木质部导管空穴化研究中的几个热点问题

 导管的空穴化和栓塞化现象是目前国际上水分生理生态研究的一个热点。该文对导管空穴化和栓塞化研究中出现的几个热点问题进行了概括和总结。1）在研究导管空穴化的实验手段上，超声波传感器探测法具有一定的局限性；目前至少存在4种可能的原因来解释木质部压力探针法（XPP）和压力室法所测得的导管水柱张力不一致的现象；近来出现的核磁共振成像法可以进行导管空穴化的无损伤检测。2）导管解剖学特征与形成空穴的可能性之间的关系可能与树种相关。3）导管空穴化引起气孔关闭的作用机制目前还不太清楚。4）植物防止空穴化产生的能力与适应干旱能力之间的关系还没有定论。5）单独用根压来解释空穴化导管的重新注水机制是不全面的，还存在其它重新注水机制。

SEVERAL CONTROVERSIAL VIEWPOINTS IN STUDYING THE CAVITATION OF XYLEM VESSELS

Xylem cavitation/embolism, a topic that has received extensive attention, is generally seen as a potential threat for living plants because it lowers the conductivity of the axial water-conducting system. In this paper, several controversial viewpoints about studying the cavitation of xylem vessels were listed, and current efforts and advances in this field were reviewed. Ultrasonic sensors can be used to directly detect the occurrence of xylem cavitation; however, this method has some limitations. Almost all current methods used to study xylem cavitation/embolism, except for the ultrasonic sensor, requires destructive sampling. Recently, a noninvasive method based on high-resolution magnetic resonance imaging technique was introduced. Large discrepancies existed between the results of pressure chamber tests and xylem pressure probe, both of which were used to detect the tension of xylem vessels. There are at least four possible reasons to explain this discrepancy. The relationship between the anatomical characteristics and the ability of xylem vessel to resist cavitation is complex and it may show a species-specific response. The underlying mechanism of stomatal closure induced by xylem cavitation is still unclear. There is no general conclusion about the relationship between xylem vessel resistance to cavitation and the capacity of plants to adapt to water-stress. What about the embolized vessels refilled by water? It is risky to attribute this process solely to the mechanism of root pressure, because there is now some evidence that other mechanisms are involved. In general, further efforts are needed to clarify and resolve these uncertainties and discrepancies in order for us to better understand the cavitation of xylem vessels and the water-conducting system of plants.