Assessing environmental and physiological controls over water relations in a Scots pine (Pinus sylvestris L.) stand through analyses of stable isotope composition of water and organic matter

This study investigated the influence of meteorological, pedospheric and physiological factors on the water relations of Scots pine, as characterized by the origin of water taken up, by xylem transport as well as by carbon isotope discrimination (Delta13C) and oxygen isotope enrichment (Delta18O) of newly assimilated organic matter. For more than 1 year, we quantified delta2H and delta18O of potential water sources and xylem water as well as Delta13C and Delta18O in twig and trunk phloem organic matter biweekly, and related these values to continuously measured or modelled meteorological parameters, soil water content, stand transpiration (ST) and canopy stomatal conductance (G(s)). During the growing season, delta18O and delta2H of xylem water were generally in a range comparable to soil water from a depth of 2-20 cm. Long residence time of water in the tracheids uncoupled the isotopic signals of xylem and soil water in winter. Delta18O but not Delta13C in phloem organic matter was directly indicative of recent environmental conditions during the whole year. Delta18O could be described applying a model that included 18O fractionation associated with water exchange between leaf and atmosphere, and with the production of organic matter as well as the influence of transpiration. Phloem Delta13C was assumed to be concertedly influenced by G(s) and photosynthetically active radiation (PAR) (as a proxy for photosynthetic capacity). We conclude that isotope signatures can be used as effective tools (1) to characterize the seasonal dynamics in source and xylem water, and (2) to assess environmental effects on transpiration and G(s) of Scots pine, thus helping to understand and predict potential impacts of climate change on trees and forest ecosystems.     

Evaporative enrichment and time lags between delta18O of leaf water and organic pools in a pine stand

Understanding ecosystem water fluxes has gained increasing attention, as climate scenarios predict a drier environment for many parts of the world. Evaporative enrichment of (18)O (Delta(18)O) of leaf water and subsequent enrichment of plant organic matter can be used to characterize environmental and physiological factors that control evaporation, based on a recently established mechanistic model. In a Pinus sylvestris forest, we measured the dynamics of oxygen isotopic composition (delta(18)O) every 6 h for 4 d in atmospheric water vapour, xylem sap, leaf water and water-soluble organic matter in current (N) and previous year (N-1) needles, phloem sap, together with leaf gas exchange for pooled N and N-1 needles, and relevant micrometeorological variables. Leaf water delta(18)O showed strong diel periodicity, while delta(18)O in atmospheric water vapour and in xylem sap showed little variation. The Delta(18)O was consistently lower for N than for N-1 needles, possibly related to phenological stage. Modelled leaf water Delta(18)O showed good agreement with measured values when applying a non-steady state evaporative enrichment model including a Péclet effect. We determined the time lags between delta(18)O signals from leaf water to water-soluble foliar organic matter and to phloem sap at different locations down the trunk, which clearly demonstrated the relevance of considering these time-lag effects for carbon transport, source-sink and carbon flux partitioning studies.     

Life form-specific variations in leaf water oxygen-18 enrichment in Amazonian vegetation

Leaf water (18)O enrichment (Delta(o)) influences the isotopic composition of both gas exchange and organic matter, with Delta(o) values responding to changes in atmospheric parameters. In order to examine possible influences of plant parameters on Delta(o) dynamics, we measured oxygen isotope ratios (delta(18)O) of leaf and stem water on plant species representing different life forms in Amazonia forest and pasture ecosystems. We conducted two field experiments: one in March (wet season) and another in September (dry season) 2004. In each experiment, leaf and stem samples were collected at 2-h intervals at night and hourly during the day for 50 h from eight species including upper-canopy forest trees, upper-canopy forest lianas, and lower-canopy forest trees, a C(4) pasture grass and a C(3) pasture shrub. Significant life form-related differences were detected in (18)O leaf water values. Initial modeling efforts to explain these observations over-predicted nighttime Delta(o) values by as much as 10 per thousand. Across all species, errors associated with measured values of the delta(18)O of atmospheric water vapor (delta(v)) appeared to be largely responsible for the over-predictions of nighttime Delta(o) observations. We could not eliminate collection or storage of water vapor samples as a possible error and therefore developed an alternative, plant-based method for estimating the daily average delta(v) value in the absence of direct (reliable) measurements. This approach differs from the common assumption that isotopic equilibrium exists between water vapor and precipitation water, by including transpiration-based contributions from local vegetation through (18)O measurements of bulk leaf water. Inclusion of both modified delta(v) and non-steady state features resulted in model predictions that more reliably predicted both the magnitude and temporal patterns observed in the data. The influence of life form-specific patterns of Delta(o) was incorporated through changes in the effective path length, an important but little known parameter associated with the Péclet effect.     

