Intramolecular, compound-specific, and bulk carbon isotope patterns in C3 and C4 plants:: a review and synthesis

Studies using carbon isotope differences between C-3 and C-4 photosynthesis to calculate terrestrial productivity or soil carbon turnover assume that intramolecular isotopic patterns and isotopic shifts between specific plant components are similar in C-3 and C-4 plants. To test these assumptions, we calculated isotopic differences in studies measuring components from C-3 or C-4 photosynthesis. Relative to source sugars in fermentation, C-3-derived ethanol had less C-13 and C-3-derived CO2 had more C-13 than C-4-derived ethanol and CO2. Both results agreed with intramolecular isotopic signatures in C-3 and C-4 glucose. Isotopic shifts between plant compounds (e.g. lignin and cellulose) or tissues (e.g. leaves and roots) also differed in C-3 and C-4 plants. Woody C-3 plants allocated more carbon to C-13-depleted compounds such as lignin or lipids than herbaceous C-3 or C-4 plants. This allocation influenced C-13 patterns among compounds and tissues. Photorespiration and isotopic fractionation at metabolic branch points, coupled to different allocation patterns during metabolism for C-3 vs C-4 plants, probably influence position-specific and compound-specific isotopic differences. Differing C-13 content of mobile and immobile compounds (e.g. sugars vs lignin) may then create isotopic differences among plant pools and along transport pathways. We conclude that a few basic mechanisms can explain intramolecular, compound-specific and bulk isotopic differences between C-3 and C-4 plants. Understanding these mechanisms will improve our ability to link bulk and compound-specific isotopic patterns to metabolic pathways in C-3 and C-4 plants.