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  • 1
    ISSN: 1573-5052
    Keywords: Antioxidants ; Mechanical stress ; Oxidative stress ; Photosynthesis ; Vacuolation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract The mechanisms of protection against mechanical and oxidative stress were identified and compared in the angiosperm resurrection plants Craterostigma wilmsii, Myrothamnus flabellifolius and Xerophyta humilis. Drying-induced ultrastructural changes within mesophyll cells were followed to gain an understanding of the mechanisms of mechanical stabilisation. In all three species, water filled vacuoles present in hydrated cells were replaced by several smaller vacuoles filled with non-aqueous substances. In X. humilis, these occupied a large proportion of the cytoplasm, preventing plasmalemma withdrawal and cell wall collapse. In C. wilmsii, vacuoles were small but extensive cell wall folding occurred to prevent plasmalemma withdrawal. In M. flabellifolius, some degree of vacuolation and wall folding occurred, but neither were sufficient to prevent plasmalemma withdrawal. This membrane was not ruptured, possibly due to membrane repair at plasmodesmata junctions where tearing might have occurred. In addition, the extra-cytoplasmic compartment appeared to contain material (possibly similar to that in vacuoles) which could facilitate stabilisation of dry cells. Photosynthesis and respiration are particularly susceptible to oxidative stress during drying. Photosynthesis ceased at high water contents and it is proposed that a controlled shut down of this metabolism occurred in order to minimise the potential for photo-oxidation. The mechanisms whereby this was achieved varied among the species. In X. humilis, chlorophyll was degraded and thylakoid membranes dismantled during drying. In both C. wilmsii and M. flabellifolius, chlorophyll was retained, but photosynthesis was stopped due to chlorophyll shading from leaf folding and anthocyanin accumulation. Furthermore, in M. flabellifolius thylakoid membranes became unstacked during drying. All species continued respiration during drying to 10% relative water content, which is proposed to be necessary for energy to establish protection mechanisms. Activity of antioxidant enzymes increased during drying and remained high at low water contents in all species, ameliorating free radical damage from both photosynthesis and respiration. The nature and extent of antioxidant upregulation varied among the species. In C. wilmsii, only ascorbate peroxidise activity increased, but in M. flabellifolius and X. humilis ascorbate peroxidise, glutathione reductase and superoxide dismutase activity increased, to various extents, during drying. Anthocyanins accumulated in all species but this was more extensive in the homoiochlorophyllous types, possibly for protection against photo-oxidation.
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  • 2
    ISSN: 1573-5052
    Keywords: Electrolyte leakage ; Germination ; Glutathione reductase ; Growth ; Photochemistry ; Pigments ; Serotinous species ; UV-radiation
    Source: Springer Online Journal Archives 1860-2000
    Topics: Biology
    Notes: Abstract Dry seeds of Leucadendron laureolum (Lam.) Fourc. (Proteaceae) were exposed for different intervals (range: 7 to 84 days) to visible, UV-A and UV-B radiation of different biologically effective dose (range: 0 to 11.43 kJ m-2 d-1). Changes in seed germination, physiology and ultrastructure, and residual UV effects on seedling performance, were examined. Germination was depressed in seeds following short (7-day) exposures to UV radiation. This depression was intensified with increased UV exposure dose, and most pronounced at shorter UV-B wavelengths. Also glutathione reductase (GR) activities increased in seeds exposed to shorter UV-B wavelengths, but these were unaffected by irradiation dose level in the UV-B range. Electrolyte leakage rates from UV-irradiated seeds were unaltered, which indicated that germination depression did not result from intrinsic membrane damage. The reversal of germination depression (UV-induced dormancy) in UV-irradiated seeds by red light pointed to the possible involvement of phytochrome in this photo-response. Germination depression disappeared in seeds after 56-days irradiation, possibly due to photoreceptor damage by excess UV light. At this stage, all UV irradiated seeds, irrespective of treatment wavelength or dose level, exhibited increased electrolyte leakage rates, which indicated membrane perturbation. Also, increased GR activities were observed in irradiated seeds, but these were proportionately smaller in seeds exposed to shorter wavelength UV-B radiation (9.1 to 35.8% increase) than longer wavelength UV-A (73.4% increase) and visible (97.7% increase) radiation. This implied a metabolic limitation for scavenging of free radicals and peroxides in aging seeds exposed to UV-B radiation, which pointed to accelerated seed deterioration. It was indirectly supported by ultrastructural evidence of sub-cellular damage (lipid coagulation and plasmalemma withdrawal from cell walls) in embryonic tissues of seeds after 84 days UV-B exposure, and reflected in decreased leaf numbers, photochemical efficiencies, and foliar chlorophyll a and carotenoid levels in seedlings cultured from these seeds.
