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1.
In order to investigate mid-Cretaceous terrestrial climates of low paleolatitudes, Moroccan, Tunisian and Brazilian vertebrate apatites have been analyzed for their oxygen and carbon isotope compositions of phosphates (δ18Op) and carbonates (δ18Oc, δ13Cc). At each site, coexisting theropod dinosaurs, titanosaurid sauropods, pterosaurs, crocodilians, turtles and fish have distinct δ18Op and δ13Cc values reflecting their ecologies, diets and foraging environments. Oxygen isotope compositions of surface waters (δ18Ow) estimated from turtle and crocodile δ18Op values range from − 5.0 ± 1.0‰ to − 2.4 ± 1.0‰, which do not differ from mean annual rainwater values occurring today under inter-tropical sub-arid to arid climates. High water temperatures ranging from 21 ± 6 °C to 34 ± 2 °C deduced from fish δ18Op values are in agreement with those published for mid-Cretaceous low latitudes. Temporary or seasonal droughts are inferred from high δ18Op values of lungfish teeth, even though lower reptile δ18Op values suggest the use of distinct and most likely larger or regularly renewed bodies of water. Environmental conditions of the studied low latitude regions during the Aptian-Cenomanian interval were somewhat similar to those experienced today under semi-arid to arid tropical or equatorial climates, but with higher mean surface temperatures than present-day ones.  相似文献   

2.
Oxygen isotope compositions of phosphate (δ18Op) were measured in tooth enamel from captive and wild individuals of 8 crocodilian species. A rough linear correlation is observed between the δ18Op of all the studied species and the oxygen isotope composition of ambient water (δ18Ow). Differences in mean air temperature, diet and physiology could contribute significantly to the large scatter of δ18Op values. The combination of these parameters results in a fractionation equation for which the slope (0.82) is lower than that expected (≥ 1) from predictive model equations that assume temperature and diet as fixed parameters. Taking into account large uncertainties, the observed oxygen isotope fractionation between phosphate and ambient water does not statistically differ from that formerly established for aquatic turtles. Case studies show that δ18Op values of fossil crocodile tooth enamel can be used to discriminate between marine and freshwater living environments within a precision of about ± 2‰ only.  相似文献   

3.
Although considerable variation has been documented in tree‐ring cellulose oxygen isotope ratios (δ18Ocell) among co‐occurring species, the underlying causes are unknown. Here, we used a combination of field measurements and modelling to investigate the mechanisms behind variations in late‐wood δ18Ocell18Olc) among three co‐occurring species (chestnut oak, black oak and pitch pine) in a temperate forest. For two growing seasons, we quantified among‐species variation in δ18Olc, as well as several variables that could potentially cause the δ18Olc variation. Data analysis based on the δ18Ocell model rules out leaf water enrichment (Δ18Olw) and tree‐ring formation period (Δt), but highlights source water δ18O (δ18Osw) as an important driver for the measured difference in δ18Olc between black and chestnut oak. However, the enriched δ18Olc in pitch pine relative to the oaks could not be sufficiently explained by consideration of the above three variables only, but rather, we show that differences in the proportion of oxygen exchange during cellulose synthesis (pex) is most likely a key mechanism. Our demonstration of the relevance of some species‐specific features (or lack thereof) to δ18Ocell has important implications for isotope based ecophysiological/paleoclimate studies.  相似文献   

4.
Using both oxygen isotope ratios of leaf water (δ18OL) and cellulose (δ18OC) of Tillandsia usneoides in situ, this paper examined how short‐ and long‐term responses to environmental variation and model parameterization affected the reconstruction of the atmospheric water vapour (δ18Oa). During sample‐intensive field campaigns, predictions of δ18OL matched observations well using a non‐steady‐state model, but the model required data‐rich parameterization. Predictions from the more easily parameterized maximum enrichment model (δ18OL–M) matched observed δ18OL and observed δ18Oa when leaf water turnover was less than 3.5 d. Using the δ18OL–M model and weekly samples of δ18OL across two growing seasons in Florida, USA, reconstructed δ18Oa was ?12.6 ± 0.3‰. This is compared with δ18Oa of ?12.4 ± 0.2‰ resolved from the growing‐season‐weighted δ18OC. Both of these values were similar to δ18Oa in equilibrium with precipitation, ?12.9‰. δ18Oa was also reconstructed through a large‐scale transect with δ18OL and the growing‐season‐integrated δ18OC across the southeastern United States. There was considerable large‐scale variation, but there was regional, weather‐induced coherence in δ18Oa when using δ18OL. The reconstruction of δ18Oa with δ18OC generally supported the assumption of δ18Oa being in equilibrium with precipitation δ18O (δ18Oppt), but the pool of δ18Oppt with which δ18Oa was in equilibrium – growing season versus annual δ18Oppt – changed with latitude.  相似文献   

