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

Contents

• Summary  371
• I.  

  Introduction  372
• II.  

  Methods and terminology  373
• III.  

  Results  373
• IV.  

  Discussion  376
• V.  

  Conclusions  382

• Acknowledgements  382

• References  382

Summary

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 ¹³C and C₃-derived CO₂ had more ¹³C 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 ¹³C-depleted compounds such as lignin or lipids than herbaceous C₃ or C₄ plants. This allocation influenced ¹³C 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 ¹³C 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.