A Simple Thermocouple Psychrometer for Determining Tissue 6

Abstract

An apparatus for determining water potential of leaves (ψ) is described. It is a modification of one previously developed by van Andel for determining osmotic potentials of sap (π), for which purpose it may still be employed. A constantan—minalpha thermocouple is used for direct determination of wet-bulb temperature depression of air in moisture equilibrium with leaves. The system has advantages over others in simplicity of manufacture and electrical circuitry, and in adaptability to a range of signal reading and recording methods. It is capable of handling relatively large samples, e.g. 20 cereal leaves of total area (single side) of 300 cm2. Tests of leaf maturity and position effects in two potato crops are described. Differences between corresponding values of ψ and π, considered indicative of cell wall pressures, were less in lower leaves. In one crop, ψ was lower in relation to leaf relative water content with lower than with upper leaves and, in the other crop, with leaves from the more mature stages of growth. These findings are in accord with a higher rigidity of cell walls in lower and more mature leaves. Mean differences of ψ and π involved in the above conclusions were small, usually less than 2 bar, indicating the potential usefulness of the apparatus.