title: A Comparison of Dewpoint and Psychrometric Mode in Leaf Water Potential Measurements
Creators: name: Durand-Campero, Gladys
Publisher: name: Utah State University
Year: 1977

description: Leaf water potential of two maize plants (Zea mays L. ) two chlorophytum plants (Chlorophytwn capense , Kuntze), a schefflera (Brassaia actinophylla) and one aspen (Populus tremuloides Michx.), were measured under laboratory conditions with aluminum block in situ leaf hygrometers and with stainless steel single junction chamber hygrometer using excised entire leaves. Plants were subjected to a drying cycle. The hygrometers were controlled with a dewpoint microvoltmeter and all readouts were recorded on a chart recorder. A typical reading and control schedule included 20 second cooling before a first psychrometric reading allowing the output to return to zero, followed by 20 seconds cooling and switching to DEWPOINT function. Dewpoint was recorded for periods up to 300 seconds. Finally, the instrument was switched directly to READ function and a second psychrometric reading was recorded, again allowing the output to return to zero. The area under the psychrometric trace, measured during evaporation phase, was taken as a measurement of the amount of water condensed on the thermocouple.
It was found that the cooling coefficient (Πv) of in situ leaf  hygrometers had to be lowered, compared to Πv values found in dry air, as plant water potentials decreased. This lowering was necessary to set the reading at dewpoint temperature without serious drifting. The areas under pre- and post- dewpoint psychrometric outputs were thus nearly equal and the dewpoint could be read for extended periods, confirming that equilibrium conditions were possible.
When water potential was measured in both the psychrometric and dewpoint mode with in situ leaf hygrometers, lower water potentials were found in the dewpoint mode than in the psychrometric mode and this difference tended to increase at lower water potentials. Conversely, in the Merrill units the water potentials determined on the psychrometric mode were consistently slightly lower than those based on the dewpoint readings. The greater agreement between psychrometric and dewpoint determinations obtained with Merrill units may well be explained by a manyfold higher leaf surface area exposed to the junction as compared to the limited leaf area sampled by the in situ leaf hygrometers. A greater area would contribute to a lesser total leaf resistance influencing the psychrometric determination.
The shape of hygrometer output traces in the psychrometric mode over standard solution generally had the typical, relatively flat shoulder, while over drier leaves it often had a more or less steady decline to zero. This difference was much more pronounced with in situ leaf hygrometers than with the chamber units which sampled larger leaf area.
The data suggest that the dewpoint mode, using proper precautions, measures water potential under equilibrium or isopiestic conditions, under which epidermal resistance is not a problem. Nonisopiestic conditions occur in the psychrometric mode. It appears that, immediately after cessation of the cooling current the evaporation of water from the wet junction elevates chamber vapor pressure above that of the mesophyll. This discrepancy would be zero over standard solutions and increases with increasing leaf resistance and with smaller leaf surface in the hygrometer.