Thermocouple psychrometer for measurements of water potential in plant tissues by isopiestic method

The paper describes a thermocouple psychrometer for measurements of water potential (ψ_w) and its components—osmotic potential (ψ_{s + m}) and turgor pressure (ψ_p)—in biological objects. The isopiestic method applied in this work does not require preliminary scarification of plant material for eliminating cuticular resistance to diffusion of water vapors. The device is reliable and simple in operation owing to an original design of replaceable plungers carrying the thermocouples. A modified construction of the lid for a thermocouple chamber and the application of a cryoholder excluded the necessity of removing the sample from the chamber after ψ_w measurements prior to its freezing in liquid nitrogen and subsequent thawing for determination of ψ_{s +m}. This feature improves the accuracy of determining ψ_p, which is calculated as ψ_w − ψ_{s + m}. The device can operate with minimal quantities of plant material and allows determination of all three components (ψ_w, ψ_{s + m}, ψ_p) for the same sample.