Plasticity of xylem vulnerability to cavitation and its structural determinants in Populus tremula x alba

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