While drying, detached leaves produced ultrasound acoustic emissions (UAE) comparable to emissions from stem and twig wood. Experiments on Ilex aquifotium L. showed that the main source of these signals was cavitation in the veins, to which conduits and fibres probably both contributed. Regions of the leaf blade with abundant mesophyll and only small veins emitted few signals. More signals were counted on the adaxial side of the midrib than on the abaxial one and on the proximal third than on the distal one, in accordance with the anatomical structure. Sound attenuation was pronounced. Eight species were compared with respect to cavitation behaviour, field water relations and pressure-volume curves, and tbe data showed differences in cumulative number of events and resistance of leaves to cavitation. Data were generally in good agreement witb anatomical structure and habitat preferences. The number of signals per conduit counted on cross-sections was in some leaves much higher than unity, which suggests short xylem elements or an acoustic activity of cells other than conduits. There was no correlation between cavitation threshold or cumulative number of signals and the degree of sclerophylly; unexpectedly, there was a correlation between tbe cumulative number of signals at a water potential of-1-3 MPa and the bulk modulus of elasticity.

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Diurnal measurements of total water potential and stomatal opening were made at six sites. Pressure-volume curves were established on parallel leaf samples. In eastern Austria, the species investigated were Cornus mas L., Cornus sanguinea L., Crateagus monogyna Jacq., Sorbusaria (L.) Crantz and Viburnum lantana L. in southern France Crateagus monogyna, and in southern Turkey Crateagus monogyna and Olea europaea L. Osmotic adjustment, defined as a change in osmotic potential larger than the passive change resulting from the loss of cell water, was relatively small from day to day or week to week in mature, non-senescing leaves. Cornus sanguinea was an exception. A recently suggested method for the demonstration of diurnal active osmotic adjustment seems not to be reliable without further independent corroboration. Changes in the leaf water potential threshold for stomatal closure were either insignificant when the pressure-volume characteristics of the plant material were stable, or significant when shifts in such parameters as the turgor loss point occurred {Cornus sanguinea).

The stem diameter of adult Norway spruce trees was measured to see whether changes in xylem water potential lead to detectable radial deformation of the wood. The dendrometers used in these experiments measured only the dimensional changes of the woody cylinder (sap-and heartwood). Wood diameter was measured close to the ground and just below the living crown. After correction for thermal expansion of dendrometers and wood, diurnal variation of wood diameter ranged between 50 and 180 µm. Psychrometric measurements showed that xylem water potential varied in parallel to wood diameter. Diameter changes were always more pronounced at the higher stem position and exhibited a clear diurnal pattern. During the day, wood diameter decreased with increasing vapor pressure deficit and transpiration rate and with decreasing twig water potential. At night, the wood re-expanded but did not always reach the dimension of the previous day. Pre-dawn wood diameter decreased during periods of soil drought, a process which rapidly stopped and reversed after rain events. On several days, oscillation in wood diameter was observed during the mid-day hours. The oscillation had a period of approximately 50 min and showed a phase shift between different stem heights. All observed patterns of wood shrinkage and expansion were consistent with the hypothesis that xylem water tension leads to an elastic contraction of xylem conduits. The results demonstrate that xylem diameter is more suitable than whole-stem diameter for monitoring changes in xylem water potential.

A pressure collar, assembled around 25cm sections of 4‐year‐old willow twigs, was used to examine cavitation events under field conditions. When the air pressure inside the collar was raised to between 1–8 and 2–8MPa, ultrasound acoustic emission signals were triggered which indicated the breaking of water columns in the xylem. The hydraulic conductivity of the twig portion inside the chamber decreased markedly. As a result, water potentials and conductances in leaves at the end of the twig decreased. Similar changes were induced at comparable pressures in detached twigs. The equipment used is described in detail, and evidence is presented that the mechanism of this artificial production of emboli follows the air‐seeding principle hypothesized for natural cavitation events.