199 research outputs found
Liquid 4He near the superfluid transition in the presence of a heat current and gravity
The effects of a heat current and gravity in liquid 4He near the superfluid
transition are investigated for temperatures above and below T_lambda. We
present a renormalization-group calculation based on model F for the Green's
function in a self-consistent approximation which in quantum many-particle
theory is known as the Hartree approximation. The approach can handle a zero
average order parameter above and below T_lambda and includes effects of
vortices. We calculate the thermal conductivity and the specific heat for all
temperatures T and heat currents Q in the critical regime. Furthermore, we
calculate the temperature profile. Below T_lambda we find a second correlation
length which describes the dephasing of the order parameter field due to
vortices. We find dissipation and mutual friction of the superfluid-normal
fluid counterflow and calculate the Gorter-Mellink coefficient A. We compare
our theoretical results with recent experiments.Comment: 26 pages, 9 figure
Surprising flowering response to photoperiod: Preliminary characterization of West and Central African pearl millet germplasm
Pearl millet (Pennisetum glaucum) is considered to be a
short-day species that flowers, or flowers earlier, when
day lengths are short. A few studies with two to six
planting dates and few selected entries have been
conducted in USA (Burton 1965), Senegal (Ramond
1968), and India (Patil et al. 1978, Das 1991). However,
there is no known research on the flowering response of
pearl millet to photoperiod changes over the entire year.
Likewise, knowledge about the photoperiod-sensitivity
in West and Central African pearl millets is insufficient
Evaluation of medium maturity, high-tillering Pearl Millet population diallel in Niger
Pearl millet [Pennisetum glaucum (L.) R. Br.] is the most
important staple crop of millions of people in the semiarid
and arid regions of Asia and sub-Saharan Africa. In
1998, ICRISAT pearl millet scientists in southern and
western Africa and India initiated global pearl millet diallel crosses with the aim of promoting systematic diversification of elite regional germplasm (Bidinger et al. 2000). The project produced trait-based sets of diallel crosses among elite landrace and breeding materials from each of the three major pearl millet-growing regions. This study evaluates the potential of the medium-maturity, hightillering population diallel as new source material for use in breeding programs in the Sahelian zone of West
Africa, especially Niger
A First-Landau-Level Laughlin/Jain Wave Function for the Fractional Quantum Hall Effect
We show that the introduction of a more general closed-shell operator allows
one to extend Laughlin's wave function to account for the richer hierarchies
(1/3, 2/5, 3/7 ...; 1/5, 2/9, 3/13, ..., etc.) found experimentally. The
construction identifies the special hierarchy states with condensates of
correlated electron clusters. This clustering implies a single-particle (ls)j
algebra within the first Landau level (LL) identical to that of multiply filled
LLs in the integer quantum Hall effect. The end result is a simple generalized
wave function that reproduces the results of both Laughlin and Jain, without
reference to higher LLs or projection.Comment: Revtex. In this replacement we show how to generate the Jain wave
function explicitly, by acting with the generalized ls closed-shell operator
discussed in the original version. We also walk the reader through a
classical 1d caricature of this problem so that he/she can better understand
why 2s+1, where s is the spin, should be associated with the number of
electrons associated with the underlying clusters or composites. 11 page
Multiplication and preliminary characterization of west and central African pearl millet landraces
West Africa is a center of origin and diversity of pearl millet [Pennisetum glaucum (L.) R. Br.], but this diversity is neither well-understood nor fully accessible to and exploited by NARS breeders and farmers. The objective of the present study was to multiply and initially characterize 281 pearl millet accessions from all over West and Central Africa, with the final aim of promoting a more systematic and targeted exploitation of genetic diversity in adapted germplasm in West African pearl millet improvement programs
Improved methodologies for breeding striga-resistant sorghums
