δ 13C of CO2 respired in the dark in relation to δ13C of leaf carbohydrates in Phaseolus vulgaris L. under progressive drought

The variations in δ ¹³C in both leaf carbohydrates (starch and sucrose) and CO₂ respired in the dark from the cotyledonary leaves of Phaseolus vulgaris L. were investigated during a progressive drought. As expected, sucrose and starch became heavier (enriched in ¹³C) with decreasing stomatal conductance and decreasing p _i/ p _a during the first half (15 d) of the dehydration cycle. Thereafter, when stomata remained closed and leaf net photosynthesis was near zero, the tendency was reversed: the carbohydrates became lighter (depleted in ¹³C). This may be explained by increased p _i/ p _a but other possible explanations are also discussed. Interestingly, the variations in δ ¹³C of CO₂ respired in the dark were correlated with those of sucrose for both well-watered and dehydrated plants. A linear relationship was obtained between δ ¹³C of CO₂ respired in the dark and sucrose, respired CO₂ always being enriched in ¹³C compared with sucrose by ≈ 6‰. The whole leaf organic matter was depleted in ¹³C compared with leaf carbohydrates by at least 1‰. These results suggest that: (i) a discrimination by ≈ 6‰ occurs during dark respiration processes releasing ¹³C-enriched CO₂; and that (ii) this leads to ¹³C depletion in the remaining leaf material.     

Relationships of grain δ13C and δ18O with wheat phenology and yield under water-limited conditions

Stable carbon isotope composition (δ¹³C) of dry matter has been widely investigated as a selection tool in cereal breeding programmes. However, reports on the possibilities of using stable oxygen isotope composition (δ¹⁸O) as a yield predictor are very scarce and only in the absence of water stress. Indeed, it remains to be tested whether changes in phenology and stomatal conductance in response to water stress overrule the use of either δ¹³C or δ¹⁸O when water is limited. To answer this question, a set of 24 genotypes of bread wheat ( Triticum aestivum ) were assayed in two trials with different levels of deficit irrigation and a third trial under rainfed conditions in a Mediterranean climate (northwest Syria). Grain yield (GY) and phenology (duration from planting to anthesis and from anthesis to maturity) were recorded, and the δ¹³C and δ¹⁸O of grains were analysed to assess their suitability as GY predictors. Both δ¹³C and δ¹⁸O showed higher broad-sense heritabilities ( H ²) than GY. Genotype means of GY across trials were negatively correlated with δ¹³C, as previously reported, but not with δ¹⁸O. Both isotopes were correlated with grain filling duration, whereas δ¹⁸O was also strongly affected by crop duration from planting to anthesis. We concluded that δ¹⁸O of grains is not a proper physiological trait to breed for suboptimal water conditions, as its variability is almost entirely determined by crop phenology. In contrast, δ¹³C of grains, despite being also affected by phenology, still provides complementary information associated with GY.     

Stable isotope evidence (δ13C, δ18O) for winter feeding on seaweed by Neolithic sheep of Scotland

The antiquity of the use of seaweed to feed domestic animals was investigated through carbon ( δ ¹³C) and oxygen ( δ ¹⁸O) isotope analysis of tooth enamel bioapatite. The analysis was performed on sheep and cattle teeth from two Neolithic sites in Orkney (Scotland). At the Knap of Howar, c . 3600 bc , carbon isotopes reflect grazing on terrestrial plants throughout the year for both sheep and cattle, with no contribution of seaweed to their diet. At the Holm of Papa Westray North (HPWN), c . 3000 bc , significant contribution of seaweed to the sheep diet during winter is indicated by bioapatite δ ¹³C values as high as −5.7‰, far outside of the range of values expected for the feeding on terrestrial C₃ plants, and δ ¹⁸O values higher than expected during winter, possibly caused by ingestion of oceanic water with seaweed. Ingestion of seaweed by sheep at HPWN might have been necessitated by severe reduction of pastures during winter. Results suggest that sheep ingested fresh seaweed rather than dry fodder, perhaps directly on the shore as sheep do nowadays on North Ronaldsay. A significant difference between the two populations is the exclusive reliance on seaweed by the North Ronaldsay sheep, which have developed physiological adaptations to this diet. Contribution of seaweed to the sheep winter diet at HPWN might have been a first step towards this adaptation.     

