WATER POTENTIAL MEASUREMENT, WATER COMPARTMENTATION AND WATER FLOW IN DACTYLIS GLOMERATI L. LEAVES

Abstract

When measuring the water potential of leaf samples in a dew‐point hygrometer, it takes several hours to achieve a constant value. It is demonstrated that this is due to water potential gradients within the sample which occur only if the leaf is actively transpiring. Water potentials have been measured on the same leaf or plant using a pressure chamber, a sample dew‐point hygrometer and an in situ hygrometer. Relatively good agreement was found in plants with zero or low transpiration, but results diverged widely when transpiration was high. It is suggested that divergences occur because the methods refer to different compartments of the leaf separated by resistances to water flow. The pressure chamber measures the water potential of the xylem vessels (ψxw), the sample hygrometer gives a value (ψcw) close to the water potential of the vacuoles and the in situ hygrometer yields a value (ψaw) close to the water potential around the evaporation sites. ψawshould logically be the most negative, but this was not always the case. On the other hand, ψxw was always and up to 10 × 105 Pa less negative than ψcw. These findings question the validity of the calibration of the pressure chamber against psychrometers or hygrometers and are relevant to the topic of water pathway in leaves. Copyright © 1983, Wiley Blackwell. All rights reserved