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Combustion-based studies of the carbon-13 content of plants give only an integrated, long-term value for the isotope fractionation associated with photosynthesis. A method is described here which permits determination of this isotope fractionation in 2 to 3 hours. To accomplish tlhis, the plant is enclosed in a glass chamber, and the quantity and isotopic content of the CO2 remaining in the atmosphere are monitored during photosynthesis. Isotope fractionation studies by this method give results consistent with what is expected from combustion studies of C3, C., and Crassulacean acid metabolism plants. This method will make possible a variety of new studies of environmental and species effects in carbon isotope fractionation.Studies begun in the 1950s demonstrated that 6b3C values2 for plants are more negative than that of atmospheric C02; that is, plants contain less '3C than does atmospheric CO2 (1,5,6,16,21,22 (14,19,20 been carried out by combustion of dried leaves or other plant parts. The isotopic composition so obtained is a long-term integration of environmental and developmental effects. Early in leaf development, carbon is imported from elsewhere in the plant, and this carbon contributes to the isotopic signal. In a mature leaf, carbon is incorporated as a result of local photosynthesis, but some of this carbon is exported to elsewhere in the plant. As senescence approaches, the export rate increases and the photosynthetic rate decreases. All these phenomena contribute to the isotopic composition obtained in combustion analysis. This long-term integration undoubtedly masks a number of effects and is probably responsible for the lack of environmental and species variations cited above.A more detailed isotopic signal can probably only be obtained by short-term studies in which the isotopic composition (or fractionation) is measured over a period of a few hours. In the case of CAM plants, we have established a method for studying the isotope fractionation associated with nocturnal CO2 fixation that makes use of the isotopic composition of carbon-4 of newly formed malate (17). This isotopic information reflects the history of the plant during a short term and is more nearly independent of long-term influences. Studies have been made of the effect of temperature (8), CO2 concentration (12), and species (17)

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