Probing an nonequilibrium Einstein relation in an aging colloidal glass

We present a direct experimental measurement of an effective temperature in a colloidal glass of Laponite, using a micrometric bead as a thermometer. The nonequilibrium fluctuation-dissipation relation, in the particular form of a modified Einstein relation, is investigated with diffusion and mobility measurements of the bead embedded in the glass. We observe an unusual non-monotonic behavior of the effective temperature : starting from the bath temperature, it is found to increase up to a maximum value, and then decreases back, as the system ages. We show that the observed deviation from the Einstein relation is related to the relaxation times previously measured in dynamic light scattering experiments.Comment: 4 pages, 4 figures, corrected references, published in Phys. Rev. Lette

Phenolic Compounds in Natural Solutions of a Coniferous Forest

Phenolic compounds have been identified previously as potentially responsible for allelopathic interferences in spruce forest at high altitude. They have now been analyzed in canopy leachates, snow, and soil solutions collected from the three layers of the podsolic soil: OA, E. and B. Leachates were characterized by high tanning capacity and by p-hydroxyacetophenone (found at 10−6 M) also detected as the major (10−7 M) monomeric compound in snow. At least 10 phenolic monomers, including vanillic, p-hydroxybenzoic, and protocatechuic acids were identified in capillary waters extracted from the OA layer with their sum reaching 2 × 10−6 M. These monomers were only a small part (1%) of the total phenolics. In soil solutions, significant decreases in phenolic concentrations with depth were observed between the E and B layers, with qualitative modifications of the phenolic pattern. Spruce leachates and soil solutions exhibited high temporal variability, resulting in transitory allelopathic potential towards both aerial and subterranean parts of spruce seedlings. The occurrence of various and soluble phenolic coumpounds in the whole forest system, including p-hydroxyacetophenone, which persist from green needles to soil solutions, suggests that they could be involved in complex and interactive processes occurring during organic matter accumulation on the coniferous forest floo

Generalized Einstein Relation in an aging colloidal glass

20 pages, 6 figures; revised versionInternational audienceWe present an experimental and theoretical investigation of the Generalized Einstein Relation (GER), a particular form of a fluctuation-dissipation relation, in an out-of-equilibrium visco-elastic fluid. Micrometer beads, used as thermometers, are immersed in an aging colloidal glass to provide both fluctuation and dissipation measurements. The deviations from the Generalized Einstein Relation are derived as a function of frequency and aging time. The observed deviations from GER are interpreted as directly related to the change in the glass relaxation times with aging time. In our scenario, deviations are observed in the regime where the observation time scale is of the order of a characteristic relaxation time of the glass

Approche agronomique de l'allélopathie

International audienceL’allélopathie est l’objet d’un nombre croissant de recherches. Des progrès importants ont été réalisés dans la compréhension des mécanismes à l’origine des phénomènes observés, et dans « l’établissement de la preuve » que ces phénomènes n’étaient ni dus à un artefact expérimental, ni confondus avec d’autres phénomènes. Cependant, peu de tentatives ont été menées dans l’optique d’une maîtrise agronomique de ces phénomènes. Une meilleure connaissance des relations entre pratiques agricoles et allélopathie serait nécessaire afin de valoriser cette dernière dans des stratégies de protection intégrée des cultures, et/ou de mieux maîtriser les « effets précédent » des cultures. Un recensement des connaissances actuellement disponibles est effectué, insistant notamment sur les facteurs responsables des variations de potentiel phytotoxique, et sur leur contrôle en vue de l’application agronomique de l’allélopathie. Plusieurs orientations pour une approche agronomique de l’allélopathie sont proposées, en particulier : replacer les mécanismes de l’allélopathie dans le contexte du fonctionnement du champ cultivé, identifier les mécanismes-clés qui varient en fonction des pratiques, et étudier les effets d’une gamme de pratiques agricoles sur ces mécanismes ; vérifier l’occurrence du phénomène en parcelles agricoles en s’appuyant sur des études analytiques ; appréhender simultanément l’allélopathie et les autres dimensions agronomiques majeures de l’activité agricole, en particulier l’élaboration du rendement des cultures

Experimental Observation of Spatially Localized Dynamo Magnetic Fields

International audienceWe report the first experimental observation of a spatially localized dynamo magnetic field, a common feature of astrophysical dynamos and convective dynamo simulations. When the two propellers of the von Ka ́rma ́n sodium experiment are driven at frequencies that differ by 15%, the mean magnetic field's energy measured close to the slower disk is nearly 10 times larger than the one close to the faster one. This strong localization of the magnetic field when a symmetry of the forcing is broken is in good agreement with a prediction based on the interaction between a dipolar and a quadrupolar magnetic mode

The variety mixture strategy assessed in a G × G experiment with rice and the blast fungus Magnaporthe oryzae

Frequent and devastating epidemics of parasites are one of the major issues encountered by modern agriculture. To manage the impact of pathogens, resistant plant varieties have been selected. However, resistances are overcome by parasites requiring the use of pesticides and causing new economical and food safety issues. A promising strategy to maintain the epidemic at a low level and hamper pathogen's adaptation to varietal resistance is the use of mixtures of varieties such that the mix will form a heterogeneous environment for the parasite. A way to find the good combination of varieties that will actually constitute a heterogeneous environment for pathogens is to look for genotype × genotype (G × G) interactions between pathogens and plant varieties. A pattern in which pathogens have a high fitness on one variety and a poor fitness on other varieties guarantees the efficiency of the mixture strategy. In the present article, we inoculated 18 different genotypes of the fungus Magnaporthe oryzae on three rice plant varieties showing different levels of partial resistance in order to find a variety combination compatible with the requirements of the variety mixture strategy, i.e., showing appropriate G × G interactions. We estimated the success of each plant-fungus interaction by measuring fungal fitness and three fungal life history traits: infection success, within-host growth, sporulation capacity. Our results show the existence of G × G interactions between the two varieties Ariete and CO39 on all measured traits and fungal fitness. We also observed that these varieties have different resistance mechanisms; Ariete is good at controlling infection success of the parasite but is not able to control its growth when inside the leaf, while CO39 shows the opposite pattern. We also found that Maratelli's resistance has been eroded. Finally, correlation analyses demonstrated that not all infectious traits are positively correlated. (Résumé d'auteur