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B. M. Powell

Bio: B. M. Powell is an academic researcher from Atomic Energy of Canada Limited. The author has contributed to research in topics: Neutron diffraction & Crystal structure. The author has an hindex of 16, co-authored 36 publications receiving 1803 citations.

Papers
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Journal ArticleDOI
08 Jan 1987-Nature
TL;DR: In this article, a new hexagonal hydrate structure requiring both large and small guest molecules to stabilize the structure is reported, which is expected to be isostructural with the hexagonal clathrasil dodecasil-lH.
Abstract: Clathrate hydrates, ice-like host–guest systems containing guest molecules in cages of hydrogen-bonded water molecules exist in three well-characterized cubic forms, and a less well-characterized tetragonal form1,2 On the basis of 2H and l29Xe NMR measurements and X-ray and neutron powder diffraction results, we now report a new hexagonal hydrate structure requiring both large and small guest molecules to stabilize the structure This hydrate is expected to be isostructural with the hexagonal clathrasil dodecasil-lH (see ref 14 for clathrasil nomenclature) As for the cubic clathrate hydrates, the new hydrate structure may occur naturally

710 citations

Journal ArticleDOI
29 Mar 2001-Nature
TL;DR: It is reported that sphingosine-1-phosphate is a new calcium-mobilizing molecule in plants and evidence that this molecule is involved in the signal-transduction pathway linking the perception of abscisic acid to reductions in guard cell turgor is presented.
Abstract: Stomata form pores on leaf surfaces that regulate the uptake of CO2 for photosynthesis and the loss of water vapour during transpiration. An increase in the cytosolic concentration of free calcium ions ([Ca2+]cyt) is a common intermediate in many of the pathways leading to either opening or closure of the stomatal pore. This observation has prompted investigations into how specificity is controlled in calcium-based signalling systems in plants. One possible explanation is that each stimulus generates a unique increase in [Ca2+]cyt, or 'calcium signature', that dictates the outcome of the final response. It has been suggested that the key to generating a calcium signature, and hence to understanding how specificity is controlled, is the ability to access differentially the cellular machinery controlling calcium influx and release from internal stores. Here we report that sphingosine-1-phosphate is a new calcium-mobilizing molecule in plants. We show that after drought treatment sphingosine-1-phosphate levels increase, and we present evidence that this molecule is involved in the signal-transduction pathway linking the perception of abscisic acid to reductions in guard cell turgor.

328 citations

Journal ArticleDOI
TL;DR: In this paper, the crystal structure of α-methanol at 15K has been determined from neutron powder diffraction measurements, and the structure is orthorhombic, space group P212121.
Abstract: The crystal structure of α-methanol at 15K has been determined from neutron powder diffraction measurements. The structure is orthorhombic, space group P212121. The molecular geometry is found to be very similar to that in the gas phase, but the methyl group no longer has ideal 3-fold symmetry. The crystal is formed by infinite hydrogen-bonded chains of molecules with adjacent chains ‘pointing’ in opposite directions. The O-H … O hydrogen bonds are almost linear. No phase intermediate between the low temperature α-phase and the high temperature β-phase was found, but a new, metastable phase was discovered.

114 citations

Journal ArticleDOI
TL;DR: A theory of neutron diffraction in polycrystalline specimens of sulphur hexafluoride (SF6) and carbon tetrabromide (CBr4) was developed and applied in the interpretation of the neutron results as discussed by the authors.
Abstract: An investigation of the plastic phases of polycrystalline specimens of sulphur hexafluoride (SF6) and carbon tetrabromide (CBr4) by neutron elastic scattering and neutron diffraction experiments is reported. A theory of neutron diffraction in plastic crystals, which treats the Bragg scattering and the diffuse scattering from a unified point of view, is developed and applied in the interpretation of the neutron results. The Debye-Scherrer peaks are analysed, both by a cumulant expansion technique and a cubic harmonic analysis, to determine the crystal structures of the plastic phase which are found to be body-centred cubic (space group Im3m) for SF6 and face-centred cubic (space group Fm3m) for CBr4. The bond-orientation distribution function, f(Ω), has maxima in the directions for SF6 and in the directions for CBr4. Since, in both cases, f(Ω) is appreciably different from zero for all orientations, it is apparent that significant thermal reorientation takes place in both these crystals. The tr...

