Modulated structures in electroconvection in nematic liquid crystals

Motivated by experiments in electroconvection in nematic liquid crystals with homeotropic alignment we study the coupled amplitude equations describing the formation of a stationary roll pattern in the presence of a weakly-damped mode that breaks isotropy. The equations can be generalized to describe the planarly aligned case if the orienting effect of the boundaries is small, which can be achieved by a destabilizing magnetic field. The slow mode represents the in-plane director at the center of the cell. The simplest uniform states are normal rolls which may undergo a pitchfork bifurcation to abnormal rolls with a misaligned in-plane director.We present a new class of defect-free solutions with spatial modulations perpendicular to the rolls. In a parameter range where the zig-zag instability is not relevant these solutions are stable attractors, as observed in experiments. We also present two-dimensionally modulated states with and without defects which result from the destabilization of the one-dimensionally modulated structures. Finally, for no (or very small) damping, and away from the rotationally symmetric case, we find static chevrons made up of a periodic arrangement of defect chains (or bands of defects) separating homogeneous regions of oblique rolls with very small amplitude. These states may provide a model for a class of poorly understood stationary structures observed in various highly-conducting materials ("prechevrons" or "broad domains").Comment: 13 pages, 13 figure

Phase Behavior of Bent-Core Molecules

Recently, a new class of smectic liquid crystal phases (SmCP phases) characterized by the spontaneous formation of macroscopic chiral domains from achiral bent-core molecules has been discovered. We have carried out Monte Carlo simulations of a minimal hard spherocylinder dimer model to investigate the role of excluded volume interations in determining the phase behavior of bent-core materials and to probe the molecular origins of polar and chiral symmetry breaking. We present the phase diagram as a function of pressure or density and dimer opening angle $\psi$. With decreasing $\psi$, a transition from a nonpolar to a polar smectic phase is observed near $\psi = 167^{\circ}$, and the nematic phase becomes thermodynamically unstable for $\psi < 135^{\circ}$. No chiral smectic or biaxial nematic phases were found.Comment: 4 pages Revtex, 3 eps figures (included

Winterweizen: Bilanz aus 15 Jahren Sortenprüfung unter extensiven Anbaubedingungen

Damit eine neue Weizensorte in der Schweiz oder im Ausland auf den Markt kommen kann, muss sie eine Reihe von Tests bestehen und beweisen, dass sie bereits angebauten Sorten bezüglich Kornertrag, Qualität oder Krankheitsresistenz überlegen ist. In diesem als Sortenprüfung bezeichneten Verfahren, wird «vom Guten das Beste» ausgewählt. Ist es möglich, diesem Verfahren einen Wert zu geben? Um eine Antwort auf diese Frage zu finden, wurden die Ergebnisse zu allen während der vergangenen 15 Jahre getesteten Sorten untersucht. Es wurden drei Ansätze ausgewählt, um eine Bilanz über die Sortenprüfung bei Winterweizen zu ziehen: Zuerst wurde die Zahl der getesteten Sorten mit der Zahl der Sorten verglichen, die in den Nationalen Sortenkatalog und in die Listen der empfohlenen Sorten aufgenommen wurden. Anschliessend wurde die Entwicklung der Leistung verschiedener Sorten im Lauf der Zeit berechnet. Schliesslich wurde mit einem ökonomischen Ansatz die Grössenordnung des wirtschaftlichen Mehrwerts geschätzt, der durch die Auswahl im Rahmen der Sortenprüfung und durch die Identifikation der besten Sorten erzielt wurde (Vergleich der Bruttoeinnahmen der drei besten getesteten Sorten jedes Jahres mit den durchschnittlichen Bruttoeinnahmen der aktuellen Sorten). Die Ergebnisse heben den Mehrwert hervor, der durch die Sortenprüfung für die ganze Produktionskette von der Züchtung über die Produktion und Verarbeitung bis hin zu den Konsumentinnen und Konsumenten erzeugt wird

Label-free chemically specific imaging in planta with stimulated Raman scattering microscopy.

The growing world population puts ever-increasing demands on the agricultural and agrochemical industries to increase agricultural yields. This can only be achieved by investing in fundamental plant and agrochemical research and in the development of improved analytical tools to support research in these areas. There is currently a lack of analytical tools that provide noninvasive structural and chemical analysis of plant tissues at the cellular scale. Imaging techniques such as coherent anti-Stokes Raman scattering (CARS) and stimulated Raman scattering (SRS) microscopy provide label-free chemically specific image contrast based on vibrational spectroscopy. Over the past decade, these techniques have been shown to offer clear advantages for a vast range of biomedical research applications. The intrinsic vibrational contrast provides label-free quantitative functional analysis, it does not suffer from photobleaching, and it allows near real-time imaging in 3D with submicrometer spatial resolution. However, due to the susceptibility of current detection schemes to optical absorption and fluorescence from pigments (such as chlorophyll), the plant science and agrochemical research communities have not been able to benefit from these techniques and their application in plant research has remained virtually unexplored. In this paper, we explore the effect of chlorophyll fluorescence and absorption in CARS and SRS microscopy. We show that with the latter it is possible to use phase-sensitive detection to separate the vibrational signal from the (electronic) absorption processes. Finally, we demonstrate the potential of SRS for a range of in planta applications by presenting in situ chemical analysis of plant cell wall components, epicuticular waxes, and the deposition of agrochemical formulations onto the leaf surface

Higher predation risk for insect prey at low latitudes and elevations

Biotic interactions underlie ecosystem structure and function, but predicting interaction outcomes is difficult. We tested the hypothesis that biotic interaction strength increases toward the equator, using a global experiment with model caterpillars to measure predation risk. Across an 11,660-kilometer latitudinal gradient spanning six continents, we found increasing predation toward the equator, with a parallel pattern of increasing predation toward lower elevations. Patterns across both latitude and elevation were driven by arthropod predators, with no systematic trend in attack rates by birds or mammals. These matching gradients at global and regional scales suggest consistent drivers of biotic interaction strength, a finding that needs to be integrated into general theories of herbivory, community organization, and life-history evolution