Evaluation of a Thermocouple Hygrometer for Measuring Leaf Water Potential In Situ1

Abstract Several methods for measuring leaf water potential in situ with thermocouple psychrometers or hygrometers have recently become available. Disadvantages of these methods include difficulty in construction or use, or excessive modification of the leaf environment. Comparisons between in situ and other measurements are generally lacking. We constructed an aluminum hygrometer which is relatively easy to build and use and covers only about 1.5 cm2 of leaf area. This hygrometer was tested in the laboratory and the field and readings were compared with those made with a pressure chamber for several plant species. The leaf hygrometer worked well in both field and laboratory situations. Comparisons between readings with the leaf hygrometer and a pressure chamber gave regression lines with slopes of 1, but the average pressure chamber reading was 0.4 to 1 bar lower than that obtained with the leaf hygrometer. The leaf hygrometer appeared to respond to plant water potential changes in less than 30 minutes. The equivalent error due to temperature gradients within the chamber was found to be less than 0.4 bar. Measurement precision with this technique appears to be about 1 bar or better.