Acoustic emission analysis and experiments with physical model systems reveal a peculiar nature of the xylem tension.

Advanced acoustic emission analysis, special microscopic examinations and experiments with physical model systems give reasons for the assumption that the tension in the water conducting system of vascular plants is caused by countless minute gas bubbles strongly adhering to the hydrophobic lignin domains of the xylem vessel walls. We ascertained these bubbles for several species of temperate deciduous trees and conifers. It is our hypothesis that the coherent bubble system of the xylem conduits operates as a force-transmitting medium that is capable of transporting water in traveling peristaltic waves. By virtue of the high elasticity of the gas bubbles, the hydro-pneumatic bubble system is capable of cyclic storing and releasing of energy. We consider the abrupt regrouping of the wall adherent bubble system to be the origin of acoustic emissions from plants. For Ulmus glabra, we recorded violent acoustic activity during both transpiration and re-hydration. The frequency spectrum and the waveforms of the detected acoustic emissions contradict traditional assumptions according to which acoustic emissions are caused by cavitation disruption of the stressed water column. We consider negative pressure in terms of the cohesion theory to be mimicked by the tension of the wall adherent bubble system.