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  • 1
    ISSN: 1612-1112
    Keywords: Column liquid chromatography ; Calix[6]arene ; Positional isomers ; PAHs ; Nucleosides
    Source: Springer Online Journal Archives 1860-2000
    Topics: Chemistry and Pharmacology
    Notes: Summary For the first time calix[6]arene has been chemically combined with silica gel via a longer spacer to prepare calix[6]arene-bonded silica gel stationary phase for high-performance liquid chromatography (HPLC). The separation of positional isomers and polycyclic aromatic hydrocarbons on the calix[6]arene-bonded phase was achieved with methanol-water as mobile phase. Some nucleosides were also separated on the bonded phase. The reversed-phase chromatographic performance of the bonded phase was studied. The results showed that the calix[6]arene-bonded phase was highly hydrophobic. A possible separation mechanism has been proposed.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1520-5118
    Source: ACS Legacy Archives
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition , Process Engineering, Biotechnology, Nutrition Technology
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Diel variation in specific hydraulic conductivity (ks) was recorded in petioles of two savanna tree species, Schefflera macrocarpa and Caryocar brasiliense, from central Brazil. These two species have compound leaves with long petioles (10–30 cm). In both species, petiole ks decreased sharply with increasing transpiration rates and declining leaf water potentials (ψL) during the morning. Petiole ks increased during the afternoon while the plants were still transpiring and the water in the non-embolized vessels was still under tension. Dye experiments confirmed that in both species diel variation in ks was associated with embolism formation and repair. When transpiration was prevented in individual leaves, their petiole ks and water potential remained close to their maximum values during the day. When minimum daily ψL on selected branches was experimentally lowered by 0.2–0.6 MPa, the rate of ks recovery during the afternoon was slower in comparison with control branches. Several field manipulations were performed to identify potential mechanisms involved in the refilling of embolized petiole vessels. Removal of the cortex or longitudinal incisions in the cortex prevented afternoon recovery of ks and refilling of embolized vessels. When distilled water was added to petiole surfaces that had been abraded to partially remove the cuticle, ks increased sharply during the morning and early afternoon. Evidence of starch to sugar conversion in the starch sheath cells surrounding the vascular bundles of the petioles was observed during periods of rapid transpiration when the abundance of starch granules in the starch sheath cells surrounding the vascular bundles decreased. Consistent with this, petiole sugar content was highest in the early afternoon. The most parsimonious explanation of the field observations and the experimental results was that an increase in osmotically active solutes in cells outside the vascular bundles at around midday leads to water uptake by these cells. However, the concurrent increase in tissue volume is partially constrained by the cortex, resulting in a transient pressure imbalance that may drive radial water movement in the direction of the embolized vessels, thereby refilling them and restoring water flow. This study thus presents evidence that embolism formation and repair are two distinct phenomena controlled by different variables. The degree of embolism is a function of tension, and the rate of refilling a function of internal pressure imbalances.
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  • 4
    ISSN: 1432-1939
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Natural abundances of oxygen-18, carbon-14, carbon-13 and deuterium were measured inStylites andicola, a terrestrial vascular plant lacking stomates, and two terrestrial plant species having stomates that grew nearby. No substantial differences in oxygen-18 and carbon-13 abundances were observed among the three species. The deuterium concentration inStylites was much higher than in the other species, confirming a previous report of CAM inStylites. Stylites was depleted in carbon-14 relative to the present day atmosphere, consistent with the proposal that it fixes CO2 derived from decomposing peat.
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  • 5
    ISSN: 1432-1939
    Keywords: Clusia ; Ficus ; Strangler ; CAM ; Photosynthesis ; Hemiepiphyte
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Hemiepiphytic species in the genera Clusia and Ficus were investigated to study their mode of photosynthetic metabolism when growing under natural conditions. Despite growing sympatrically in many areas and having the same growth habit, some Clusia species show Crassulacean acid metabolism (CAM) whereas all species of Ficus investigated are C3. This conclusion is based on diurnal CO2 fixation patterns, diurnal stomatal conductances, diurnal titratable acidity fluctuations, and δ13C isotope ratios. Clusia minor, growing in the savannas adjacent to Barinas, Venezuela, shows all aspects of Crassulacean acid metabolism (CAM) on the basis of nocturnal gas exchange, stomatal conductance, total titratable acidity, and carbon isotope composition when measured during the dry season (February 1986). During the wet season (June 1986), the plants shifted to C3-type gas exchange with all CO2 uptake occurring during the daylight hours. The carbon isotope composition of new growth was-28 to-29‰ typical of C3 plants.
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  • 6
    Electronic Resource
    Electronic Resource
    Springer
    Oecologia 123 (2000), S. 297-311 
    ISSN: 1432-1939
    Keywords: Key words Ecosystem CO2 exchange ; Ecosystem discrimination ; Oxygen-18 ; Carbon-13 ; Evapotranspiration
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract  Stable isotopes are a powerful research tool in environmental sciences and their use in ecosystem research is increasing. In this review we introduce and discuss the relevant details underlying the use of carbon and oxygen isotopic compositions in ecosystem gas exchange research. The current use and potential developments of stable isotope measurements together with concentration and flux measurements of CO2 and water vapor are emphasized. For these applications it is critical to know the isotopic identity of specific ecosystem components such as the isotopic composition of CO2, organic matter, liquid water, and water vapor, as well as the associated isotopic fractionations, in the soil-plant- atmosphere system. Combining stable isotopes and concentration measurements is very effective through the use of ”Keeling plots.” This approach allows the identification of the isotopic composition and the contribution of ecosystem, or ecosystem components, to the exchange fluxes with the atmosphere. It also allows the estimation of net ecosystem discrimination and soil disequilibrium effects. Recent modifications of the Keeling plot approach permit examination of CO2 recycling in ecosystems. Combining stable isotopes with dynamic flux measurements requires precision in isotopic sampling and analysis, which is currently at the limit of detection. Combined with the micrometeorological gradient approach (applicable mostly in grasslands and crop fields), stable isotope measurements allow separation of net CO2 exchange into photosynthetic and soil respiration components, and the evapotranspiration flux into soil evaporation and leaf transpiration. Similar applications in conjunction with eddy correlation techniques (applicable to forests, in addition to grasslands and crop fields) are more demanding, but can potentially be applied in combination with the Keeling plot relationship. The advance and potential in using stable isotope measurements should make their use a standard component in the limited arsenal of ecosystem-scale research tools.
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  • 7
    ISSN: 1432-1939
    Keywords: Photosynthesis ; Hemiepiphyte ; Crassulacean acid metabolism ; Carbon isotope ratios
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Summary Clusia rosea Jacq. is a hemiepiphyte having Crassulacean Acid Metabolism (CAM). In its natural habitat Clusia begins its life cycle as an epiphyte and eventually becomes a rooted tree. These two stages of the life cycle of Clusia represent markedly different water regimes. Our CO2 exchange, stomatal conductance, titratable acidity, and stable carbon isotope ratio measurements indicate that Clusia has a flexible photosynthetic mode, where CO2 is fixed mostly via CAM during its epiphytic stage, when water availability is low, and via both CAM and C3 during its rooted stage.
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