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

Abstract

Review SummaryStudies 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. © New Phytologist (2004) 161 : 371-385

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