A new stem hygrometer, corrected for temperature gradients and calibrated against the pressure bomb

Abstract

Abstract A simple stem hygrometer for attachment to a bared section of sapwood or a cross‐sectional cut end of a shoot is described. Two welded chromelconstantan thermocouples inside the chamber, one touching the sample and the other in the chamber air, allowed measurement of and correction for the temperature gradient between the sample and the dewpoint measuring junction. The instrument was attached to the cut end of an apical shoot of Thuja occidentalis L. protuding from a Scholander‐Hammel pressure bomb. Cut‐end water potential (ψ hyg ), measured using the stem hygrometer, was compared to xylem pressure potential (ψ xp ) while the latter was manipulated in the pressure bomb. After an initial equilibration time of 3–4 h, hygrometer equilibrium values were achieved within 1.5–4.0 min of changing ψ xp in the pressure bomb. The half‐time (ψ 1/2 ) for vapour pressure equilibration was 15–40 s. Stable temperature gradients between the sample and dewpoint measuring junction of 0.01–0.1°C were measured. Correcting ψ hyg for the temperature gradient resulted in excellent agreement with ψ xp .