Acoustic emissions from transpiring plants – new results and conclusions

Using Ulmus glabra as an example we revealed that acoustic emissions from plants do not necessarily occur in conjunction with water stress. The waveforms in the various signals show that acoustic emissions may possibly be generated by still unknown hydraulic events, more complex than cavitation. Using a special microscopic technique we established the presence of minute gas bubbles attached firmly to the xylem vessel walls of the water-conducting conduits. The micro-bubbles accumulate especially to the poorly wettable lignin domains of the vessel pits. Our conclusion is that minute gas bubbles at the xylem vessel walls are a normal feature of actively water-conducting conduits. By means of physical model systems we ascertained that by virtue of the cohesion of water the entire system of wall adherent bubbles acts as a forcetransmitting medium that creates the xylem tension. During transpiration the wall adherent gas bubbles expand, during refilling they contract. Since the bubble system always reacts intermittently this creates acoustic emissions both during transpiration and refilling. Our hypothesis is that under a gradient of the wall attraction forces of the bubble system the water column moves in a similar fashion as under a gradient of actual negative pressure. The outward appearance of xylem tension caused by the bubble system is deceptively similar to that caused by real negative pressure. Our investigations have resulted in an alternative concept of water transport in plants.