5.
The oxygen isotope composition of cellulose (δ18OCel) archives hydrological and physiological information. Here, we assess previously unexplored direct and interactive effects of the δ18O of CO218OCO2), nitrogen (N) fertilizer supply and vapour pressure deficit (VPD) on δ18OCel, 18O‐enrichment of leaf water (Δ18OLW) and cellulose (Δ18OCel) relative to source water, and pexpx, the proportion of oxygen in cellulose that exchanged with unenriched water at the site of cellulose synthesis, in a C4 grass (Cleistogenes squarrosa). δ18OCO2 and N supply, and their interactions with VPD, had no effect on δ18OCel, Δ18OLW, Δ18OCel and pexpx. Δ18OCel and Δ18OLW increased with VPD, while pexpx decreased. That VPD‐effect on pexpx was supported by sensitivity tests to variation of Δ18OLW and the equilibrium fractionation factor between carbonyl oxygen and water. N supply altered growth and morphological features, but not 18O relations; conversely, VPD had no effect on growth or morphology, but controlled 18O relations. The work implies that reconstructions of VPD from Δ18OCel would overestimate amplitudes of VPD variation, at least in this species, if the VPD‐effect on pexpx is ignored. Progress in understanding the relationship between Δ18OLW and Δ18OCel will require separate investigations of pex and px and of their responses to environmental conditions.  相似文献   

6.
Measurements of the carbon (δ13Cm) and oxygen (δ18Om) isotope composition of C3 plant tissue provide important insights into controls on water‐use efficiency. We investigated the causes of seasonal and inter‐annual variability in water‐use efficiency in a grassland near Lethbridge, Canada using stable isotope (leaf‐scale) and eddy covariance measurements (ecosystem‐scale). The positive relationship between δ13Cm and δ18Om values for samples collected during 1998–2001 indicated that variation in stomatal conductance and water stress‐induced changes in the degree of stomatal limitation of net photosynthesis were the major controls on variation in δ13Cm and biomass production during this time. By comparison, the lack of a significant relationship between δ13Cm and δ18Om values during 2002, 2003 and 2006 demonstrated that water stress was not a significant limitation on photosynthesis and biomass production in these years. Water‐use efficiency was higher in 2000 than 1999, consistent with expectations because of greater stomatal limitation of photosynthesis and lower leaf ci/ca during the drier conditions of 2000. Calculated values of leaf‐scale water‐use efficiency were 2–3 times higher than ecosystem‐scale water‐use efficiency, a difference that was likely due to carbon lost in root respiration and water lost during soil evaporation that was not accounted for by the stable isotope measurements.  相似文献   

7.
We measured the carbon and oxygen isotopic composition of stem cellulose of Pinus sylvestris, Picea abies, Fagus sylvatica and Fraxinus excelsior. Several sites along a transect of a small valley in Switzerland were selected which differ in soil moisture conditions. At every site, six trees per species were sampled, and a sample representing a mean value for the period from 1940 to 1990 was analysed. For all species, the mean site δ13C and δ18O of stem cellulose are related to the soil moisture availability, whereby higher isotope ratios are found at drier sites. This result is consistent with isotope fractionation models when assuming enhanced stomatal resistance (thus higher δ13C of incorporated carbon) and increased oxygen isotope enrichment in the leaf water (thus higher δ18O) at the dry sites. δ18 O-δ13C plots reveal a linear relationship between the carbon and oxygen isotopes in cellulose. To interpret this relationship we developed an equation which combines the above-mentioned fractionation models. An important new parameter is the degree to which the leaf water enrichment is reflected in the stem cellulose. In the combined model the slope of the δ18O-δ13C plot is related to the sensitivity of the pi/pa of a plant to changing relative humidity.  相似文献   

