Discrimination against 13C during photosynthesis is a well-characterised phenomenon. It leaves behind distinct signatures in organic matter of plants and in the atmosphere. The former is depleted in 13C, the latter is enriched during periods of preponderant photosynthetic activity of terrestrial ecosystems. The intra-annual cycle and latitudinal gradient in atmospheric 13C resulting from photosynthetic and respiratory activities of terrestrial plants have been exploited for the reconstruction of sources and sinks through deconvolution by inverse modelling. Here, we compile evidence for widespread post-photosynthetic fractionation that further modifies the isotopic signatures of individual plant organs and consequently leads to consistent differences in delta13C between plant organs. Leaves were on average 0.96 per thousand and 1.91 per thousand more depleted than roots and woody stems, respectively. This phenomenon is relevant if the isotopic signature of CO2-exchange fluxes at the ecosystem level is used for the reconstruction of individual sources and sinks. It may also modify the parameterization of inverse modelling approaches if it leads to different isotopic signatures of organic matter with different residence times within the ecosystems and to a respiratory contribution to the average difference between the isotopic composition of plant organic matter and the atmosphere. We discuss the main hypotheses that can explain the observed inter-organ differences in delta13C.

The carbon isotope composition (␦ 13 C) of CO 2 produced in darkness by intact French bean (Phaseolus vulgaris) leaves was investigated for different leaf temperatures and during dark periods of increasing length. The ␦ 13 C of CO 2 linearly decreased when temperature increased, from Ϫ19‰ at 10°C to Ϫ24‰ at 35°C. It also progressively decreased from Ϫ21‰ to Ϫ30‰ when leaves were maintained in continuous darkness for several days. Under normal conditions (temperature not exceeding 30°C and normal dark period), the evolved CO 2 was enriched in 13 C compared with carbohydrates, the most 13 C-enriched metabolites. However, at the end of a long dark period (carbohydrate starvation), CO 2 was depleted in 13 C even when compared with the composition of total organic matter. In the two types of experiment, the variations of ␦ 13 C were linearly related to those of the respiratory quotient. This strongly suggests that the variation of ␦ 13 C is the direct consequence of a substrate switch that may occur to feed respiration; carbohydrate oxidation producing 13 C-enriched CO 2 and ␤-oxidation of fatty acids producing 13 C-depleted CO 2 when compared with total organic matter (Ϫ27.5‰). These results are consistent with the assumption that the ␦ 13 C of dark respired CO 2 is determined by the relative contributions of the two major decarboxylation processes that occur in darkness: pyruvate dehydrogenase activity and the Krebs cycle.Photosynthetic CO 2 assimilation of C 3 plants discriminates against 13 CO 2 so that organic matter is, on average, 20‰ depleted in 13 C compared with atmospheric carbon dioxide (for recent review, see Brugnoli and Farquhar, 2000). Respiratory carbon fluxes in light (i.e. photorespiration and "day" respiration) are often assumed to be negligible or weakly fractionating processes. However, the carbon isotope signature of organic matter may be modified by nighttime respiration depending on the ␦ 13 C of the evolved CO 2 because respiratory carbon lost by many plants has been shown to be within 30% to 60% of the carbon fixed through photosynthesis (Evans, 1993; Amthor, 2000).In vitro studies using protoplasts have shown that respired CO 2 isotope composition is identical to that of the Suc supplied to the culture medium, indicating that no fractionation occurs during respiration in the dark (Lin and Ehleringer, 1997). A similar result was also obtained in long-term experiments with animals, where the isotope composition of CO 2 expired by mice (Mus musculus) reflected that of the diet (Perkins and Speakman, 2001). In contrast, it has been shown previously that CO 2 produced by respiration in the dark is 6‰ 13 C enriched when compared with Suc in intact French bean (Phaseolus vulgaris) leaves (Duranceau et al., 1999). Similar results were also obtained in Nicotiana sylvestris and sunflower (Helianthus annuus), although CO 2 was less 13 C enriched with ␦ 13 C values of 4‰ and 3‰, respectively (Ghashghaie et al., 2001). Moreover, it has been demonstrated that the ␦ 13 C value of CO 2 evolved in the dar...

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