Oxygen isotope composition of phloem sap in relation to leaf water in Ricinus communis

We measured the oxygen isotope composition of both the water and dry matter components of phloem sap exported from photosynthesising Ricinus communis L. leaves. The 18O / 16O composition of exported dry matter matched almost exactly that expected for equilibrium with average lamina leaf water (leaf water exclusive of water associated with primary veins) with an isotope effect of αo=1.027, where αo=Ro / Rw , and Ro and Rw are 18O / 16O of organic molecules and water, respectively. Average lamina leaf water was enriched by 14–22‰ compared with source water under our experimental conditions, and depleted by 4–7‰, compared with evaporative site water. This showed that it is the average lamina leaf water 18O / 16O signal that is exported from photosynthesising leaves rather than a signal more closely related to that of evaporative site water or source water. Additionally, we found that water exported in phloem sap from photosynthesising leaves was enriched compared with source water; the mean phloem water enrichment observed for leaf petioles was 4.0 ± 1.5‰ (mean ± 1 s.d., n = 27). Phloem water collected from stem bases was also enriched compared with source water. However, the enrichment was approximately 0.8 times that observed for leaf petioles, suggesting some mixing between enriched phloem water and unenriched xylem water occurred during translocation. Results validated the assumption that organic molecules exported from photosynthesising leaves are enriched by 27‰ compared with average lamina leaf water. Furthermore, results suggest that the potential influence of enriched phloem water should be considered when interpreting the 18O / 16O signatures of plant organic material and plant cellulose.

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On the 16O/18O isotope effect associated with photosynthetic O2 production

Functional Plant Biology ◽

10.1071/fp07168 ◽

2007 ◽

Vol 34 (11) ◽

pp. 1049 ◽

Cited By ~ 5

Author(s):

Guillaume Tcherkez ◽

Graham D. Farquhar

Keyword(s):

Oxygen Isotope ◽

Isotope Effect ◽

Classical Theory ◽

Isotope Composition ◽

Isotope Effects ◽

Oxygen Isotope Composition ◽

Source Water ◽

18O Isotope ◽

O2 Production

While photosynthetically evolved O2 has been repeatedly shown to have nearly the same oxygen isotope composition as source water so that there is no corresponding 16O/18O isotope effect, some recent 18O-enrichment studies suggest that a large isotope effect may occur, thus feeding a debate in the literature. Here, the classical theory of isotope effects was applied to show that a very small isotope effect is indeed expected during O2 production. Explanations of the conflicting results are briefly discussed.

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Nocturnal stomatal conductance and implications for modelling δ18O of leaf-respired CO2 in temperate tree species

Functional Plant Biology ◽

10.1071/fp05118 ◽

2005 ◽

Vol 32 (12) ◽

pp. 1107 ◽

Cited By ~ 43

Author(s):

Margaret M. Barbour ◽

Lucas A. Cernusak ◽

David Whitehead ◽

Kevin L. Griffin ◽

Matthew H. Turnbull ◽

...

Keyword(s):

Stomatal Conductance ◽

Oxygen Isotope ◽

Tree Species ◽

Ricinus Communis ◽

Ecosystem Respiration ◽

Isotope Composition ◽

Theoretical Work ◽

Oxygen Isotope Composition ◽

Flux Model ◽

The One

Variation in the oxygen isotope composition of within-canopy CO2 has potential to allow partitioning of the ecosystem respiratory flux into above- and below-ground components. Recent theoretical work has highlighted the sensitivity of the oxygen isotope composition of leaf-respired CO2 (δRl) to nocturnal stomatal conductance. When the one-way flux model was tested on Ricinus communis L. large enrichments in δRl were observed. However, most species for which the isotope flux partitioning technique has been or would be applied (i.e. temperate tree species) are much more conservative users of water than R. communis. So, high stomatal conductance and very high enrichment of δRl observed may not be typical for temperate tree species. Using existing gas-exchange measurements on six temperate tree species, we demonstrate significant water loss through stomata for all species (i.e. statistically significantly greater than cuticular loss alone) at some time for some leaves during the night. δRl values predicted by the one-way flux model revealed that δRl might be very much more enriched than when the net flux alone is considered, particularly close to sunrise and sunset. Incorporation of the one-way flux model into ecosystem respiration partitioning studies will affect model outputs and interpretation of variation in the oxygen isotope composition of atmospheric CO2.

