Hydrogen and oxygen isotope ratios of tree ring cellulose for field-grown riparian trees

The isotopic composition of tree ring cellulose was obtained over a 2-year period from small-diameter riparian-zone trees at field sites that differed in source water isotopic composition and humidity. The sites were located in Utah (cool and low humidity), Oregon (cool and high humidity), and Arizona (warm and low humidity) with source water isotope ratio values of –125/–15‰ (δD/δ¹⁸O), –48/–6‰, and –67/–7‰, respectively. Monthly environmental measurements included temperature and humidity along with measurements of the isotope ratios in atmospheric water vapor, stream, stem, and leaf water. Small riparian trees used only stream water (both δD and δ¹⁸O of stem and stream water did not differ), but δ values of both atmospheric water vapor and leaf water varied substantially between months. Differences in ambient temperature and humidity conditions between sites contributed to substantial differences in leaf water evaporative enrichment. These leaf water differences resulted in differences in the δD and δ¹⁸O values of tree ring cellulose, indicating that humidity information was recorded in the annual rings of trees. These environmental and isotopic measurements were used to test a mechanistic model of the factors contributing to δD and δ¹⁸O values in tree ring cellulose. The model was tested in two parts: (a) a leaf water model using environmental information to predict leaf water evaporative enrichment and (b) a model describing biochemical fractionation events and isotopic exchange with medium water. The models adequately accounted for field observations of both leaf water and tree ring cellulose, indicating that the model parameterization from controlled experiments was robust even under uncontrolled and variable field conditions. Received: 7 April 1999 / Accepted: 8 December 1999     

Leaf carbon isotope discrimination and nitrogen content for riparian trees along elevational transects

 Leaf carbon isotope discrimination (Δ), seasonal estimates of the leaf-to-air water vapor gradient on a molar basis (ω), and leaf nitrogen contents were examined in three riparian tree species (Populus fremontii, P. angustifolia, and Salix exigua) along elevational transects in northern and southern Utah USA (1500–2670 m and 600–1820 m elevational gradients, respectively). The ω values decreased with elevation for all species along transects. Plants growing at higher elevations exhibited lower Δ values than plants at lower elevations (P. fremontii, 22.9‰ and 19.5‰, respectively; P. angustifolia, 23.2‰ and 19.2‰, respectively; and S.␣exigua, 21.1‰ and 19.1‰, respectively). Leaf nitrogen content increased with elevation for all species, suggesting that photosynthetic capacity at a given intercellular carbon dioxide concentration was greater at higher elevations. Leaf Δ and nitrogen content values were highly correlated, implying that leaves with higher photosynthetic capacities also had lower intercellular carbon dioxide concentrations. No significant interannual differences were detected in carbon isotope discrimination. Received: 25 February 1996 / Accepted: 8 September 1996     

Aerial decay influence on the stable oxygen and carbon isotope ratios in tree ring cellulose

Sub-fossil wood is often affected by the decaying process that introduces uncertainties in the measurement of oxygen and carbon stable isotope composition in cellulose. Although the cellulose stable isotopes are widely used as climatic proxies, our understanding of processes controlling their behavior is very limited. We present here a comparative study of stable oxygen and carbon isotope ratios in tree ring cellulose in decayed and non-decayed wood samples of Swiss stone pine (Pinus cembra) trees. The intra-ring stable isotope variability (around the circumference of a single ring) was between 0.1 and 0.5‰ for δ¹⁸O values and between 0.5 and 1.6‰ for δ¹³C values for both decayed and non-decayed wood. Observed intra-tree δ¹⁸O variability is less than that reported in the literature (0.5–1.5‰), however, for δ¹³C it is larger than the reported values (0.7–1.2‰). The inter-tree variability for non-decayed wood ranges between 1.1 and 2.3‰ for δ¹⁸O values, and between 2 and 4.7‰ for δ¹³C values. The inter-tree differences for δ¹⁸O values are similar to those reported in the literature (1–2‰ for oxygen and 1–3‰ for carbon) but are larger for δ¹³C values. We have found that the differences for δ¹⁸O and δ¹³C values between decayed and non-decayed wood are smaller than the variation among different trees from the same site, suggesting that the decayed wood can be used for isotopic paleoclimate research.     

