Respiratory Carbon Metabolism following Illumination in Intact French Bean Leaves Using 13C/12C Isotope Labeling

Abstract

The origin of the carbon atoms in the CO 2 respired by French bean (Phaseolus vulgaris) leaves in the dark has been studied using 13 C/ 12 C isotopes as tracers. The stable isotope labeling was achieved through a technical device that uses an open gas-exchange system coupled online to an elemental analyzer and linked to an isotope ratio mass spectrometer. The isotopic analysis of the CO 2 respired in the dark after a light period revealed that the CO 2 was labeled, but the labeling level decreased progressively as the dark period increased. The pattern of disappearance depended on the amount of carbon fixed during the labeling and indicated that there were several pools of respiratory metabolites with distinct turnover rates. We demonstrate that the carbon recently assimilated during photosynthesis accounts for less than 50% of the carbon in the CO 2 lost by dark respiration and that the proportion is not influenced by leaf starvation in darkness before the labeling. Therefore, most of the carbon released by dark respiration after illumination does not come from new photosynthates.Photosynthesis provides the carbohydrate substrate upon which plants depend. By contrast, glycolysis and respiration are responsible for the release of energy stored in carbohydrates. However, the amount of carbon assimilated during photosynthesis and immediately respired in leaves is unknown. In general, the respiratory carbon metabolism and its relation to recent photosynthates in leaves are poorly understood.For many years, studies on carbon metabolism after a period of light used carbon isotopes as tracers. 14 C labeling in the light was first applied in pioneering studies by Calvin and others to identify the fate of the carbon atoms fixed by photosynthesis (for review, see Rabinowitch, 1956). Subsequent studies using pulsechase techniques with 14 C were mainly used to examine allocation, that is, the partitioning of assimilates between distinct parts of the plants (Pearen and Hume, 1981;Fondy and Geiger, 1982;Soja et al., 1989;Kuzyakov et al., 2001) or the distribution of the 14 C in different metabolites of the same organ (Sharkey et al., 1985;Li et al., 1992). Refixation of CO 2 produced in the light by photorespiration is probably not a problem in the above studies, although it can obscure the results when studying fluxes in a given pathway, i.e. the photorespiratory pathway, where knowledge of the specific radioactivity of the carbon feeding the pathway must be accurately known (Biehler and Fock, 1996).In darkened leaves, the measurements of 14 C labeling in respired CO 2 are scarce: Birecka et al. (1969) found that the radioactivity of the respired CO 2 from the main shoot increased in wheat plants deprived of the ear, indicating that carbon losses in the last period of wheat development are due to more intensive respiration; Bort et al. (1996), feeding ears and flag leaves of durum wheat (Triticum durum) with 14 C-labeled Suc and trapping the 14 CO 2 released by respiration, concluded that the apparent refixation of res...

Carbon allocation and carbon isotope fluxes in the plant-soil-atmosphere continuum: a review

“…Only 43 % of respiration was directly driven by current photosynthates, thus pointing to the importance of short-term storage pools with half-lives of a few hours to more than a day. This finding is in agreement with observations made by Nogués et al (2004) for French bean, showing that the leaf respiratory substrate is a mixture in which current photosynthates are not the main components. Changes in the N supply (Lehmeier et al, 2010), but presumably also in other environmental conditions, can change the mean residence time of the respiratory substrate pool mainly due to different contributions from storage.…”

Respiratory metabolism of illuminated leaves depends on CO 2 and O 2 conditions

“…It is equal to 0.6 previous ϩ 0.4 fixed after 2-3 h in the light under ordinary CO2/O2 conditions (during which 200 -400 mmol C m Ϫ2 has been fixed), and 0.5 previous ϩ 0.5 fixed after 2-3 h in the light under high CO2 conditions (during which 400 -800 mmol C m Ϫ2 has been fixed) (34). It should be noted that possible variations in these coefficients only introduce very slight errors in the estimate of the 13 C-enriched substrate decarboxylation rnight, because of the strong 13 C enrichment of the substrate (that is, the p value is always very small compared with global or s and may be neglected).…”

Diel variations in the carbon isotope composition of respired CO<sub>2</sub> and associated carbon sources: a review of dynamics and mechanisms

“…3), and thus cannot be solely responsible for the diel variations in δ 13 C res , when one major respiratory pool consisting of one compound class is assumed to fuel respiration. Another aspect of substrate-induced variations (M1) might be related to the use of different respiratory substrates: M1.4: A switch between respiratory sources of different storage pools or substrate types including, soluble sugar, starch, lipids or amino acids, or stored and fresh assimilates with different isotopic signature could account for variation in δ 13 C res Nogués et al, 2004; Fig. 2; M1.4).…”