An Analysis of the Water Potential Isotherm in Plant Tissue I. The Theory

Abstract

The water potential isotherm is the function relating the water potential (specific free energy of water) in a given system to its water content The theory of this function in plant tissues, including its partition into the hydrostatic, osmotic, and matric components, is developed in 3 stages: the derivation of the water potential and its components from simple variables, for an "ideal", homogeneous tissue. It is shown that solute-matrix interactions cause a possibility of ambiguity in the definitions of the osmotic and matric components; the treatment of the theoretical dependence of each component on the water content. Certain simplifying assumptions about the osmotic and the hydrostatic potentials are critically examined; the extension of the theory to the real plant tissue, which is multiphasic and heterogeneous. The components of water potential as derived from measurements on such a tissue in bulk are expressed in terms of the components in the individual phases. One of the results is that heterogeneity causes considerable difficulties in the exact interpretation of the measurable "bulk" components. The partition of water content into the "matrix-bound" and "solution" fractions, as an alternative to the partition of water potential into its matric and osmotic components, is considered.