Limited uptake of summer precipitation by bigtooth maple (Acer grandidentatum Nutt) and Gambel's oak (Quereus gambelii Nutt)

Winter and spring precipitation that saturates to deep soil layers precedes summer droughts in the Intermountain West. Occasional summer convection storms relieve summer drought, but are infrequent and unreliable from year to year, leading to the hypothesis that dominant tree species might not invest limited carbon reserves to surface roots to take up summer precipitation in these regions. We compared the hydrogen (D) and oxygen (¹⁸O) isotope ratios of winter, spring and summer precipitation to that of xylem sap water in Acer grandidentatum and Quercus gambelii, two dominant trees of this region. By this method we could identify water sources utilized throughout the growing season. Xylem D and ¹⁸O values changed significantly when each species leafed-out; this change was not associated with changes in either soil or plant water status (as measured by predawn and midday water potentials). This shift is apparently related increased transpirational flux, which may flush out residual stem water from the previous growing season. D values of xylem sap of both species matched winter precipitation input values throughout most of the summer, indicating a reliance on deep-soil moisture sources throughout the growing season. Mature Q. gambelii did not take up summer precipitation, whereas A. grandidentatum responded slightly to the largest summer rain event. Small trees of both species, particularly A. grandidentatum, showed a limited uptake of summer rains.     

Benefit, cost and water-use efficiency of arbuscular mycorrhizal durum wheat grown under drought stress

 Arbuscular mycorrhizal fungi (AMF) living symbiotically with host plants enhance plant growth by improving the acquisition of mineral nutrients and water relations. This study determined the effects of AMF inoculation on growth, benefit/cost and water-use efficiency (grams dry matter produced per kilogram water evapotranspired) in two durum wheat genotypes (drought sensitive and drought tolerant) under water-stressed and well-watered conditions. Plants were grown in a low-P silty clay (Typic Xerochrept) soil mix in a greenhouse. Shoot and root dry matter (DM) and root AMF colonization were higher for well-watered than for water-stressed plants. The mycorrhizal plants were more water-use efficient than nonmycorrhizal plants. Shoot DM differences between mycorrhizal and nonmycorrhizal plants represent the benefit derived by plants from AMF-root associations. Shoot DM differences between mycorrhizal and nonmycorrhizal plants under similar conditions of water treatment represent the cost to the plant of AMF-root associations. Values of benefit/cost for AMF-root associations were highest when plants were water-stressed and decreased under well-watered conditions. Genotypic differences in calculated costs and benefits were pronounced. Benefit/cost analysis may be helpful in evaluating host plant genotypes in order to optimize efficiencies of AMF symbiosis under different environmental conditions. Accepted: 4 April 1998     

Assessing the effects of nitrogen deposition and climate on carbon isotope discrimination and intrinsic water‐use efficiency of angiosperm and conifer trees under rising CO2 conditions

The objective of this study is to globally assess the effects of atmospheric nitrogen deposition and climate, associated with rising levels of atmospheric CO₂, on the variability of carbon isotope discrimination (Δ¹³C), and intrinsic water‐use efficiency (iWUE) of angiosperm and conifer tree species. Eighty‐nine long‐term isotope tree‐ring chronologies, representing 23 conifer and 13 angiosperm species for 53 sites worldwide, were extracted from the literature, and used to obtain long‐term time series of Δ¹³C and iWUE. Δ¹³C and iWUE were related to the increasing concentration of atmospheric CO₂ over the industrial period (1850–2000) and to the variation of simulated atmospheric nitrogen deposition and climatic variables over the period 1950–2000. We applied generalized additive models and linear mixed‐effects models to predict the effects of climatic variables and nitrogen deposition on Δ¹³C and iWUE. Results showed a declining Δ¹³C trend in the angiosperm and conifer species over the industrial period and a 16.1% increase of iWUE between 1850 and 2000, with no evidence that the increased rate was reduced at higher ambient CO₂ values. The temporal variation in Δ¹³C supported the hypothesis of an active plant mechanism that maintains a constant ratio between intercellular and ambient CO₂ concentrations. We defined linear mixed‐effects models that were effective to describe the variation of Δ¹³C and iWUE as a function of a set of environmental predictors, alternatively including annual rate (N_rate) and long‐term cumulative (N_cum) nitrogen deposition. No single climatic or atmospheric variable had a clearly predominant effect, however, Δ¹³C and iWUE showed complex dependent interactions between different covariates. A significant association of N_rate with iWUE and Δ¹³C was observed in conifers and in the angiosperms, and N_cum was the only independent term with a significant positive association with iWUE, although a multi‐factorial control was evident in conifers.     

