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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.

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