Measurement And Analysis Of Ultrasonic Acoustic Emissions In Xylem Sap

It has been dozens of years since the ultrasonic acoustic emissions (UAE) from the cavitation in xylem sap were discovered. It is important for studying plant physiology of water to measure and analyse the UAE signals. In recent years, researchers have done a lot of studies on the relationship between the UAE signals from cavitation in xylem sap and water potential、temperature、light、 transpiration rate and so on. Moreover some researchers have found that UAE signals can also be detected from embolism repairing process. But the mechanism of UAEs form cavitation and embolism repairing process is still not clear.In this paper,we first measured and analyzed the UAEs from the slices of Platycladus orientalis, Acer truncatum and Robinia pseudoacacia when losing water and concluded that the characteristic of these three tree species UAE signals were similar. The signal frequency distribution of about80%in450to550kHz range and the ringing counts of UAE signals were less than10.Besides. The typical waves of UAE signal vibrate by the way of convergent oscillation. We also found that there were UAE signals during the embolism repairing process by measuring the slices during the process of rehydration and the characteristic of UAEs from the embolism repairing process is similar to that from the process of cavitation.This study focuses on the characteristics and mechanism of ultrasonic acoustic emissions from the xylem sap. A bubble motion equation based on the Rayleigh-Plesset equation was established and solved. The equilibrium radius was calculated. By solving the equation with a four-order Runge-Kutta method, we found that the radii of the bubbles were greater or less than the equilibrium radius when disturbed by other changes, such as changes of xylem sap pressure; the bubbles would get back to the stable equilibrium by the way of convergent oscillation. That was the source of UAEs.During the process of embolism repairing the bubbles contracted and got smaller, when xylem water potential rises. But the embolisms can’t be repaired by only one time contraction. When the pressure rose, the bubbles would continue to contract until they became tiny stable ones. And then the embolisms could be repaired. In the process of repeated bubble contraction, the radii of bubbles didn’t decrease monotonely but by the way of convergent oscillation and this was one of the sources of AE signals.