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Embolism Formation during Freezing in the Wood of Picea abies¹

Institut für Botanik, Universität Innsbruck, A–6020 Innsbruck, Austria (S.M.); University of Natural Resources and Applied Life Sciences, Department of Integrative Biology, Institute of Botany, A–1180 Vienna, Austria (S.B.K.); and Unité Mixte de Recherche Physiologie Intégrée de l'Arbre Fruitier et Forestier, Institut National de la Recherche Agronomique/Université Blaise Pascal, Site de Crouelle, 63039 Clermont-Ferrand, France (H.C., T.A.)

Freeze-thaw events can cause embolism in plant xylem. According to classical theory, gas bubbles are formed during freezing and expand during thawing. Conifers have proved to be very resistant to freeze-thaw induced embolism, because bubbles in tracheids are small and redissolve during thawing. In contrast, increasing embolism rates upon consecutive freeze-thaw events were observed that cannot be explained by the classical mechanism. In this study, embolism formation during freeze-thaw events was analyzed via ultrasonic and Cryo-scanning electron microscope techniques. Twigs of Picea abies L. Karst. were subjected to up to 120 freeze-thaw cycles during which ultrasonic acoustic emissions, xylem temperature, and diameter variations were registered. In addition, the extent and cross-sectional pattern of embolism were analyzed with staining experiments and Cryo-scanning electron microscope observations. Embolism increased with the number of freeze-thaw events in twigs previously dehydrated to a water potential of –2.8 MPa. In these twigs, acoustic emissions were registered, while saturated twigs showed low, and totally dehydrated twigs showed no, acoustic activity. Acoustic emissions were detected only during the freezing process. This means that embolism was formed during freezing, which is in contradiction to the classical theory of freeze-thaw induced embolism. The clustered pattern of embolized tracheids in cross sections indicates that air spread from a dysfunctional tracheid to adjacent functional ones. We hypothesize that the low water potential of the growing ice front led to a decrease of the potential in nearby tracheids. This may result in freezing-induced air seeding.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Stefan Mayr (stefan.mayr{at}uibk.ac.at).

www.plantphysiol.org/cgi/doi/10.1104/pp.106.085704

^* Corresponding author; e-mail stefan.mayr{at}uibk.ac.at; fax 0043–512–507–2715.

Received June 23, 2006; accepted October 9, 2006; published October 13, 2006.