Bulk leaf δ18O and δ13C reflect the intensity of intraspecific competition for water in a semi-arid tussock grassland

We investigated the extent to which plant water and nutrient status are affected by intraspecific competition intensity and microsite quality in a monodominant tussock grassland. Leaf gas exchange and stable isotope measurements were used to assess the water relations of Stipa tenacissima tussocks growing along a gradient of plant cover and soil depth in a semi-arid catchment of Southeast Spain. Stomatal conductance and photosynthetic rate decreased with increasing intensity of competition during the wet growing season, leading to foliar δ ¹⁸O and δ ¹³C enrichment. A high potential for runoff interception by upslope neighbours exerted strong detrimental effects on the water and phosphorus status of downslope S. tenacissima tussocks. Foliar δ ¹⁵N values became more enriched with increasing soil depth. Multiple stepwise regression showed that competition potential and/or rhizosphere soil depth accounted for large proportions of variance in foliar δ ¹³C, δ ¹⁸O and δ ¹⁵N among target tussocks (57, 37 and 64%, respectively). The results presented here highlight the key role that spatial redistribution of resources (water and nutrients) by runoff plays in semi-arid ecosystems. It is concluded that combined measurement of δ ¹³C, δ ¹⁸O and nutrient concentrations in bulk leaf tissue can provide insight into the intensity of competitive interactions occurring in natural plant communities.     

Tracing carbon and oxygen isotope signals from newly assimilated sugars in the leaves to the tree-ring archive

The analysis of δ ¹³C and δ ¹⁸O in tree-ring archives offers retrospective insights into environmental conditions and ecophysiological processes. While photosynthetic carbon isotope discrimination and evaporative oxygen isotope enrichment are well understood, we lack information on how the isotope signal is altered by downstream metabolic processes.
In Pinus sylvestris , we traced the isotopic signals from their origin in the leaf water ( δ ¹⁸O) or the newly assimilated carbon ( δ ¹³C), via phloem sugars to the tree-ring, over a time-scale that ranges from hours to a growing season.
Seasonally, variable ¹³C enrichment of sugars related to phloem loading and transport did lead to uncoupling between δ ¹³C in the tree-ring, and the c _i/ c _a ratio at the leaf level. In contrast, the oxygen isotope signal was transferred from the leaf water to the tree-ring with an expected enrichment of 27‰, with time-lags of approximately 2 weeks and with a 40% exchange between organic oxygen and xylem water oxygen during cellulose synthesis.
This integrated overview of the fate of carbon and oxygen isotope signals within the model tree species P. sylvestris provides a novel physiological basis for the interpretation of δ ¹³C and δ ¹⁸O in tree-ring ecology.     

How to account for the lipid effect on carbon stable-isotope ratio (δ13C): sample treatment effects and model bias

This study investigated the impact of lipid extraction, CaCO₃ removal and of both treatments combined on fish tissue δ¹³C, δ¹⁵N and C:N ratio. Furthermore, the suitability of empirical δ¹³C lipid normalization and correction models was examined. δ¹⁵N was affected by lipid extraction (increase of up to 1·65‰) and by the combination of both treatments, while acidification alone showed no effect. The observed shift in δ¹⁵N represents a significant bias in trophic level estimates, i.e. lipid-extracted samples are not suitable for δ¹⁵N analysis. C:N and δ¹³C were significantly affected by lipid extraction, proportional to initial tissue lipid content. For both variables, rates of change with lipid content (ΔC:N and Δδ¹³C) were species specific. All tested lipid normalization and correction models produced biased estimates of fish tissue δ¹³C, probably due to a non-representative database and incorrect assumptions and generalizations the models were based on. Improved models need a priori more extensive and detailed studies of the relationships between lipid content, C:N and δ¹³C, as well as of the underlying biochemical processes.     