Non-steady state effects in diurnal 180 discrimination by Picea sitchensis branches in the field

We report diurnal variations in 18O discrimination (18 delta) during photosynthesis (18 delta A) and respiration (18 delta R) of Picea sitchensis branches measured in branch chambers in the field. These observations were compared with predicted 18 delta (18 delta pred) based on concurrent measurements of branch gas exchange to evaluate steady state and non-steady state (NSS) models of foliage water 18O enrichment for predicting the impact of this ecosystem on the Delta 18O of atmospheric CO2. The non-steady state approach substantially improved the agreement between 18 delta pred and observed 18 delta (18 delta obs) compared with the assumption of isotopic steady state (ISS) for the Delta 18O signature of foliage water. In addition, we found direct observational evidence for NSS effects: extremely high apparent 18 delta values at dusk, dawn and during nocturnal respiration. Our experiments also show the importance of bidirectional foliage gas exchange at night (isotopic equilibration in addition to the net flux). Taken together, neglecting these effects leads to an underestimation of daily net canopy isofluxes from this forest by up to 30%. We expect NSS effects to be most pronounced in species with high specific leaf water content such as conifers and when stomata are open at night or when there is high relative humidity, and we suggest modifications to ecosystem and global models of delta 18O of CO2.     

A transgenic approach to understanding the influence of carbonic anhydrase on C18OO discrimination during C4 photosynthesis

The oxygen isotope composition of atmospheric CO(2) is an important signal that helps distinguish between ecosystem photosynthetic and respiratory processes. In C(4) plants the carbonic anhydrase (CA)-catalyzed interconversion of CO(2) and bicarbonate (HCO(3)(-)) is an essential first reaction for C(4) photosynthesis but also plays an important role in the CO(2)-H(2)O exchange of oxygen as it enhances the rate of isotopic equilibrium between CO(2) and water. The C(4) dicot Flaveria bidentis containing genetically reduced levels of leaf CA (CA(leaf)) has been used to test whether changing leaf CA activity influences online measurements of C(18)OO discrimination (Delta(18)O) and the proportion of CO(2) in isotopic equilibrium with leaf water at the site of oxygen exchange (theta). The Delta(18)O in wild-type F. bidentis, which contains high levels of CA relative to the rates of net CO(2) assimilation, was less than predicted by models of Delta(18)O. Additionally, Delta(18)O was sensitive to small decreases in CA(leaf). However, reduced CA activity in F. bidentis had little effect on net CO(2) assimilation, transpiration rates (E), and stomatal conductance (g(s)) until CA levels were less than 20% of wild type. The values of theta determined from measurements of Delta(18)O and the (18)O isotopic composition of leaf water at the site of evaporation (delta(e)) were low in the wild-type F. bidentis and decreased in transgenic plants with reduced levels of CA activity. Measured values of theta were always significantly lower than the values of theta predicted from in vitro CA activity and gas exchange. The data presented here indicates that CA content in a C(4) leaf may not represent the CA activity associated with the CO(2)-H(2)O oxygen exchange and therefore may not be a good predictor of theta during C(4) photosynthesis. Furthermore, uncertainties in the isotopic composition of water at the site of exchange may also limit the ability to accurately predict theta in C(4) plants.     