Predicting leaf wax n‐alkane 2H/1H ratios: controlled water source and humidity experiments with hydroponically grown trees confirm predictions of Craig–Gordon model

The extent to which both water source and atmospheric humidity affect δ²H values of terrestrial plant leaf waxes will affect the interpretations of δ²H variation of leaf waxes as a proxy for hydrological conditions. To elucidate the effects of these parameters, we conducted a long‐term experiment in which we grew two tree species, Populus fremontii and Betula occidentalis, hydroponically under combinations of six isotopically distinct waters and two different atmospheric humidities. We observed that leaf n‐alkane δ²H values of both species were linearly related to source water δ²H values, but with slope differences associated with differing humidities. When a modified version of the Craig–Gordon model incorporating plant factors was used to predict the δ²H values of leaf water, all modelled leaf water values fit the same linear relationship with n‐alkane δ²H values. These observations suggested a relatively constant biosynthetic fractionation factor between leaf water and n‐alkanes. However, our calculations indicated a small difference in the biosynthetic fractionation factor between the two species, consistent with small differences calculated for species in other studies. At present, it remains unclear if these apparent interspecies differences in biosynthetic fractionation reflect species‐specific biochemistry or a common biosynthetic fractionation factor with insufficient model parameterization.     

New QTLs identified for plant water status, water-soluble carbohydrate and osmotic adjustment in a barley population grown in a growth-chamber under two water regimes

Quantitative trait locus (QTL) analysis was carried out with 167 recombinant inbred lines (RILs) of barley derived from a cross between Tadmor and Er/Apm to identify the genomic regions controlling traits related to plant water status and osmotic adjustment (OA). The experiment was conducted in a growth chamber using a random incomplete block design (nine blocks). Relative water content (RWC) and leaf osmotic potential (ψ_π) were measured at 100% and 14% of the field capacity on 105 RILs in each block. In addition, the water-soluble carbohydrate concentration (WSC) was measured in the four first-blocks. The leaf osmotic potential at full turgor (ψ_π100), the water-soluble carbohydrate concentration at full turgor (WSC100), and also OA, the accumulation of water-soluble carbohydrates (dWSC100), the contribution of a change in water content to OA (CWC) and of the net solute accumulation to OA (SA) have also been calculated. In a previous paper (Teulat et al. 1998), 12 QTLs were identified for RWC, ψ_π, ψ_π100 and OA with adjusted means (block effects and pot-within-block effects fixed) with an incomplete genetic map. In the present paper, a more-saturated and improved map is described. A new QTL analysis as been performed with adjusted means. The new QTLs identified for previous evaluated traits, as well as the QTLs for the new traits, are presented. Eight additional regions (22 QTLs) were identified which increased to 13 the total number of chromosomal regions (32 QTLs) controlling traits related to plant water status and/or osmotic adjustment in this barley genetic background. The results emphasise the value of the experimental design employed for the evaluation of traits difficult to assess in genetic studies. The putative target regions for drought-tolerance improvement are discussed combining arguments on the consistency of QTLs and, when possible, the physiological value of QTLs (trait relevance, syntenic relationships and clustering of QTLs). Received: 8 March 2000 / Accepted: 18 October 2000     

Establishing pathways of energy flow for insect predators using stable isotope ratios: field and laboratory evidence

 Quantifying pathways of energy transfer between plants, pests, and beneficial insects is a necessary step toward maintaining pest stable agroecosystems in the absence of chemical subsidies. A diet switching experiment utilizing a predatory ladybird beetle, Hippodamia variegata (Goeze), evaluated the use of naturally occurring stable C and N isotopes as an economically feasible and safe method for quantifying pathways of energy flow within agroecosystems. Stable isotope values of the ladybird beetle Coleomegilla maculata lengi (Timberlake) collected from an agroecosystem were used to estimate the relative amount of C and N derived from agricultural plants and incorporated by ladybird beetles based on mass balance equations. At the beginning of the diet-switching experiment δ¹³C and δ¹⁵N values of H. variegata (–12.0‰ and 6.3‰, respectively) differed by –0.2‰ and 2.9‰ from the aphids that were provided exclusively as their diet. These data are consistent with previous estimates of trophic level isotope effects. After switching the diet of H. variegata to an alternative food, isotope values of H. variegata gradually shifted toward expected values for individuals fed this diet (–22.9‰ and 8.8‰ for δ¹³C and δ¹⁵N values, respectively). Isotope values of another ladybird beetle, C. maculata, collected from the field indicated that in May, alfalfa and maize (pollen) obtained in the previous year contributed 32% and 68% of the C or N to the diets of these individuals and in August, 52%, 6%, and 42% of the C or N assimilated by these insects was derived from alfalfa, wheat, and maize, respectively. These data are consistent with expectations based on the relative abundance of C. maculata in various crops during the season. The field and laboratory data are a clear indication that isotope values are sensitive to dietary changes on a relatively short time scale (days) and provide a strong basis for the use stable C and N isotope to trace energy flow patterns of these beneficial organisms within agroecosystems. Received: 17 November 1995 / Accepted: 20 June 1996     

