Search

International audience; The use of MRI in the plant sciences has traditionally been limited due to the immobility of the devices, and restrictions with regard to sample size and shape. To overcome these limitations and to be able to study plants directly in their natural environment, we evaluated the capacities of a portable, unilateral magnet: The Nuclear Magnetic Resonance Mobile Universal Surface Explorer (NMR-MOUSE), designed by Blümich et al. [1]. This device permits measuring the water NMR signal in a 100-micrometer slice, and by means of a built-in lift, the magnet can be moved to record an NMR profile within a depth of approximately 25-millimeters from the surface of the device. We tested its capacity to measure tree water content by following the dehydration dynamic of thirty cut branches from six different species and two different functional types. We also tested the device on four in-situ trees. There was a linear correlation between the integral of the MRI profiles obtained and the water content of the branches and trees. This correlation was present regardless of tree species or functional type. Using the profiles obtained on the branches, we then tested the capacity of the device to differentiate the conductive tissues, i.e. the xylem and phloem fluxes. The MRI profiles of the branches presented distinct peaks which corresponded to the xylem and phloem tissues, whose location was validated with x-ray microtomography imaging. In conclusion, the NMR-MOUSE is a promising candidate for measuring tree water dynamics in the field.

International audience

International audience; Wood is a complex cellular material because of its heterogeneity and variability. The first level of variability is that of the species, which controls the organization of the woody plane, which is specific to it. But within the same species, with the same genetic baggage, each tree has its own mode of growth, of wood formation, which are then strongly dependent on the region, the plot, in which it develops but also on the local environment of the individual. Finally, each year, each season imposes its own environmental conditions of growth which will modulate the biological processes. In addition, there are the local specificities of the wood linked to particular developments such as knots, the presence of reaction wood, etc.But, finally, even among those who work with wood on a daily basis, few have the opportunity to spend hours under a microscope to appreciate all this complexity and diversity of structures. And yet, many teachers, in high school, university or in a wood science engineering school, wish to have their students observe these fascinating structures.There is now a theoretically simple solution, a little more complex to implement, which makes it possible to put in the hands of all, an object, exact copy of an anatomical structure of wood in 3D.; Le bois est un matériau cellulaire complexe par son hétérogénéité et sa variabilité. Le premier niveau de variabilité est celui de l’espèce, laquelle pilote l’organisation du plan ligneux, qui lui est spécifique. Mais au sein d’une même espèce, au même bagage génétique, chaque arbre a son propre mode de croissance, de formation du bois, lesquels sont alors fortement dépendant de la région, de la placette, dans laquelle il se développe mais aussi de l’environnement local de l’individu. Enfin, chaque année, chaque saison impose ses conditions environnementales de croissance qui vont moduler les processus biologiques. A cela se rajoutent les spécificités locales du bois liées à des développements particuliers tels que les noeuds, la présence de bois de réaction, etc.Mais, finalement, même parmi ceux qui travaillent le bois quotidiennement, peu sont ceux qui ont l’occasion de passer des heures sous un microscope pour apprécier toute cette complexité et cette diversité de structures. Et nombreux sont pourtant les enseignants, en lycée, à l’université, dans une école d’ingénieur de sciences du bois, qui souhaitent faire observer ces fascinantes structures à leurs élèves.Il existe maintenant une solution théoriquement simple, un peu plus complexe à mettre en oeuvre, qui permet de mettre dans les mains de tous, un objet, copie conforme d’une structure anatomique de bois en 3D.

International audience

International audience

La microtomographie à rayons X, ou ‘’scanner à rayons X’’, est une technique non destructive permettant de visualiser l’intérieur comme l’extérieur d’un objet en 3 dimensions. Son principe repose sur l’absorption d'un faisceau de rayons X par la matière, en enregistrant le rayonnement transmis après traversée d'un échantillon. Les données collectées permettent alors de reconstruire une image 3D. La microtomographie à rayons X permet ainsi d’accéder au cœur de l’échantillon, et d’y observer toute particularité, anomalie, vide ou inclusion à une échelle qui peut atteindre le micron et sans découpe ou préparation particulière. Au sein de l’UMR PIAF (INRA, Clermont-Ferrand), nous avons acquis une expertise sur l’observation et la caractérisation d’échantillons végétaux vivants ou morts (arbres, graminées, composites et fossiles d’origine végétale,…). Quelques exemples d’observations de routine réalisées au sein du laboratoire: architecture hydraulique et embolie des plantes, , feuilles, aiguilles,…

Climate change can strongly impact the forest ecosystems and droughts can lead to dieback events. The xylem vulnerability to cavitation is a key parameter in the drought resistance of the trees. Indeed, water stress tolerant species are less vulnerable to cavitation than water stress intolerant species. Despite a low genetic variability at the intra-specific scale, there is an environmental plasticity for this trait which could permit to the trees to acclimatize to changing climate conditions. However, our knowledge on hydraulic trait plasticity, especially vulnerability to cavitation, are weak. The objective of this thesis study is to identify the structural determinants of the plasticity of the vulnerability to cavitation. We characterized the structural modifications of xylem related to modifications of vulnerability to cavitation in poplars growing in contrasted environmental conditions leading to contrasted hydraulic responses (differences of measured water potential, differences in cavitation resistance). The structure modifications will be analyzed at three scales: xylem, vessels and intervessel pits. The latter are cell wall structures that provide hydraulic connectivity between vessels, and are also key structures of vulnerability to cavitation. The changes in the dimensions of bordered pit chambers have been characterized by transmitted electron microscopy (TEM). Other scales have been analyzed with various technics, such as scanning electron microscopy (SEM), optical microscopy and silicon injections in vessels. This allowed us to show a relationship between xylem organization (vessel grouping index) and vessel structure (diameters, surface occupied by pits) on plasticity. High-resolution X-ray tomography analysis would permit the identification of the most resistant structures and the propagation mechanisms of embolism in acclimatized structures.

National audience