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

Abstract

Summary: 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 13 C and C 3‐derived CO 2 had more 13 C than C 4‐derived ethanol and CO 2. 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 13 C‐depleted compounds such as lignin or lipids than herbaceous C 3 or C 4 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 C 3 vs C 4 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 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. Contents Summary 371 I. Introduction 372 II. Methods and terminology 373 III. Results 373 IV. Discussion 376 V. Conclusions 382 Acknowledgements 382 References 382