C4 photosynthetic isotope exchange in NAD-ME- and NADP-ME-type grasses

Monitoring photosynthetic isotope exchange is an important tool for predicting the influence of plant communities on the global carbon cycle in response to climate change. C(4) grasses play an important role in the global carbon cycle, but their contribution to the isotopic composition of atmospheric CO(2) is not well understood. Instantaneous measurements of (13)CO(2) (Delta(13)C) and C(18)OO (Delta(18)O) isotope exchange in five NAD-ME and seven NADP-ME C(4) grasses have been conducted to investigate the difference in photosynthetic CO(2) isotopic fractionation in these subgroups. As previously reported, the isotope composition of the leaf material (delta(13)C) was depleted in (13)C in the NAD-ME compared with the NADP-ME grasses. However, Delta(13)C was not different between subtypes at high light, and, although Delta(13)C increased at low light, it did so similarly in both subtypes. This suggests that differences in leaf delta(13)C between the C(4) subtypes are not caused by photosynthetic isotope fractionation and leaf delta(13)C is not a good indicator of bundle sheath leakiness. Additionally, low carbonic anhydrase (CA) in C(4) grasses may influences Delta(13)C and should be considered when estimating the contribution of C(4) grasses to the global isotopic signature of atmospheric CO(2). It was found that measured Delta(18)O values were lower than those predicted from leaf CA activities and Delta(18)O was similar in all species measured. The Delta(18)O in these C(4) grasses is similar to low Delta(18)O previously measured in C(4) dicots which contain 2.5 times the leaf CA activity, suggesting that leaf CA activity is not a predictor of Delta(18)O in C(4) plants.     

Oxygen isotope enrichment of organic matter in Ricinus communis during the diel course and as affected by assimilate transport

The oxygen isotope composition in leaf water and organic compounds in different plant tissues is useful for assessing the physiological performance of plants in their environment, but more information is needed on Delta(18)O variation during a diel course. Here, we assessed Delta(18)O of the organic matter in leaves, phloem and xylem in stem segments, and fine roots of Ricinus communis during a full diel cycle. Enrichment of newly assimilated organic matter in equilibrium with leaf water was calculated by applying a nonsteady-state evaporative enrichment model. During the light period, Delta(18)O of the water soluble organic matter pool in leaves and phloem could be explained by a 27 per thousand enrichment compared with leaf water enrichment. Leaf water enrichment influenced Delta(18)O of phloem organic matter during the night via daytime starch synthesis and night-time starch remobilization. Phloem transport did not affect Delta(18)O of phloem organic matter. Diel variation in Delta(18)O in organic matter pools can be modeled, and oxygen isotopic information is not biased during transport through the plant. These findings will aid field studies that characterize environmental influences on plant water balance using Delta(18)O in phloem organic matter or tree rings.     

Internal conductance to CO(2) diffusion and C(18)OO discrimination in C(3) leaves

(18)O discrimination in CO(2) stems from the oxygen exchange between (18)O-enriched water and CO(2) in the chloroplast, a process catalyzed by carbonic anhydrase (CA). A proportion of this (18)O-labeled CO(2) escapes back to the atmosphere, resulting in an effective discrimination against C(18)OO during photosynthesis (Delta(18)O). By constraining the delta(18)O of chloroplast water (delta(e)) by analysis of transpired water and the extent of CO(2)-H(2)O isotopic equilibrium (theta(eq)) by measurements of CA activity (theta(eq) = 0.75-1.0 for tobacco, soybean, and oak), we could apply measured Delta(18)O in a leaf cuvette attached to a mass spectrometer to derive the CO(2) concentration at the physical limit of CA activity, i.e. the chloroplast surface (c(cs)). From the CO(2) drawdown sequence between stomatal cavities from gas exchange (c(i)), from Delta(18)O (c(cs)), and at Rubisco sites from Delta(13)C (c(c)), the internal CO(2) conductance (g(i)) was partitioned into cell wall (g(w)) and chloroplast (g(ch)) components. The results indicated that g(ch) is variable (0.42-1.13 mol m(-2) s(-1)) and proportional to CA activity. We suggest that the influence of CA activity on the CO(2) assimilation rate should be important mainly in plants with low internal conductances.     

