Photosynthetic Fractionation of Carbon Isotopes

During photosynthetic CO2 fixation fractionation of stable carbon isotopes occurs and, consequently, plants are generally depleted in the heavier isotope 13C. Carbon isotope discrimination (Δ) is a measure of this process and depends on fractionation during diffusion and during enzymatic carboxylation reactions. Discrimination during photosynthesis has a significant, though relatively small, effect on the isotopic composition of atmospheric CO2 both at regional and global level; hence stable isotopes find relevant applications in the study of the global carbon cycle. In addition to variation in Δ among plants with different photosynthetic pathways, large variations are found within plant groups, resulting from genetic and environmental influences on the ratio of partial pressures of CO2 at the sites of carboxylation and that in the free turbulent atmosphere. Experimental evidences confirming the theory of carbon isotope discrimination and known complications are discussed. Carbon isotope composition also varies among different metabolites, compartments and plant organs as a result of fractionation during secondary metabolism and variation in the ratio of diffusional and carboxylation limitations. Special emphases are given to measurements of Δ in different carbon pools such as bulk dry matter, cellulose, starch and sucrose, with different turnover rates and different integration of p1/p8 and to the links with water-use efficiency. The application of carbon isotope discrimination to physiological and ecophysiological studies and to selection of genotypes with improved water-use efficiency and drought tolerance and the recent progress in this field are reviewed.