7,803 research outputs found
The effects of leaf litter treatments, post-harvest urea and omission of early season fungicide sprays on the overwintering of apple scab on Bramley’s Seedling grown in a maritime environment.
peer-reviewedThe theory that orchards with zero or low levels of apple scab post harvest do not
need scab protection at the start of the next growing season was evaluated under Irish
conditions. In addition, a range of post-harvest orchard sanitation practices (application
of urea to rot overwintering leaves, mowing the orchard or total leaf removal in
February) were also evaluated. Due to the high summer rainfall in Ireland (compared
to all other European apple growing areas) and the severe susceptibility of the apple
cultivar Bramley’s Seedling to scab (Venturia inaequalis), neither clean orchards in
the autumn nor sanitation practices were sufficient to eliminate the requirement for
full fungicide protection programmes at the start of the following growing season.
Post harvest applications of urea proved difficult due to late harvesting of pollinator
fruit for the juice market and wet weather. Total removal of leaf litter from plots
prior to the commencement of growth did not significantly reduce disease incidence.
Regardless of orchard cleanliness in autumn, missing the first fungicide application
in the spring always reduced yield.This work was funded by the Department
of Agriculture and Rural Development, Northern Ireland
Explicit tensor network representation for the ground states of string-net models
The structure of string-net lattice models, relevant as examples of
topological phases, leads to a remarkably simple way of expressing their ground
states as a tensor network constructed from the basic data of the underlying
tensor categories. The construction highlights the importance of the fat
lattice to understand these models.Comment: 5 pages, pdf figure
Ultrasensitivity in phosphorylation-dephosphorylation cycles with little substrate
Cellular decision-making is driven by dynamic behaviours, such as the preparations for sunrise enabled by circadian rhythms and the choice of cell fates enabled by positive feedback. Such behaviours are often built upon ultrasensitive responses where a linear change in input generates a sigmoidal change in output. Phosphorylation-dephosphorylation cycles are one means to generate ultrasensitivity. Using bioinformatics, we show that in vivo levels of kinases and phosphatases frequently exceed the levels of their corresponding substrates in budding yeast. This result is in contrast to the conditions often required by zero-order ultrasensitivity, perhaps the most well known means for how such cycles become ultrasensitive. We therefore introduce a mechanism to generate ultrasensitivity when numbers of enzymes are higher than numbers of substrates. Our model combines distributive and non-distributive actions of the enzymes with two-stage binding and concerted allosteric transitions of the substrate. We use analytical and numerical methods to calculate the Hill number of the response. For a substrate with [Formula: see text] phosphosites, we find an upper bound of the Hill number of [Formula: see text], and so even systems with a single phosphosite can be ultrasensitive. Two-stage binding, where an enzyme must first bind to a binding site on the substrate before it can access the substrate's phosphosites, allows the enzymes to sequester the substrate. Such sequestration combined with competition for each phosphosite provides an intuitive explanation for the sigmoidal shifts in levels of phosphorylated substrate. Additionally, we find cases for which the response is not monotonic, but shows instead a peak at intermediate levels of input. Given its generality, we expect the mechanism described by our model to often underlay decision-making circuits in eukaryotic cells
Introduction to Categories and Categorical Logic
The aim of these notes is to provide a succinct, accessible introduction to
some of the basic ideas of category theory and categorical logic. The notes are
based on a lecture course given at Oxford over the past few years. They contain
numerous exercises, and hopefully will prove useful for self-study by those
seeking a first introduction to the subject, with fairly minimal prerequisites.
The coverage is by no means comprehensive, but should provide a good basis for
further study; a guide to further reading is included. The main prerequisite is
a basic familiarity with the elements of discrete mathematics: sets, relations
and functions. An Appendix contains a summary of what we will need, and it may
be useful to review this first. In addition, some prior exposure to abstract
algebra - vector spaces and linear maps, or groups and group homomorphisms -
would be helpful.Comment: 96 page
Preliminary evaluation of infrared and radar imagery, Washington and Oregon coasts
Airborne infrared and radar photography of Oregon and Washington coastal region
Dynamical Expansion of H II Regions from Ultracompact to Compact Sizes in Turbulent, Self-Gravitating Molecular Clouds
The nature of ultracompact H II regions (UCHRs) remains poorly determined. In
particular, they are about an order of magnitude more common than would be
expected if they formed around young massive stars and lasted for one dynamical
time, around 10^4 yr. We here perform three-dimensional numerical simulations
of the expansion of an H II region into self-gravitating, radiatively cooled
gas, both with and without supersonic turbulent flows. In the laminar case, we
find that H II region expansion in a collapsing core produces nearly spherical
shells, even if the ionizing source is off-center in the core. This agrees with
analytic models of blast waves in power-law media. In the turbulent case, we
find that the H II region does not disrupt the central collapsing region, but
rather sweeps up a shell of gas in which further collapse occurs. Although this
does not constitute triggering, as the swept-up gas would eventually have
collapsed anyway, it does expose the collapsing regions to ionizing radiation.
We suggest that these regions of secondary collapse, which will not all
themselves form massive stars, may form the bulk of observed UCHRs. As the
larger shell will take over 10^5 years to complete its evolution, this could
solve the timescale problem. Our suggestion is supported by the ubiquitous
observation of more diffuse emission surrounding UCHRs.Comment: accepted to ApJ, 40 pages, 13 b/w figures, changes from v1 include
analytic prediction of radio luminosity, better description of code testing,
and many minor changes also in response to refere
The Influence of Metallicity on Star Formation in Protogalaxies
In cold dark matter cosmological models, the first stars to form are believed
to do so within small protogalaxies. We wish to understand how the evolution of
these early protogalaxies changes once the gas forming them has been enriched
with small quantities of heavy elements, which are produced and dispersed into
the intergalactic medium by the first supernovae. Our initial conditions
represent protogalaxies forming within a fossil H II region, a previously
ionized region that has not yet had time to cool and recombine. We study the
influence of low levels of metal enrichment on the cooling and collapse of
ionized gas in small protogalactic halos using three-dimensional, smoothed
particle hydrodynamics (SPH) simulations that incorporate the effects of the
appropriate chemical and thermal processes. Our previous simulations
demonstrated that for metallicities Z < 0.001 Z_sun, metal line cooling alters
the density and temperature evolution of the gas by less than 1% compared to
the metal-free case at densities below 1 cm-3) and temperatures above 2000 K.
Here, we present the results of high-resolution simulations using particle
splitting to improve resolution in regions of interest. These simulations allow
us to address the question of whether there is a critical metallicity above
which fine structure cooling from metals allows efficient fragmentation to
occur, producing an initial mass function (IMF) resembling the local Salpeter
IMF, rather than only high-mass stars.Comment: 3 pages, 2 figures, First Stars III conference proceeding
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