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Published on October 13, 2006; 10.1104/pp.106.085704

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Received June 23, 2006
Accepted October 9, 2006

Embolism Formation during Freezing in the Wood of Picea abies

Stefan Mayr *, Hervé Cochard , Thierry Améglio , and Silvia B. Kikuta

University of Innsbruck, Institute of Botany, Sternwartestr. 15, A-6020 Innsbruck, Austria
UMR PIAF, Site INRA de Crouel, Cedex 2, 63039 Clermont-Ferrand, France
University of Natural Resources and Applied Life Sciences, Department of Integrative Biology, Institute of Botany, Gregor Mendel-Str. 33, A-1180 Vienna, Austria

* Corresponding author; email: stefan.mayr{at}uibk.ac.at.

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 re-dissolve during thawing. In contrast, increasing embolism rates upon consecutive freeze-thaw events were observed, that cannot be explained by the classical mechanism. In the present study, embolism formation during freeze-thaw events was analysed via ultrasonic and Cryo-SEM 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 analysed with staining experiments and Cryo-SEM 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 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 embolised tracheids in cross sections indicates that air spread from a dysfunctional tracheid to adjacent functional ones. We hypothesise 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.






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