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Gas exchange patterns and nocturnal acid accumulation were examined in four species of Clusia under simulated field conditions in the laboratory. Clusia alata and C. major had midday stomatal closure, substantial net CO exchange ([Formula: see text]) during the night, and the highest water use efficiency (WUE). C. venosa showed a pattern similar to a C plant, with nighttime stomatal closure, while C. minor maintained positive [Formula: see text] continuously throughout a 24-h period. However, large changes in titratable acidity, which closely matched changes in citrate and malate levels, indicated that Crassulacean acid metabolism (CAM) is active in all four species. C. venosa showed dawn-dusk oscillations in titratable acidity that were higher than the values reported for other C-CAM intermediates, while the nighttime acid accumulation of 998 mol m observed in C. major is unsurpassed by any other CAM plant. Moreover, the dawn-dusk changes in citrate levels of over 65 mol m in C. alata and C. minor, and over 120 mol m in C. major, are 3-6 times higher than values reported for other CAM plants. Although these oscillations in citrate levels were quite large, and the nighttime dark respiration rates were high, the O budget analysis suggestes that only part of the reducing power generated by the synthesis of citric acid enters the respiratory chain. Dawn-dusk changes in malate levels were just over 50 mol m for C. venosa but over 300 mol m for C. major. Between 28% (C. major) and 89% (C. venosa) of the malate accumulated during the night was derived from recycled respiratory CO. These daily changes in malate and citrate levels also contributed significantly to changes in leaf sap osmolality. This variability in CO uptake patterns, the recycling of nighttime respiratory CO, and the high WUE may have contributed to the successful invasion of Clusia into a wide range of habitats in the tropics.

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Night‐time citrate accumulation has been proposed as a response to stress in CAM plants. To address this hypothesis, gas exchange patterns and nocturnal acid accumulation in three species of Clusia were investigated under controlled conditions with regard to water stress and responses to low and high photosynthetic photon flux density (PPFD). Under high PPFD, leaves of Clusia nocturnally accumulated large amounts of both malic and citric acids. Under low PPFD and well‐watered conditions, substantial night‐time citrate accumulation persisted, whereas malate accumulation was close to zero. Malate accumulation and night‐time CO2 uptake from the atmosphere declined in all three species during prolonged drought periods, whereas citrate accumulation remained similar or increased. Recycling of respiratory CO2 was substantial for both well‐watered and water‐stressed plants. The suggestion that citrate accumulation is energetically more favourable than malate accumulation is not supported if the source of CO2 for the formation of malate is respiratory CO2. However, the breakdown of citric acid to pyruvate in the light period releases three molecules of CO2, while the breakdown of malic acid releases only one CO2 per pyruvate formed. Thus, citric acid should be more effective than malic acid as a mechanism to increase CO2 concentration in the mesophyll and may help to prevent photoinhibition. Organic acid accumulation also affected the vacuolar pH, which reached values of 2·6–3·0 at dawn. At these pH values, the transport of 2H+/ATP is still feasible, suggesting that it is the divalent form of citrate which is being transported in the vacuoles. Since citrate is a well‐known buffer, and Clusia spp. show the largest day‐night changes in organic acid levels measured in any CAM plant, it is possible that citrate increases the buffer capacity of the vacuoles. Indeed, malate and titratable acidity levels are positively related to citrate levels. Moreover, Clusia species that show the highest nocturnal accumulation of organic acids are also the ones that show the greatest changes in citric acid levels.

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.