Intramolecular, compound-specific, and bulk carbon isotope patterns in C3 and C4 plants: a review and synthesis

Historical and contemporary stable isotope tracer approaches to studying mammalian protein metabolism

Over a century ago, Frederick Soddy provided the first evidence for the existence of isotopes; elements that occupy the same position in the periodic table are essentially chemically identical but differ in mass due to a different number of neutrons within the atomic nucleus. Allied to the discovery of isotopes was the development of some of the first forms of mass spectrometers, driven forward by the Nobel laureates JJ Thomson and FW Aston, enabling the accurate separation, identification, and quantification of the relative abundance of these isotopes. As a result, within a few years, the number of known isotopes both stable and radioactive had greatly increased and there are now over 300 stable or radioisotopes presently known. Unknown at the time, however, was the potential utility of these isotopes within biological disciplines, it was soon discovered that these stable isotopes, particularly those of carbon (13 C), nitrogen (15 N), oxygen (18 O), and hydrogen (2 H) could be chemically introduced into organic compounds, such as fatty acids, amino acids, and sugars, and used to "trace" the metabolic fate of these compounds within biological systems. From this important breakthrough, the age of the isotope tracer was born. Over the following 80 yrs, stable isotopes would become a vital tool in not only the biological sciences, but also areas as diverse as forensics, geology, and art. This progress has been almost exclusively driven through the development of new and innovative mass spectrometry equipment from IRMS to GC-MS to LC-MS, which has allowed for the accurate quantitation of isotopic abundance within samples of complex matrices. This historical review details the development of stable isotope tracers as metabolic tools, with particular reference to their use in monitoring protein metabolism, highlighting the unique array of tools that are now available for the investigation of protein metabolism in vivo at a whole body down to a single protein level. Importantly, it will detail how this development has been closely aligned to the technological development within the area of mass spectrometry. Without the dedicated development provided by these mass spectrometrists over the past century, the use of stable isotope tracers within the field of protein metabolism would not be as widely applied as it is today, this relationship will no doubt continue to flourish in the future and stable isotope tracers will maintain their importance as a tool within the biological sciences for many years to come. © 2016 The Authors. Mass Spectrometry Reviews Published by Wiley Periodicals, Inc. Mass Spec Rev.

Drought Sensitivity of the Carbon Isotope Composition of Leaf Dark-Respired CO2 in C3 (Leymus chinensis) and C4 (Chloris virgata and Hemarthria altissima) Grasses in Northeast China

Whether photosynthetic pathway differences exist in the amplitude of nighttime variations in the carbon isotope composition of leaf dark-respired CO2 (δ13Cl) and respiratory apparent isotope fractionation relative to biomass (ΔR,biomass) in response to drought stress is unclear. These differences, if present, would be important for the partitioning of C3-C4 mixed ecosystem C fluxes. We measured δ13Cl, the δ13C of biomass and of potential respiratory substrates and leaf gas exchange in one C3 (Leymus chinensis) and two C4 (Chloris virgata and Hemarthria altissima) grasses during a manipulated drought period. For all studied grasses, δ13Cl decreased from 21:00 to 03:00 h. The magnitude of the nighttime shift in δ13Cl decreased with increasing drought stress. The δ13Cl values were correlated with the δ13C of respiratory substrates, whereas the magnitude of the nighttime shift in δ13Cl strongly depended on the daytime carbon assimilation rate and the range of nighttime variations in the respiratory substrate content. The ΔR,biomass in the C3 and C4 grasses varied in opposite directions with the intensification of the drought stress. The contribution of C4 plant-associated carbon flux is likely to be overestimated if carbon isotope signatures are used for the partitioning of ecosystem carbon exchange and the δ13C of biomass is used as a substitute for leaf dark-respired CO2. The detected drought sensitivities in δ13Cl and differences in respiratory apparent isotope fractionation between C3 and C4 grasses have marked implications for isotope partitioning studies at the ecosystem level.

Nitrogen and carbon isotopic dynamics of subarctic soils and plants in southern Yukon Territory and its implications for paleoecological and paleodietary studies

We examine here the carbon and nitrogen isotopic compositions of bulk soils (8 topsoil and 7 subsoils, including two soil profiles) and five different plant parts of 79 C3 plants from two main functional groups: herbs and shrubs/subshrubs, from 18 different locations in grasslands of southern Yukon Territory, Canada (eastern shoreline of Kluane Lake and Whitehorse area). The Kluane Lake region in particular has been identified previously as an analogue for Late Pleistocene eastern Beringia. All topsoils have higher average total nitrogen δ15N and organic carbon δ13C than plants from the same sites with a positive shift occurring with depth in two soil profiles analyzed. All plants analyzed have an average whole plant δ13C of -27.5 ± 1.2 ‰ and foliar δ13C of -28.0 ± 1.3 ‰, and average whole plant δ15N of -0.3 ± 2.2 ‰ and foliar δ15N of -0.6 ± 2.7 ‰. Plants analyzed here showed relatively smaller variability in δ13C than δ15N. Their average δ13C after suitable corrections for the Suess effect should be suitable as baseline for interpreting diets of Late Pleistocene herbivores that lived in eastern Beringia. Water availability, nitrogen availability, spacial differences and intra-plant variability are important controls on δ15N of herbaceous plants in the study area. The wider range of δ15N, the more numerous factors that affect nitrogen isotopic composition and their likely differences in the past, however, limit use of the modern N isotopic baseline for vegetation in paleodietary models for such ecosystems. That said, the positive correlation between foliar δ15N and N content shown for the modern plants could support use of plant δ15N as an index for plant N content and therefore forage quality. The modern N isotopic baseline cannot be applied directly to the past, but it is prerequisite to future efforts to detect shifts in N cycling and forage quality since the Late Pleistocene through comparison with fossil plants from the same region.

Relationship of leaf oxygen and carbon isotopic composition with transpiration efficiency in the C4 grasses Setaria viridis and Setaria italica

Leaf carbon and oxygen isotope ratios can potentially provide a time-integrated proxy for stomatal conductance (gs) and transpiration rate (E), and can be used to estimate transpiration efficiency (TE). In this study, we found significant relationships of bulk leaf carbon isotopic signature (δ13CBL) and bulk leaf oxygen enrichment above source water (Δ18OBL) with gas exchange and TE in the model C4 grasses Setaria viridis and S. italica. Leaf δ13C had strong relationships with E, gs, water use, biomass, and TE. Additionally, the consistent difference in δ13CBL between well-watered and water-limited plants suggests that δ13CBL is effective in separating C4 plants with different availability of water. Alternatively, the use of Δ18OBL as a proxy for E and TE in S. viridis and S. italica was problematic. First, the oxygen isotopic composition of source water, used to calculate leaf water enrichment (Δ18OLW), was variable with time and differed across water treatments. Second, water limitations changed leaf size and masked the relationship of Δ18OLW and Δ18OBL with E. Therefore, the data collected here suggest that δ13CBL but not Δ18OBL may be an effective proxy for TE in C4 grasses.