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  • 3
    ISSN: 0749-1581
    Keywords: 1H NMR ; 13C NMR ; 19F NMR ; β-Diketones ; Tautomerism ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: Multinuclear NMR experiments on fluorinated β-diketones have furnished information on the various tautomers present in solution and their proportions. 2-Trifluoroacetyl-1-indalone in CDCI3 solution exists in the enol form, as a mixture of exocyclic and endocyclic cis-enols with the exocyclic form favoured, but in DMF-d7 solution the equilibrium shifts to 40% enol, 60% hydrate with the enol and hydrate being in slow exchange on the NMR time scale. 2-Trifluoroacetyl-1-tetralone in CDCI3 solution exists only as the enol forms, approximately equally populated, but in DMF-d7, unlike the indalone, no hydrate is formed although there is now 6% of the keto form present in slow exchange. The experimental results were compared with calculated data produced using molecular orbital simulations. These agree with the NMR results and predict that the tetralone enol tautomers are both significantly populated at room temperature but that the indalone exists primarily as the exocyclic enol. The oxygen-oxygen internuclear distance in the hydrogen bond was estimated from the OH proton chemical shift based on a calibration from solid-state data from model compounds, and agrees well with the predictions using the molecular mechanics approach.
    Additional Material: 2 Tab.
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  • 4
    ISSN: 0749-1581
    Keywords: NMR ; Relaxation ; Selective pulses ; Distance ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: An efficient method of measuring proton spin-lattice cross-relaxation rates in small molecules is proposed. The method involves measuring the difference in relaxation rates in a non-selective experiment and in one with all resonances except one of interest inverted. This procedure yields cross-relaxation rates between this nucleus and all other nuclei. These rates are independent of the effects of non-excited heteronuclei and of non-dipolar contributions and can be used to determine relative inter-proton distances. The method was tested using 7-azaindole and compared favourably with the standard NOE method for estimating proton proximities requiring less data acquisition time.
    Additional Material: 1 Ill.
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  • 5
    Electronic Resource
    Electronic Resource
    Chichester : Wiley-Blackwell
    Organic Magnetic Resonance 32 (1994), S. 231-234 
    ISSN: 0749-1581
    Keywords: NMR probe ; Nematic phase ; Temperature stability ; Temperature gradient ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The measurement of the 1H NMR spectrum of a sealed sample of benzene dissolved in a nematic liquid crystal provides a convenient and sensitive test of both NMR probe temperature stability and of the temperature gradient across the sample.
    Additional Material: 4 Ill.
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  • 6
    ISSN: 0749-1581
    Keywords: Ethanediol ; NMR ; Coupling constants ; Conformation ; Inverse detection ; Gradients ; Chemistry ; Analytical Chemistry and Spectroscopy
    Source: Wiley InterScience Backfile Collection 1832-2000
    Topics: Chemistry and Pharmacology
    Notes: The 1H NMR spectrum of [13C]ethane-1,2-diol was measured at natural abundance using the 1H—13C inverse-detected HMQC method with magnetic field gradients. A limited solvent and temperature dependence study was undertaken and the vicinal 1H—1H coupling constants were related to those in model compounds. It is shown that the trans rotamer is significantly populated, in contradiction to some recent theoretical and experimental studies.
    Additional Material: 1 Ill.
