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Physical State of Water in Plant Xylem Vessels¹

The vapor pressure psychrometer was used as a tool to study the physical state of water in plant xylem vessels. The experimental procedure involved measuring the change in diffusion pressure deficit (DPD) of corn and tomato plants when the stem was cut. When the DPD was greater than 4 bars in tomatoes and 28 bars in corn, the water in xylem vessels no longer appeared to flow in response to hydrostatic pressure gradients. The limiting value of DPD increased as the xylem radius decreased. A mechanism is suggested which describes the physical state and the movement of water through xylem tissue under high DPD. The proposal is based on the pressure difference across a curved air-water interface and on the concept of an electrostatic double layer with its associated osmotic pressure.

Key Words: transpiration • DPD • xylem sap

² ~Research Soil Scientist, Research Investigations Leader, and Mathematician, respectively, Snake River Conservation Research Center, Kimberly, Idaho 83341.

Received for publication May 27, 1967.