Bulk C, H, O, and fatty acid C stable isotope analyses for purity assessment of vegetable oils from the southern and northern hemispheres

RATIONALE

The carbon, hydrogen and oxygen stable isotope composition (δ¹³ C, δ² H, and δ¹⁸ O values) of plants and their products is linked to photosynthetic fractionation, environmental factors and agricultural practices. Therefore, they contribute to determining the purity of commercial vegetable oils and may provide information on their geographical origin.

METHODS

Maize, olive, sunflower, groundnut, soybean and rice oils differing in sites of growth in the southern and northern hemispheres were characterized by bulk oil stable isotope ratios (δ¹³ C_bulk , δ² H_bulk , and δ¹⁸ O_bulk values), fatty acid (FA) concentrations and δ¹³ C_FA values using elemental analysis/isotope ratio mass spectrometry, gas chromatography/mass spectrometry, gas chromatography/flame ionization detection and gas chromatography/combustion/isotope ratio mass spectrometry. Principal component analysis was applied to examine the inherent structure of the data.

RESULTS

The δ¹³ C_bulk values of maize oils (-18.4 to -14.9 ‰) are typical for C₄ plants, and those of olive (-30.2 to -28.2 ‰), sunflower (-30.2 to -29.2 ‰), groundnut (-29.3 ‰), soybean (-30.6 ‰), and rice (-34.5 ‰) oils are typical for C₃ plants. The δ² H_bulk values vary from -161 to -132 ‰ for maize oils and -171 to -109 ‰ for C₃ oils. The δ¹⁸ O_bulk values of all oils vary between 15.2 and 38.9 ‰. The major δ¹³ C_FA differences (>5 ‰) within plant species render the inter-C₃ -species comparison difficult. These differences are explained in terms of variations in the lipid biosynthetic pathways and blend of vegetable oils of different FA composition and δ¹³ C_FA values. The samples from the southern hemisphere are generally enriched in ¹³ C compared with those from the northern hemisphere. Differences between the southern and northern hemispheres were observed in δ² H (p < 0.001) and δ¹⁸ O_bulk (p = 0.129) values for all C₃ oils, and in δ¹³ C_18:1 (p = 0.026) and δ¹⁸ O_bulk (p = 0.160) values for maize oils.

CONCLUSIONS

The results of this study show that combining bulk and molecular stable isotope ratios, FA compositions and their statistical analysis helps the characterization of the geographic origin of oils. This methodology can be used to detect and source impurities in valuable vegetable oils commercialized worldwide. Copyright © 2016 John Wiley & Sons, Ltd.

Ecophysiological responses of Betula pendula, Pinus uncinata and Rhododendron ferrugineum in the Catalan Pyrenees to low summer rainfall

Climate change is producing modifications in the intensity and frequency of rainfall in some regions of the planet. According to predictions, annual rainfall distribution in Western Europe will result in an increase in episodes of drought, thereby negatively affecting nutrient availability. Since high mountain systems will be particularly vulnerable, the physiological and nutritional responses to changes in summer rainfall were monitored over the course of two consecutive summers on three species, which are representative of subalpine forests: birch (Betula pendula Roth.), rhododendron (Rhododendron ferrugineum L.) and mountain pine (Pinus uncinata Mill.). Birch was especially susceptible to scarce precipitation showing alterations in leaf morphology and a decline in net photosynthesis (A) due to stomatal closure, which led to photoinhibition and to early leaf senescence as shown by the photosynthetic nitrogen-use efficiency (PNUE), carbon/nitrogen (C/N) ratio, foliar N and ¹³C isotope discrimination (Δ¹³C) results. The Δ¹³C of the soluble fraction is a good estimator of intrinsic water-use efficiency in this species. Rhododendron and mountain pine had sclerophyllous leaves, as indicated by leaf mass per area, Δ¹³C, PNUE and C/N results. Rhododendron was particularly affected by short periods of scarce rainfall, which negatively affected gas exchange and photochemistry, and reduced the remobilization of leaf N and P. Mountain pine was the most tolerant species since alterations of gas exchange, photochemistry and Δ¹³C were not observed. Its highest investment of N in RuBisCo and highest potassium, iron and magnesium leaf concentration contributed to the highest A rates observed.

Afforestation impacts microbial biomass and its natural (13)C and (15)N abundance in soil aggregates in central China

We investigated soil microbial biomass and its natural abundance of δ(13)C and δ(15)N in aggregates (>2000μm, 250-2000μm, 53-250μm and <53μm) of afforested (implementing woodland and shrubland plantations) soils, adjacent croplands and open area (i.e., control) in the Danjiangkou Reservoir area of central China. The afforested soils averaged higher microbial biomass carbon (MBC) and nitrogen (MBN) levels in all aggregates than in open area and cropland, with higher microbial biomass in micro-aggregates (<250μm) than in macro-aggregates (>2000μm). The δ(13)C of soil microbial biomass was more enriched in woodland soils than in other land use types, while δ(15)N of soil microbial biomass was more enriched compared with that of organic soil in all land use types. The δ(13)C and δ(15)N of microbial biomass were positively correlated with the δ(13)C and δ(15)N of organic soil across aggregates and land use types, whereas the (13)C and (15)N enrichment of microbial biomass exhibited linear decreases with the corresponding C:N ratio of organic soil. Our results suggest that shifts in the natural (13)C and (15)N abundance of microbial biomass reflect changes in the stabilization and turnover of soil organic matter (SOM) and thereby imply that afforestation can greatly impact SOM accumulation over the long-term.

Sources of variability in fatty acid (FA) biomarkers in the application of compound-specific stable isotopes (CSSIs) to soil and sediment fingerprinting and tracing: A review

Determining soil redistribution and sediment budgets in watersheds is often challenging. One of the methods for making such determinations employs soil and sediment fingerprinting techniques, using sediment properties such as geochemistry, fallout radionuclides, and mineral magnetism. These methods greatly improve the estimation of erosion and deposition within a watershed, but are limited when determining land use-based soil and sediment movement. Recently, compound-specific stable isotopes (CSSIs), which employ fatty acids naturally occurring in the vegetative cover of soils, offer the possibility of refining fingerprinting techniques based on land use, complementing other methods that are currently in use. The CSSI method has been met with some success; however, challenges still remain with respect to scale and resolution due to a potentially large degree of biological, environmental and analytical uncertainty. By better understanding the source of tracers used in CSSI work and the inherent biochemical variability in those tracers, improvement in sample design and tracer selection is possible. Furthermore, an understanding of environmental and analytical factors affecting the CSSI signal will lead to refinement of the approach and the ability to generate more robust data. This review focuses on sources of biological, environmental and analytical variability in applying CSSI to soil and sediment fingerprinting, and presents recommendations based on past work and current research in this area for improving the CSSI technique. A recommendation, based on current information available in the literature, is to use very-long chain saturated fatty acids and to avoid the use of the ubiquitous saturated fatty acids, C16 and C18.

