Evaluation of the Pressure Chamber Technique for Estimating Plant Water Potential of Forest Tree Species

Abstract

Abstract The pressure chamber method for rapid field or laboratory estimation of plant water stress was evaluated, using the thermocouple psychrometer technique. The psychrometer is believed to provide the most accurate measurements of leaf water potential available when certain precautions are taken. The pressure chamber method estimates xylem pressure potential. Comparisons were made by bringing seedlings or large branches into the laboratory and making measurements on leaf samples in the psychrometer and on single leaves or stem tips in the pressure chamber. Most comparisons on Pinus taeda L., P. strobus L., Picea engelmannii Parry, and Liriodendron tulipifera L. agreed within 5 bars. With Quercus rubra L. and Q. alba L., however, the observed pressure potentials were as much as 16 bars lower than leaf water potential. These discrepancies prevent using the values obtained by the pressure chamber technique as direct estimates of leaf water potential for the species studied. The most appropriate way to use the pressure chamber method in evaluating plant water stress is to first determine calibration curves for each species relating the estimate of xylem pressure potential to known leaf water potential.