Pressure probe technique for measuring water relations of cells in higher plants

A new method is described for continuously measuring cell turgor pressure (P), hydraulic conductivity (L(p)), and volumetric elastic modulus (epsilon) in higher plant cells, using a pressure probe. This technique permits volume changes, DeltaV, and turgor pressure changes, DeltaP, to be determined with an accuracy of 10(-5) to 10(-6) mul and 3 to 5.10(-2) bar, respectively.The main principle of the new method is the same as the pressure probe developed by Zimmermann and Steudle in which pressure is transmitted to a pressure transducer by means of an oil-filled capillary introduced into the cell. In order to use the pressure probe for small tissue cells, the effective compressible volume of the apparatus has to be sufficiently small in comparison to the volume of the cell itself. This is achieved by accurately fixing the oil/cell sap boundary in the very tip of the microcapillary by means of an electronic feedback mechanism, so that the effective volume of the apparatus is reduced to about 2 to 10% of the cell volume. In this way also, errors arising from compressibility of the apparatus and temperature fluctuations can be excluded.Measurements on tissues cells of Capsicum annuum fruits yield epsilon values of 2 to 25 bar. Furthermore, epsilon can be shown to be a function of both cell turgor pressure and cell volume; epsilon increases with increasing turgor pressure and is higher in larger cells.