Vulnerability curves from conifer sapwood sections exposed over solutions with known water potentials.

The cohesion-tension (CT) theory requires stability of liquid water in conducting elements under high tensions. This stability has been measured using different methods, some of which yielded contradictory results. In this study a method is presented to establish known tensions in the water inside conifer tracheids, to detect cavitation events under these conditions and to construct vulnerability curves. Tangential sapwood sections of Juniperus virginiana L. were placed closely over the surface of NaCl solutions with water potentials ranging from -0.91 to -7.57 MPa. Water potentials were measured with a thermocouple hygrometer in contact with the section, and ultrasound acoustic emissions (UAE) from the sections were registered with an ultrasound transducer. The emission rate of signals increased with the concentration of the solution. Exposure of 100 microm sections in the airspace over a solution provided optimal conditions for the rupture of the water column: many tracheid walls bordered on air, and water in the lumen came under high tension. Nevertheless, the water remained in the metastable liquid state for periods of many hours. The vulnerability obtained from simultaneous measurements of water potentials and ultrasound acoustic emissions on sapwood sections was substantially higher than from conventionally measured curves of detached branches. It is argued that the isolation of tracheids in a massive organ as well as the rate of potential decline will influence the probability of cavitations at a given water potential and thus the parameters of the vulnerability curve.