14,012 research outputs found
The Functional Significance of Black-Pigmented Leaves: Photosynthesis, Photoprotection and Productivity in Ophiopogon planiscapus ‘Nigrescens’
Black pigmented leaves are common among horticultural cultivars, yet are extremely rare across natural plant populations. We hypothesised that black pigmentation would disadvantage a plant by reducing photosynthesis and therefore shoot productivity, but that this trait might also confer protective benefits by shielding chloroplasts against photo-oxidative stress. CO2 assimilation, chlorophyll a fluorescence, shoot biomass, and pigment concentrations were compared for near isogenic green- and black-leafed Ophiopogon planiscapus ‘Nigrescens’. The black leaves had lower maximum CO2 assimilation rates, higher light saturation points and higher quantum efficiencies of photosystem II (PSII) than green leaves. Under saturating light, PSII photochemistry was inactivated less and recovered more completely in the black leaves. In full sunlight, green plants branched more abundantly and accumulated shoot biomass quicker than the black plants; in the shade, productivities of the two morphs were comparable. The data indicate a light-screening, photoprotective role of foliar anthocyanins. However, limitations to photosynthetic carbon assimilation are relatively small, insufficient to explain the natural scarcity of black-leafed plants
Near mean-field behavior in the generalized Burridge-Knopoff earthquake model with variable range stress transfer
Simple models of earthquake faults are important for understanding the
mechanisms for their observed behavior in nature, such as Gutenberg-Richter
scaling. Because of the importance of long-range interactions in an elastic
medium, we generalize the Burridge-Knopoff slider-block model to include
variable range stress transfer. We find that the Burridge-Knopoff model with
long-range stress transfer exhibits qualitatively different behavior than the
corresponding long-range cellular automata models and the usual
Burridge-Knopoff model with nearest-neighbor stress transfer, depending on how
quickly the friction force weakens with increasing velocity. Extensive
simulations of quasiperiodic characteristic events, mode-switching phenomena,
ergodicity, and waiting-time distributions are also discussed. Our results are
consistent with the existence of a mean-field critical point and have important
implications for our understanding of earthquakes and other driven dissipative
systems.Comment: 24 pages 12 figures, revised version for Phys. Rev.
Quantum Hall effect in narrow graphene ribbons
The edge states in the integer quantum Hall effect are known to be
significantly affected by electrostatic interactions leading to the formation
of compressible and incompressible strips at the boundaries of Hall bars. We
show here, in a combined experimental and theoretical analysis, that this does
not hold for the quantum Hall effect in narrow graphene ribbons. In our
graphene Hall bar, which is only 60 nm wide, we observe the quantum Hall effect
up to Landau level index k=2 and show within a zero free-parameter model that
the spatial extent of the compressible and incompressible strips is of a
similar magnitude as the magnetic length. We conclude that in narrow graphene
ribbons the single-particle picture is a more appropriate description of the
quantum Hall effect and that electrostatic effects are of minor importance.Comment: RevTex, 5 pages, 4 figures (matches published version
Simulation of the Burridge-Knopoff Model of Earthquakes with Variable Range Stress Transfer
Simple models of earthquake faults are important for understanding the
mechanisms for their observed behavior, such as Gutenberg-Richter scaling and
the relation between large and small events, which is the basis for various
forecasting methods. Although cellular automaton models have been studied
extensively in the long-range stress transfer limit, this limit has not been
studied for the Burridge-Knopoff model, which includes more realistic friction
forces and inertia. We find that the latter model with long-range stress
transfer exhibits qualitatively different behavior than both the long-range
cellular automaton models and the usual Burridge-Knopoff model with nearest
neighbor springs, depending on the nature of the velocity-weakening friction
force. This result has important implications for our understanding of
earthquakes and other driven dissipative systems.Comment: 4 pages, 5 figures, published on Phys. Rev. Let
Nearby Microlensing Events - Identification of the Candidates for the SIM
The Space Interferometry Mission (SIM) is the instrument of choice when it
comes to observing astrometric microlensing events where nearby, usually
high-proper-motion stars (``lenses''), pass in front of more distant stars
(``sources''). Each such encounter produces a deflection in the source's
apparent position that when observed by SIM can lead to a precise mass
determination of the nearby lens star. We search for lens-source encounters
during the 2005-2015 period using Hipparcos, ACT and NLTT to select lenses, and
USNO-A2.0 to search for the corresponding sources, and rank these by the SIM
time required for a 1% mass measurement.
For Hipparcos and ACT lenses, the lens distance and lens-source impact
parameter are precisely determined so the events are well characterized. We
present 32 candidates beginning with a 61 Cyg A event in 2012 that requires
only a few minutes of SIM time. Proxima Centauri and Barnard's star each
generate several events. For NLTT lenses, the distance is known only to a
factor of 3, and the impact parameter only to 1''. Together, these produce
uncertainties of a factor ~10 in the amount of SIM time required. We present a
list of 146 NLTT candidates and show how single-epoch CCD photometry of the
candidates could reduce the uncertainty in SIM time to a factor of ~1.5.Comment: ApJ accepted, 31 pages (inc. 5 tables), 5 figures. t SIM refine
Observations of a propagating vortex in a tidal current
While observing the hydrodynamics and geomorphology of the entrance to Burrill Lake, a small estuary on the south coast of New South Wales, Australia, a striking vortex phenomenon was observed. This vortex is described and interpreted
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