Inherent Tracers for Carbon Capture and Storage in Sedimentary Formations: Composition and Applications

Inherent tracers-the "natural" isotopic and trace gas composition of captured CO2 streams-are potentially powerful tracers for use in CCS technology. This review outlines for the first time the expected carbon isotope and noble gas compositions of captured CO2 streams from a range of feedstocks, CO2-generating processes, and carbon capture techniques. The C-isotope composition of captured CO2 will be most strongly controlled by the feedstock, but significant isotope fractionation is possible during capture; noble gas concentrations will be controlled by the capture technique employed. Comparison with likely baseline data suggests that CO2 generated from fossil fuel feedstocks will often have δ(13)C distinguishable from storage reservoir CO2. Noble gases in amine-captured CO2 streams are likely to be low concentration, with isotopic ratios dependent on the feedstock, but CO2 captured from oxyfuel plants may be strongly enriched in Kr and Xe which are potentially valuable subsurface tracers. CO2 streams derived from fossil fuels will have noble gas isotope ratios reflecting a radiogenic component that will be difficult to distinguish in the storage reservoir, but inheritance of radiogenic components will provide an easily recognizable signature in the case of any unplanned migration into shallow aquifers or to the surface.

Carbon Isotope Composition of Carbohydrates and Polyols in Leaf and Phloem Sap of Phaseolus vulgaris L. Influences Predictions of Plant Water Use Efficiency

The use of carbon isotope abundance (δ(13)C) to assess plant carbon acquisition and water use has significant potential for use in crop management and plant improvement programs. Utilizing Phaseolus vulgaris L. as a model system, this study demonstrates the occurrence and sensitivity of carbon isotope fractionation during the onset of abiotic stresses between leaf and phloem carbon pools. In addition to gas exchange data, compound-specific measures of carbon isotope abundance and concentrations of soluble components of phloem sap were compared with major carbohydrate and sugar alcohol pools in leaf tissue. Differences in both δ(13)C and concentration of metabolites were found in leaf and phloem tissues, the magnitude of which responded to changing environmental conditions. These changes have inplications for the modeling of leaf-level gas exchange based upon δ(13)C natural abundance. Estimates of δ(13)C of low molecular weight carbohydrates and polyols increased the precision of predictions of water use efficiency compared with those based on bulk soluble carbon. The use of this technique requires consideration of the dynamics of the δ(13)C pool under investigation. Understanding the dynamics of changes in δ(13)C during movement and incorporation into heterotrophic tissues is vital for the continued development of tools that provide information on plant physiological performance relating to water use.

Effects of heat and drought stress on post-illumination bursts of volatile organic compounds in isoprene-emitting and non-emitting poplar

Over the last decades, post-illumination bursts (PIBs) of isoprene, acetaldehyde and green leaf volatiles (GLVs) following rapid light-to-dark transitions have been reported for a variety of different plant species. However, the mechanisms triggering their release still remain unclear. Here we measured PIBs of isoprene-emitting (IE) and isoprene non-emitting (NE) grey poplar plants grown under different climate scenarios (ambient control and three scenarios with elevated CO2 concentrations: elevated control, periodic heat and temperature stress, chronic heat and temperature stress, followed by recovery periods). PIBs of isoprene were unaffected by elevated CO2 and heat and drought stress in IE, while they were absent in NE plants. On the other hand, PIBs of acetaldehyde and also GLVs were strongly reduced in stress-affected plants of all genotypes. After recovery from stress, distinct differences in PIB emissions in both genotypes confirmed different precursor pools for acetaldehyde and GLV emissions. Changes in PIBs of GLVs, almost absent in stressed plants and enhanced after recovery, could be mainly attributed to changes in lipoxygenase activity. Our results indicate that acetaldehyde PIBs, which recovered only partly, derive from a new mechanism in which acetaldehyde is produced from methylerythritol phosphate pathway intermediates, driven by deoxyxylulose phosphate synthase activity.

Carbon isotopes and water use efficiency in C4 plants

Drought is a major agricultural problem worldwide. Therefore, selection for increased water use efficiency (WUE) in food and biofuel crop species will be an important trait in plant breeding programs. The leaf carbon isotopic composition (δ(13)Cleaf) has been suggested to serve as a rapid and effective high throughput phenotyping method for WUE in both C3 and C4 species. This is because WUE, leaf carbon discrimination (Δ(13)Cleaf), and δ(13)Cleaf are correlated through their relationships with intercellular to ambient CO2 partial pressures (Ci/Ca). However, in C4 plants, changing environmental conditions may influence photosynthetic efficiency (bundle-sheath leakiness) and post-photosynthetic fractionation that will potentially alter the relationship between δ(13)Cleaf and Ci/Ca. Here we discuss how these factors influence the relationship between δ(13)Cleaf and WUE, and the potential of using δ(13)Cleaf as a meaningful proxy for WUE.

Changes and their possible causes in δ13C of dark-respired CO2 and its putative bulk and soluble sources during maize ontogeny

The issues of whether, where, and to what extent carbon isotopic fractionations occur during respiration affect interpretations of plant functions that are important to many disciplines across the natural sciences. Studies of carbon isotopic fractionation during dark respiration in C3 plants have repeatedly shown respired CO2 to be (13)C enriched relative to its bulk leaf sources and (13)C depleted relative to its bulk root sources. Furthermore, two studies showed respired CO2 to become progressively (13)C enriched during leaf ontogeny and (13)C depleted during root ontogeny in C3 legumes. As such data on C4 plants are scarce and contradictory, we investigated apparent respiratory fractionations of carbon and their possible causes in different organs of maize plants during early ontogeny. As in the C3 plants, leaf-respired CO2 was (13)C enriched whereas root-respired CO2 was (13)C depleted relative to their putative sources. In contrast to the findings for C3 plants, however, not only root- but also leaf-respired CO2 became more (13)C depleted during ontogeny. Leaf-respired CO2 was highly (13)C enriched just after light-dark transition but the enrichment rapidly decreased over time in darkness. We conclude that (i) although carbon isotopic fractionations in C4 maize and leguminous C3 crop roots are similar, increasing phosphoenolpyruvate-carboxylase activity during maize ontogeny could have produced the contrast between the progressive (13)C depletion of maize leaf-respired CO2 and (13)C enrichment of C3 leaf-respired CO2 over time, and (ii) in both maize and C3 leaves, highly (13)C enriched leaf-respired CO2 at light-to-dark transition and its rapid decrease during darkness, together with the observed decrease in leaf malate content, may be the result of a transient effect of light-enhanced dark respiration.

