Unexplained hydrogen isotope offsets complicate the identification and quantification of tree water sources in a riparian forest

We investigated plant water sources of an emblematic refugial population of Fagus sylvatica (L.) in the Ciron river gorges in south-western France using stable water isotopes. It is generally assumed that no isotopic fractionation occurs during root water uptake, so that the isotopic composition of xylem water effectively reflects that of source water. However, this assumption has been called into question by recent studies that found that, at least at some dates during the growing season, plant water did not reflect any mixture of the potential water sources. In this context, highly resolved datasets covering a range of environmental conditions could shed light on possible plant–soil fractionation processes responsible for this phenomenon. In this study, the hydrogen (δ2H) and oxygen (δ18O) isotope compositions of all potential tree water sources and xylem water were measured fortnightly over an entire growing season. Using a Bayesian isotope mixing model (MixSIAR), we then quantified the relative contribution of water sources for F. sylvatica and Quercus robur (L.) trees. Based on δ18O data alone, both species used a mix of top and deep soil water over the season, with Q. robur using deeper soil water than F. sylvatica. The contribution of stream water appeared to be marginal despite the proximity of the trees to the stream, as already reported for other riparian forests. Xylem water δ18O could always be interpreted as a mixture of deep and shallow soil waters, but the δ2H of xylem water was often more depleted than the considered water sources. We argue that an isotopic fractionation in the unsaturated zone and/or within the plant tissues could underlie this unexpected relatively depleted δ2H of xylem water, as already observed in halophytic and xerophytic species. By means of a sensitivity analysis, we found that the estimation of plant water sources using mixing models was strongly affected by this δ2H depletion. A better understanding of what causes this isotopic separation between xylem and source water is urgently needed.</p

Combined Ring Inversion and Side Group Rotation in Geminal Diphosphoryl Substituted Pyrrolinoxyl Radicals: ESR Analysis of Chemical Exchange between Four Nonequivalent Sites

The ESR spectra of three diphosphorylated pyrrolidinoxyl radicals have been studied over a large temperature range. While for the trans 2,5-disubstituted compound 1 no line width alternation was found, for the 2,2-disubstituted compounds 2 and 3, dramatic changes in the spectra were observed as a function of the temperature. These changes were explained by a four-site chemical exchange model including both ring inversion and hindered rotation around the carbon-phosphorus bonds. For radical 3, the presence of additional 5,5-dimethyl substitution can completely block rotations around carbon-phosphorus bonds for certain ring geometries, while for other ring conformations chemical exchange still occurs through combined inversionrotation processes. An effective two-site model composed of nonequivalent sites and a superposition model composed of a pair of exchanging conformers and a pair of nonexchanging conformers were used to simulate spectrum variations. A satisfactory fit was obtained over the entire temperature range investigated. From the temperature dependence of the exchange frequencies the potential barriers for ring inversion (27 kJ/mol), for combined inversion-rotation (11 kJ/mol), and for rotation around the carbon-phosphorus bonds (14 kJ/mol) were estimated

Genèse médiévale de l'anthroponymie moderne. Tome II-2 : Persistances du nom unique

En 1986-87, un groupe de médiévistes français s'est attaché à étudier, dans les cartulaires, quand et comment s'est élaboré le système anthroponymique moderne, prénom et patronyme, au cours des xie et xiie siècles. Après cette première phase centrée sur les points communs et les nuances régionales de cette évolution, l'enquête s'est élargie, en 1989 et 1990, aux autres pays européens qui feront l'objet de publications ultérieures et aux « résistances », dans l'espace français, à cette double dénomination. Une région résistante : la Bretagne, un mode de désignation spécifique, pour les clercs par leur fonction, pour les femmes par une relation familiale : ce sont les modalités de ces évolutions différentes qui font l'objet du volume II des Rencontres d'Azay-le-Ferron, publié aujourd'hui en deux tomes, par le même groupe de chercheurs. Le même principe, de comparaison régionale fondée sur un questionnement identique des sources, y est appliqué. Les « prénoms » des clercs sont-ils différents de ceux des laïcs ? Les femmes sont-elles identifiées par la famille où elles sont nées ou par celle à laquelle elles se sont alliées par mariage ? L'analyse de ces usages spécifiques permet de comprendre comment fonctionne, entre la norme et le concret, la désignation écrite des individus entre le xie et le xive siècle

Neuropatia experimental por DDT: análise de nervo por microdissecção de fibras

Estudou-se o nervo gênito-femural do rato albino submetido a intoxicação crônica por DDT, administrado por 180 dias na dose de 5 mg/kg de peso via oral. Os resultados mostraram proporção anormal de fibras tipo C, sendo sugerido ocorrer degeneração tipo axonal determinada pelo DDT

Structure and Spectromagnetic Properties of the Superoxide Radical Adduct of DMPO in Water: Elucidation by Theoretical Investigations

International audienceIn the field of spin trapping chemistry, the design of more efficient radical traps can be assisted by the development of theoretical methods able to give a quantitative evaluation of the electron pm-magnetic resonance (EPR) spectrum features of the spin-adduct radical, even before initiating the experimental work. The superoxide radical adduct of the 5,5-dimethyl-1-pyrroline-N-oxide nitrone (DMPO-OOH) has been reported in a huge number of papers devoted to the study of the oxidative stress. Here, we present for the first time the theoretical study of DMPO-OOH in an explicit water solution, based on the combined QM/MM//MD protocol we recently proposed, featuring a full coupling between the solute and all the explicit water molecules. Our results show that the DMPO-OOH EPR spectrum, whose interpretation is still debated, can be explained in the light of two sites in chemical exchange, in agreement with the most recent experimental data. Moreover, we demonstrate that each site consists of an equilibrium between the two main 5-membered ring conformations of DMPO-OOH. We provide also an analysis of the solvent contribution to the hyperfine coupling constants (hcc's) as well as an exhaustive study of the possible relationship between the hcc's and the main structural characteristics of DMPO-OOH. Our QM/MM//MD protocol appears thus to be an accurate theoretical tool allowing the investigation of the magnetic properties of large nitroxide spin adducts in complex environments