Communication Dans Un Congrès Année : 2007

Diurnal enrichment of leaf respired d 13 CO 2 -its possible implications at a larger scale

Christiane Werner
  • Fonction : Auteur

Résumé

Stable isotope signatures of CO 2 respired from ecosystems (d 13 C R) have successfully been used to analyze processes involved in ecosystem carbon fluxes and to partition the opposing respiratory and photosynthetic fluxes. Ecosystem respiration is a complex response of different respiratory sources, including autotrophic (leaves, stem, roots) and heterotrophic components (e.g. soil microorganisms). Until recently, apparent fractionation during dark respiration has been considered negligible and, thus, has been disregarded in ecosystem models. However, there is now evidence on substantial fractionation in d 13 C of leaf-respired CO 2 (δ 13 C res) in leaves of many C 3 species and two studies performed under natural conditions show pronounced diurnal variations of δ 13 C res in three tree species (up to 7 and 10‰). We obtained in our lab major differences between functional groups, confirming large diurnal enrichment in δ 13 C res of 8‰ in Quercus ilex, while no enrichment occurred in herbaceous species (1). The dynamics in δ 13 C res remain to be investigated in more species and for the different ecosystem compartments in addition with their influence in the integrated signal of ecosystem respiration (δ 13 C R). The short-term temporal variability in δ 13 C R has been disregarded in most ecosystem studies despite its potential large implications for the sampling protocols used to collect nocturnal Keeling-plots, partially because of technological difficulties since high time-resolved measurements and large samplings are required. Only recently it has been shown that d 13 C R can exhibit substantial changes even on a hourly time scale (i.e. 4‰ during one night (2)). The fast, precise and low-cost method developed in our group enables automatic assessment of δ 13 C res within 5 minutes (1) , which overcomes these difficulties. Taking advantage of this technique, δ 13 C res signatures were analysed in around twenty different species grown under natural or controlled conditions, with a large diurnal enrichment of δ 13 C res (4 to 8‰) in evergreen, slow growing or aromatic species. Herbaceous, fast growing plants and trees from temperate regions do not exhibit this enrichment. Our working hypothesis is that large diurnal enrichment occurs in species with a high investment in secondary metabolism, e.g. for synthesis of secondary compounds, such as lignins, aromatic phenols, isoprenoids or defence components, due to apparent fractionation in the different respiratory pathways. This hypothesis is based on a shift in the ratio of CO 2 produced during the initial carboxylation of pyruvate to the oxidation of acetyl-CoA in the Krebs cycle. Due to the non-statistical distribution of 13 C in this molecule, enriched d 13 CO 2 is released during glycolysis by pyruvate dehydrogenase (the hypothesis being that this pathway is favored in evergreen species during the day), while 13 C-depleted acetyl-CoA enters the Krebs cycle. If acetyl-CoA intermediates are fully decomposed during growth respiration, net isotopic effects on d 13 C res will not be observed. These functional differences in fractionation in δ 13 C res of mature leaves could be thus a strong indication that allocation (e.g. growth vs. storage or maintenance) may play a central role in the enrichment of δ 13 C res. Measurements on growing leaves show a lower enrichment compared to mature leaves (3‰ vs 8‰ in H. halimifolium), reinforcing this hypothesis. Moreover, the observed diurnal enrichment in δ 13 C res is linearly linked to cumulative light interception and cumulative assimilation rates for all species. This could indicate that with increased metabolite pool, a higher proportion would be diverted to secondary compound synthesis. Indeed, when those plants are grown under lower light intensity or when dark periods are inserted in the diurnal cycle, the enrichment is decreased almost proportionally with the light quantity received.

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Dates et versions

hal-02979136 , version 1 (27-10-2020)

Identifiants

  • HAL Id : hal-02979136 , version 1

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Pierrick Priault, Christiane Werner. Diurnal enrichment of leaf respired d 13 CO 2 -its possible implications at a larger scale. Mini symposium on the use of stable isotopes in tree physiology and forest ecology, May 2007, Nancy, France. ⟨hal-02979136⟩
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