Determination of the relative uptake of ground vs. surface water by Populus deltoides during phytoremediation

The use of plants to remediate polluted groundwater is becoming an attractive alternative to more expensive traditional techniques. In order to adequately assess the effectiveness of the phytoremediation treatment, a clear understanding of water-use habits by the selected plant species is essential. We examined the relative uptake of surface water (i.e., precipitation) vs. groundwater by mature Populus deltoides by applying irrigation water at a rate equivalent to a 5-cm rain event. We used stable isotopes of hydrogen (D) and oxygen (18O) to identify groundwater and surface water (irrigation water) in the xylem sap water. Pretreatment isotopic ratios of both deuterium and 18O, ranked from heaviest to lightest, were irrigation water > groundwater > xylem sap. The discrepancy in preirrigation isotopic signatures between groundwater and xylem sap suggests that in the absence of a surface source of water (i.e., between rain events) there is an unknown amount of water being extracted from sources other than groundwater (i.e., soil surface water). We examined changes in volumetric soil water content (%), total hourly sapflux rates, and trichloroethene (TCE) concentrations. Following the irrigation treatment, volumetric soil water increased by 86% and sapflux increased by as much as 61%. Isotopic signatures of the xylem sap became substantially heavier following irrigation, suggesting that the applied irrigation water was quickly taken up by the plants. TCE concentrations in the xylem sap were diluted by an average of 21% following irrigation; however, dilution was low relative to the increase in sapflux. Our results show that water use by Populus deltoides is variable. Hence, studies addressing phytoremediation effectiveness must account for the relative proportion of surface vs. groundwater uptake.     

Understanding plant drought resistance in a Mediterranean coastal sand dune ecosystem: differences between native and exotic invasive species

Aims Mediterranean coastal dunes are habitats of great conservation interest, with a distinctive and rich flora. In the last century, Acacia spp., native from Australia, have been introduced in Portugal, with the objective of stabilizing sand dunes, and since have become dominant in numerous sand dune habitats. This invasion process led to the reduction of native plant species richness, changed soil characteristics and modified habitat's microclimatic characteristics. The aim of this research was to typify and compare, in Mediterranean sand dune ecosystems, the ecophysiological responses to drought of Helichrysum italicum and Corema album, two native species, and Acacia longifolia, an exotic invasive species. We addressed the following specific objectives: (i) to compare water relations and water use efficiencies, (ii) to evaluate water stress, (iii) to assess water use strategies and water sources used by plants and (iv) to evaluate the morphological adaptations at leaf and phyllode level. Methods In order to obtain an integrative view of ecophysiological patterns, water relations and performance measuring methods have been applied: predawn (ψ PD) and midday (ψ MD) water potential, chlorophyll a fluorescence, oxygen isotopic composition of xylem, rain and groundwater (δ 18 O) and leaf carbon isotopic discrimination (Δ 13 C). The leaf characteristics of the three species, as well as the histochemistry of non-glandular trichome cell walls, were also studied to identify morpho-traits related to drought resistance.Important findings The results support our initial hypothesis: although A. longifolia clearly possesses a degree of resistance to water stress, such ability is provided by a different water strategy, when compared to native species. Natives relied on morphological adaptations to restrict water loss, whereas the invasive species adjusted the water uptake as a way to balance their limited ability of restricting water loss. We corroborate that woody native species (i) have a conservative water-saving strategy and minor seasonal variations relative to invasive species, (ii) use enriched water sources during drought periods, indicating different water sources and root systems comparing with invasive species and (iii) present drought leaf morpho-functional adaptations related with limiting water loss. Comparing the physiological performance of invasive and native species can offer causal explanations for the relative success of alien plant invasions on sand dunes ecosystems.     