Dietary options and behavior suggested by plant biomarker evidence in an early human habitat

The availability of plants and freshwater shapes the diets and social behavior of chimpanzees, our closest living relative. However, limited evidence about the spatial relationships shared between ancestral human (hominin) remains, edible resources, refuge, and freshwater leaves the influence of local resources on our species' evolution open to debate. Exceptionally well-preserved organic geochemical fossils--biomarkers--preserved in a soil horizon resolve different plant communities at meter scales across a contiguous 25,000 m(2) archaeological land surface at Olduvai Gorge from about 2 Ma. Biomarkers reveal hominins had access to aquatic plants and protective woods in a patchwork landscape, which included a spring-fed wetland near a woodland that both were surrounded by open grassland. Numerous cut-marked animal bones are located within the wooded area, and within meters of wetland vegetation delineated by biomarkers for ferns and sedges. Taken together, plant biomarkers, clustered bone debris, and hominin remains define a clear spatial pattern that places animal butchery amid the refuge of an isolated forest patch and near freshwater with diverse edible resources.

Isotopic reconstructions of habitat change surrounding the extinction of Oreopithecus, the last European ape

OBJECTIVE

Oreopithecus bambolii was the last hominoid to survive in Europe. The purpose of this investigation was to reconstruct, through stable isotope analyses, Oreopithecus' habitat, subsistence behavior, and changes in habitat that may have led to its extinction.

METHODS

Carbon and oxygen stable isotopes from inorganic carbonate in tooth enamel from Oreopithecus and its contemporaneous faunas from localities in Tuscany and Sardinia were sampled. Also the fauna from localities in Tuscany shortly after Oreopithecus went extinct were sampled.

RESULTS

Results indicated that Oreopithecus, compared with most modern hominoids, inhabited forests that probably had a more open canopy. At Tuscan localities, Oreopithecus yields some of the highest carbon isotope values but some of the lowest oxygen, suggesting a diet that may have included tubers or aquatic vegetation. Relatively higher oxygen values in Sardinia suggested that its diet included arboreal foods as well. Among modern and fossil hominoids, Oreopithecus only resembled chimpanzees living outside of rainforests. It also resembled Ardipithecus in carbon isotope values, suggesting possible similarities in feeding strategies concordant with shared skeletal features between Oreopithecus and early hominins. Isotope values from post-Oreopithecus faunas indicated a shift to more forested conditions, unlike other hominoid extinctions associated with loss of forest.

CONCLUSIONS

Isotopic reconstructions of Oreopithecus' habitat and changes associated with its extinction indicated that its paleoecology was unique among hominoids. However, these reconstructions also suggested that like other hominoids, Oreopithecus was susceptible to changes in seasonality of precipitation, and it may have used wetlands as a buffer to seasonal regimes. Am J Phys Anthropol 160:254-271, 2016. © 2016 Wiley Periodicals, Inc.

Eutrophication-Driven Hypoxia in the East China Sea off the Changjiang Estuary

Coastal hypoxia is an increasingly recognized environmental issue of global concern to both the scientific community and the general public. We assessed the relative contributions from marine and terrestrially sourced organic matter that were responsible for oxygen consumption in a well-studied seasonal coastal hypoxic zone, the East China Sea off the Changjiang Estuary. Our fieldwork was conducted in August 2011 during reinstatement of a subsurface hypoxia, when we observed a continuous decline of dissolved oxygen along with production of dissolved inorganic carbon resulting from organic carbon remineralization. On the basis of a three end-member mixing model and determinations of the stable isotopic compositions of dissolved inorganic carbon (δ(13)CDIC), the end product of particulate organic carbon (POC) degradation, we quantified the δ(13)C value of the remineralized organic carbon (δ(13)COCx), which was -18.5 ± 1.0‰. This isotopic composition was very similar to the δ(13)C of marine sourced POC produced in situ (-18.5 ± 0.3‰) rather than that of the terrestrially sourced POC (-24.4 ± 0.2‰). We concluded that marine-sourced organic matter, formed by eutrophication-induced marine primary production, was the dominant oxygen consumer in the subsurface hypoxic zone in the East China Sea off the Changjiang Estuary.

Analytical method for simultaneous determination of bulk and intramolecular (13) C-isotope compositions of acetic acid

RATIONALE

Headspace solid-phase microextraction (HS-SPME) combined with gas chromatography/pyrolysis-gas chromatography/combustion-isotope ratio mass spectrometry (GC/Py-GC/C-IRMS) was developed for the simultaneous determination of the intramolecular and molecular carbon-isotopic composition (δ(13) C value) of acetic acid.

METHODS

The δ(13) C values of carboxyl and methyl carbon were standardized using calibration curves constructed from the regression between the measured δ(13) C values and the δ(13) C values of working standards determined in a previous study. We applied this developed HS-SPME-GC/Py-GC/C-IRMS technique to commercial vinegars.

RESULTS

In one injection analysis, the bulk and intramolecular δ(13) C values of pure acetic acid standards can be obtained. The repeatability (1σ) of the bulk δ(13) C values is within ±0.4‰, and that of the δ(13) Ccarboxyl and δ(13) Cmethyl values is within ±0.6‰. The intramolecular δ(13) C values of acetic acid in vinegars exhibit a similar pattern. The average Δδ value (δ(13) CCOOH - δ(13) CCH3 ) is 4.3 ± 2.0‰.

CONCLUSIONS

The approach presented herein for the molecular and intramolecular δ(13) C determination of acetic acid avoids switching between configuration systems and thereby reduces systematic errors. It is expected to be useful for examining isotope fractionation associated with processes related to organic acid (bio)transformations.

iTRAQ-based quantitative proteomic analysis gives insight into sexually different metabolic processes of poplars under nitrogen and phosphorus deficiencies

Male and female poplars (Populus cathayana Rehd.) respond differently to nitrogen (N) and phosphorus (P) deficiencies. In this study, an iTRAQ-based quantitative proteomic analysis was performed. N and P deficiencies caused 189 and 144 proteins to change in abundance in males and 244 and 464 in females, respectively. Compared to N- and P-deficient males, both N- and P-deficient females showed a wider range of changes in proteins that are involved in amino acid, carbohydrate and protein metabolism, and the sexual differences were significant. When comparing the effects of N- and P-deficiencies, N-deficient females expressed more changes in proteins that are involved in stress responses and gene expression regulation, while P-deficient females showed more changes in proteins that are involved in energy and lipid metabolism, stress responses and gene expression regulation. The quantitative RT-PCR analysis of stress-related proteins showed that males have a better expression correlation between mRNA and protein levels than do females. This study shows that P. cathayana females are more sensitive and have more rapid metabolic mechanisms when responding to N and P deficiencies than do males, and P deficiency has a wider range of effects on females than does N deficiency.

