A Comparison of Methods for the Determination of Water Status in Quercus ilex L.

Summary Water saturation deficit (WSD), osmotic potential of the cell sap (Ψo) and total water potential (Ψt) were determined in drying shoots and saplings of the Mediterranean evergreen oak, Quercus ilex L. Reliability as well as time and material requirements were assessed. Thermocouple psychrometry and the pressure chamber technique offer great advantages over refractometry of exchange solutions for the determination of Ψt. The relationship between WSD, Ψo and Ψt was evaluated by means of the «pressure-volume curve« technique in the modification of Talbot et al. (1975). While the direct comparison of the cryoscopically determined osmotic potential and the psychrometrically determined total water potential suggests the occurrence of negative turgor, this evidence appears doubtful after a scrutiny of the pressure-volume curves. Some advantages of this evaluation technique are emphasized: osmotic relations may be analyzed in organs where the cryoscopic technique cannot be applied, e.g. in stem disks; linear regressions may be calculated for data from cryoscopy and vapour pressure osmometry as well as from different Ψt-techniques; and curves may be extrapolated to high WSDs, whereas direct determinations are not feasible with most of methods due to technical limitations.