Ice formation and propagation in the xylem of plants is a complex process. During freezing of xylem sap, gases dissolved in liquid sap are forced out of the ice lattice due to their low solubility in ice, and supersaturation of xylem sap as well as low water potential (Ѱ) are induced at the ice–liquid interface. Supersaturation of gases near the ice front may lead to bubble formation and potentially to cavitation and/or to burst of gases driven out from the branch. In this study, we investigated the origin and dynamics of freezing-related gas bursts and ultrasonic acoustic emissions (AEs), which are suggested to indicate cavitation. Picea abies (L.) H. Karst. and Salix caprea L. branch segments were exposed to frost cycles in a temperature test chamber, and CO₂ efflux (indicating gas bursts) and AEs were recorded. On freezing, two-thirds of the observed gas bursts originated from the xylem and only one-third from the bark. Simultaneously with gas bursts, AEs were detected. Branch Ѱ affected both gas bursts and AEs, with high gas burst in saturated and dry samples but relevant AEs only in the latter. Repeated frost cycles led to decreasing gas burst volumes and AE activity. Experiments revealed that the expanding ice front in freezing xylem was responsible for observed gas bursts and AEs, and that branch Ѱ influenced both processes. Results also indicated that gas bursts and cavitation are independently induced by ice formation, though both may be relevant for bubble dynamics during freezing.

中文翻译：

---

传播的冰锋引起冻结木质部产生气体爆炸和超声波发射

植物木质部中冰的形成和传播是一个复杂的过程。在木质部汁液冻结期间，溶解在液体汁液中的气体由于其在冰中的低溶解性而被迫离开冰晶格，并且木质部液汁的过饱和度以及低的水势（Ѱ）在冰-液界面处产生。靠近冰​​面的气体过饱和会导致气泡形成，并可能导致气蚀和/或从分支驱出的气体爆裂。在这项研究中，我们调查了与冷冻有关的气体爆发和超声声发射（AE）的起源和动力学，这些结果表明存在气蚀现象。云杉云杉（L.）H. 和Salix caprea L.分支段在温度测试室内暴露于霜冻循环中，CO ₂记录流出（指示气体爆发）和AE。冻结时，观察到的三分之二的气体爆发来自木质部，只有三分之一来自树皮。同时爆发气体，检测到AE。分支Ѱ同时影响了气体爆发和AE，饱和和干燥样品中的气体爆发较高，而仅在后者中具有相关的AE。反复的霜冻循环导致气体爆炸量和AE活性降低。实验表明，在冻结的木质部中膨胀的冰锋是造成观测到的气体爆发和声发射的原因，并且分支branch影响了这两个过程。结果还表明，气体爆炸和空化是由冰的形成独立引起的，尽管两者都可能与冻结过程中的气泡动力学有关。