In situ studies of Crassulacean acid metabolism in several sympatric species of tropical trees of the genus Clusia


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The genus Clusia is characterized by the presence of C3, C3,-CAM and CAM species growing sympatrically in many areas of the neotropics, The aim of this study was to investigate the photosynthetic behaviour of such sympatric species, and the effect that differences in photosynthetic metabolism and light levels have on their leaf characteristics, nutritional status, carbon and water economy. In the two field sites in Northern Venezuela, the apparent C3C. multiflora had the lowest leaf water content, leaf dry weight, N and P levels, when compared with the four other Clusia species that showed CAM activity, as measured by dawn/dusk changes in malate levels. Respiratory CO2., recycling probably accounted for 19 to 76% of nocturnal malate accumulation in the species with CAM activity. The measured δ13C values of C. minor were similar to values of a C3 plant, although about half of the CO2 uptake from the atmosphere occurred during night-time. Under these conditions, C. multiflora had larger carbon gain and lower night-time respiration rates than C, minor in spite of a lower water use efficiency. Light levels affected leaf concentrations of nitrogen, phosphorus and chlorophyll, as well as leaf water content and leaf dry weight of both species. Sun-exposed leaves of both species had higher night-time respiration rates than shade leaves. Leaves of C. minor under full sun exposure had a higher proportion of their total net CO2 uptake during daytime compared with shaded or partly shaded leaves, although night-time citrate and malate accumulation were depressed. However, even under these conditions, the daylight decarboxylation of both acids could potentially maintain substantial photosynthetic rates during the midday stomatal closure period that characterizes CAM plants. Soluble sugars and starch are apparently the precursors of night-time citrate and malate synthesis. The predominant utilization of soluble sugars for the formation of both organic acids moderated large dawn/dusk changes in vacuolar osmotic pressure.