Dynamic of cavitation, embolism and water fluxes in trees during freeze-thaw cycle analysed by in vivo visualization
Résumé
Freeze-thaw cycles are inducing major hydraulic changes due to liquid to ice transition within tree stems. The very low water potential at the ice-liquid interface is crucial as it may cause irreversible water fluxes and cavitation in xylem sap conduits, which impacts whole tree water relations. We therefore investigated water fluxes induced by ice formation during freeze-thaw cycles in angiosperm tree stems using four complementary approaches: Microdendrometer, Magnetic Resonance Imaging (MRI), X-ray microtomography and Ultrasonic acoustic Emissions (UEs) analysis. When temperature dropped, ice nucleation occurred likely in cambial area inducing water redistribution within the stem toward the ice front. Dehydration of bark living cells leading to drastic shrinkage of this tissue was observed, as well as high tension within wood reaching the cavitation threshold in sap vessels (Charrier et al., 2014). UEs, strictly emitted during freezing, indicated cavitation events (i.e. bubbles formation) in the xylem sap (Ponomarenko et al., 2014). However, xylem embolism formation (i.e. bubbles expansion) was observed only on thawing via X-ray microtomography. The observations indicated drastic dehydration of bark cells and cavitation events in conduits during freezing, whereas embolism occurred during thawing. UEs were detected during freezing and not directly related to embolism formation. These results enabled new insights into the complex process and the dynamics of water movements and ice formation during freeze thaw cycle in branches of trees.