Seasonal and annual changes in leaf δ13C in two co-occurring Mediterranean oaks: relations to leaf growth and drought progression

1. Changes of δ¹³C and its relation to leaf development, biochemical content and water stress were monitored over a 2 year period in two co-occurring Mediterranean oak species: the deciduous Quercus pubescens and the evergreen Quercus ilex .
2. The time course of leaf δ¹³C showed different patterns in the two species. Young Q. pubescens leaves had a high δ¹³C and a marked decrease occurred during leaf growth. In contrast, leaves at budburst and maturity did not differ significantly in the case of Q. ilex . We suggest that the difference between δ¹³C of young leaves was linked to differential use of reserves of carbon compounds in the two species.
3. δ¹³C values of mature leaves were negatively correlated with minimum seasonal values of predawn water potential, suggesting that a functional adjustment to water resources occurred.
4. There was a significant correlation between individual δ¹³C values for two successive years. This interannual dependence showed that δ¹³C rankings between trees were constant through time.     

Leaf δ13C reflects ecosystem patterns and responses of alpine plants to the environments on the Tibetan Plateau

Leaf δ¹³C is an indicator of water-use efficiency and provides useful information on the carbon and water balance of plants over longer periods. Variation in leaf δ¹³C between or within species is determined by plant physiological characteristics and environmental factors. We hypothesized that variation in leaf δ¹³C values among dominant species reflected ecosystem patterns controlled by large-scale environmental gradients, and that within-species variation indicates plant adaptability to environmental conditions. To test these hypotheses, we collected leaves of dominant species from six ecosystems across a horizontal vegetation transect on the Tibetan Plateau, as well as leaves of Kobresia pygmaea (herbaceous) throughout its distribution and leaves of two coniferous tree species ( Picea crassifolia, Abies fabri ) along an elevation gradient throughout their distribution in the Qilian Mountains and Gongga Mountains, respectively. Leaf δ¹³C of dominant species in the six ecosystems differed significantly, with values for evergreen coniferous13C values of the dominant species and of K. pygmaea were negatively correlated with annual precipitation along a water gradient, but leaf δ¹³C of A. fabri was not significantly correlated with precipitation in habitats without water-stress. This confirms that variation of δ¹³C between or within species reflects plant responses to environmental conditions. Leaf δ¹³C of the dominant species also reflected water patterns on the Tibetan Plateau, providing evidence that precipitation plays a primary role in controlling ecosystem changes from southeast to northwest on the Tibetan Plateau.     

Seasonal Variation of δ^13C of Four Tree Species： A Biological Integrator of Environmental Variables

Foliar δ^13C values, an indicator of long-term intercellular carbon dioxide concentration and, thus, of long-term water use efficiency （WUE） in plants, were measured for Pinus massoniana Lamb., P. elliottii Engelm., Cunninghamia laceolata （Lamb.） Hook., and Schima superba Gardn. et Champ. in a restored forest ecosystem in the Jiazhu River Basin. Seasonal variation and the relationship between the foliar δ^13C values of the four species and environmental factors （monthly total precipitation, monthly average air temperature, relative humidity, atmospheric pressure, and monthly total solar radiation and evaporation） were investigated. The monthly δ^13C values and WUE of the four species increased with increasing precipitation, air temperature, solar radiation, and evaporation, whereas δ^13C values of the four species decreased with increasing relative humidity and atmospheric pressure. Despite significant differences in δ^13C seasonal means for the four species, our results demonstrate a significant convergence in the responses of δ^13C values and WUE to seasonal variations in environmental factors among the species investigated and that the δ^13C signature for each species gives a strong indication of environmental variables.     