The relationship between needle sugar carbon isotope ratios and tree rings of larch in Siberia

Significant gaps still exist in our knowledge about post-photosynthetic leaf level and downstream metabolic processes and isotopic fractionations. This includes their impact on the isotopic climate signal stored in the carbon isotope composition (δ(13)C) of leaf assimilates and tree rings. For the first time, we compared the seasonal δ(13)C variability of leaf sucrose with intra-annual, high-resolution δ(13)C signature of tree rings from larch (Larix gmelinii Rupr.). The trees were growing at two sites in the continuous permafrost zone of Siberia with different growth conditions. Our results indicate very similar low-frequency intra-seasonal trends of the sucrose and tree ring δ(13)C records with little or no indication for the use of 'old' photosynthates formed during the previous year(s). The comparison of leaf sucrose δ(13)C values with that in other leaf sugars and in tree rings elucidates the cause for the reported (13)C-enrichment of sink organs compared with leaves. We observed that while the average δ(13)C of all needle sugars was 1.2‰ more negative than δ(13)C value of wood, the δ(13)C value of the transport sugar sucrose was on an average 1.0‰ more positive than that of wood. Our study shows a high potential of the combined use of compound-specific isotope analysis of sugars (leaf and phloem) with intra-annual tree ring δ(13)C measurements for deepening our understanding about the mechanisms controlling the isotope variability in tree rings under different environmental conditions.

Carbon Isotope Composition of Nighttime Leaf-Respired CO2 in the Agricultural-Pastoral Zone of the Songnen Plain, Northeast China

Variations in the carbon isotope signature of leaf dark-respired CO2 (δ13CR) within a single night is a widely observed phenomenon. However, it is unclear whether there are plant functional type differences with regard to the amplitude of the nighttime variation in δ13CR. These differences, if present, would be important for interpreting the short-term variations in the stable carbon signature of ecosystem respiration and the partitioning of carbon fluxes. To assess the plant functional type differences relating to the magnitude of the nighttime variation in δ13CR and the respiratory apparent fractionation, we measured the δ13CR, the leaf gas exchange, and the δ13C of the respiratory substrates of 22 species present in the agricultural-pastoral zone of the Songnen Plain, northeast China. The species studied were grouped into C3 and C4 plants, trees, grasses, and herbs. A significant nocturnal shift in δ13CR was detected in 20 of the studied species, with the magnitude of the shift ranging from 1‰ to 5.8‰. The magnitude of the nighttime variation in δ13CR was strongly correlated with the daytime cumulative carbon assimilation, which suggests that variation in δ13CR were influenced, to some extent, by changes in the contribution of malate decarboxylation to total respiratory CO2 flux. There were no differences in the magnitude of the nighttime variation in δ13CR between the C3 and C4 plants, as well as among the woody plants, herbs and graminoids. Leaf respired CO2 was enriched in 13C compared to biomass, soluble carbohydrates and lipids; however the magnitude of enrichment differed between 8 pm and 4 am, which were mainly caused by the changes in δ13CR. We also detected the plant functional type differences in respiratory apparent fractionation relative to biomass at 4 am, which suggests that caution should be exercised when using the δ13C of bulk leaf material as a proxy for the δ13C of leaf-respired CO2.

Losses of soil carbon by converting tropical forest to plantations: erosion and decomposition estimated by δ(13) C

Indonesia lost more tropical forest than all of Brazil in 2012, mainly driven by the rubber, oil palm, and timber industries. Nonetheless, the effects of converting forest to oil palm and rubber plantations on soil organic carbon (SOC) stocks remain unclear. We analyzed SOC losses after lowland rainforest conversion to oil palm, intensive rubber, and extensive rubber plantations in Jambi Province on Sumatra Island. The focus was on two processes: (1) erosion and (2) decomposition of soil organic matter. Carbon contents in the Ah horizon under oil palm and rubber plantations were strongly reduced up to 70% and 62%, respectively. The decrease was lower under extensive rubber plantations (41%). On average, converting forest to plantations led to a loss of 10 Mg C ha(-1) after about 15 years of conversion. The C content in the subsoil was similar under the forest and the plantations. We therefore assumed that a shift to higher δ(13) C values in plantation subsoil corresponds to the losses from the upper soil layer by erosion. Erosion was estimated by comparing the δ(13) C profiles in the soils under forest and under plantations. The estimated erosion was the strongest in oil palm (35 ± 8 cm) and rubber (33 ± 10 cm) plantations. The (13) C enrichment of SOC used as a proxy of its turnover indicates a decrease of SOC decomposition rate in the Ah horizon under oil palm plantations after forest conversion. Nonetheless, based on the lack of C input from litter, we expect further losses of SOC in oil palm plantations, which are a less sustainable land use compared to rubber plantations. We conclude that δ(13) C depth profiles may be a powerful tool to disentangle soil erosion and SOC mineralization after the conversion of natural ecosystems conversion to intensive plantations when soils show gradual increase of δ(13) C values with depth.

Dynamic carbon allocation into source and sink tissues determine within-plant differences in carbon isotope ratios

Organs of C3 plants differ in their C isotopic signature (δ13C). In general, leaves are 13C-depleted relative to other organs. To investigate the development of spatial δ13C patterns, we induced different C allocation strategies by reducing light and nutrient availability for 12 months in the Mediterranean shrub Halimium halimifolium L. We measured morphological and physiological traits and the spatial δ13C variation among seven tissue classes during the experiment. A reduction of light (Low-L treatment) increased aboveground C allocation, plant height and specific leaf area. Reduced nutrient availability (Low-N treatment) enhanced C allocation into fine roots and reduced the spatial δ13C variation. In contrast, control and Low-L plants with high C allocation in new leaves showed a high δ13C variation within the plant (up to 2.5‰). The spatial δ13C variation was significantly correlated with the proportion of second-generation leaves from whole-plant biomass (R2=0.46). According to our results, isotope fractionation in dark respiration can influence the C isotope composition of plant tissues but cannot explain the entire spatial pattern seen. Our study indicates a foliar depletion in 13C during leaf development combined with export of relatively 13C-enriched C by mature source leaves as an important reason for the observed spatial δ13C pattern.

