A Vapour-pressure Instrument for the Measurement of Leaf and Soil Water Potential

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Abstract
A previously described vapour-pressure osmometer has been adapted for the measurement of depression of water potential (DPD) in samples of leaf tissue and soil. A pile of about 30 leaf annuli are contained in an air-tight metal capsule through the lid of which passes a pipette. A drop of water hangs from the tip of the pipette into the central cavity of the pile of leaf annuli. The drop is drawn bath into the pipette from time to time and the rate of evaporation from it measured directly. The water-potential depression is linearly related to the rate of evaporation. The instrument is calibrated empincally. A period of a hours is necessary to attain vapour equilibnum before readings are taken The readings then occupy less than 30 minutes. For the calibration data, using filter paper moistened with NaCl solution, the 5 percent inverse tolerance limits for single readings of water-potential depression were ±0.26 atm. With leaf material the errors appear to be about the same, although release of cell sap at the cut edges of the disks may cause an overestimation by as much as 0.5 atm. with turgid leaf material. Lining the capsule with leaf strips gave erroneous results. Using a suitably modified capsule the instrument can be used for soil-water potential measurement. Equilibration takes only 30 minutes so that a reading can be completed within an hour.

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