8.
Here we report bone phosphate oxygen (δ18Op) values from perinates/neonates and infants (<3.5 years; n = 32); children (4–12 years; n = 12); unsexed juveniles (16–18 years; n = 2); and adult bones (n = 17) from Wharram Percy, North Yorkshire, England, in order to explore the potential of this method to investigate patterns of past breastfeeding and weaning. In prior studies, δ15N and δ13C analyses of bone collagen have been utilized to explore weaning age in this large and well‐studied assemblage, rendering this material highly appropriate for the testing and development of this alternative method targeting the inorganic phase of bone. Data produced reveal 18O‐enrichment in the youngest perinatal/neonatal and infant samples, and an association between age and bone δ18Op (and previously‐published δ15N values), with high values in both these isotope systems likely due to breastfeeding. After the age of 2–3 years, δ18Op values are lower, and all children between the ages of 4 and 12, along with the vast majority of sub‐adults and adults sampled (aged 16 to >50 years), have δ18Op values consistent with the consumption of local modern drinking water. The implications of this study for the reconstruction of weaning practices in archaeological populations are discussed, including variations observed with bone δ15Ncoll and δ18Op co‐analysis and the influence of culturally‐modified drinking water and seasonality. The use of this method to explore human mobility and palaeoclimatic conditions are also discussed with reference to the data presented. Am J Phys Anthropol 157:226–241, 2015. © 2015 Wiley Periodicals, Inc.  相似文献   

9.
Almost no δ18O data are available for leaf carbohydrates, leaving a gap in the understanding of the δ18O relationship between leaf water and cellulose. We measured δ18O values of bulk leaf water (δ18OLW) and individual leaf carbohydrates (e.g. fructose, glucose and sucrose) in grass and tree species and δ18O of leaf cellulose in grasses. The grasses were grown under two relative humidity (rH) conditions. Sucrose was generally 18O‐enriched compared with hexoses across all species with an apparent biosynthetic fractionation factor (εbio) of more than 27‰ relative to δ18OLW, which might be explained by isotopic leaf water and sucrose synthesis gradients. δ18OLW and δ18O values of carbohydrates and cellulose in grasses were strongly related, indicating that the leaf water signal in carbohydrates was transferred to cellulose (εbio = 25.1‰). Interestingly, damping factor pexpx, which reflects oxygen isotope exchange with less enriched water during cellulose synthesis, responded to rH conditions if modelled from δ18OLW but not if modelled directly from δ18O of individual carbohydrates. We conclude that δ18OLW is not always a good substitute for δ18O of synthesis water due to isotopic leaf water gradients. Thus, compound‐specific δ18O analyses of individual carbohydrates are helpful to better constrain (post‐)photosynthetic isotope fractionation processes in plants.  相似文献   

10.
Offspring fitness generally improves with increasing egg size. Yet, eggs of most aquatic organisms are small. A common but largely untested assumption is that larger embryos require more oxygen than they can acquire through diffusion via the egg surface, constraining egg size evolution. However, we found no detrimental effects of large egg size on embryo growth and survival under hypoxic conditions. We tested this in the broad-nosed pipefish, Syngnathus typhle, whose males provide extensive care (nourishment, osmoregulation and oxygenation) to their young in a brood pouch on their bodies. We took advantage of this species'' pronounced variation in egg size, correlating positively with female size, and tested the effect of hypoxia (40% dissolved oxygen) versus fully oxygenated (100%) water on embryo size and survival of large versus small eggs after 18 days of paternal brooding. Egg size did not affect embryo survival, regardless of O2 treatment. While hypoxia affected embryo size negatively, both large and small eggs showed similar reductions in growth. Males in hypoxia ventilated more and males with large eggs swam more, but neither treatment affected their position in the water column. Overall, our results call into question the most common explanation for constrained egg size evolution in aquatic environments.  相似文献   

