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Search Results: 1 - 10 of 190687 matches for " G. Neumann "
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Root exudation and root development of lettuce (Lactuca sativa L.cv. Tizian) as affected by different soils
Günter Neumann
Frontiers in Microbiology , 2014, DOI: 10.3389/fmicb.2014.00002
Abstract: Development and activity of plant roots exhibits high adaptive variability. Although it is well-documented, that physicochemical soil properties can strongly influence root morphology and root exudation, particularly under field conditions, a comparative assessment is complicated by the impact of additional factors, such as climate and cropping history. To overcome these limitations, in this study, field soils originating from an unique experimental plot system with three different soil types, which were stored at the same field site for ten years and exposed to the same agricultural management practice, were used for an investigation on effects of soil type on root development and root exudation. Lettuce (Lactuca sativa L. cv. Tizian) was used as a model plant, grown under controlled environmental conditions in a minirhizotrone system equipped with root observation windows (rhizoboxes). Root exudates were collected by placing sorption filters onto the root surface followed by subsequent extraction and GC-MS profiling of the trapped compounds. Surprisingly, even in absence of external stress factors with known impact on root exudation, such as pH extremes, water and nutrient limitations/toxicities or soil structure effects (use of sieved soils), root growth characteristics (root length, fine root development) as well as profiles of root exudates were strongly influenced by the soil type used for plant cultivation. The results coincided well with differences in rhizosphere bacterial communities, detected in field-grown lettuce plants cultivated on the same soils (Schreiter et al., this issue). The findings suggest that the observed differences may be the result of plant interactions with the soil-specific microbiomes.
Compact compatible topologies for graphs with small cycles
Victor Neumann-Lara,Richard G. Wilson
International Journal of Mathematics and Mathematical Sciences , 2005, DOI: 10.1155/ijmms.2005.2195
Abstract: A topology τ on the vertices of a comparability graph G is said to be compatible with G if each subgraph H of G is graph-connected if and only if it is a connected subspace of (G,τ). In two previous papers we considered the problem of finding compatible topologies for a given comparability graph and we proved that the nonexistence of infinite paths was a necessary and sufficient condition for the existence of a compact compatible topology on a tree (that is to say, a connected graph without cycles) and we asked whether this condition characterized the existence of a compact compatible topology on a comparability graph in which all cycles are of length at most n. Here we prove an extension of the above-mentioned theorem to graphs whose cycles are all of length at most five and we show that this is the best possible result by exhibiting a comparability graph in which all cycles are of length 6, with no infinite paths, but which has no compact compatible topology.
A study of the elemental reactions involved in the initiation of the polymerization of tetrahydrofuran induced by the photosensitization of a triphenylsulphonium salt by perylene
Neumann, Miguel G.;Rodrigues, Maira R.;
Journal of the Brazilian Chemical Society , 2003, DOI: 10.1590/S0103-50532003000100013
Abstract: the photopolymerization of thf was studied in the presence of a triphenylsulphonium salt and perylene. the mechanism involves an electron transfer from the excited singlet state of perylene to triphenylsulphonium ions, resulting in perylene cation radicals that react with tetrahydrofuran. the calculated apparent yield of cation radicals follows the same general trend than the measured polymerization rates.
Fotopolimeriza??o: princípios e métodos
Rodrigues, Máira R.;Neumann, Miguel G.;
Polímeros , 2003, DOI: 10.1590/S0104-14282003000400013
Abstract: the interest for polymer photochemistry has been growing in the last decades as a consequence not only of the great number of new applications as well as due to the repercussion of economical, technical and ecological characters. the photopolymerization - photocuring processes have received special attention due to the countless and important applications in the area of materials. the kinetics of these processes needs fast and precise analytic methods to be evaluated. among the conventional analytic methods, the most used are gravimetry and the determination of macroscopic properties, as resistance to traction, deformation and hardness. however, for a better insight in the reactions it is convenient to use more advanced analytic methods, in order to study the photopolymerization kinetics in real time. dilatometry, raman spectroscopy, calorimetry, and real time fourier transform infrared spectroscopy are some of the techniques that have been used. activity in the field of photopolymerization continues to expand in many areas and recently, the interest in photoinitiation systems involving the initiation of free-radical polymerization with visible light irradiation has been retaken. additionally, studies on cationic photopolymerization induced by visible light started again to be studied because of the large variety of monomers that can be photopolymerized by via cation initiation, but not by free radicals, such as oxiranes and vinyl ethers.