108 citations


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Journal ArticleDOI
TL;DR: Salt and drought stress signal transduction consists of ionic and osmotic homeostasis signaling pathways, detoxification (i.e., damage control and repair) response pathways, and pathways for growth regulation.
Abstract: Salt and drought stress signal transduction consists of ionic and osmotic homeostasis signaling pathways, detoxification (i.e., damage control and repair) response pathways, and pathways for growth regulation. The ionic aspect of salt stress is signaled via the SOS pathway where a calcium-responsive SOS3-SOS2 protein kinase complex controls the expression and activity of ion transporters such as SOS1. Osmotic stress activates several protein kinases including mitogen-activated kinases, which may mediate osmotic homeostasis and/or detoxification responses. A number of phospholipid systems are activated by osmotic stress, generating a diverse array of messenger molecules, some of which may function upstream of the osmotic stress-activated protein kinases. Abscisic acid biosynthesis is regulated by osmotic stress at multiple steps. Both ABA-dependent and -independent osmotic stress signaling first modify constitutively expressed transcription factors, leading to the expression of early response transcriptional activators, which then activate downstream stress tolerance effector genes.

5,328 citations

Journal ArticleDOI
20 Nov 2003-Nature
TL;DR: Natural gas hydrates have an important bearing on flow assurance and safety issues in oil and gas pipelines, they offer a largely unexploited means of energy recovery and transportation, and could play a significant role in past and future climate change.
Abstract: Natural gas hydrates are solid, non-stoichiometric compounds of small gas molecules and water. They form when the constituents come into contact at low temperature and high pressure. The physical properties of these compounds, most notably that they are non-flowing crystalline solids that are denser than typical fluid hydrocarbons and that the gas molecules they contain are effectively compressed, give rise to numerous applications in the broad areas of energy and climate effects. In particular, they have an important bearing on flow assurance and safety issues in oil and gas pipelines, they offer a largely unexploited means of energy recovery and transportation, and they could play a significant role in past and future climate change.

2,419 citations

Journal ArticleDOI
TL;DR: The responses of plants to salt and water stress are described, the regulatory circuits which allow plants to cope with stress are presented, and how the present knowledge can be applied to obtain tolerant plants is discussed.
Abstract: Agricultural productivity worldwide is subject to increasing environmental constraints, particularly to drought and salinity due to their high magnitude of impact and wide distribution. Traditional breeding programs trying to improve abiotic stress tolerance have had some success, but are limited by the multigenic nature of the trait. Tolerant plants such as Craterostigma plantagenium, Mesembryanthemum crystallinum, Thellungiella halophila and other hardy plants could be valuable tools to dissect the extreme tolerance nature. In the last decade, Arabidopsis thaliana, a genetic model plant, has been extensively used for unravelling the molecular basis of stress tolerance. Arabidopsis also proved to be extremely important for assessing functions for individual stress-associated genes due to the availability of knock-out mutants and its amenability for genetic transformation. In this review, the responses of plants to salt and water stress are described, the regulatory circuits which allow plants to cope wit...

2,351 citations

Journal ArticleDOI
TL;DR: Abscisic acid regulates many agronomically important aspects of plant development, including the synthesis of seed storage proteins and lipids, the promotion of seed desiccation tolerance and dormancy, and the inhibition of the phase transitions from embryonic to germinative growth and from.
Abstract: Abscisic acid (ABA) regulates many agronomically important aspects of plant development, including the synthesis of seed storage proteins and lipids, the promotion of seed desiccation tolerance and dormancy, and the inhibition of the phase transitions from embryonic to germinative growth and from

2,039 citations

Journal ArticleDOI
TL;DR: The evolutionarily conserved actions of the sphingolipid metabolite, sphingosine-1-phosphate (S1P), in yeast, plants and mammals have shown that it has important functions.
Abstract: The evolutionarily conserved actions of the sphingolipid metabolite, sphingosine-1-phosphate (S1P), in yeast, plants and mammals have shown that it has important functions. In higher eukaryotes, S1P is the ligand for a family of five G-protein-coupled receptors. These S1P receptors are differentially expressed, coupled to various G proteins, and regulate angiogenesis, vascular maturation, cardiac development and immunity, and are important for directed cell movement.

2,008 citations