An Analysis of the Water Potential Isotherm in Plant Tissue

Abstract

The water potential isotherms of leaves of carob (a sclerophyllic xerophyte), plane tree (a mesophyte), and saltbush (a semisucculent xero-halophyte) were measured by vapour equilibration with filter paper. The isotherm of the living tissue was partitioned into components by measuring the isotherms of killed tissue and of isolated matrix fractions. Empirical functions were fitted by regression to each of the components. The isotherm of the matrix fractions fitted best to a function of the form 'P = -a/w2+b/w and the isotherms of killed tissue, whether before or after subtraction of the matrix, to a function of the form 'P = -a/w2-b/w. The first term indicates non-ideality of the tissue solution. The water potential difference between living and killed tissue, which is an approximation of the hydro-static potential, was far from linear with water content; either a quadratic function or two discontinuous linear ones could be fitted to it. Negative hydrostatic poten-tials were measured, the highest values (20 atm) being attained in carob. A hyster-etic component was measured both in the entire tissue and in the matrix fractions.