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• W1814090210 abstract "The ability to distinguish between ‘light’ and ‘heavy’ forms of carbon dioxide in the atmosphere has proven to be a useful tool in the study of plant ecology and the role of terrestrial ecosystems in the global carbon cycle. Plant processes imprint unique isotopic signatures on atmospheric CO2 such that measurements of the ratio of heavy to light carbon in CO2 can provide a means of detecting plant physiological processes integrated over whole canopies, ecosystems, or even regions. To interpret these measurements, a mechanistic understanding of the processes that affect the proportion of 12CO2 vs 13CO2 in the atmosphere is required, such as the well-known model of ‘discrimination’ against 13CO2 in photosynthesis caused by preferential diffusion and enzymatic reaction with 12CO2 (Farquhar et al., 1982). In this issue, Hymus et al. (pp. 377–384) show that short-term variations in isotopes of respiratory CO2 under natural conditions may be as large as variations in photosynthesis, and may contain additional information about plant metabolic pathways that have not been fully explored. ‘Measurements of the carbon isotope composition of plant respiration may provide insight into how photosynthate is allocated into synthesis of various compounds and metabolism in different species and in response to changes in the environment’ Photosynthetic discrimination causes plant material to be highly depleted in 13C relative to the atmosphere, particularly for plants that utilize the C3 photosynthetic pathway. When this carbon is released back to the atmosphere, it is also highly depleted in 13C, providing a useful tracer of ecosystem respiration. Using a simple mixing model first described by Keeling (1958, 1961), it is possible to use atmospheric measurements to estimate the carbon isotope composition of ecosystem respiration (often abbreviated as δ13Cr) at night when there is no photosynthetic uptake. This approach has been utilized in a number of ecosystems around the world to gain insight into processes that affect plant gas exchange integrated over time (see review by Pataki et al., 2003). Inherent in the interpretation of these measurements has been the assumption that discrimination against heavy isotopes during respiration is negligible such that the isotopic composition of respiration reflects the isotopic composition of the respiratory substrate. An increasing number of studies have recently challenged this assumption, which has become a topic of some debate. In particular, a common finding is that the isotopic composition of foliar respiration may be quite enriched in heavy carbon relative to all measured leaf compounds that may serve as respiratory substrates (e.g. Duranceau et al., 1999; Ghashghaie et al., 2001; Tcherkez et al., 2003; Xu et al., 2004). Most of these studies have been conducted with potted plants or controlled environments; Hymus et al. show that this effect is also observed under field conditions. By excising leaves and measuring the isotopic composition of dark respiration repeatedly over a diurnal period in two Mediterranean oak forests, they found that foliar respiration became progressively heavier over the course of the day, particularly in the upper canopy. Changes in the isotopic composition of the respiratory substrate caused by diurnal changes in photosynthetic discrimination are a possible explanation for this pattern; however, sugars, starch, lipids and cellulose extracted from excised leaves did not show much diurnal variation in isotopic composition. Despite this, cumulative leaf CO2 uptake measured in cuvettes was well correlated with isotopic enrichment of respiratory CO2 in the upper canopy. Early measurements of the isotopic composition of respiration showed a great deal of variability relative to substrates (see reviews by O’Leary, 1981; Farquhar et al., 1982). In addition, Lin and Ehleringer (1997) showed that the isotopic composition of CO2 evolved from isolated plant protoplasts was similar to that of their sugar substrates. This led many to conclude that isotope effects in respiration were negligible and that the isotopic composition of respiratory CO2 was predominantly influenced by photosynthetic effects on the isotopic composition of substrates, particularly sugars. In a review, Ghashghaie et al. (2003) discussed several studies that measured respiratory CO2, leaf sugars and other potential respiratory substrates directly and found poor correlations in isotopic composition. They suggested that changes in metabolic pathways from reactions that oxidize lipids to those that synthesize lipids could at least partially explain observed variations in isotopes of respiratory CO2. This was supported by Tcherkez et al. (2003), who showed that isotope effects in respiration were correlated with the respiratory quotient (RQ): the ratio of carbon dioxide evolved to oxygen consumed in respiration. RQ values are known to be related to respiratory substrate, such as the oxidation of sucrose (RQ = 1) vs highly reduced compounds such as lipids (RQ < 1). Plant respiratory pathways provide energy for metabolism, but they also provide