Isotopes reveal contrasting water use strategies among coexisting plant species in a Mediterranean ecosystem

Variation in the stable carbon and oxygen isotope composition (δ(13) C, Δ(18) O) of co-occurring plant species may reflect the functional diversity of water use strategies present in natural plant communities. We investigated the patterns of water use among 10 coexisting plant species representing diverse taxonomic groups and life forms in semiarid southeast Spain by measuring their leaf δ(13) C and Δ(18) O, the oxygen isotope ratio of stem water and leaf gas exchange rates. Across species, Δ(18) O was tightly negatively correlated with stomatal conductance (g(s) ), whereas δ(13) C was positively correlated with intrinsic water use efficiency (WUE(i) ). Broad interspecific variation in Δ(18) O, δ(13) C and WUE(i) was largely determined by differences in g(s) , as indicated by a strong positive correlation between leaf δ(13) C and Δ(18) O across species The 10 co-occurring species segregated along a continuous ecophysiological gradient defined by their leaf δ(13) C and Δ(18) O, thus revealing a wide spectrum of stomatal regulation intensity and contrasting water use strategies ranging from 'profligate/opportunistic' (high g(s) , low WUE(i) ) to 'conservative' (low g(s) , high WUE(i) ). Coexisting species maintained their relative isotopic rankings in 2?yr with contrasting rainfall, suggesting the existence of species-specific 'isotopic niches' that reflect ecophysiological niche segregation in dryland plant communities.     

Effects of environmental parameters, leaf physiological properties and leaf water relations on leaf water delta18O enrichment in different Eucalyptus species

Stable oxygen isotope ratios (delta18O) have become a valuable tool in the plant and ecosystem sciences. The interpretation of delta18O values in plant material is, however, still complicated owing to the complex interactions among factors that influence leaf water enrichment. This study investigated the interplay among environmental parameters, leaf physiological properties and leaf water relations as drivers of the isotopic enrichment of leaf water across 17 Eucalyptus species growing in a common garden. We observed large differences in maximum daily leaf water delta18O across the 17 species. By fitting different leaf water models to these empirical data, we determined that differences in leaf water delta18O across species are largely explained by variation in the Péclet effect across species. Our analyses also revealed that species-specific differences in transpiration do not explain the observed differences in delta18O while the unconstrained fitting parameter 'effective path length' (L) was highly correlated with delta18O. None of the leaf morphological or leaf water related parameters we quantified in this study correlated with the L values we determined even though L was typically interpreted as a leaf morphological/anatomical property. A sensitivity analysis supported the importance of L for explaining the variability in leaf water delta18O across different species. Our investigation highlighted the importance of future studies to quantify the leaf properties that influence L. Obtaining such information will significantly improve our understanding of what ultimately determines the delta18O values of leaf water across different plant species.     

A new measurement technique reveals temporal variation in delta18O of leaf-respired CO2

The oxygen isotope composition of CO(2) respired by Ricinus communis leaves (delta(18)O(R)) was measured under non-steady-state conditions with a temporal resolution of 3 min using a tunable diode laser (TDL) absorption spectrometer coupled to a portable gas exchange system. The SD of delta(18)O measurement by the TDL was +/- 0.2 per thousand and close to that of traditional mass spectrometers. Further, delta(18)O(R) values at isotopic steady state were comparable to those obtained using traditional flask sampling and mass spectrometric techniques for R. communis grown and measured in similar environmental conditions. As well as higher temporal resolution, the online TDL method described here has a number of advantages over mass spectrometric techniques. At isotopic steady state among plants grown at high light, the "one-way flux" model was required to accurately predict delta(18)O(R). A comparison of measurements and the model suggests that plants grown under low-light conditions have either a lower proportion of chloroplast CO(2) that isotopically equilibrates with chloroplast water, or more enriched delta(18)O of CO(2) in the chloroplast that has not equilibrated with local water. The high temporal resolution of isotopic measurements allowed the first measurements of delta(18)O(R) when stomatal conductance was rapidly changing. Under non-steady-state conditions, delta(18)O(R) varied between 50 and 220 per thousand for leaves of plants grown under different light and water environments, and varied by as much as 100 per thousand within 10 min for a single leaf. Stomatal conductance ranged from 0.001 to 1.586 mol m(-2) s(-1), and had an important influence on delta(18)O(R) under non-steady-state conditions not only via effects on leaf water H(2) (18)O enrichment, but also via effects on the rate of the one-way fluxes of CO(2) into and out of the leaf.     

