Comparative study of the C-3/CAM intermediate species Clusia parviflora Saldanha et Engl. and the obligate CAM species Clusia hilariana Schlecht. growing sympatrically exposed and shaded in the coastal Restinga of Brazil

The C-3/CAM intermediate species, Clusia parviflora Saldanha et Engl., and the obligate CAM species Clusia hilariana Schlecht., occur sympatrically in the coastal sand dune vegetation of the Restinga of Brazil. Their photosyn thetic activity at an exposed and at a shaded site was compared by measurin g gas exchange (porometry), chlorophyll a fluorescence parameters, organic acid levels (malic and citric) and carbon isotope ratios. At the shaded sit e, low photosynthetic photon flux densities (PPFD) strongly restricted phot osynthetic activity. However, C. parviflora could readily make use of light flecks. At the exposed site, C. parviflora was much less affected by photo inhibition than C. hilariana. The CAM species showed higher apparent rates of linear photosynthetic electron transport (ETR) and higher effective quan tum yield of PSII (Delta F/F-m') than did C. parviflora during high insolat ion in the middle of the day, i.e., the time of Phase III of CAM. Neverthel ess, it suffered much more severe acute photoinhibition that was not revers ible after 20 min of darkening during this time, and even some chronic phot oinhibition not reversible overnight. Comparative studies of sympatric phys iotypes with different modes of photosynthesis of a given leaf morphotype, as available in the genus Clusia, challenge some CAM dogmas, e.g., CAM may not always be superior at exposed sites and may not always provide better p hotoprotection at high PPFD. However, the idea that C-3/CAM plasticity allo ws occupation of a wider range of habitats is supported.