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

A previously described vapour-pressure osmometer has been adaptedfor the measurement of depression of water potential (DPD) insamples of leaf tissue and soil. A pile of about 30 leaf annuliare contained in an air-tight metal capsule through the lidof which passes a pipette. A drop of water hangs from the tipof the pipette into the central cavity of the pile of leaf annuli.The drop is drawn bath into the pipette from time to time andthe rate of evaporation from it measured directly. The water-potentialdepression is linearly related to the rate of evaporation. Theinstrument is calibrated empincally. A period of a hours isnecessary to attain vapour equilibnum before readings are takenThe readings then occupy less than 30 minutes. For the calibration data, using filter paper moistened withNaCl solution, the 5 percent inverse tolerance limits for singlereadings of water-potential depression were ±0.26 atm.With leaf material the errors appear to be about the same, althoughrelease of cell sap at the cut edges of the disks may causean 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 usedfor soil-water potential measurement. Equilibration takes only30 minutes so that a reading can be completed within an hour.