18O pattern and biosynthesis of natural plant products

Oxygen atoms in plant products originate from CO(2), H(2)O and O(2), precursors with quite different delta18O values. Furthermore their incorporation by different reactions implies isotope effects. On this base the resulting non-statistical 18O distributions in natural compounds are discussed. The delta18O value of cellulose is correlated to that of the leaf water, and the observed 18O enrichment (approximately +27 per thousand) is generally attributed to an equilibrium isotope effect between carbonyl groups and water. However, as soluble and heterotrophically synthesised carbohydrates show other correlations, a non-statistical 18O distribution - originating from individual biosynthetic reactions - is postulated for carbohydrates. Similarly, the delta18O values of organic acids, carbonyl compounds, alcohols and esters indicate water-correlated, but individual 18O abundances (e.g. O from acyl groups approximately +19% above water), depending upon origin and biosyntheses. Alcoholic groups introduced by monooxygenase reactions, e.g. in sterols and phenols, show delta18O values near +5 per thousand, in agreement with an assumed isotope fractionation factor of approximately 1.02 on the reaction with atmospheric oxygen (delta18O=+23.5 per thousand). Correspondingly, a "thermodynamically ordered isotope distribution" is only observed for oxygen in some functional groups correlated to an origin from CO(2) and H(2)O, not from O(2). The individual isotopic increments of functional groups permit the prediction of global delta18O values of natural compounds on the basis of their biosynthesis.     

Water sources of dominant species in three alpine ecosystems on the Tibetan Plateau, China

Plant water sources were estimated by two or three compartment linear mixing models using hydrogen and oxygen isotope （δD and δ^18O） values of different components such as plant xylem water, precipitation and river water as well as soil water on the Tibetan Plateau in the summer of 2005. Four dominant species （Quercus aquifolioides, Pinus tabulaeformis, Salix rehderiana and Nitraria tangutorum） in three typical ecosystems （forest, shrub and desert） were investigated in this study. Stable isotope ratios of the summer precipitations and the soil water presented variations in spatial and temporal scales. δ^18O values of N. tangutorum xylem water were constant in the whole growth season and very similar to those of deep soil water. Water sources for all of the plants came from both precipitations and soil water. Plants switched rapidly among different water sources when environmental water conditions changed. Rainwater had different contributions to the plants, which was influenced by amounts of precipitation. The percentage of plant xylem water derived from rainwater rose with an increase in precipitation. Water sources for broad-leaved and coniferous species were different although they grew in the same environmental conditions. For example, the broad-leaved species Q. aquifolioides used mainly the water from deep soil, while 92.5% of xylem water of the coniferous species P. tabulaeformis was derived from rainwater during the growth season. The study will be helpful for us to fully understand responses of species on the Tibetan Plateau to changes in precipitation patterns, and to assess accurately changes of vegetation distribution in the future.     

降雨对高寒沙地不同林龄中间锦鸡儿水分利用特征的影响

为明确不同林龄中间锦鸡儿水分来源对降雨的响应,利用稳定同位素技术测定青海共和盆地不同林龄的中间锦鸡儿(4 a、9 a、17 a和31 a)在降雨前后土壤水、木质部水、地下水和雨水的δ²H、δ¹⁸O组成,运用Iso-Source模型计算植物对各潜在水源的利用比例。结果表明: 各林龄中间锦鸡儿的浅层(0～40 cm)土壤水δ²H、δ¹⁸O组成对降雨的响应最明显;降雨前后各林龄中间锦鸡儿的木质部水、土壤水分和地下水同位素值均位于当地大气降水线(LMWL)的右下侧,其截距和斜率均小于LMWL和全球大气降水线(GMWL),且降雨后中间锦鸡儿的木质部水和土壤水的同位素组成更趋近LMWL;降雨前,4 a和9 a中间锦鸡儿主要利用浅层土壤水,17 a中间锦鸡儿主要利用中层(40～90 cm)土壤水,31 a中间锦鸡儿主要利用深层土壤水;降雨后,各林龄中间锦鸡儿的吸水层位均为降雨补充的浅层土壤水;各林龄对地下水的利用率仅有1.8%～11.9%。说明不同林龄中间锦鸡儿的水分来源对降雨的响应方式相同,均优先利用降雨补充的浅层土壤水,对地下水的利用减少。     