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  • 7
    ISSN: 1573-5087
    Keywords: Avicennia marina ; desiccation-sensitive ; homoiohydrous ; recalcitrant ; seed development ; storage/LEA/heat-stable proteins
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract In the recalcitrant seeds of Avicennia marina, protein content and the rates of protein synthesis increase during histodifferentiation. This is similar to the situation in desiccation tolerant seeds. During the stage of reserve accumulation the protein content and rates of synthesis remain constant and there is no de novo synthesis of proteins which might qualify as storage proteins. There is also no change in the nature of proteins present in either axis or cotyledonary tissues during development or germination. Similarly, fluorographs of axis proteins show only very limited changes in the patterns of protein synthesis during development and germination, at least until the onset of root growth. Heat-stable proteins are present from an early developmental stage. However, no late embryogenic abundant (LEA) proteins are synthesised during the late stages of development, indicating that seedling establishment is independent of such maturation proteins. It is suggested that the lack of desiccation tolerance of A. marina seeds might be related to the absence of desiccation-related LEAs. Although the rate of protein synthesis increases during germination, protein metabolism appears to remain qualitatively the same as that occurring during development. The present results suggest that in these desiccation sensitive seeds, protein metabolism characterising development changes imperceptibly into that of germination.
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  • 8
    ISSN: 1573-5087
    Keywords: actinomycin-D ; cyclohexamide ; desiccation-tolerance ; poikilochlorophyllous ; rehydration ; Xerophyta humilis
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract Xerophyta humilis (Bak.) Dur. and Schinz is a poikilochlorophyllous resurrection plant in that it is tolerant of considerable water loss (〈 5% relative water content [RWC]) and thylakoid membranes are dismantled and chlorophyll is lost during dehydration. In this paper we examined the processes associated with recovery from desiccation upon rehydration. Dried leaf explants were rehydrated in water (control) or in solutions of actinomycin-D or cyclohexamide in order to determine to what extent initial recovery was dependant on de novo transcription and translation respectively. Our results suggest that considerable protection of subcellular organisation and components of metabolism occurs during drying such that the initial recovery of metabolism on rehydration is virtually independent of de novo transcription of nuclear genes. However recovery does require the synthesis of new proteins. The plasmalemma remains intact and macromolecular synthesis is not required for maintenance of its integrity. Messenger RNA's for chlorophyll biosynthesis appear to be stored in a stable form in the dried leaves and are translated on rehydration. Similarly most of the mRNA's necessary for recovery of electron transport in the chloroplast (as determined by measuring the quantum efficiency of photosystem II [FV/FM] using chlorophyll fluorescence) appear to be stabily present in the dried leaves. However, for total recovery of FV/FM new genomic transcription is necessary.
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  • 9
    ISSN: 1573-5087
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
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  • 10
    ISSN: 1573-5087
    Keywords: anti-oxidant enzymes ; anthocyanins ; chlorophyll ; free radicals ; homoiochlorophyllous ; poikilochlorophyllous
    Source: Springer Online Journal Archives 1860-2000
    Topics: Agriculture, Forestry, Horticulture, Fishery, Domestic Science, Nutrition
    Notes: Abstract Mechanisms of avoidance and protection against light damage were studied in the resurrection plants Craterostigma wilmsii and Xerophyta viscosa. In C. wilmsii, a combination of both physical and chemical changes appeared to afford protection against free radical damage. During dehydration leaves curled inwards, and the abaxial surface became exposed to light. The tissue became purple/brown in colour, this coinciding with a three-fold increase in anthocyanin content and a 30% decline in chlorophyll content. Thus light-chlorophyll interactions are progressively reduced as chlorophyll became masked by anthocyanins in abaxial layers and shaded in the adaxial layers. Ascorbate peroxidase (AP) activity increased during this process but declined when the leaf was desiccated (5% RWC). During rehydration leaves uncurled and the potential for normal light-chlorophyll interaction was possible before full hydration had occurred. Superoxide dismutase (SOD) and glutathione reductase (GR) activities increased markedly during this stage, possibly affording free radical protection until full hydration and metabolic recovery had occurred. In contrast, the leaves of X. viscosa did not curl, but light-chlorophyll interactions were minimised by the loss of chlorophyll and dismantling of thylakoid membranes. During dehydration, free radical protection was afforded by a four-fold increase in anthocyanin content and increased activities of AP, GR and SOD. These declined during rehydration. It is suggested that potential free radical damage may be avoided by the persistence of anthocyanins during the period of thylakoid membrane re-assembly and full chlorophyll restitution which only occurred once the leaves were fully rehydrated.
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