The influence of diet and water on the stable oxygen and hydrogen isotope composition of Chironomidae (Diptera) with paleoecological implications

Stable oxygen and hydrogen isotope analyses of fossil aquatic organisms, such as the chitinous head capsules of chironomid larvae (Chironomidae: Diptera), are promising proxies for inferring paleoecological conditions. In order for analyses of stable oxygen (δ¹⁸O) and hydrogen isotope ratios (δ²H) of fossil chironomid head capsules to be used effectively in paleoecological research, it is necessary to understand the factors controlling their stable oxygen and hydrogen composition. We cultured chironomid larvae in two isotopically distinct waters under controlled, replicated laboratory conditions. Chironomid larvae were fed on identical diets, to examine the degree to which water and diet influence the δ¹⁸O and δ²H of these organisms. We used a two-end member mixing model to determine the proportional contributions of oxygen and hydrogen from water to the oxygen and hydrogen of chironomid larvae. Our experiment demonstrated that 69.0 ± 0.4% of oxygen and 30.8 ± 2.6% of hydrogen in chironomid larvae are derived from habitat water. Our results show that oxygen isotopes from chironomid remains can better constrain past habitat water isotopic changes compared to hydrogen, due to 69% of the chironomid oxygen being influenced by habitat water. Our data add to a small but growing suite of comparative data on the sources of oxygen and hydrogen in animal tissues, and provide the first such analyses from aquatic insects.     

Application of the Marcus theory for analysis of the temperature dependence of the reactions leading to photosynthetic water oxidation: results and implications

The temperature dependence of donor side reactions was analysed within the framework of the Marcus theory of nonadiabatic electron transfer. The following results were obtained for PS II membrane fragments from spinach: (1) the reorganisation energy of P680^+? reduction by Y_Z is of the order of 0.5?eV in samples with a functionally fully competent water oxidising complex (WOC); (2) destruction of the WOC by Tris-washing gives rise to a drastic increase of λ to values of the order of 1.6?eV; (3) the reorganisation energies of the oxidation steps in the WOC are dependent, on the redox states S _i with values of about 0.6?eV for the reactions Y_Z ^OX S ₀→Y_Z S ₁ and Y_Z ^OX S ₁→Y_Z S ₂, 1.6?eV for the reaction Y_Z ^OX S ₂→Y_Z S ₃ and 1.1?eV (above a characteristic temperature u_c of about 6??°C) for the reaction Y_Z ^OX S ₃→→Y_Z S ₀+O₂. Using an empirical rate constant-distance relationship, the van der Waals distance between Y_Z and P680 was found to be about 10?Å, independent of the presence or absence of the WOC, whereas the distance between Y_Z and the manganese cluster in the WOC was ≥15?Å. Based on the calculated activation energies the environment of Y_Z is inferred to be almost "dry" and hydrophobic when the WOC is intact but becomes enriched with water molecules after WOC destruction. Furthermore, it is concluded that the transition S ₂→S ₃ is an electron transfer reaction gated by a conformational change, i.e. it comprises significant structural changes of functional relevance. Measurements of kinetic H/D isotope exchange effects support the idea that none of these reactions is gated by the break of a covalent O-H bond. The implications of these findings for the mechanism of water oxidation are discussed.     

A polarographic, oxygen-selective, vibrating-microelectrode system for the spatial and temporal characterisation of transmembrane oxygen fluxes in plants

 A simple procedure is described for the fabrication of micrometer to nanometer-scale platinum electrodes to be used in a vibrating oxygen-selective system. The electrode was prepared by etching a fine platinum wire and insulating it with an electrophoretic paint. The dimensions allowed this electrode to be used with the “vibrating probe technique” in exploratory studies aimed at mapping and measuring the patterns of net influxes as well as effluxes of oxygen in Olea europaea L. leaves and roots with spatial and temporal resolutions of a few microns and a few seconds, respectively. The magnitude and spatial localisation of O₂ influxes in roots was characterised by two distinct peaks. The first, in the division zone, averaged 38 ± 5 nmol m⁻² s⁻¹; the second, in the elongation region, averaged 68 ± 6 nmol m⁻² s⁻¹. Long-term records of oxygen influx in the elongation region of the root showed an oscillatory regime characterised by a fast oscillation with periods of about 8–9 min. In leaves, the system allowed the measurement of real-time changes in O₂ evolution following changes in light. Furthermore, it was possible to obtain “topographical” images of the photosynthetically generated oxygen diffusing through different stomata from a region of the leaf of 120 μm × 120 μm. The combination of topographic and electrochemical information at the micrometer scale makes the system an efficient tool for studying biological phenomena involving oxygen diffusion. Received: 12 November 1999 / Accepted: 1 February 2000