Fractionation of δ15N and δ13C for Atlantic salmon and its intestinal cestode Eubothrium crassum

Stable isotopes of nitrogen (δ¹⁵N) and carbon (δ¹³C) were measured for Atlantic salmon Salmo salar and their intestinal cestode, Eubothrium crassum , sharing the same diet. Atlantic salmon muscle tissues were enriched in ¹⁵N and depleted in ¹³C compared to their prey (sprat Sprattus sprattus sprattus ) and their intestinal cestode. There was no significant difference in δ¹⁵N or δ¹³C between E. crassum and the sprat. Differences in nutrient uptake and intestine physiology between Atlantic salmon and E. crassum are discussed, as well as how these may give rise to different fractionations of stable isotopes between a host and its parasites. Furthermore, Atlantic salmon contained a significantly higher lipid content than their prey, which may partly explain differences in δ¹³C values between the host and its cestode. In addition, cestodes inhabiting lipid-rich hosts were also lipid rich. Larger Atlantic salmon were enriched in ¹⁵N compared to smaller fish. Cestodes inhabiting large hosts were also enriched in ¹⁵N compared to parasites living in smaller hosts. The last two results were explained by larger fish possibly feeding from a higher trophic level, or from larger and older prey, that resulted in both a higher lipid content and an enrichment in ¹⁵N.     

ISOTOPIC ANALYSIS OF δ13C, δ15N, AND δ34S "A FEEDING TALE" IN TEETH OF THE LONGBEAKED COMMON DOLPHIN, DELPHINUS CAPENSIS

Stable isotopic analyses of carbon, nitrogen, and sulfur were performed on teeth of different ages and sexes of the longbeaked common dolphin, Delphinus capensis, from the Gulf of California. Similarities in diet are suggested between the sexes, with no significant differences in isotopic compositions being observed. Differences in the δ¹³C, δ¹⁵N, and δ³⁴S signatures were found among the age groups (nursing calf, juvenile, subadult, and adult). These data suggest that this species is generally a coastal feeder, and that it changes its feeding habits with increasing age, drawing more nutrition from higher trophic level organisms later in life.     

δ13C variability of benthic algae: effects of water colour via modulation by stream current

1. Increased water motion is expected to reduce boundary layer diffusion resistance of autotrophs, thereby enabling greater isotopic discrimination against ¹³C such that lower δ¹³C values (ratio of ¹³C : ¹²C) should ensue. A field test of this hypothesis was undertaken by sampling benthic algae in streams of differing current speed.
2. Contrary to the expected negative relationship between δ¹³C and water motion, filamentous benthic algae were found to exhibit higher δ¹³C values in rapid water.
3. Under conditions of low current in the streams studied, concentrations of dissolved organic carbon as measured by water colour are elevated through the microbial decomposition of largely terrestrial organic matter. Photoassimilation of this respired carbon by benthic filamentous algae generates ¹³C‐depletion and lower δ¹³C values, and appears to be substantial enough in the streams used in the present study to override the competing influence of water motion on boundary layer thickness.     

δ13C of wood in growth-rings indicates cambial activity of drought-stressed trees of Eucalyptus globulus

1. Growth, density and δ¹³C of wood and leaf area were measured in two adjacent stands of 6 year-old Eucalyptus globulus growing in the 600–700 mm year^–1 rainfall region of south-western Australia. Study sites were identical except for differences in the availability of water owing to physical properties of soil profiles and location of sites within the landscape.
2. Abundance of ¹³C (expressed as δ¹³C) in wood of trees growing on the drought-prone site (– 24·8‰±1·4) was greater than in other trees (– 25·8‰±1·2, P <0·001) throughout the 6 years and, with further development, the δ¹³C signatures of wood may become useful indices of drought-susceptibility in plantations within a few years of establishment. The seasonal pattern of δ¹³C of wood appeared to reflect seasonal variation in water availability and duration of cambial activity.
3. Basic density of wood of trees growing on the more drought-prone site (496±14·0 kg m^–3) was reduced compared to other trees (554±5·3 kg m^–3, P <0·001). δ¹³C of wood across boundaries of growth-rings suggested that drought stopped cambial activity resulting in less production of late wood and less dense wood.
4. The stand growing on the drought-prone site had reduced growth, wood yield and leaf area but identical specific leaf area. Annual growth was correlated with the previous season's rainfall. Together, these results suggested that within the same evaporative climate, drought reduces growth primarily by reducing leaf area and that there is a lag between onset of drought and reduced productivity.     