Oxygen isotope composition of CAM and C3 Clusia species: non-steady-state dynamics control leaf water 18O enrichment in succulent leaves

Leaf gas exchange and leaf water (18)O enrichment (Delta(18)O(L)) were measured in three Clusia species under field conditions during dry and wet seasons and in Miconia argentea during the dry season in the Republic of Panama. During the dry season, all three Clusia species used crassulacean acid metabolism (CAM); during the wet season Clusia pratensis operated in the C(3) mode, while Clusia uvitana and Clusia rosea used CAM. Large departures from isotopic steady state were observed in daytime Delta(18)O(L) of the Clusia species, especially during the dry season. In contrast, daytime Delta(18)O(L) was near isotopic steady state in the C(3) tree M. argentea. Across the full data set, non-steady-state predictions explained 49% of variation in observed Delta(18)O(L), whereas steady-state predictions explained only 14%. During the wet season, when Delta(18)O(L) could be compared with Clusia individuals operating in both C(3) and CAM modes, steady-state and non-steady-state models gave contrasting predictions with respect to interspecific variation in daytime Delta(18)O(L). The observed Delta(18)O(L) pattern matched that predicted for the non-steady state. The results provided a clear example of how non-steady-state control of leaf water (18)O dynamics can shift the slope of the relationship between transpiration rate and daytime Delta(18)O(L) from negative to positive.     

Stable isotopic observation of water use sources of Pinus sylvestris var. mongolica in Horqin Sandy Land, China

Since the late 1950s, Mongolian pine (Pinus sylvestris var. mongolica) has been widely planted for vegetation restoration in arid and semi-arid areas in North China. We used stable isotope signals from precipitation, soil water, and xylem water of Mongolian pine trees, which were planted in early 1980s on sand dunes in the east-southern margin of Horqin Sandy Land, to identify water uptake sources of this tree. (1) Stable ¹⁸O isotope composition of the xylem water exhibited little seasonality, suggesting that the trees use a relatively stable water source; (2) the water source of the pine trees primarily came from a soil depth of 20–60 cm (sampling depth up to 60 cm in this study) and the trees might use groundwater when soil moisture became extremely low; and (3) there was not much difference in water sources used by the pine trees grown at the top of the fixed dune and in the inter dune lowland, although these two sites had 3–8 m elevation difference. This study suggests that it is critical for Mongolian pine trees to access relatively reliable and stable water sources to grow in sandy land habitats, and timely recharging of rainwater to the trees’ rooting depth is requisite for avoiding and/or reducing their degradation caused by water shortage.     

Isotopic composition of the delta-18O--delta-13C from the otoliths of reef fish from Taiaro (Tuamotu, French Polynesia): isotopic and biological implications

Nuclei (larval stage) and outer parts (adult stage) of fish otoliths from the Taiaro closed lagoon (French Polynesia) and adjacent ocean have been analysed for the C-O isotopic compositions. delta 18O values of the nuclei of both populations indicate that isotopic equilibrium is reached. This implies that the lagoonal fish population has done its complete biological cycle in the lagoon and represents an adaptation in a closed system. delta 18O values of the outer parts show a slight isotopic disequilibrium (< 0.2@1000) interpreted in term of vital effect. All the delta 13C values exhibit a strong isotopic disequilibrium related to metabolic activity.     

Oxygen and nitrogen isotopic fractionations during human respiration

Both oxygen and nitrogen isotope compositions (delta18O and delta15N) of exhaled air from 10 individuals were measured. Results show linear relations between isotope variation and the fraction of O2 used during the respiration process. The isotopic influence of physiological parameters such as smoking habits, age, haemoglobin count, oxygen fixation rate or physical exercise was assessed. Among them, only smoking habits do not have any effect on delta18O. Delta15N differences between inhaled and exhaled air may indicate an active (but minor) role for nitrogen during the human respiration process. Nevertheless, nitrogen fractionation is homogenous among all the individuals, which is coherent with the fact that nitrogen metabolism is controlled by the intestinal bacterial activity.