11.
Kim, J.K., Khim, B.‐K., Woo, K.S., & Yoon, S.H. 2009: Records of palaeo‐seawater condition from oxygen‐isotope profiles of early Pleistocene fossil molluscs from the Seoguipo Formation (Korea). Lethaia, Vol. 43, pp. 170–181. High‐resolution δ18O profiles of early Pleistocene fossil molluscs (Mizuhopecten tokyoensis hokurikuensis) from the shallow‐marine sedimentary Seoguipo Formation (Korea) were found to exhibit distinct annual cycles identified by their unique seasonality (δ18O amplitude). A direct comparison of fossil δ18O profiles with that of living shells (Amusium japonicum japonicumi) indicated that the palaeoseawater conditions differed from present‐day seawater. Specifically, the positive δ18O shift in the isotope profile of the fossil specimens relative to that of the living mollusc shell reflected that palaeotemperature was lower than that today. However, a comparison of the coldest palaeotemperatures (determined from the heaviest δ18O values of fossil shells), with the present‐day winter temperatures indicates that temperature variation alone cannot account for the entire positive δ18O offset. These findings indicate that variation in the seawater δ18Ow values plays a dominant role in the biogenic carbonate precipitation of fossils. Thus, the fossil shells obtained from stratigraphic units suggest different palaeoenvironmental conditions, including lower temperatures and 18O‐enriched glacial seawater, when compared with the present‐day conditions. The Seoguipo Formation records at least five cycles of relative sea‐level fluctuations, with changes in fossil δ18O amplitudes separated by sequence boundaries likely to reflect variations of unique palaeoseawater condition, although the oxygen‐isotope profile of fossil molluscs appears to provide a snap‐shot of the palaeoclimatic signature. □Early Pleistocene, mollusc fossils, oxygen isotope, palaeoenvironment, seawater temperature.  相似文献   

12.
Organic and inorganic stable isotopes of lacustrine carbonate sediments are commonly used in reconstructions of ancient terrestrial ecosystems and environments. Microbial activity and local hydrological inputs can alter porewater chemistry (e.g., pH, alkalinity) and isotopic composition (e.g., δ18Owater, δ13CDIC), which in turn has the potential to impact the stable isotopic compositions recorded and preserved in lithified carbonate. The fingerprint these syngenetic processes have on lacustrine carbonate facies is yet unknown, however, and thus, reconstructions based on stable isotopes may misinterpret diagenetic records as broader climate signals. Here, we characterize geochemical and stable isotopic variability of carbonate minerals, organic matter, and water within one modern lake that has known microbial influences (e.g., microbial mats and microbialite carbonate) and combine these data with the context provided by 16S rRNA amplicon sequencing community profiles. Specifically, we measure oxygen, carbon, and clumped isotopic compositions of carbonate sediments (δ18Ocarb, δ13Ccarb, ?47), as well as carbon isotopic compositions of bulk organic matter (δ13Corg) and dissolved inorganic carbon (DIC; δ13CDIC) of lake and porewater in Great Salt Lake, Utah from five sites and three seasons. We find that facies equivalent to ooid grainstones provide time‐averaged records of lake chemistry that reflect minimal alteration by microbial activity, whereas microbialite, intraclasts, and carbonate mud show greater alteration by local microbial influence and hydrology. Further, we find at least one occurrence of ?47 isotopic disequilibrium likely driven by local microbial metabolism during authigenic carbonate precipitation. The remainder of the carbonate materials (primarily ooids, grain coatings, mud, and intraclasts) yield clumped isotope temperatures (T(?47)), δ18Ocarb, and calculated δ18Owater in isotopic equilibrium with ambient water and temperature at the time and site of carbonate precipitation. Our findings suggest that it is possible and necessary to leverage diverse carbonate facies across one sedimentary horizon to reconstruct regional hydroclimate and evaporation–precipitation balance, as well as identify microbially mediated carbonate formation.  相似文献   

13.
The oxygen isotope ratios of tree ring cellulose have a great potential as proxy for the oxygen isotope ratios of source water, which is related to climate. However, source water isotopic signatures can be masked by plant physiological and biochemical processes during cellulose synthesis. To minimize biochemical effects in the recording of source water, we modified the cellulose molecule to phenylglucosazone, which only has oxygen attached to carbon 3–6 (OC3–6) of the cellulose glucose moieties, thus eliminating the oxygen attached to carbon 2 of the cellulose glucose moieties (OC-2). Here we developed a method to use small amounts of inter and intra-annual tree ring cellulose for phenylglucosazone synthesis. Using this new method we tested if the oxygen isotope ratios of source water reconstructed from tree ring phenylglucosazone (δ18OswPG) and the observed source water (δ18Oswobs) would have a better agreement than those reconstructed from the tree ring cellulose molecule. Annual tree ring samples were obtained from Pinus sylvestris (1997–2003) (Finland) and Picea abies (1971–1992) (Switzerland) and intra-annual tree ring samples were obtained from Pinus radiata (October 2004–March 2006) (New Zealand), each near a meteorological station where precipitation and relative humidity (RH) were measured periodically. The δ18O of tree ring cellulose and tree ring phenylglucosazone for each of the three species were then used to back calculate the δ18O of source water according to a previous published empirical equation. As expected, the δ18O of tree ring phenylglucosazone was superior than cellulose in the reconstruction of source water available to the plant. Deviation between δ18OswPG and δ18Oswobs was in part correlated with variation in atmospheric relative humidity (RH) which was not observed for the cellulose molecule. We conclude that this new method can be applicable to inter and intra-annual tree ring studies and that the use of the tree ring phenylglucosazone will significantly improve the quality of paleoclimate studies.  相似文献   

