Field measurement of leaf water potential with a temperature-corrected in situ thermocouple psychrometer

Abstract. The construction and evaluation of a temperature-corrected in situ thermocouple psychrometer for measurement of leaf water potential (Ψ) is described. The instrument utilized two chromel-constantan thermocouples which allowed for detection of both the psychrometric zero offset and the temperature differential between the sample and the Peltier measuring junction. The psychrometer was subjected to stable temperature gradients while in contact with reference solutions of sodium chloride, and the effects of thermal gradients were quantified. Regression analysis indicated that temperature differentials were responsible for errors in water potential determinations of approximately –7.73 MPa°C⁻¹. When installed on leaves of field-grown cotton ( Gossypium hirsutum L.), corn ( Zea mays L.) and soybean ( Glycine max L. Merr) the instrument detected temperature differentials up to 0.1°C (–6.0 μV) which were associated with relatively small shifts in psychrometric zero offsets (–0.05––0.75 μV). Results indicated that substantial errors in apparent Ψ were caused by non-isothermal conditions between the leaf and the psychrometer measuring junction. The relative magnitude of these errors could be quantified and the corrected results showed good agreement with conventional psychrometric determination of Ψ on excised samples during a diurnal cycle.