The influence of leaf thickness on the CO2 transfer conductance and leaf stable carbon isotope ratio for some evergreen tree species in Japanese warm-temperate forests

1. The influence of leaf thickness on internal conductance for CO₂ transfer from substomatal cavity to chloroplast stroma ( g _i) and carbon isotope ratio (δ¹³C) of leaf dry matter was investigated for some evergreen tree species from Japanese temperate forests. g _i was estimated based on the combined measurements of gas exchange and concurrent carbon isotope discrimination.
2. Leaves with thicker mesophyll tended to have larger leaf dry mass per area (LMA), larger surface area of mesophyll cells exposed to intercellular air spaces per unit leaf area ( S _mes) and smaller volume ratio of intercellular spaces to the whole mesophyll (mesophyll porosity).
3. g _i of these leaves was correlated positively to S _mes but negatively to mesophyll porosity. The variation in g _i among these species would be therefore primarily determined by variation of the conductance in liquid phase rather than that in gas phase.
4. δ¹³C was positively correlated to mesophyll thickness and leaf nitrogen content on an area basis. However, g _i values did not correlate to δ¹³C. These results suggest that difference in δ¹³C among the species was not caused by the variation in g _i, but mainly by the difference in long-term photosynthetic capacity.
5. Comparison of our results with those of previous studies showed that the correlation between leaf thickness and g _i differed depending on leaf functional types (evergreen, deciduous or annual). Differences in leaf properties among these functional types were discussed.     

Trophic structure of a boreal forest arthropod community revealed by stable isotope (δ13C, δ15N) analyses

Terrestrial arthropods are important components of boreal ecosystems but relatively little is known about their trophic structure within communities. We measured δ¹³C and δ¹⁵N values in a broad range of arthropod taxa (Coleoptera, Diptera, Ephemeroptera, Homoptera, Hymenoptera, Lepidoptera, Odonata, Orthoptera, Araneae) from boreal forest in Prince Albert National Park, Saskatchewan, Canada. Isotopic measurements supported previous conventional investigations on foraging niches based on stomach content analysis and direct feeding observations but additional, new information was also obtained using the stable isotope approach. Significant differences were observed in both δ¹⁵N and δ¹³C values between various orders and families or superfamilies within orders. Higher variance in stable isotope values for scavengers (e.g. carrion beetles; Coleoptera, Silphidae) and generalists (e.g. ground beetles; Coleoptera, Carabidae) was found compared to specialists (e.g. grasshoppers; Orthoptera). Consistent isotopic differences between terrestrial and aquatic species were not found. However, aquatic insect δ¹³C values tended to be lower than those of their terrestrial counterparts. We discuss the potential for using stable isotope methods to reconstruct trophic linkages and interaction involving arthropods.     

Using the δ13C and δ15N of whitefish scales for retrospective ecological studies: changes in isotope signatures during the restoration of Lake Geneva, 1980–2001

The scales of whitefish Coregonus lavaretus were used in place of dorsal muscle, which necessitates killing the fish, to study food webs from the δ¹³C and δ¹⁵N isotopic ratios in the organic fraction. As scales are composed of both organic and calcified fractions, a protocol for scale decalcification was first devised. The δ¹³C and δ¹⁵N values of the decalcified scales were then shown to be closely correlated to those of the dorsal muscle, demonstrating that scales could be used in place of muscle to study food webs. Changes in the δ¹³C of whitefish were determined from a scale collection that extended over the period during which the trophic state of Lake Geneva was recovering.     

