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.