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Articles

Diffusional Contribution to Carbon Isotope Fractionation during Dark CO₂ Fixation in CAM Plants ¹

Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706, Department of Biochemistry, University of Wisconsin, Madison, Wisconsin 53706, Department of Environmental Biology, Research School of Biological Sciences, Australian National University, Canberra 2601, Australia

A mathematical model is developed which can be used to predict in vivo carbon isotope fractionations associated with carbon fixation in plants in terms of diffusion, CO₂ hydration, and carboxylation components. This model also permits calculation of internal CO₂ concentration for comparison with results of gas-exchange experiments. The isotope fractionations associated with carbon fixation in Kalanchoë daigremontiana and Bryophyllum tubiflorum have been measured by isolation of malic acid following dark fixation and enzymic determination of the isotopic composition of carbon-4 of this material. Corrections are made for residual malic acid, fumarase activity, and respiration. Comparison of these data with calculations from the model indicates that the rate of carbon fixation is limited principally by diffusion, rather than by carboxylation. Processes subsequent to the initial carboxylation also contribute to the over-all isotopic composition of the plant.

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