Anatomical determinants of plasticity of xylem resistance to cavitation in Populus tremula x alba

Xylem resistance to cavitation is a key parameter in drought resistance of trees. Indeed, water stress tolerant species are less vulnerable to cavitation than water stress intolerant species. At the within species level, phenotypic variability was reported for the xylem cavitation resistance, mainly based on plasticity among environmental growth conditions. This raises the question of genetic and structural determinants of this plasticity. The objective of the present work is to identify the structural determinants controlling plasticity of xylem resistance to cavitation. Young poplars trees were grown in contrasted water availability or light exposure in order to induce acclimation in hydraulic properties. We then characterized the structural modifications of their acclimated xylem at three levels: xylem organization, vessels dimensions and pits shape. We used different microscopy techniques including Transmission Electron Microscope (T.E.M.), Scanning Electron Microscope (M.E.B.) and Light Microscope – on the same individual trees. We specially investigated the intervessel pits that provide hydraulic connections between vessels while they prevent air embolism spreading. Moreover, a local approach using direct observation of embolism spreading at the cellular level via X-ray micro-CT allowed us to explore the structural determinants of vessel resistance to cavitation.