PAR both species responded dire ctly to changes in incident radiation, reflected by decreases in photo n use efficiency (Phi PSII) and the intrinsic photochemical efficiency (F-V/F-M), together with high and reversible quenching of excess ligh t by the means of radiationless or thermal dissipation (q(N)). Both sp ecies, with such markedly different carboxylation characteristics achi eve similar rates of electron transport and maintain photosynthetic in tegrity. Under field conditions, however the severity of a prolonged d ry season caused the CAM species to become deciduous, whereas the ''C3 -like'' species remained healthy. This suggests that the widely expect ed advantages of CAM do not extend to tolerance of extreme environment al conditions, in contrast to the more C3-like of these sympatric spec ies.