13C isotopic fractionation during biodegradation of agricultural wastes

Significant differences in δ(13)C signatures occur within and between plant tissues and their constituent biochemical entities, and also within and between heterotrophic bacteria and fungi and their metabolic products. Furthermore, (13)C isotopic fractionation occurs during the biodegradation of organic molecules as seen in the substrate, respired CO(2) and the microbial biomass, which could be related to substrate composition and/or microbial metabolism. The (13)C isotopic fractionation observed during the decomposition of a single defined C substrate appears to be due to the intra-molecular heterogeneity in (13)C in the substrate and to (13)C isotopic fractionation during microbial metabolism. Very limited data suggest that the latter may be quantitatively more important than the former. Studies with defined fungi in culture media have highlighted the complexities associated with the interpretation of the observed patterns of (13)C isotopic fractionation when a single defined C source is added to the culture medium which itself contains one or more C sources. Techniques involving (13)C enrichment or paired treatments involving an equivalent C(3)- and C(4)-derived substrate have been devised to overcome the problem of background C in the culture medium and (13)C isotopic fractionation during metabolism. Studies with complex substrates have shown an initial (13)C depletion phase in respired CO(2) followed by a (13)C enrichment phase which may or may not be followed by another (13)C depletion phase. Basic studies involving an integrated approach are required to gain a new insight into (13)C isotopic fractionation during organic residue decomposition, by simultaneous measurements of δ(13)C in all C moieties. New analytical tools to measure real-time changes in δ(13)CO(2) and the intra-molecular δ(13)C distribution within plant biochemical entities offer new opportunities for unravelling the complex interactions between substrate and microbial metabolism with respect to (13)C isotopic fractionation during biodegradation.

Bias in estimated online leaf carbon isotope discrimination due to woody tissues

Branch or shoot chamber measurements integrate over both foliar and woody tissue carbon dioxide (CO2) fluxes and their associated influences on the carbon isotopic composition of atmospheric/headspace CO2. Here, we quantified the bias introduced by woody tissue carbon isotope fluxes on apparent leaf (13)C discrimination (Δ(13)Capparent) estimates, using laser spectroscopy under controlled conditions. CO2 efflux from woody tissues of defoliated beech saplings in the dark was strongly related to temperature (R(2) = 0.78), which served as the basis to model light-dependent woody tissue photosynthesis (R(2) = 0.72). We then quantified the contributions of leaf and woody tissues to leaf Δ(13)Capparent of foliated beech saplings in the light. Unbiased foliar Δ(13)C was 1.1 to 4.9‰ lower than leaf Δ(13)Capparent, depending on photosynthetic rates of woody tissues. Therefore, we strongly recommend accounting for isotope-related bias due to woody tissues when estimating leaf Δ(13)Capparent based on branch or shoot chamber measurements.

Multi-factorial in vivo stable isotope fractionation: causes, correlations, consequences and applications

Many physical and chemical processes in living systems are accompanied by isotope fractionation on H, C, N, O and S. Although kinetic or thermodynamic isotope effects are always the basis, their in vivo manifestation is often modulated by secondary influences. These include metabolic branching events or metabolite channeling, metabolite pool sizes, reaction mechanisms, anatomical properties and compartmentation of plants and animals, and climatological or environmental conditions. In the present contribution, the fundamentals of isotope effects and their manifestation under in vivo conditions are outlined. The knowledge about and the understanding of these interferences provide a potent tool for the reconstruction of physiological events in plants and animals, their geographical origin, the history of bulk biomass and the biosynthesis of defined representatives. It allows the use of isotope characteristics of biomass for the elucidation of biochemical pathways and reaction mechanisms and for the reconstruction of climatic, physiological, ecological and environmental conditions during biosynthesis. Thus, it can be used for the origin and authenticity control of food, the study of ecosystems and animal physiology, the reconstruction of present and prehistoric nutrition chains and paleaoclimatological conditions. This is demonstrated by the outline of fundamental and application-orientated examples for all bio-elements. The aim of the review is to inform (advanced) students from various disciplines about the whole potential and the scope of stable isotope characteristics and fractionations and to provide them with a comprehensive introduction to the literature on fundamental aspects and applications.

Interactive effects of elevated CO2 and nitrogen deposition on fatty acid molecular and isotope composition of above- and belowground tree biomass and forest soil fractions

Atmospheric carbon dioxide (CO2) and reactive nitrogen (N) concentrations have been increasing due to human activities and impact the global carbon (C) cycle by affecting plant photosynthesis and decomposition processes in soil. Large amounts of C are stored in plants and soils, but the mechanisms behind the stabilization of plant- and microbial-derived organic matter (OM) in soils are still under debate and it is not clear how N deposition affects soil OM dynamics. Here, we studied the effects of 4 years of elevated (13C-depleted) CO2 and N deposition in forest ecosystems established in open-top chambers on composition and turnover of fatty acids (FAs) in plants and soils. FAs served as biomarkers for plant- and microbial-derived OM in soil density fractions. We analyzed above- and belowground plant biomass of beech and spruce trees as well as soil density fractions for the total organic C and FA molecular and isotope (δ13C) composition. FAs did not accumulate relative to total organic C in fine mineral fractions, showing that FAs are not effectively stabilized by association with soil minerals. The δ13C values of FAs in plant biomass increased under high N deposition. However, the N effect was only apparent under elevated CO2 suggesting a N limitation of the system. In soil fractions, only isotope compositions of short-chain FAs (C16+18) were affected. Fractions of 'new' (experimental-derived) FAs were calculated using isotope depletion in elevated CO2 plots and decreased from free light to fine mineral fractions. 'New' FAs were higher in short-chain compared to long-chain FAs (C20-30), indicating a faster turnover of short-chain compared to long-chain FAs. Increased N deposition did not significantly affect the quantity of 'new' FAs in soil fractions, but showed a tendency of increased amounts of 'old' (pre-experimental) C suggesting that decomposition of 'old' C is retarded by high N inputs.

(13)C discrimination between diet, faeces, milk and milk components

Stable isotope analysis is a fundamental tool in food origin and authenticity testing. Its use in livestock production requires knowledge of isotope discrimination between product and diet. Here, we report (13)C discrimination ((13)Δ) for milk, milk components (fat, casein and lactose) and faeces in eight lactating dairy cows, which grazed pasture or were fed fresh pasture herbage in the stall. Cows were supplemented with grain maize at 1.72 kg d(-1) (dry matter). Feed components were collected daily, and faeces, milk fat, casein, lactose and whole milk 4 times per week during an 8-week-long sampling period. Carbon isotope composition (δ(13)C) of each sample was analysed. δ(13)C was lowest in milk fat (-29.8‰) and highest in casein (-26.4‰). Compared to the diet, whole milk was depleted in (13)C ((13)Δ = 0.4‰) due to a strong (13)C-depletion of fat ((13)Δ = 2.2‰), which was not fully compensated by the (13)C-enrichment of casein ((13)Δ = -1.1‰) and lactose ((13)Δ = -0.7‰). Faeces were also depleted in (13)C ((13)Δ =1.7‰). Influences of feeding environment (stall vs. pasture) and herbage quality were minor (<0.4‰). A review of literature data shows large variation between studies. We consider that the present results are superior, as they are based on a much larger data set regarding the number of cows and milkings (total n = 256) with greater detail in analyses of diet and milk products. Also, the study covered both stall- and pasture-feeding scenarios in realistic settings with long periods of equilibration. This is the first comprehensive analysis of (13)C discrimination between diet and all main milk components (as well as faeces). Thus, the results will improve the use of stable isotope analyses in regard to authenticity testing and proof of origin.

