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Crassulacean acid metabolism (CAM) plants are dependent on the organic acids that accumulate overnight in the vacuoles as a source of CO(2) during the daylight deacidification period, when stomata are closed and high irradiances generally prevail. We performed an integrative analysis of diurnal changes in gas exchange, chlorophyll fluorescence parameters and organic acid decarboxylation to understand the adjustments in photochemical and non-photochemical processes during the different CAM phases in Clusia hilariana Schlecht., a dominant tree species in the sandy coastal plains of southeastern Brazil. A linear relationship was obtained between the quantum yields of photochemical and non-photochemical quenching, irrespective of the CAM phase and prevailing irradiance. Degradation of malic and citric acids during the midday stomatal closure period could lead to potential CO(2) fixation rates of 23 &mgr;mol m(-2) s(-1), whereas CO(2) losses, measured as CO(2) evolution, corresponded to about 3% of this value. Thus, decarboxylation of malate and citrate provided high internal CO(2) concentrations during phase III of CAM, even though the stomata were closed, allowing optimal utilization of light energy, as indicated by the non-saturating electron transport rates (ETR) in the light response curves, with highest rates of ETR occurring at midday in the diurnal curves. At the transition from phase III to IV of CAM, depletion of internal CO(2) sources and low stomatal conductances, which restricted the supply of exogenous CO(2), reduced the demand for photochemical energy to drive carbon assimilation. This was compensated by increases in thermal energy dissipation as indicated by higher rates of non-photochemical quenching, while high irradiances still prevailed. Shifts in the CAM phases and changes in protective thermal dissipation potential allowed C. hilariana to match changes in PPFD patterns for leaves of different orientations. Evidence that most of the decline in photochemical efficiency was probably related to the fast-relaxing component of non-photochemical quenching is provided by the high values of the quantum yield of photosystem II after 20 min of relaxation in darkness, and an almost complete recovery after sunset. These adjustments in photosynthetic machinery minimized the danger of photo-inhibition in C. hilariana, which is commonly found in fully exposed habitats.

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: Clusia multiflora H. B. K., an obligate C3 species and Clusia minor L. a C3/CAM intermediate species, are two physio‐types of a similar morphotype. They can sympatrically occupy secondary savanna sites exposed to high insolation in the tropics. In C. multiflora severe stress, i.e., switching shade‐grown plants to high light plus drought, resulted in leaves browning or yellowing and becoming necrotic. However, in long‐term light stress C. multiflora was able to grow new leaves with their photosynthetic apparatus fit for high light conditions. Shade‐grown C. minor readily overcame switching to high light conditions and drought, responding by a rapid change from C3 photosynthesis to CAM. Decreasing soil led to increased abscisic acid levels in the leaves of C. minor, however CAM induction was not directly related to this and was mainly determined by increased PPFD. Both species were capable of rapid accumulation of zea‐xanthin for acute photoprotection following high PPFD exposure. The maximum capacity for zeaxanthin accumulation was larger in C. minor, but under steady high PPFD it only partially made use of this capacity, relying on high internal CO2 concentrations of Phase Ill of CAM, in addition to zeaxanthin, for acute photo‐protection. Thus, by different means the two species perform well under high light conditions. However, C. multiflora needs time for development of adapted leaves under such stress conditions while the more flexible C. minor can readily switch from low light to high light conditions.