Mayr Stefan, Gruber Andreas, Bauer Helmut
Institut f. Botanik, Universität Innsbruck, Sternwartestr. 15, 6020 Innsbruck, Austria.
Planta. 2003 Jul;217(3):436-41. doi: 10.1007/s00425-003-0997-4. Epub 2003 Mar 6.
Freezing and thawing lead to xylem embolism when gas bubbles caused by ice formation expand during the thaw process. However, previous experimental studies indicated that conifers are resistant to freezing-induced embolism, unless xylem pressure becomes very negative during the freezing. In this study, we show that conifers experienced freezing-induced embolism when exposed to repeated freeze-thaw cycles and simultaneously to drought. Simulating conditions at the alpine timberline (128 days with freeze-thaw events and thawing rates of up to 9.5 K h(-1) in the xylem of exposed twigs during winter), young trees of Norway spruce [Picea abies (L.) Karst.] and stone pine (Pinus cembra L.) were exposed to 50 and 100 freeze-thaw cycles. This treatment caused a significant increase in embolism rates in drought-stressed samples. Upon 100 freeze-thaw cycles, vulnerability thresholds (50% loss of conductivity) were shifted 1.8 MPa (Norway spruce) and 0.8 MPa (stone pine) towards less negative water potentials. The results demonstrate that freeze-thaw cycles are a possible reason for winter-embolism in conifers observed in several field studies. Freezing-induced embolism may contribute to the altitudinal limits of conifers.
当结冰形成的气泡在解冻过程中膨胀时，冻融作用会导致木质部栓塞。然而，先前的实验研究表明，针叶树对冻融诱导的栓塞具有抗性，除非在结冰过程中木质部压力变得非常负。在本研究中，我们发现针叶树在经历反复冻融循环并同时遭遇干旱时会出现冻融诱导的栓塞。模拟高山林线的条件（冬季暴露树枝的木质部有128天的冻融事件，解冻速率高达9.5 K h(-1)），将挪威云杉[Picea abies (L.) Karst.]和瑞士石松（Pinus cembra L.）的幼树暴露于50次和100次冻融循环中。这种处理导致干旱胁迫样本中的栓塞率显著增加。经过100次冻融循环后，脆弱性阈值（导水率损失50%）向水势较低的方向移动了1.8 MPa（挪威云杉）和0.8 MPa（瑞士石松）。结果表明，冻融循环可能是在一些野外研究中观察到的针叶树冬季栓塞的一个原因。冻融诱导的栓塞可能导致针叶树的海拔分布上限。