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Accuracy and reproducibility of the triple oxygen isotope measurement of silicate micro-samples by laser-fluorination-IRMS

10.5194/egusphere-egu2020-14088 ◽

2020 ◽

Author(s):

Martine Couapel ◽

Corinne Sonzogni ◽

Anne Alexandre ◽

Florence Sylvestre

Keyword(s):

Oxygen Isotope ◽

Isotope Composition ◽

Parent Body ◽

Leaf Water ◽

Oxygen Isotope Composition ◽

High Time Resolution ◽

Trace Back ◽

San Carlos Olivine ◽

Isotope Ratio Mass Spectrometer ◽

Laser Fluorination

Recent studies showed that the 17O-excess of plant leaf biogenic silicates (phytoliths) can be used to quantify the atmospheric relative humidity occurring during leaf water transpiration. The 17O-excess vs &#8706;18O signature of phytoliths can also be used to trace back to the signature of leaf water. In a similar way, the signature of lacustrine diatoms is expected to record the signature of the lake water in which they formed. Therefore, the triple oxygen isotope composition of biogenic silicates extracted from well-dated sedimentary cores may bring new insights for past climate and hydrological reconstructions. However, for high time-resolution reconstructions, we need to be able to measure microsamples (300 to 800 &#181;g) of biogenic silica. In another context, the triple oxygen isotope composition of micro-meteorites constitutes an efficient tool to determine their parent-body. In this case too, micro-samples need to be handled.Here we report the results of new &#8706;18O and &#8706;17O measurements of macro- and micro-samples of international and laboratory silicate standards (e.g. NBS28 quartz, San Carlos Olivine, Boulang&#233; quartz, MSG phytoliths and PS diatoms). Molecular O2 is extracted from silica and purified in a laser-fluorination line, passed through a 114&#176;C slush to condense potential interfering gasses and sent to the dual-inlet Isotope Ratio Mass Spectrometer (IRMS) Thermo-Scientific Delta V. In order to get sufficient 34/32 and 33/32 signals for microsamples the O2 gas is concentrated within the IRMS in an additional auto-cooled 800 ml microvolume tube filled with silica gel. Accuracy and reproducibility of the &#8706;18O, &#8706;17O and 17O excess measurements are assessed. Attention is payed to determine the concentration from which O2 gas yields offsets in &#8706;18O, &#8706;17O and 17O-excess are measured and whether these offsets are reproducible and can be corrected for.

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Factors controlling the triple oxygen isotope composition of grass leaf water and phytoliths: insights for paleo-environmental reconstructions

10.5194/egusphere-egu2020-6641 ◽

2020 ◽

Author(s):

Anne Alexandre ◽

Clément Outrequin ◽

Christine Vallet-Coulomb ◽

Amaelle Landais ◽

Clément Piel ◽

...

Keyword(s):

Relative Humidity ◽

Vapor Pressure ◽

Oxygen Isotope ◽

Meteoric Water ◽

Isotope Composition ◽

Gross Primary Production ◽

Leaf Water ◽

Oxygen Isotope Composition ◽

Phase Changes ◽

Isotope Signature

The oxygen isotope signature of leaf water is used to trace several processes at the soil-plant-atmosphere interface. During photosynthesis, it is transferred to the oxygen isotope signature of atmospheric CO2 and O2, which can be used for reconstructing past changes in gross primary production. The oxygen isotope signature of leaf water additionally imprints leaf organic and mineral compounds, such as phytoliths, used as paleoclimate and paleovegetation proxies when extracted from sedimentary materials.Numerous experimental and modelling studies were dedicated to constrain the main parameters responsible for changes in the &#948;18O of leaf water. Although these models usually correctly depict the main trends of 18O-enrichment of the leaf water when relative humidity decreases, the calculated absolute values often depart from the observed ones by several &#8240;. Moreover, the &#948;18O of leaf water absorbed by plants is dependent on the &#948;18O value of meteoric and soil waters that can vary by several &#8240; at different space and time scales. These added uncertainties make our knowledge of the parameters responsible for changes in the &#948;18O of leaf water and phytoliths flawed.Changes in the triple oxygen isotope composition of leaf water, expressed by the 17O-excess, are controlled by fewer variables than changes in &#948;18O. In meteoric water the 17O-excess varies slightly as it is weakly affected by temperature or phase changes during air mass transport. This makes the soil water fed by meteoric water and the atmospheric vapour in equilibrium with meteoric water changing little from a place to another. Hence the 17O-excess of leaf water is essentially controlled by the evaporative fractionation. The latest depends on the ratio of vapor pressure in the air to vapor pressure in the stomata intercellular space, close to relative humidity. Leaf water evaporative fractionation can lead to 17O-excess negative values that can exceed most of surficial water ones.Here we present the outcomes of several recent growth chamber and field studies, for the purpose of i) refining the grass leaf water and phytoliths &#948;18O and 17O-excess modelling, ii) assessing whether the &#948;18O and 17O-excess of grass leaf water can be reconstructed from phytoliths, and iii) examining the precision of the 17O-excess of phytoliths as a new proxy for past changes in continental atmospheric relative humidity. Atmospheric continental relative humidity is an important climate parameter poorly constrained in global climate models. A model-data comparison approach, applicable beyond the instrumental period, is essential to progress on this issue. However, there is currently a lack of proxies allowing quantitative reconstruction of past continental relative humidity. The 17O-excess signature of phytoliths could fill this gap.

