The essentials of direct xylem pressure measurement

This paper discusses the essentials of the oil-filled pressure probe techni que in the measurement of negative xylem pressures, focusing in particular on the technique and physics underlying our recent, successful experiment w hich has rekindled the debate on the validity of the Cohesion-Tension theor y. We illustrate a number of general problems associated with the cell pres sure probe and xylem pressure probe techniques, and propose appropriate cri teria for micropipette construction. We enumerate factors dealing with the cavitation problem and suggest methods for eliminating air seeds in the sys tem. We introduce reliable criteria for the,successful measurement of xylem pressure, and emphasize the importance of the probe pressure relaxation te st. Several problems regarding the controversy over the Cohesion-Tension th eory are also discussed. We discuss the correlation between xylem pressure and the transpiration rate, the existence of absolute negative xylem pressu re in intact plants, the most negative values of xylem pressure measured by the pressure probe, the agreement between the pressure probe and pressure bomb techniques, and the vulnerability to cavitation (tensile strength) of pressure probes.