2,781 research outputs found
Regulation of nitrogen uptake and assimilation: Effects of nitrogen source, root-zone pH, and aerial CO2 concentration on growth and productivity of soybeans
An important feature of controlled-environment crop production systems such as those to be used for life support of crews during space exploration is the efficient utilization of nitrogen supplies. Making decisions about the best sources of these supplies requires research into the relationship between nitrogen source and the physiological processes which regulate vegetative and reproductive plant growth. Work done in four areas within this research objective is reported: (1) experiments on the effects of root-zone pH on preferential utilization of NO3(-) versus NH4(+) nitrogen; (2) investigation of processes at the whole-plant level that regulate nitrogen uptake; (3) studies of the effects of atmospheric CO2 and NO3(-) supply on the growth of soybeans; and (4) examination of the role of NO3(-) uptake in enhancement of root respiration
Nitrogen uptake and utilization by intact plants
The results of experiments support the proposed conceptual model that relates nitrogen uptake activity by plants as a balanced interdependence between the carbon-supplying function of the shoot and the nitrogen-supplying function of the roots. The data are being used to modify a dynamic simulation of plant growth, which presently describes carbon flows through the plant, to describe nitrogen uptake and assimilation within the plant system. Although several models have been proposed to predict nitrogen uptake and partitioning, they emphasize root characteristics affecting nutrient uptake and relay on empirical methods to describe the relationship between nitrogen and carbon flows within the plant. Researchers, on the other hand, propose to continue to attempt a mechanistic solution in which the effects of environment on nitrogen (as well as carbon) assimilation are incorporated through their direct effects on photosynthesis, respiration, and aging processes
Book Review: State Supreme Courts in State and Nation. by G. Alan Tarr and Mary Cornelia Aldis Porter.
Book review: State Supreme Courts in State and Nation. By G. Alan Tarr and Mary Cornelia Aldis Porter. New Haven, Conn.: Yale University Press. 1988. Pp. 288. Reviewed by: Michael C. Tolley
Exact Wave Solutions to 6D Gauged Chiral Supergravity
We describe a broad class of time-dependent exact wave solutions to 6D gauged
chiral supergravity with two compact dimensions. These 6D solutions are
nontrivial warped generalizations of 4D pp-waves and Kundt class solutions and
describe how a broad class of previously-static compactifications from 6D to 4D
(sourced by two 3-branes) respond to waves moving along one of the
uncompactified directions. Because our methods are generally applicable to any
higher dimensional supergravity they are likely to be of use for finding the
supergravity limit of time-dependent solutions in string theory. The 6D
solutions are interesting in their own right, describing 6D shock waves induced
by high energy particles on the branes, and as descriptions of the near-brane
limit of the transient wavefront arising from a local bubble-nucleation event
on one of the branes, such as might occur if a tension-changing phase
transition were to occur.Comment: 22 pages, 1 figure. Minor clarifications added. Accepted in JHE
On Black Holes in Massive Gravity
In massive gravity the so-far-found black hole solutions on Minkowski space
happen to convert horizons into a certain type of singularities. Here we
explore whether these singularities can be avoided if space-time is not
asymptotically Minkowskian. We find an exact analytic black hole (BH) solution
which evades the above problem by a transition at large scales to self-induced
de Sitter (dS) space-time, with the curvature scale set by the graviton mass.
This solution is similar to the ones discovered by Koyama, Niz and Tasinato,
and by Nieuwenhuizen, but differs in detail. The solution demonstrates that in
massive GR, in the Schwarzschild coordinate system, a BH metric has to be
accompanied by the St\"uckelberg fields with nontrivial backgrounds to prevent
the horizons to convert into the singularities. We also find an analogous
solution for a Reissner-Nordstr\"om BH on dS space. A limitation of our
approach, is that we find the solutions only for specific values of the two
free parameters of the theory, for which both the vector and scalar
fluctuations loose their kinetic terms, however, we hope our solutions
represent a broader class with better behaved perturbations.Comment: 17 LateX page
Fur Trappers in the Northwest Territories: An Econometric Analysis of the Factors Influencing Participation
Commercial fur trapping, once the primary economic activity of the aboriginal inhabitants of the Northwest Territories, now accounts for only a small fraction of the income received by native people. Many adult native males do continue to engage in commercial trapping, nevertheless, though with varying degrees of commitment. A review of the recent literature reveals a wide variety of suggested motivations for this continuing involvement. Through the use of econometric techniques we are able to analyze the motivations of two distinct sub-groups of trappers. One group, accounting for about 15% of those who trap, has a substantial commitment to the activity and is motivated primarily by the income-earning potential of fur sales. The second group, which includes approximately 85% of the participants, consists of those whose participation is best explained by the lack of alternative employment opportunities.
String Propagation through a Big Crunch/Big Bang Transition
We consider the propagation of classical and quantum strings on cosmological
space-times which interpolate from a collapsing phase to an expanding phase. We
begin by considering the classical propagation of strings on space-times with
isotropic and anisotropic cosmological singularities. We find that cosmological
singularities fall into two classes, in the first class the string evolution is
well behaved all the way up to the singularity, whilst in the second class it
becomes ill-defined. Then assuming the singularities are regulated by string
scale corrections, we consider the implications of the propagation through a
`bounce'. It is known that as we evolve through a bounce, quantum strings will
become excited giving rise to `particle transmutation'. We reconsider this
effect, giving qualitative arguments for the amount of excitation for each
class. We find that strings whose physical wavelength at the bounce is less
that inevitably emerge in highly excited states, and that in
this regime there is an interesting correspondence between strings on
anisotropic cosmological space-times and plane waves. We argue that long
wavelength modes, such as those describing cosmological perturbations, will
also emerge in mildly excited string scale mass states. Finally we discuss the
relevance of this to the propagation of cosmological perturbations in models
such as the ekpyrotic/cyclic universe.Comment: 15 page
Scaling Solutions to 6D Gauged Chiral Supergravity
We construct explicitly time-dependent exact solutions to the field equations
of 6D gauged chiral supergravity, compactified to 4D in the presence of up to
two 3-branes situated within the extra dimensions. The solutions we find are
scaling solutions, and are plausibly attractors which represent the late-time
evolution of a broad class of initial conditions. By matching their near-brane
boundary conditions to physical brane properties we argue that these solutions
(together with the known maximally-symmetric solutions and a new class of
non-Lorentz-invariant static solutions, which we also present here) describe
the bulk geometry between a pair of 3-branes with non-trivial on-brane
equations of state.Comment: Contribution to the New Journal of Physics focus issue on Dark
Energy; 28 page
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