Carbon isotope discrimination: leaves vs. roots

What is it about?

Carbon-12 (12C) and Carbon-13 (13C) are the two stable isotopes of carbon, with 12C amounting to approximately 98.9% of all natural, non-radioactive, carbon on Earth and 13C making up the rest. Until recently, it was thought that differences in the amount of 13C present in respired CO2 were caused by plants discriminating between 12C and 13C during photosynthesis. DISCRIMINATION AFTER PHOTOSYNTHESIS In general, the leaves of plants contain less of the 13C isotope compared with all other plant organs. Because of this, it is now thought that discrimination between 12C and 13C also takes place after photosynthesis fixes CO2 in the leaves. This so-called post-photosynthetic discrimination is contrary to previous theories. OPPOSITE CARBON ISOTOPE DISCRIMINATION This review collected evidence of ‘opposite carbon isotope discrimination’ during respiration in both the roots and leaves of herbaceous species. In general, the CO2 respired by leaves contains a higher proportion of 13C than the organic matter in the leaf – and the CO2 respired by roots contains a lower proportion of 13C than the root material. This could help explain the different levels of 12C and 13C in carbon isotopic type in roots and leaves. IMPLICATIONS This research will help to determine the different carbon isotopes present in ecosystems that have multiple sources of respiratory CO2 and enable respiratory CO2 to be attributed to its carbon sources. This will lead to a better understanding of the origin of different carbon isotopes present in the soil.

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