Stable isotopes in tree rings: towards a mechanistic understanding of isotope fractionation and mixing processes from the leaves to the wood

The mechanistic understanding of isotope fractionation processes is increasing but we still lack detailed knowledge of the processes that determine the isotopic composition of the tree-ring archive over the long term. Especially with regard to the path from leaf photosynthate production to wood formation, post-assimilation fractionations/processes might cause at least a partial decoupling between the leaf isotope signals that record processes such as stomatal conductance, transpiration and photosynthesis, and the wood or cellulose signals that are stored in the paleophysiological record. In this review, we start from the rather well understood processes at the leaf level such as photosynthetic carbon isotope fractionation, leaf water evaporative isotope enrichment and the issue of the isotopic composition of inorganic sources (CO2 and H2O), though we focus on the less explored 'downstream' processes related to metabolism and transport. We further summarize the roles of cellulose and lignin as important chemical constituents of wood, and the processes that determine the transfer of photosynthate (sucrose) and associated isotopic signals to wood production. We cover the broad topics of post-carboxylation carbon isotope fractionation and of the exchange of organic oxygen with water within the tree. In two case studies, we assess the transfer of carbon and oxygen isotopic signals from leaves to tree rings. Finally we address the issue of different temporal scales and link isotope fractionation at the shorter time scale for processes in the leaf to the isotopic ratio as recorded across longer time scales of the tree-ring archive.

Quantifying the contribution of grape hexoses to wine volatiles by high-precision [U¹³C]-glucose tracer studies

Many fermentation volatiles important to wine aroma potentially arise from yeast metabolism of hexose sugars, but assessing the relative importance of these pathways is challenging due to high endogenous hexose substrate concentrations. To overcome this problem, gas chromatography combustion isotope ratio mass spectrometry (GC-C-IRMS) was used to measure high-precision (13)C/(12)C isotope ratios of volatiles in wines produced from juices spiked with tracer levels (0.01-1 APE) of uniformly labeled [U-(13)C]-glucose. The contribution of hexose to individual volatiles was determined from the degree of (13)C enrichment. As expected, straight-chain fatty acids and their corresponding ethyl esters were derived almost exclusively from hexoses. Most fusel alcohols and their acetate esters were also majority hexose-derived, indicating the importance of anabolic pathways for their formation. Only two compounds were not derived primarily from hexoses (hexanol and isobutyric acid). This approach can be extended to other food systems or substrates for studying precursor-product relationships.

Carbon isotope discrimination as a tool to explore C4 photosynthesis

Photosynthetic carbon isotope discrimination is a non-destructive tool for investigating C4 metabolism. Tuneable diode laser absorption spectroscopy provides new opportunities for making rapid, concurrent measurements of carbon isotope discrimination and CO2 assimilation over a range of environmental conditions, and this has facilitated the use of carbon isotope discrimination as a probe of C4 metabolism. In spite of the significant progress made in recent years, understanding how photosynthetic carbon isotope discrimination measured concurrently with gas exchange relates to carbon isotope composition of leaf and plant dry matter remains a challenge that requires resolution if this technique is to be successfully applied as a screening tool in crop breeding and phylogenetic research. In this review, we update our understanding of the factors and assumptions that underlie variations in photosynthetic carbon isotope discrimination in C4 leaves. Closing the main gaps in our understanding of carbon isotope discrimination during C4 photosynthesis may help advance research aimed at developing higher productivity and efficiency in key C4 food, feed, and biofuel crops.

Opposite carbon isotope discrimination during dark respiration in leaves versus roots - a review

In general, leaves are (13) C-depleted compared with all other organs (e.g. roots, stem/trunk and fruits). Different hypotheses are formulated in the literature to explain this difference. One of these states that CO2 respired by leaves in the dark is (13) C-enriched compared with leaf organic matter, while it is (13) C-depleted in the case of root respiration. The opposite respiratory fractionation between leaves and roots was invoked as an explanation for the widespread between-organ isotopic differences. After summarizing the basics of photosynthetic and post-photosynthetic discrimination, we mainly review the recent findings on the isotopic composition of CO2 respired by leaves (autotrophic organs) and roots (heterotrophic organs) compared with respective plant material (i.e. apparent respiratory fractionation) as well as its metabolic origin. The potential impact of such fractionation on the isotopic signal of organic matter (OM) is discussed. Some perspectives for future studies are also proposed .

Comparison of isotopic variability in proteinaceous tissues of a domesticated herbivore: a baseline for zooarchaeological investigation

RATIONALE

A variety of metabolic, dietary and climatic influences on isotopic variation have been established in mammalian hair. The relevance of these factors to collagen isotopic composition is unknown, but would be of great interest to zooarchaeological analyses of faunal skeletal tissue.

METHODS

The relationships between carbon (δ(13)C), nitrogen (δ(15)N), non-exchangeable hydrogen (δ(2)H) and oxygen (δ(18)O) values of defatted, demineralised and gelatinised bone collagen and defatted wool keratin from two sheep flocks (n = 20, 5) in the UK were investigated, including testing for the effects of nutritional plane, sex, pregnancy and season of sample collection. The sulfur composition (δ(34)S values) was also investigated for tissues from the smaller flock.

RESULTS

Bulk collagen was enriched in (13)C over bulk keratin by 2.0 - 2.7‰ and in (2)H by 29 - 40‰ but depleted in (18)O relative to keratin by 1.8‰. Differences in δ(15)N values were within experimental error. The collagen samples were generally more enriched in (34)S than keratin, but this was very variable. Pregnancy, sex and season, but not nutritional plane, significantly affected isotope values but did not change overall keratin-collagen relationships.

CONCLUSIONS

This dataset provides a baseline measure of variability and comparability for isotopic investigations into origin and husbandry conditions in archaeological sheep tissues, both collagen and keratin.

