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Unambiguous measurements of plant water potential are necessary for the study of crop response to water deficits. Our purpose is to describe procedures for field measurement of alfalfa ( Medicago sativa L.) water potential using the pressure chamber and a commercial in situ leaf dew‐point hygrometer. Valid pressure chamber measurements require protection against transpiration following sample excision, immediate measurement, and low chamber pressurization rates so that false endpoints can be circumvented. Valid hygrometer measurements require scrupulous hygrometer hygiene, hygrometer placement to avoid shading of the exposed leaflets on the measured leaf and insulating hygrometers against rapid temperature changes. When compared on the same plant, the pressure chamber and hygrometer measured similar water potentials. When sampling comparisons were made on different plants, the water potential measured with the hygrometer was always greater than that measured with the pressure chamber. The good agreement between methods on the same plant was due to the pressure chamber measuring the ψ of the leaf to which the hygrometer was attached. The higher ψ measured by the hygrometer when different plants were sampled was caused by the hygrometer covering one leaflet of the alfalfa leaf and decreasing leaf transpiration. Use of the hygrometer to determine the ψ of a fully‐exposed leaf can result in overestimates of more than 2 bars. Using either method to determine small ψ differences between field‐grown alfalfa plants at a given time may be impossible due to the large between‐plant variability.
Brown et al. (Sun,) studied this question.