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Leaf-scale quantification of the effect of photosynthetic gas exchange on Δ17O of atmospheric CO2

10.5194/bg-2020-91 ◽

2020 ◽

Author(s):

Getachew Agmuas Adnew ◽

Thijs L. Pons ◽

Gerbrand Koren ◽

Wouter Peters ◽

Thomas Röckmann

Keyword(s):

Gas Exchange ◽

Oxygen Isotope ◽

Mole Fraction ◽

Isotope Composition ◽

Leaf Water ◽

Oxygen Isotope Composition ◽

Atmospheric Co2 ◽

C3 Plants ◽

Back Diffusion ◽

Photosynthetic Gas Exchange

Abstract. Understanding the processes that affect the triple oxygen isotope composition of atmospheric CO2 during gas exchange can help constrain the interaction and fluxes between the atmosphere and the biosphere. We conducted leaf cuvette experiments under controlled conditions, using three plant species. The experiments were conducted at two different light intensities and using CO2 with different 17O-excess. The oxygen isotope composition of CO2 was used to estimate cm, the mole fraction of CO2 at the CO2-H2O exchange site. Our results demonstrate that two key factors determine the effect of gas exchange on the Δ17O of atmospheric CO2. The relative difference between Δ17O of the CO2 entering the leaf and the CO2 in equilibrium with leaf water, and the back-diffusion flux of CO2 from the leaf to the atmosphere, which can be quantified by the cm/ca ratio where ca is the CO2 mole fraction in the surrounding air. At low cm/ca ratio the discrimination is governed mainly by diffusion into the leaf, and at high cm/ca ratio by back-diffusion of CO2 that has equilibrated with the leaf water. Plants with a higher cm/ca ratio modify the Δ17O of atmospheric CO2 more strongly than plants with a lower cm/ca ratio. Based on the leaf cuvette experiments, the global value for discrimination against Δ17O of atmospheric CO2 during the photosynthetic gas exchange is estimated to be −0.57+/−0.14 ‰ using cm/ca values of 0.3 and 0.7 for C4 and C3 plants, respectively. The main uncertainties in this global estimate arise from variation in cm/ca ratios among plants and growth conditions.

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Stable oxygen isotope composition of plant tissue: a review

Functional Plant Biology ◽

10.1071/fp06228 ◽

2007 ◽

Vol 34 (2) ◽

pp. 83 ◽

Cited By ~ 365

Author(s):

Margaret M. Barbour

Keyword(s):

Oxygen Isotope ◽

Plant Tissue ◽

Organic Molecules ◽

Isotope Composition ◽

Measurement Techniques ◽

Isotope Analysis ◽

Theoretical Models ◽

Oxygen Isotope Composition ◽

Stable Oxygen Isotope ◽

Stable Oxygen

With the development of rapid measurement techniques, stable oxygen isotope analysis of plant tissue is poised to become an important tool in plant physiological, ecological, paleoclimatic and forensic studies. Recent advances in mechanistic understanding have led to the improvement of process-based models that accurately predict variability in the oxygen isotope composition of plant organic material (δ18Op). δ18Op has been shown to reflect the isotope composition of soil water, evaporative enrichment in transpiring leaves, and isotopic exchange between oxygen atoms in organic molecules and local water in the cells in which organic molecules are formed. This review presents current theoretical models describing the influences on δ18Op, using recently published experimental work to outline strengths and weaknesses in the models. The potential and realised applications of the technique are described.

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