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

Abstract

A mathematical model was developed which can be used to predict in vivo C isotope fractions associated with C 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 fractions associated with carbon fixation in Kalanchoe daigremontiana and Bryophyllum tubiflorum were measured by isolation of malic acid following dark fixation and enzymic determination of the isotopic composition of carbon-4 of this material. Corrections were made for residual malic acid, fumarase activity and respiration. Comparison of these data with calculation from the model indicates that the rate of C fixation is limited principally by diffusion, rather than by carboxylation. Processes subsequent to the initial carboxylation also contribute to the overall isotopic composition of the plant.