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Response of Leaf Water Potential to Pressure Changes at the Root Surface of Corn Plants¹

The nature of the hydraulic continuum between the roots and leaves of intact plants requires further study to improve our understanding of the effect of pressure changes. A large pressure chamber was designed to apply hydrostatic pressure to the roots of intact corn (Zea mays L.) plants. The chamber permitted application of up to 6 bars pressure and allowed for in situ watering of the plants. Changes in leaf water potential resulting from changes in pressure applied to the roots were monitored in situ using a thermocouple dewpoint hygrometer. Responses of leaf water potential were consistently between 0.94 and 1.19 times the change in applied pressure, regardless of whether the pressure was being increased or decreased. When pressure was applied to very turgid plants leaf water potential increased until it reached a value equal to the osmotic pressure of expressed xylem sap. Some time after this value was reached water droplets appeared on the leaf surface. The experiments showed that the hydraulic system within intact plants acts as a true continuum in that pressure changes at one end of the system are faithfully, manifested at the other end of the system. If such changes are not observed some additional mechanism must be operating.

Key Words: Pressure chamber • Psychrometer • Water relations • Transpiration

² Researchers, Dep. of Land Resource Science, Univ. of Guelph, Guelph, Ontario, and researcher, Dep. of Crop Science, Univ. of Guelph. Present address R.A.N. Dep. of Agriculture South Perth, Western Australia).

Received for publication November 1, 1975.

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