C4 plants use fluctuating light less efficiently than do C3 plants: a study of growth, photosynthesis and carbon isotope discrimination

Plants in the field are commonly exposed to fluctuating light intensity, caused by variable cloud cover, self-shading of leaves in the canopy and/or leaf movement due to turbulence. In contrast to C3 plant species, only little is known about the effects of dynamic light (DL) on photosynthesis and growth in C4 plants. Two C4 and two C3 monocot and eudicot species were grown under steady light or DL conditions with equal sum of daily incident photon flux. We measured leaf gas exchange, plant growth and dry matter carbon isotope discrimination to infer CO2 bundle sheath leakiness in C4 plants. The growth of all species was reduced by DL, despite only small changes in steady-state gas exchange characteristics, and this effect was more pronounced in C4 than C3 species due to lower assimilation at light transitions. This was partially attributed to increased bundle sheath leakiness in C4 plants under the simulated lightfleck conditions. We hypothesize that DL leads to imbalances in the coordination of C4 and C3 cycles and increasing leakiness, thereby decreasing the quantum efficiency of photosynthesis. In addition to their other constraints, the inability of C4 plants to efficiently utilize fluctuating light likely contributes to their absence in such environments as forest understoreys.

Environmental and physiological determinants of carbon isotope discrimination in terrestrial plants

Stable carbon isotope ratios (δ(13) C) of terrestrial plants are employed across a diverse range of applications in environmental and plant sciences; however, the kind of information that is desired from the δ(13) C signal often differs. At the extremes, it ranges between purely environmental and purely biological. Here, we review environmental drivers of variation in carbon isotope discrimination (Δ) in terrestrial plants, and the biological processes that can either damp or amplify the response. For C3 plants, where Δ is primarily controlled by the ratio of intercellular to ambient CO2 concentrations (ci /ca ), coordination between stomatal conductance and photosynthesis and leaf area adjustment tends to constrain the potential environmentally driven range of Δ. For C4 plants, variation in bundle-sheath leakiness to CO2 can either damp or amplify the effects of ci /ca on Δ. For plants with crassulacean acid metabolism (CAM), Δ varies over a relatively large range as a function of the proportion of daytime to night-time CO2 fixation. This range can be substantially broadened by environmental effects on Δ when carbon uptake takes place primarily during the day. The effective use of Δ across its full range of applications will require a holistic view of the interplay between environmental control and physiological modulation of the environmental signal.

Resource translocation drives δ(13) C fractionation during recovery from disturbance in giant kelp, Macrocystis pyrifera

Resource allocation and translocation are fundamental physiological functions for autotrophs. The mobilization and use of resources drive population dynamics by regulating growth and recovery of individuals, but also influences ecosystem-level processes such as primary productivity and carbon cycling. This study provides the first observation of translocation-driven gradients of δ(13) C in macroalgae, a critically important phenomenon recognized in vascular plants for decades. A ~10‰ δ(13) C increase in new giant kelp (Macrocystis pyrifera) fronds relative to mature canopy blades was produced after 5 weeks following a biomass removal experiment, more than twice the variation typical for macroalgae. The observed δ(13) C patterns are consistent with tissue enrichment following resource translocation in vascular plants. The analogous source-sink relationships and consistent translocation patterns in Macrocystis and vascular plants suggest that translocation of stored resources is critical for structuring productivity and recovery from disturbance in important, habitat-forming macroalgae such as kelps and fucoids.

Carbon and nitrogen isotopic survey of northern peruvian plants: baselines for paleodietary and paleoecological studies

The development of isotopic baselines for comparison with paleodietary data is crucial, but often overlooked. We review the factors affecting the carbon (δ(13)C) and nitrogen (δ(15)N) isotopic compositions of plants, with a special focus on the carbon and nitrogen isotopic compositions of twelve different species of cultivated plants (n = 91) and 139 wild plant species collected in northern Peru. The cultivated plants were collected from nineteen local markets. The mean δ(13)C value for maize (grain) was -11.8±0.4 ‰ (n = 27). Leguminous cultigens (beans, Andean lupin) were characterized by significantly lower δ(15)N values and significantly higher %N than non-leguminous cultigens. Wild plants from thirteen sites were collected in the Moche River Valley area between sea level and ∼4,000 meters above sea level (masl). These sites were associated with mean annual precipitation ranging from 0 to 710 mm. Plants growing at low altitude sites receiving low amounts of precipitation were characterized by higher δ(15)N values than plants growing at higher altitudes and receiving higher amounts of precipitation, although this trend dissipated when altitude was >2,000 masl and MAP was >400 mm. For C(3) plants, foliar δ(13)C was positively correlated with altitude and precipitation. This suggests that the influence of altitude may overshadow the influence of water availability on foliar δ(13)C values at this scale.

Site-specific responses to short-term environmental variation are reflected in leaf and phloem-sap carbon isotopic abundance of field grown Eucalyptus globulus

The carbon isotopic composition (δ(13) C) of plant material has been used extensively as an indirect measure of carbon fixation per volume of water used. More recently, the δ(13) C of phloem sap (δ(13) C(phl) ) has been used as a surrogate measure of short-term, canopy scale δ(13) C. Using a combination of δ(13) C physiological, structural and chemical indices from leaves and phloem sap of Eucalyptus globulus at sites of contrasting water availability, we sought to identify short-term, canopy scale resource limitations. Results illustrate that δ(13) C(phl) offers valid reflections of short-term, canopy scale values of leaf δ(13) C and tree water status. Under conditions limited by water, leaf and phloem sap photoassimilates differ in (13) C abundance of a magnitude large enough to significantly influence predictions of water use efficiency. This pattern was not detected among trees with adequate water supply indicating fractionation into heterotrophic tissues that may be sensitive to plant water status. Trees employed a range of physiological, biochemical and structural adaptations to acclimate to resource limitation that differed among sites providing a useful context upon which to interpret patterns in δ(13) C. Our results highlight that such easily characterized properties are ideal for use as minimally invasive tools to monitor growth and resilience of plants to variations in resource availability.

A trophic study of the sympatric Amazonian freshwater turtles Podocnemis unifilis and Podocnemis expansa (Testudines, Podocnemidae) using carbon and nitrogen stable isotope analyses

The Yellow-spotted River Turtle ( Podocnemis unifilis Troschel, 1848) and the South American River Turtle (Podocnemis expansa (Schweigger, 1812)) are two turtles species that are widely distributed and have ecological, economic, and cultural importance in the Amazon basin. Although sympatric regarding most of their distribution, few studies have addressed the coexistence of these two species. To examine this, we analyzed the trophic level and the primary carbon source from the diets of both species in Baixo Araguaia, Tocantins, Brazil, using stable isotope analyses of carbon (δ13C) and nitrogen (δ15N). We also verified possible intraspecific variations (related to sex and body mass) in the trophic levels and primary carbon sources of their diets. Podocnemis unifilis had higher values of δ15N than P. expansa, averaging 7.59‰ and 5.06‰, respectively, a difference which may indicate a possible trophic change owing to exploiting different food resources. No differences were found between the two species in relation to δ13C (mean values of –26.2‰ and –26.1‰, respectively). The similarity between δ13C values suggests that the sources of their basal feeding are the same, consisting mainly of C3 plants. There was no intraspecific variation in the values of δ13C and δ15N.