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  • 1
    Electronic Resource
    Electronic Resource
    Open Learning or Open Access? Some questions from the institutional viewpoint (1979)
    Oxford, UK : Blackwell Publishing Ltd
    British journal of educational technology 10 (1979), S. 0 
    Details
    ISSN: 1467-8535
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Education
    Notes: This paper looks at the current emphasis in open learning from the institutional point of view as the eventual provider. It outlines the current trends towards distance learning, considers the implications of them for institutional organization and compares them with other forms of opening up learning opportunities for individuals. In particular the problems and advantages of various college study schemes are outlined and the implications for individual institutions discussed. The paper draws on what evidence is available but does not attempt to provide solutions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1467-8535.1979.tb00376.x
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    Paper (German National Licenses)
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  • 2
    Electronic Resource
    Electronic Resource
    Root Signals and the Regulation of Growth and Development of Plants in Drying Soil (1991)
    Palo Alto, Calif. : Annual Reviews
    Annual Review of Plant Physiology and Plant Molecular Biology 42 (1991), S. 55-76 
    Details
    ISSN: 1040-2519
    Source: Annual Reviews Electronic Back Volume Collection 1932-2001ff
    Topics: Biology
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1146/annurev.pp.42.060191.000415
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  • 3
    Electronic Resource
    Electronic Resource
    An analysis of the dynamic response of stomatal conductance to a reduction in humidity over leaves of Cedrella odorata (1999)
    Oxford, UK : Blackwell Science Ltd.
    Plant, cell & environment 22 (1999), S. 0 
    Details
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Single leaves of 3-month-old Cedrella odorata seedlings were exposed to a step reduction in the ambient dew point. The resultant time series of dynamic variations in leaf surface water vapour concentration, leaf surface water vapour concentration gradient, transpiration rate and stomatal conductance to water vapour, are analysed using the data-based mechanistic (DBM) modelling methodology of Young (e.g. Young & Lees 1992; Minchin et al. 1996 ). It is shown that the identified second-order, dynamic model between transpiration rate (as the input) and stomatal conductance (as the output) provides an appropriate, physiologically meaningful, description of the system. In particular, the dynamic relationship between these two variables is remarkably linear and can be resolved in terms of two parallel, first-order, subsystems; a model which complements the results of Cowan (1977) for cotton. The model is also compared with the recently published simulation model of Haefner, Buckley & Mott (1997).
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-3040.1999.00446.x
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  • 4
    Electronic Resource
    Electronic Resource
    Regulation of tomato fruit growth by epidermal cell wall enzymes (1998)
    Oxford, UK : Blackwell Science Ltd, UK
    Plant, cell & environment 21 (1998), S. 0 
    Details
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Water relations of tomato fruit and the epidermal and pericarp activities of the putative cell wall loosening and tightening enzymes Xyloglucan endotransglycosylase (XET) and peroxidase were investigated, to determine whether tomato fruit growth is principally regulated in the epidermis or pericarp. Analysis of the fruit water relations and observation of the pattern of expansion of tomato fruit slices in vitro, has shown that the pericarp exerts tissue pressure on the epidermis in tomato fruit, suggesting that the rate of growth of tomato fruit is determined by the physical properties of the epidermal cell walls. The epidermal activities of XET and peroxidase were assayed throughout fruit development. Temporal changes in these enzyme activities were found to correspond well with putative cell wall loosening and stiffening during fruit development. XET activity was found to be proportional to the relative expansion rate of the fruit until growth ceased, and a peroxidase activity weakly bound to the epidermal cell wall appeared shortly before cessation of fruit expansion. No equivalent peroxidase activity was detected in pericarp tissue of any age. It is therefore plausible that the expansion of tomato fruit is regulated by the combined action of these enzyme activities in the fruit epidermis.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.1365-3040.1998.00308.x
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  • 5
    Electronic Resource
    Electronic Resource
    Xylem-transported abscisic acid: the relative importance of its mass and its concentration in the control of stomatal aperture (1993)
    Oxford, UK : Blackwell Publishing Ltd
