22,805 research outputs found
Role of magnesium in carbon partitioning and alleviating photooxidative damage
Magnesium (Mg) deficiency exerts a major influence on the partitioning of
drymatter and carbohydrates between shoots and roots. One of the very early
reactions of plants to Mg deficiency stress is themarked increase in the shootto-
root dry weight ratio, which is associated with a massive accumulation of
carbohydrates in source leaves, especially of sucrose and starch. These higher
concentrations of carbohydrates in Mg-deficient leaves together with the
accompanying increase in shoot-to-root dry weight ratio are indicative of
a severe impairment in phloem export of photoassimilates from source
leaves. Studies with common bean and sugar beet plants have shown that
Mg plays a fundamental role in phloem loading of sucrose. At a very early
stage of Mg deficiency, phloem export of sucrose is severely impaired, an
effect that occurs before any noticeable changes in shoot growth, Chl
concentration or photosynthetic activity. These findings suggest that accumulation
of carbohydrates in Mg-deficient leaves is caused directly by Mg
deficiency stress and not as a consequence of reduced sink activity. The role
of Mg in the phloem-loading process seems to be specific; resupplying Mg for
12 or 24 h to Mg-deficient plants resulted in a very rapid recovery of sucrose
export. It appears that the massive accumulation of carbohydrates and related
impairment in photosynthetic CO2 fixation in Mg-deficient leaves cause an
over-reduction in the photosynthetic electron transport chain that potentiates
the generation of highly reactive O2 species (ROS). Plants respond to Mg
deficiency stress by marked increases in antioxidative capacity of leaves,
especially under high light intensity, suggesting that ROS generation is
stimulated by Mg deficiency in chloroplasts. Accordingly, it has been found
that Mg-deficient plants are very susceptible to high light intensity. Exposure
of Mg-deficient plants to high light intensity rapidly induced leaf chlorosis
and necrosis, an outcome that was effectively delayed by partial shading of
the leaf blade, although the Mg concentrations in different parts of the leaf
blade were unaffected by shading. The results indicate that photooxidative
damage contributes to development of leaf chlorosis under Mg deficiency,
suggesting that plants under high-light conditions have a higher physiological
requirement for Mg. Maintenance of a high Mg nutritional status of plants is,
thus, essential in the avoidance of ROS generation, which occurs at the
expense of inhibited phloem export of sugars and impairment of CO2
fixation, particularly under high-light conditions
Leptonic Pion Decay And Physics Beyond The Electroweak Standard Model
The ratio of branching ratios in leptonic pion decay is a powerfully
sensitive probe of new interactions beyond the electroweak standard model. This
is due to the chirality suppression of the standard model amplitude for the
decay, which results in a precise prediction for the ratio, and suppressed
amplitudes for new contributions to interfere with. We calculate, including QCD
corrections, the contributions to arising from a broad selection of
standard model extensions to which it is sensitive, including: R-parity
violating interactions in supersymmetric theories, theories with light
(electroweak scale) leptoquark degrees of freedom, non-minimal models of extra
doublet Higgs bosons, models in which the quarks and leptons are composite both
with and without supersymmetry, and models with strong TeV scale gravitational
interactions. Comparing with existing measurements of we provide
limits on each of these classes of models; our calculations also represent
state of the art theoretical benchmarks against which the results from the
upcoming round of leptonic pion decay experiments may be compared.Comment: 31 pages, 3 figure
Performance of treated and untreated asymmetric polysulfone hollow fiber membrane in series and cascade module configurations for CO2/CH4 gas separation system
This study investigates the effects of one-, two- and three-stage membrane system configurations in series arrangement for theCO2/CH4 separation for both untreated and treated membranes. Asymmetric polysulfone hollow fiber membranes were fabricated from 33 wt.% of polysulfone polymer using dry/wet phase inversion process. The produced membranes were characterized by pure gas permeation experiments, Differential Scanning Calorimetry (DSC), Scanning Electron Microscopy (SEM), density measurement and Thermogravimetric Analysis (TGA). For both untreated and treated membranes, the pressure-normalized flux of CO2 decreased with increasing of the membrane stages. In addition, the selectivities of asymmetric hollow fiber membrane showed a more constant trend with feed pressure. Treated membrane exhibited lower pressure-normalized flux than untreated membranes due to skin layer densification which increased the transport resistance, thus lead to the reduction in pressure-normalized fluxes. Among all the three configurations studied, two-stage membrane configuration showed the most constant trend in term of selectivity. However, three-stage cascade configuration produced the highest CO2/CH4 selectivity especially when tested at low feed pressure range. Effect of stage cut on feed pressure showed an increasing trend with increasing of CO2 and CH4 feed pressure for all configurations. This is due to the increase of the permeation driving force, which caused the passage of larger amounts of more permeable gas through the membrane. This study showed that, three-stage cascade configuration exhibited the smallest stage cut values thus produced higher purity of CO2 in permeate stream
New Algebraic Formulation of Density Functional Calculation
This article addresses a fundamental problem faced by the ab initio
community: the lack of an effective formalism for the rapid exploration and
exchange of new methods. To rectify this, we introduce a novel, basis-set
independent, matrix-based formulation of generalized density functional
theories which reduces the development, implementation, and dissemination of
new ab initio techniques to the derivation and transcription of a few lines of
algebra. This new framework enables us to concisely demystify the inner
workings of fully functional, highly efficient modern ab initio codes and to
give complete instructions for the construction of such for calculations
employing arbitrary basis sets. Within this framework, we also discuss in full
detail a variety of leading-edge ab initio techniques, minimization algorithms,
and highly efficient computational kernels for use with scalar as well as
shared and distributed-memory supercomputer architectures
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A netnography study to uncover the underlying dimensions of customer experience with resort brands
The interest in customer experience has increased at a phenomenal rate. However, research to capture the true meaning of the concept is limited. This study aims to address the question of what are the underlying dimensions that constitute the construct of customer experience. The netnography method is utilized to validate a priori concepts that have been identified in the literature within the context of resort-hotel brands in a Sharm El Sheikh resort in Egypt. The results identified eight dimensions; comfort, educational, hedonic, novelty, recognition, relational, safety and sense of beauty which are consistent with major studies on experience
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