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Field Use of Screen-Caged Thermocouple Psychrometers in Sample Chambers¹

These investigations attempt to answer questions still remaining concerning the use of thermocouple psychrometers for measurement of leaf water potential when laboratory facilities are located some distance from the field. Our results show that with the necessary precautions, field use of thermocouple psychrometers provides a practical and useful method for measurement of plant water potential components. Wheat [Triticum aestivum (L.) em. Thell.] leaf samples taken in the field were sealed in sample chambers containing screencaged thermocouple psychrometers, and transported 30 km to the laboratory. Thermal equilibrium was attained in 4 min, and vapor pressure equilibrium in 4 h, after placement of the sample chambers in a constant temperature waterbath. Osmotic potential measurements could be delayed for up to 24 h as the equilibrium value remained constant after 4 h in the waterbath. Equilibration times were the same for different wheat species and cultivars tested and were unaffected by tissue water content. The temperature of the sample chamber during transit from the field to a laboratory did not affect the vapor pressure equilibration time or the water potential measured. Water potentials were however affected if transit time exceeded 2 h. Significant water loss (233 kPa in 15 s) was found to occur immediately after leaf excision before the leaf sample was sealed in the chamber. Procedures which take account of all these affects enable reliable measurements of leaf water potential components to be obtained when laboratory facilities are located some distance from the field. Leaf water potential values compared favorably with those in the literature and gave a near linear (1:1) relationship with measurements made using a Scholander pressure chamber. The osmotic potential at incipient plasmolysis as measured by the pressure-volume method compared favorably with the osmotic potential as measured with psychrometers.

Key Words: Leaf water potential • Leaf osmotic potential • Water stress • Triticum aestivum (L.) em. Thell. • Pressure chamber

² Agricultural researcher, Soil and Irrigation Res. Inst.; senior plant physiologist, Water Research Commission; and senior lecturer, Dep. of Soil Science and Agrometeorology, Univ. of Natal, Pietermaritzburg, Republic of South Africa, respectively

Received for publication October 25, 1982.

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