SEVERAL CONTROVERSIAL VIEWPOINTS IN STUDYING THE CAVITATION OF XYLEM VESSELS
木质部导管空穴化研究中的几个热点问题

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.