ABSTRACT
Measurements of water potential (c) and its components are being increasingly used to characterize plant and soil water relations. The psychrometric technique has a number of advantages compared to other methods including the ability to facilitate a large number of samples and also allows the determination of the components of c. Reviews of psychrometry in soil and plant water relations have been published,[1-3] the use and construction of these instruments have been documented,[1,4,5] and concerns about their use for measuring soil and plant c have been addressed.[6]
THEORY OF THERMOCOUPLE PSYCHROMETERS
The Concept of Water Potential
The use of thermodynamic principles to express the water relations of soil and plant tissue[1,2] is well recognized. The free energy of water in the soil-plantatmosphere continuum influences both the movement of water along energy gradients and water availability in the plant. Application of these concepts has proven extremely meaningful, since the chemical potential of water and dissolved solutes greatly affects cell growth.[2] The chemical potential of water is related to the change in the free energy of the system and can be expressed in terms of the partial water vapor pressure.[7] In an isothermal system, the volumetric c (MPa) is given by the Kelvin equation:
c ¼ ðRT=VwÞlnðe=e0Þ ð1Þ
where R is the universal gas constant (8.3143 106 m3 MPa mol1 K1), T the absolute temperature (K), Vw the partial molar volume of pure water (1.805 105 m3 mol1), and e and e0 are the partial and saturated vapor pressures of water (relative humidity of the air in the psychrometer chamber expressed as a fraction). Therefore, the c of a system can be determined if the equilibrium water vapor pressure (e/e0) is measured at a known temperature and pressure. The thermocouple psychrometer is based upon this concept and upon the principle that the
vapor pressure above a solution or segment of plant tissue is related to its water potential according to Eq. (1).