A fotoquímica no Brasil
Neumann, Miguel G.;Quina, Frank H.;
Química Nova , 2002, DOI: 10.1590/S0100-40422002000800007
Abstract: an historical overview is provided of the origins, growth and development of the field of photochemistry in brazil, with special emphasis on the last three decades.
Fotopolimeriza o: princípios e métodos
Rodrigues Máira R.,Neumann Miguel G.
Polímeros , 2003,
Abstract: O interesse na Fotoquímica de Polímeros tem crescido nas últimas décadas devido n o somente ao grande número de novas aplica es como também à repercuss o do ponto de vista econ mico, técnico e ecológico. A fotopolimeriza o/fotocura tem recebido a aten o especial devido às incontáveis aplica es e a sua importancia na área de Materiais. A cinética destes processos necessita de métodos analíticos precisos e rápidos para ser avaliada. Entre os métodos analíticos convencionais, os mais usados s o a gravimetria e a medida de propriedades macroscópicas, como resistência à tra o, à deforma o e dureza. Entretanto, para determina es mais analíticas é conveniente usar métodos que permitam estudar a cinética de fotopolimeriza o em tempo real. Neste sentido têm sido usadas técnicas como dilatometria, espectroscopia Raman, calorimetria, e espectroscopia de infravermelho por transformada de Fourier em tempo real. A atividade no campo da polimeriza o continua a se expandir em muitas áreas e, recentemente, o interesse nos sistemas do fotoinicia o envolvendo a polimeriza o via radicais livres, iniciada por luz visível, tem recebido aten o especial. Recentemente, estudos sobre fotopolimeriza o cati nica induzida pela luz visível come aram a ser pesquisados novamente devido à grande variedade de mon meros que s o polimerizáveis por via cati nica mas n o por via radicar, como, por exemplo, oxiranas e éteres vinílicos.
A fotoquímica no Brasil
Neumann Miguel G.,Quina Frank H.
Química Nova , 2002,
Abstract: An historical overview is provided of the origins, growth and development of the field of photochemistry in Brazil, with special emphasis on the last three decades.
A study of the elemental reactions involved in the initiation of the polymerization of tetrahydrofuran induced by the photosensitization of a triphenylsulphonium salt by perylene
Neumann Miguel G.,Rodrigues Maira R.
Journal of the Brazilian Chemical Society , 2003,
Abstract: The photopolymerization of THF was studied in the presence of a triphenylsulphonium salt and perylene. The mechanism involves an electron transfer from the excited singlet state of perylene to triphenylsulphonium ions, resulting in perylene cation radicals that react with tetrahydrofuran. The calculated apparent yield of cation radicals follows the same general trend than the measured polymerization rates.
Phonon Properties of Knbo3 and Ktao3 from First-Principles Calculations
A. V. Postnikov,T. Neumann,G. Borstel
Physics , 1994, DOI: 10.1103/PhysRevB.50.758
Abstract: The frequencies of transverse-optical $\Gamma$ phonons in KNbO$_3$ and KTaO$_3$ are calculated in the frozen-phonon scheme making use of the full-potential linearized muffin-tin orbital method. The calculated frequencies in the cubic phase of KNbO$_3$ and in the tetragonal ferroelectric phase are in good agreement with experimental data. For KTaO$_3$, the effect of lattice volume was found to be substantial on the frequency of the soft mode, but rather small on the relative displacement patterns of atoms in all three modes of the $T_{1u}$ symmetry. The TO frequencies in KTaO$_3$ are found to be of the order of, but somehow higher than, the corresponding frequencies in cubic KNbO$_3$.
The Aharonov-Bohm effect: the role of tunneling and associated forces
G. C. Hegerfeldt,J. T. Neumann
Mathematics , 2008, DOI: 10.1088/1751-8113/41/15/155305
Abstract: Through tunneling, or barrier penetration, small wavefunction tails can enter a finitely shielded cylinder with a magnetic field inside. When the shielding increases to infinity the Lorentz force goes to zero together with these tails. However, it is shown, by considering the radial derivative of the wavefunction on the cylinder surface, that a flux dependent force remains. This force explains in a natural way the Aharonov-Bohm effect in the idealized case of infinite shielding.
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