14.
We examined the isotopic parameters in two C3 species (Artemisia diffusa H. Krasch and Tamarix hispida Willd.) and a C4 species [Haloxylon aphyllum (Minkw.) Iljin.] growing or planted in soils with different levels of salinity in a Central Asian desert. The oxygen isotope ratios of stem water (δ18Ostem) in T. hispida and H. aphyllum distributed in high-salinity zones were similar to the δ18O of artesian water (δ18Oartesian) and different from that in A. diffusa distributed in lower-salinity zones. This indicates that T. hispida and H. aphyllum depend on water with low salinity in the deeper soil layer, whereas A. diffusa depends on water in the shallower soil layer that would be affected by salt accumulation. The carbon isotope composition of leaf organic matter (δ13Com) and oxygen isotope enrichment in leaf organic matter above stem water (Δ18Oom) were lower in A. diffusa than in the other species. The responses of δ13Com and Δ18Oom to soil salinity observed for T. hispida suggest that the species decreased its transpiration rate and increased its intrinsic water-use efficiency in response to increasing soil salinity. The δ13Com and Δ18Oom of H. aphyllum were higher than those of the C3 species, and were not correlated with soil salinity, suggesting that H. aphyllum reduced its salt uptake by decreasing transpiration—even though it was able to access less saline water in the deeper soil layer. These results indicate that the water-use strategy of desert plants in high-salinity environments can be assessed based on their carbon and oxygen isotope ratios.  相似文献   

15.
Stable oxygen isotope ratio of leaf water (δ18OL) yields valuable information on many aspects of plant–environment interactions. However, current understanding of the mechanistic controls on δ18OL does not provide complete characterization of effective path length (L) of the Péclet effect, – a key component of the leaf water model. In this study, we collected diurnal and seasonal series of leaf water enrichment and estimated L in six field‐grown angiosperm and gymnosperm tree species. Our results suggest a pivotal role of leaf transpiration rate (E) in driving both within‐ and across‐species variations in L. Our observation of the common presence of an inverse scaling of L with E in the different species therefore cautions against (1) the conventional treatment of L as a species‐specific constant in leaf water or cellulose isotope (δ18Op) modelling; and (2) the use of δ18Op as a proxy for gs or E under low E conditions. Further, we show that incorporation of a multi‐species LE scaling into the leaf water model has the potential to both improve the prediction accuracy and simplify parameterization of the model when compared with the conventional approach. This has important implications for future modelling of oxygen isotope ratios.  相似文献   

16.
The oxygen isotope signature of sulphate (δ18Osulphate) is increasingly used to study nutritional fluxes and sulphur transformation processes in a variety of natural environments. However, mechanisms controlling the δ18Osulphate signature in soil–plant systems are largely unknown. The objective of this study was to determine key factors, which affect δ18Osulphate values in soil and plants. The impact of an 18O‐water isotopic gradient and different types of fertilizers was investigated in a soil incubation study and a radish (Raphanus sativus L.) greenhouse growth experiment. Water provided 31–64% of oxygen atoms in soil sulphate formed via mineralization of organic residues (green and chicken manures) while 49% of oxygen atoms were derived from water during oxidation of elemental sulphur. In contrast, δ18Osulphate values of synthetic fertilizer were not affected by soil water. Correlations between soil and plant δ18Osulphate values were controlled by water δ18O values and fertilizer treatments. Additionally, plant δ34S data showed that the sulphate isotopic composition of plants is a function of S assimilation. This study documents the potential of using compound‐specific isotope ratio analysis for investigating and tracing fertilization strategies in agricultural and environmental studies.  相似文献   