    Plant, cell & environment 16 (1993), S. 0 
    Details
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Abscisic acid (ABA) fed in pulses to the petioles of detached cherry leaves in enclosed leaf chambers, caused a reduction in leaf conductance. The degree of inhibition was analysed with respect to the amount of ABA fed and to concentration of ABA in the feeding solution. Regression analysis of the data showed both variables to have a significant effect on leaf conductance. A hypothetical maximum ABA concentration occurring in the leaf apoplast was calculated for each pulse from a simple model. This variable explained more of the variance within the data than either the amount or the applied concentration variable. A value for the rate at which ABA is removed from the apoplast is derived from the experimental data using the model. A second experiment attempted to evaluate this rate directly, by measuring the rate of catabolism of labelled ABA within the leaf. The results suggested a half-life of 36 min for the initial rate of decay. This figure is similar to that derived from the model, the importance of ABA-metabolism for the control of leaf conductance is discussed in the context of root-to-shoot communication by ABA in the xylem stream.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-3040.1993.tb00892.x
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  • 6
    Electronic Resource
    Electronic Resource
    Xylem ABA controls the stomatal conductance of field-grown maize subjected to soil compaction or soil drying (1992)
    Oxford, UK : Blackwell Publishing Ltd
    Plant, cell & environment 15 (1992), S. 0 
    Details
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Stomatal conductance of individual leaves was measured in a maize field, together with leaf water potential, leaf turgor, xylem ABA concentration and leaf ABA concentration in the same leaves. Stomatal conductance showed a tight relationship with xylem ABA, but not with the current leaf water status or with the concentration of ABA in the bulk leaf. The relationship between stomatal conductance and xylem [ABA] was common for variations in xylem [ABA] linked to the decline with time of the soil water reserve, to simultaneous differences between plants grown on compacted, non-compacted and irrigated soil, and to plant-to-plant variability. Therefore, this relationship is unlikely to be fortuitous or due to synchronous variations. These results suggest that increased concentration of ABA in the xylem sap in response to stress can control the gas exchange of plants under field conditions.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-3040.1992.tb01473.x
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  • 7
    Electronic Resource
    Electronic Resource
    Changes in the concentration of ABA in xylem sap as a function of changing soil water status can account for changes in leaf conductance and growth (1990)
    Oxford, UK : Blackwell Publishing Ltd
    Plant, cell & environment 13 (1990), S. 0 
    Details
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Abstract. Maize seedlings (Zea mays L. John Innes F1 hybrid) were grown in a greenhouse in l-m-long tubes of soil. When the plants were well established, water was withheld from half of the tubes. Control plants were watered every day during the 20-d experimental period. The soil drying treatment resulted in a substantial restriction of stomatal conductance and a limitation in shoot growth, even though there was no detectable difference in the water relations of watered and unwatered plants. From day 7 of the soil drying treatment, xylem ABA concentrations (measured using the sap exuded from detopped plants) were substantially increased in unwatered plants compared to values recorded with sap from plants watered every day. Measurements of water potential through the profile of unwatered soil suggest that xylem ABA concentrations reflects the extent of soil drying. Leaf ABA content was a much less sensitive indicator of the effect of soil drying and during the whole of experimental period there was no significant difference between ABA concentration in leaves of well watered and unwatered plants. In a second set of experiments, ABA was fed to part of the roots of potted maize plants to manipulate xylem ABA concentration. These manipulations suggested that the increases in ABA concentration in xylem sap, which resulted from soil drying, were adequate to explain the observed variation in stomatal conductance and might also explain the restriction in leaf growth rate. These results are discussed in the light of recent work which suggests that stomatal responses to soil drying are partly attributable to an as-yet unidentified inhibitor of stomatal opening.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-3040.1990.tb01312.x
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  • 8
    Electronic Resource
    Electronic Resource
    Abscisic acid produced in dehydrating roots may enable the plant to measure the water status of the soil (1989)
    Oxford, UK : Blackwell Publishing Ltd
    Plant, cell & environment 12 (1989), S. 0 
    Details
