INTRODUCTION

For technical reasons, it is very difficult to measure negative pressures in the xylem of transpiring plants directly. An artificial osmotic cell has been constructed using reverse osmosis membranes to study phenomena of negative pressure (Steudle & Heydt, 1988; Zhu, Steudle & Beck, 1989). The cell has been also used as an analytical device (osmotic sensor; Steudle & Stumpf, 1989) to measure the concentration of certain solutes in solutions. The sensivity and selectivity of the osmometer could be increased by coupling the osmotic process to a chemical reaction.

MATERIAL AND METHODS

Reverse osmosis membranes were used to separate the cell interior from the medium. ‘Cell turgor’ was measured with the aid of a pressure transducer and was recorded continuously. The system was calibrated in the range of positive and negative pressures by applying gas pressures to both sides of the transducer membrane. During the experiments, the calibration and the proper function of the transducer could be checked (Steudle & Heydt, 1988). The thickness of the osmotic cell was 0.1 to 0.2 mm and the diameter 4 mm. The cell was filled with a non-permeating solute to build up a positive turgor in the presence of hypotonic solutions. In order to create negative pressures (tensions), the external medium was replaced by hypertonic solutions of non-permeating solutes.

RESULTS AND DISCUSSION

When permeating solutes were added to the medium, biphasic changes of turgor were observed. From these pressure-time curves, the hydraulic conductivity (Lp), the permeability (Ps), and the reflection (σs) coefficients of the membrane were evaluated.