Water relations of drought hardy shrubs: osmotic potential and stomatal reactivity

Abstract. 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., Sorbus aria (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).