17.
We present new data of oxygen isotopes in marine sulfate (δ18OSO4) in pore fluid profiles through organic‐rich deep‐sea sediments from 11 ODP sites around the world. In almost all sites studied sulfate is depleted with depth, through both organic matter oxidation and anaerobic methane oxidation. The δ18OSO4 increases rapidly near the top of the sediments, from seawater values of 9 to maxima between 22 and 25, and remains isotopically heavy and constant at these values with depth. The δ18OSO4 in these pore fluid profiles is decoupled from variations in sulfur isotopes measured on the same sulfate samples (δ34SSO4); the δ34SSO4 increases continuously with depth and exhibits a shallower isotopic increase. This isotopic decoupling between the δ34SSO4 and the δ18OSO4 is hard to reconcile with the traditional understanding of bacterial sulfate reduction in sediments. Our data support the idea that sulfate or sulfite and water isotopically exchange during sulfate reduction and that some of the isotopically altered sulfur pool returns to the environment. We calculate that the rapid increase in the δ18OSO4 in the upper part of these sediments requires rates of this oxygen isotope exchange that are several orders of magnitude higher than the rates of net sulfate reduction calculated from the sulfate concentration profiles and supported by the δ34SSO4. We suggest several mechanisms by which this may occur, including ‘net‐zero’ sulfur cycling, as well as further experiments through which we can test and resolve these processes.  相似文献   

18.
Seasonal variation in δ13C and δ18O of cellulose (δ13Cc and δ18Oc) was measured within two annual rings of Pinus radiata growing at three sites in New Zealand. In general, both δ13Cc and δ18Oc increased to a peak over summer. The three sites differed markedly in annual water balance, and these differences were reflected in δ13Cc and δ18Oc. Average δ13Cc and δ18Oc from each site were positively related, so that the driest site had the most enriched cellulose. δ13Cc and δ18Oc were also related within each site, although both the slope and the closeness of fit of the relationship varied between sites. Supporting the theory, the site with the lowest average relative humidity also had the greatest change in δ18Oc‰ change in δ13Cc. Specific climatic events, such as drought or high rainfall, were recorded as a peak or a trough in enrichment, respectively. These results suggest that seasonal and between‐site variation in δ13Cc and δ18Oc are driven by the interaction between variation in climatic conditions and soil water availability, and plant response to this variation.  相似文献   

19.
Few annually dated stable isotope records exist across Oceania. Stable carbon and oxygen isotope ratios have the potential to enhance climate reconstructions currently reliant on tree ring width chronologies. The purpose of this study is to explore the sources of variability in a stable oxygen isotope chronology derived from A. selaginoides from Mount Read, Tasmania. This high elevation site receives abundant rainfall throughout the year and is ∼130 km from the Global Network of Isotopes in Precipitation (GNIP) site at Cape Grim. We crossdated 10 new tree core samples against an existing ring width chronology (954–2011 CE) and analyzed the δ18O from the individual rings for the period 1960–2018. Using high resolution (0.25 degrees) climate data and ECMWF ERA5 reanalysis data, we disentangled the effects of local climate and source region on the isotopic signatures recorded in the annual rings. In addition, we used HYSPLIT backward trajectory analysis to characterize the source region of precipitation to Mount Read and whether the source region has influence over the δ18OTR series. Median δ18OTR (n = 10) is correlated with local temperature and vapor pressure deficit in the early growing season. In addition, spatial correlations reveal that median δ18OTR is positively correlated with temperature and negatively correlated with precipitation in the source region. However, measurements of δ18OTR exhibit high inter-tree variation, particularly between 1960 and 1990. Our results indicate that this δ18OTR proxy may provide additional information about past moisture conditions during the growing season, potentially contributing to more robust reconstructions of the Southern Hemisphere climate dynamics; however, additional sampling may be necessary to resolve inter-tree variation in δ18OTR.  相似文献   

20.
Certainty regarding the degree to which organic molecules exchange oxygen with local water during plant cellulose synthesis (pex) is necessary for cellulose oxygen isotope (δ18Ocell)‐based applications in environmental and ecological studies. However, the currently accepted notion that pex is a constant of ca. 0.42 appears inconsistent with biochemical theory, which predicts that marked variation may be present in pex, in relation to variation in the turnover time (τ) of the carbohydrate pool available for cellulose synthesis. The above prediction was tested in the present study with the analysis of data collected from leaves of Ricinus communis grown in controlled environmental conditions that varied in light intensity and vapour pressure deficit. The results revealed the existence of considerable variation in both pex and τ across plants in the various growth environments. Moreover, despite uncertainties in estimates of the proportion of source water in the synthesis water (px) and of the biochemical fractionation factor (εo), our experiment yielded strong evidence that pex exhibits a significant, positive relationship with τ, consistent with biochemical theory. The observed variation in pex in association with τ has important implications for the interpretation of δ18Ocell data in environmental/ecological studies.  相似文献   

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