Patterns of gas exchange and organic acid oscillations in tropical trees of the genus Clusia

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 CO₂ exchange (\(J_{CO_2 }\)) during the night, and the highest water use efficiency (WUE). C. venosa showed a pattern similar to a C₃ plant, with nighttime stomatal closure, while C. minor maintained positive \(J_{CO_2 }\) 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 C₃-CAM intermediates, while the nighttime acid accumulation of 998 mol m⁻³ observed in C. major is unsurpassed by any other CAM plant. Moreover, the dawn-dusk changes in citrate levels of over 65 mol m⁻³ in C. alata and C. minor, and over 120 mol m⁻³ 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 O₂ 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⁻³ for C. venosa but over 300 mol m⁻³ for C. major. Between 28% (C. major) and 89% (C. venosa) of the malate accumulated during the night was derived from recycled respiratory CO₂. These daily changes in malate and citrate levels also contributed significantly to changes in leaf sap osmolality. This variability in CO₂ uptake patterns, the recycling of nighttime respiratory CO₂, and the high WUE may have contributed to the successful invasion of Clusia into a wide range of habitats in the tropics.