Studies using carbon isotope differences between C3 and C4 photosynthesis to calculate terrestrial productivity or soil carbon turnover assume that intramolecular isotopic patterns and isotopic shifts between specific plant components are similar in C3 and C4 plants. To test these assumptions, we calculated isotopic differences in studies measuring components from C3 or C4 photosynthesis. Relative to source sugars in fermentation, C3 -derived ethanol had less 13 C and C3 -derived CO2 had more 13 C than C4 -derived ethanol and CO2 . Both results agreed with intramolecular isotopic signatures in C3 and C4 glucose. Isotopic shifts between plant compounds (e.g. lignin and cellulose) or tissues (e.g. leaves and roots) also differed in C3 and C4 plants. Woody C3 plants allocated more carbon to 13 C-depleted compounds such as lignin or lipids than herbaceous C3 or C4 plants. This allocation influenced 13 C patterns among compounds and tissues. Photorespiration and isotopic fractionation at metabolic branch points, coupled to different allocation patterns during metabolism for C3 vs C4 plants, probably influence position-specific and compound-specific isotopic differences. Differing 13 C 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 C3 and C4 plants. Understanding these mechanisms will improve our ability to link bulk and compound-specific isotopic patterns to metabolic pathways in C3 and C4 plants. Contents Summary 371 I. Introduction 372 II. Methods and terminology 373 III. Results 373 IV. Discussion 376 V. Conclusions 382 Acknowledgements 382 References 382.
Keywords: C3 plants; enzymes; isotopic discrimination; metabolic modeling.