Metabolic Origin of Carbon Isotope Composition of Leaf Dark-Respired CO2 in French Bean

Abstract

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...

Do trees use reserve or newly assimilated carbon for their defense reactions? A 13C labeling approach with young Scots pines inoculated with a bark-beetle-associated fungus (Ophiostoma brunneo ciliatum)

“…Isotopic discrimination by key enzymes generates measurable isotopic gradients in pools of metabolic intermediates, resulting in end-products with different isotopic compositions [24,45]. Drought induced a marked increase of δ 13 C in both amino acids and soluble sugars of healthy phloem.…”

The successional status of tropical rainforest tree species is associated with differences in leaf carbon isotope discrimination and functional traits

“…Numbers in bold indicate significant correlation at P = 0.05. pointed to variable isotopic fractionations during dark respiration (e.g. [51]) or post-photosynthetic fractionations [1]. These fractionations, that are not included in the simple model of discrimination described in [19] and used here, could differ among species and contribute to the shift with the theoretical line.…”

In Vivo Respiratory Metabolism of Illuminated Leaves

“…The extraction procedures for starch were similar to that described by Tcherkez et al (2003). Leaf powder was suspended with 1 mL of distilled water in an Eppendorf tube (Eppendorf Scientific, Hamburg, Germany).…”

“…The respiration chamber was placed in a closed system, which was directly coupled to an elemental analyzer (EA) NA-1500 (Carlo-Erba, Milan) through a 15-mL loop, as described by Tcherkez et al (2003). Briefly, molar fractions of respiratory CO 2 were measured with an infrared gas analyzer (IRGA; Finor, Maihak, Germany) placed in the closed system that was first flushed with CO 2 -free air.…”

“…French bean (Phaseolus vulgaris) L. cv Contender plants were grown from seed in 1-L pots of potting mix in a greenhouse, as described by Tcherkez et al (2003). Minimum photosynthetic photon flux density during a 16-h photoperiod was maintained at approximately 500 mmol m 22 s 21 by supplemental lighting.…”