Natural 13C and 15N abundance of field-collected fungi and their ecological implications

The natural abundance of ¹³C and ¹⁵N was measured in basidiocarps of at least 115 species in 88 genera of ectomycorrhizal, wood-decomposing and litter-decomposing fungi from Japan and Malaysia. The natural abundance of ¹³C and ¹⁵N was also measured in leaves, litter, soil and wood from three different sites. ¹⁵N and ¹³C were enriched in ectomycorrhizal and wood-decomposing fungi, respectively, relative to their substrates. Ectomycorrhizal and wood-decomposing fungi could be distinguished on the basis of their δ¹³C and δ¹⁵N signatures. Although there was high variability in the isotopic composition of fungi, the following isotope- enrichment factors (ε, mean±SD) of the fungi relative to substrates were observed:
ε_{ectomycorrhizal fungi/litter} = 6.1±0.4‰¹⁵N
ε_{ectomycorrhizal fungi/wood} = 1.4±0.8‰¹³C
ε_{wood-decomposing fungi/wood} = −0.6±0.7‰¹⁵N
ε_{wood-decomposing fungi/wood} = 3.5±0.9‰¹³C
The basis of isotope fractionation in C metabolism from wood to wood-decomposing fungus is discussed.     

Chloroplast ultrastructure in two Crassulacean species and an F1 hybrid with differing biomass δ13C values

Abstract. The chloroplasts of two species of the Crassulaceae and their F₁ hybrid were compared by electron microscopy. The two species had contrasting leaf tissue δ¹³C values of −25°/_∞ ( Sedum greggii ) and −13°/_∞ ( Cremnophila linguifolia ), and the F₁ hybrid had a value of − 18°/_∞. S. greggii had a mean of 8.9 thylakoids per granum in contrast to C. linguifolia which had a mean of only 3.8 thylakoids per granum. The F₁ hybrid had a mean of 6.4 thylakoids per granum. Crystaloids were observed in S. greggii and the hybrid but not in C. linguifolia     

Combined monitoring of changes in δ13CH4 and archaeal community structure during mesophilic methanization of municipal solid waste

Reconstituted municipal solid waste (MSW) with varying contents of putrescible and cellulosic waste was incubated anaerobically under mesophilic conditions. Standard physicochemical parameters were monitored, together with stable isotopic signatures of produced CH₄ and CO₂. δ¹³C values for CH₄ indicated a change of methanogenic metabolism with time. CH₄ was predominantly produced from H₂/CO₂ at the beginning of the incubations. This period was associated with important shifts in archaeal communities monitored by automated ribosomal intergenic spacer analysis (ARISA) and FISH of oligonucleotidic probes targeting specifically 16S rRNA gene of various methanogenic groups. The onset of the active methane generation phase was characterized by an increase of CH₄δ¹³C, indicating a progressive shift toward an aceticlastic metabolism. When the methane production levelled off, a decrease in the isotopic signature was observed toward values characteristics of hydrogenotrophic metabolism. ARISA profiles were, however, found to be stable from the beginning of the active methane generation phase until the end of the experiment. FISH observation indicated that members of the family Methanosarcinaceae were predominant in the archaeal community during this period, suggesting that these methanogens might exhibit a high metabolic versatility during methanization of waste.     

DIET OF HARP SEALS (PAGOPHILUS GROENLANDICUS) IN NEARSHORE NORTHEAST NEWFOUNDLAND: INFERENCES FROM STABLE-CARBON (δ13C) AND NITROGEN (δ15N) ISOTOPE ANALYSES

Trophic position, and often the source of feeding of predators in food webs, can be estimated using measurements of stable isotope ratios of nitrogen and carbon in predators and their prey. Muscle samples from 60 harp seals ( Pagophilus groenlandicus ) collected during May 1995 in nearshore waters of New foundland, Canada, were analyzed for δ¹³C and δ¹⁵N values. These values were compared with those for 63 prey samples representing seven species generally collected near the same area. Using diet-tissue isotopic fractionation factors derived from previous studies using captive animals, we infer a greater dependence of harp seals on lower trophic-level prey during April compared with results expected from exclusive diets of Atlantic cod ( Gadus morhua ), Atlantic herring ( Clupea harengus ), Greenland halibut ( Reinhardtius hippoglossoides ), or northern shrimp ( Pandalus borealis ). Our mean δ¹⁵N value for harp seals is lower than previous findings for seals collected on the winter whelping patch and may be a function of interannual or seasonal differences in diet. Subadult seals (aged 1-4 yr) had significantly lower δ¹⁵N values than adults (5 + yr), suggesting that older seals were feeding at a slightly higher trophic level.