carbon skeletons for synthesis of plant compounds from photosynthate. Synthesis of lipids is particularly of interest in discussing isotope effects of respiration because of the known, large isotope effects, both positional and enzymatic, in the oxidation of pyruvate to acetyl-CoA. If acetyl-CoA intermediates are fully oxidized in the Krebs cycle, net isotope effects on CO2 will not be observed, as there can be no isotopic discrimination if all of a particular substrate is converted to a product. However, if some of this substrate is synthesized into fatty acids, amino acids and other compounds, a large isotope effect may result as the products are highly depleted in heavy carbon. Conversely, to conserve mass balance, the CO2 evolved from the pyruvate dehydrogenase reaction must be isotopically enriched (Fig. 1). This reaction is the only known process that may explain the large enrichments of up to 7 reported by Hymus et al. and others. Simplified schematic of respiratory and biosynthetic pathways that lead to CO2 evolution that is enriched vs depleted in 13C relative to sugars. Synthesis of fatty acids and certain amino acids will result in release of enriched respiratory CO2, whereas full oxidation of acetyl-CoA or β oxidation of fatty acids will result in both enriched and depleted CO2, likely with smaller net isotopes effects relative to sugars. Modified from Tcherkez et al. (2003). At first glance, these findings complicate interpretation of the isotopic composition of plant respiration at larger scales. The ability to estimate carbon isotope ratios in respiration with both measurements and models has been an integral component of both ecosystem and global carbon cycle science. At the ecosystem scale, the isotopic composition of respiration has been applied toward partitioning ecosystem fluxes between photosynthetic and respiratory components (e.g. Yakir & Wang, 1996), whereas at the global scale the net impact of ecosystem production on CO2 isotopes in the global atmosphere has been utilized to distinguish ocean from terrestrial carbon sinks (e.g. Francey et al., 1995). Studies at the ecosystem scale have shown that δ13Cr is dynamic in response to conditions of varying temperature, humidity, soil moisture, etc. on the scale of days, weeks and months (e.g. Bowling et al., 2002). Changes in photosynthetic discrimination due to the effects of environmental conditions on stomatal conductance and photosynthetic rate have been proposed as the central mechanism underlying these variations. Now, it appears that large diurnal changes in δ13Cr such as those reported in Hymus et al. may be expected under field conditions owing to shifts in plant metabolic and biosynthetic pathways as well as CO2 diffusion and uptake. In particular, their study suggests that large discrepancies between the isotopic composition of sugars and respiratory CO2 are observed late in the day when photosynthetic products have accumulated in the upper canopy and when, presumably, carbon is shunted to biosynthetic pathways. The implications of these findings for studies of whole-plant and whole-ecosystem respiration as well as regional to global carbon cycle studies remain to be seen. Klumpp et al. (2005) found that while shoot respiration was enriched relative to shoot biomass, root respiration was depleted relative to root biomass, which resulted in negligible differences between δ13C of biomass and respiratory CO2 on a whole-plant basis. Further studies of this kind should be conducted under a variety of environmental conditions to determine if these results are applicable to whole plants in the field. However, even if further experiments reveal that this simplifying assumption cannot always be made, isotope effects in respiration are more than just a ‘complication’ in the study of plant and ecosystem carbon cycles. The work of Hymus et al. and others has shown that measurements of the carbon isotope composition of plant respiration may provide insight into how photosynthate is allocated into synthesis of various compounds and metabolism in different species and in response to changes in the environment. Temporal and spatial patterns of autotrophic respiration remain difficult to predict, and an improved understanding of shifts in metabolic pathways such as liposynthesis and lipid oxidation in the Krebs cycle may improve our ability to quantify growth and maintenance respiration in response to temperature and other environmental changes of interest. Although isotope fractionation in respiration has emerged as something of a contentious issue in stable isotope ecology, an alternative view is that the dynamic isotopic composition of respiratory CO2 provides an opportunity to gain insight into the flow of carbon through plants, and will therefore continue to be an important element of plant physiological and ecological studies." @default.
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• W1814090210 date "2005-07-05" @default.
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• W1814090210 title "Emerging topics in stable isotope ecology: are there isotope effects in plant respiration?" @default.
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