Water Potential in Excised Leaf Tissue

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Abstract
Leaf water potential (.PSI.leaf) determinations were made on excised leaf samples using a commercial dew point hygrometer (Wescor Inc., Logan, Utah) and a thermocouple psychometer operated in the isopiestic mode. With soybean leaves (Glycine max L.), there was good agreement between instruments; equilibration times were 2-3 h. 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 .PSI.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 6-8. Spuriously high .PSI.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.