Changes in Water Potential During Pressure Bomb Measurement1

Potential errors in the pressure bomb measurement of water potential are important to a wide range of users. We report here specifically on changes in water potential within the chamber associated with measurement technique. Individual soybean leaves (Glycine max L..) were measured repetitively while left in the chamber for varying amounts of time, both with and without a plastic covering. Thermocouples were used to monitor leaf and air temperature during compression. For uncovered leaves, initially at −3 to −5 bars, water potential decreased as much as 1 bar during a single measurement due to drying associated with high tissue temperatures during the compression. Passing the entering air through water in the bottom of the chamber did not limit either the temperature increase or desiccation. When the leaves were wrapped in more than one layer of plastic, they were effectively insulated from the temperature increase in the chamber and decreases in water potential were reduced to 0.1 to 0.2 bars per measurement. Changes in water potential with time were negligible when desiccation was prevented. We conclude that the pressure bomb can yield highly reproducible measurements if the leaves are covered to prevent desiccation.