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Abstract. Maize plants were grown in 1-m-long tubes of John Innes No. 2 potting compost. From the start of the experimental period, half of the plants were unwatered. Stomatal conductance of these plants was restricted 6 d after last watering and continued to decline thereafter. This was despite the fact that as a result of solute accumulation, unwatered plants showed consistently higher leaf turgors than well-watered plants. Leaf water potentials of unwatered plants were not significantly lower than those of plants that were watered well. Main seminal and nodal roots showed solute regulation in drying soil and continued to grow even in the driest soil, and plants growing in drying soil showed consistently higher root dry weights than did well-watered plants, water potentials and turgors of the tips of fine roots in the upper part of the column decreased as the soil dried. Soil drying below a water content of around 0–25 g cm−3 (a bulk soil water potential of between -0.2 and -0.3 MPa) resulted in a substantial increase in the ABA content of roots. As soil columns dried progressively from the top, ABA content increased in roots deeper and deeper in the soil. These responses suggest that ABA produced by dehydrating roots and which was subsequently transported to the shoots provided a sensitive indication of the degree of soil drying.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-3040.1989.tb01918.x
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  • 9
    Electronic Resource
    Electronic Resource
    ABA-based chemical signalling: the co-ordination of responses to stress in plants (2002)
    Oxford, UK : Blackwell Science Ltd
    Plant, cell & environment 25 (2002), S. 0 
    Details
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: There is now strong evidence that the plant hormone abscisic acid (ABA) plays an important role in the regulation of stomatal behaviour and gas exchange of droughted plants. This regulation involves both long-distance transport and modulation of ABA concentration at the guard cells, as well as differential responses of the guard cells to a given dose of the hormone. We will describe how a plant can use the ABA signalling mechanism and other chemical signals to adjust the amount of water that it loses through its stomata in response to changes in both the rhizospheric and the aerial environment. The following components of the signalling process can play an important part in regulation: (a) ABA sequestration in the root; (b) ABA synthesis versus catabolism in the root; (c) the efficiency of ABA transfer across the root and into the xylem; (d) the exchange of ABA between the xylem lumen and the xylem parenchyma in the shoot; (e) the amount of ABA in the leaf symplastic reservoir and the efficiency of ABA sequestration and release from this compartment as regulated by factors such as root and leaf-sourced changes in pH; (f) cleavage of ABA from ABA conjugates in the leaf apoplast; (g) transfer of ABA from the leaf into the phloem; (h) the sensitivity of the guard cells to the [ABA] that finally reaches them; and lastly (i) the possible interaction between nitrate stress and the ABA signal.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1046/j.0016-8025.2001.00824.x
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  • 10
    Electronic Resource
    Electronic Resource
    The relationship between leaf growth and ABA accumulation in the grass leaf elongation zone (1996)
    Oxford, UK : Blackwell Publishing Ltd
    Plant, cell & environment 19 (1996), S. 0 
    Details
    ISSN: 1365-3040
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Biology
    Notes: Detached barley (Hordeum vulgare L.) shoots, maintained at different air temperatures and VPDs, were fed ABA via the sub-crown internode in a leaf elongation assay. Analysis of variance of leaf elongation rate (LER) showed significant effects of temperature (T), fed [ABA] and the interaction T × [ABA]. However, the interaction became non-significant when LER was modelled against the [ABA] of the elongation zone, [EZ-ABA]When detached barley shoots were fed sap from droughted maize (Zea mays L.) plants, sap [ABA] could not explain the growth inhibitory activity. Measurement of [EZ-ABA] accounted for this ‘unexplained’ growth inhibition.The detached shoot experiments indicated that [EZ-ABA], and not xylem sap [ABA], was an appropriate explanatory variable to measure in droughted plants. However, ABA accumulation in the elongation zone could not explain a 35% growth reduction in intact droughted plants; thus we considered an interaction of water status and ABA.Using a coleoptile growth assay, we applied mild osmotic stresses (ψ=0 to −0.06 MPa) and 10−4 mol m−3 ABA. Individually, these treatments did not inhibit growth. However, osmotic stress and ABA applied together significantly reduced growth. This interaction may be an important mechanism in explaining leaf growth inhibition of droughted plants.
    Type of Medium: Electronic Resource
    URL: http://dx.doi.org/10.1111/j.1365-3040.1996.tb00211.x
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