Carbon isotope fractionation in plants

Plants with the C₃, C₄, and crassulacean acid metabolism (CAM) photosynthetic pathways show characteristically different discriminations against ¹³C during photosynthesis. For each photosynthetic type, no more than slight variations are observed within or among species. CAM plants show large variations in isotope fractionation with temperature, but other plants do not. Different plant organs, subcellular fractions and metabolises can show widely varying isotopic compositions. The isotopic composition of respired carbon is often different from that of plant carbon, but it is not currently possible to describe this effect in detail. The principal components which will affect the overall isotope discrimination during photosynthesis are diffusion of CO₂, interconversion of CO₂ and HCO^?₃, incorporation of CO₂ by phosphoenolpyruvate carboxylase or ribulose bisphosphate carboxylase, and respiration. Theisotope fractionations associated with these processes are summarized. Mathematical models are presented which permit prediction of the overall isotope discrimination in terms of these components. These models also permit a correlation of isotope fractionations with internal CO₂ concentrations. Analysis of existing data in terms of these models reveals that CO₂ incorporation in C₃ plants is limited principally by ribulose bisphosphate carboxylase, but CO₂ diffusion also contributes. In C₄ plants, carbon fixation is principally limited by the rate of CO₂ diffusion into the leaf. There is probably a small fractionation in C₄ plants due to ribulose bisphosphate carboxylase.