Parasitic flowering weeds of the genus Striga (Scrophulariaceae) cause substantial losses in sorghum [Sorghum bicolor (L.) Moench] production in sub-Saharan Africa. Striga-resistant sorghum cultivars could be a major component of integrated striga management, if resistance was available in adapted, productive germplasm. In this paper we review methodologies for breeding striga-resistant sorghums. The agar-gel assay is an excellent tool to screen host genotypes in the laboratory for low production of the striga seed germination stimulant. Further laboratory assays are needed which allow the non-destructive, rapid and inexpensive evaluation of individual plants for additional resistance mechanisms. Field screening for striga resistance is hampered by high microvariability in African soils, heterogeneity of natural infestations, and concomitant large environmental effects on striga emergence. An improved field testing methodology should include one or several of the following practices: field inoculation with striga seeds; appropriate experimental design including elevated replication number; specific plot layout; use of appropriate susceptible and resistant checks; evaluation in adjacent infested and uninfested plots; and the use of selection indices derived from emerged striga counts, striga vigor, and grain yield or a host plant damage score. Due to the extreme variability of the parasite and significant genotype×environment interaction effects, multi-locational screening is recommended to obtain materials with stable performance. Additional strategies include: careful definition of the target environments; determination of the most important selection traits in each target environment; characterization of crop germplasm and improvement of available sources of resistance for better agronomic performance; transfer and pyramiding of resistance genes into adapted, farmer-selected cultivars; development of striga-resistant parent lines for hybrid or synthetic cultivars; and development of random-mating populations with multiple sources of resistance. The development of marker-assisted selection techniques for broad-based, polygenic striga resistance is underway. This approach is particularly promising because striga resistance tests are difficult, expensive, and sometimes unreliable; the parasite is quarantined; and some resistance genes are recessive. Transgenic, herbicide-tolerant sorghums could contribute to an immediate, cost-effective control of striga by herbicides, but such cultivars are not yet available. The selection of sorghum cultivars with specific adaptation to integrated striga management approaches could contribute to sustainable sorghum production in striga-infested areas of sub-Saharan Afric
Mechanisms of adaptation to climate variability in West African pearl millet landraces – a preliminary
Landraces are generally expected to possess specific
mechanisms of adaptation to their growing environments.
In West and Central Africa (WCA), growing conditions
of pearl millet (Pennisetum glaucum) are characterized,
among other hazards, by highly variable beginnings and
endings of the rainy season, and unpredictable drought
stress at any time during the growing season. Adaptation
to such unpredictable variable environment can be due to
individual and/or populational buffering (Allard and
Bradshaw 1964), two mechanisms initially defined by
Lerner (1954) as developmental and genetic
homeostasis. Individual buffering may be favored by
phenotypic plasticity. Photoperiod-sensitive flowering is
an example of phenotypic plasticity that can enhance
adaptation to variable planting dates followed due to a
scattered beginning of the rainy season in a region, as
typical for WCA. It enhances simultaneous flowering of
the cultivar in the target region, independent of the
individual date of planting in different fields. This has
particular advantages in terms of reducing bird damage
and insect pressure; prolonging vegetative development
in case of early planting but accelerating development in
case of late planting; therefore fitting plant development
to available rainfall patterns and resulting potentially in
increased yielding stability. Populational buffering can
be promoted by genetic heterogeneity in plant stand as
different genotypes present in the population are
specifically adapted to different environmental
conditions (Bradshaw 1965). An example is intra-varietal
variation for flowering time, which would assure that in
case of a dry spell, not all plants in the field will be
affected by drought in their most sensitive flowering
stage
Dobrushin states in the \phi^4_1 model
We consider the van der Waals free energy functional in a bounded interval
with inhomogeneous Dirichlet boundary conditions imposing the two stable phases
at the endpoints. We compute the asymptotic free energy cost, as the length of
the interval diverges, of shifting the interface from the midpoint. We then
discuss the effect of thermal fluctuations by analyzing the \phi^4_1-measure
with Dobrushin boundary conditions. In particular, we obtain a nontrivial limit
in a suitable scaling in which the length of the interval diverges and the
temperature vanishes. The limiting state is not translation invariant and
describes a localized interface. This result can be seen as the probabilistic
counterpart of the variational convergence of the associated excess free
energy.Comment: 34 page
Selection methods Part 4: Developing open-pollinated varieties using recurrent selection methods
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