Water Potential in Excised Leaf Tissue: Comparison of a Commercial Dew Point Hygrometer and a Thermocouple Psychrometer on Soybean, Wheat, and Barley

Leaf water potential (Ψ leaf ) determinations were made on excised leaf samples using a commercial dew point hygrometer (Wescor Inc., Logan, Utah) and a thermocouple psychrometer operated in the isopiestic mode. With soybean leaves ( Glycine max L.), there was good agreement between instruments; equilibration times were 2 to 3 hours. With cereals ( Triticum aestivum L. and Hordeum vulgare L.), agreement between instruments was poor for moderately wilted leaves when 7-mm-diameter punches were used in the hygrometer and 20-mm slices were used in the psychrometer, because the Ψ leaf values from the dew point hygrometer were too high. Agreement was improved by replacing the 7-mm punch samples in the hygrometer by 13-mm slices, which had a lower cut edge to volume ratio. Equilibration times for cereals were normally 6 to 8 hours. Spuriously high Ψ leaf values obtained with 7-mm leaf punches may be associated with the ion release and reabsorption that occur upon tissue excision; such errors evidently depend both on the species and on tissue water status.