Techniques and experimental approaches for the measurement of plant water status

Abstract

Living cells need to be more or less saturated with water to function normally, but they are usually incomplete in this desirable condition. The two basic parameters which describe the degree of unsaturation, i.e. the plant water deficit are (i) the water content and (ii) the energy status of the water in the cell. The water content is usually expressed as relative to that at full saturation, i.e. the relative water content or water saturation deficit, and the energy status of the water is usually expressed as the total water protential. Although the two parameters are linked in such a way that the total water potential decreases as the water content decreases, the relationship between the two, variously known as the moisture release curve, water potential isotherm or water retention characteristic, is not unique but varies with species, growth conditions and stress history (Slatyer, 1960; Jarvis and Jarvis, 1963; Altmann and Dittmer, 1966; Noy Meir and Ginzburg, 1969; Ludlow, 1976; Jones and Turner, 1978). Thus for completeness, both the water content and energy status of the water in plant tissue need to be measured. The total water potential (7') at any point in the plant can be partitioned into its components: the osmotic potential (re), turgor pressure (P), matric potential (z) and gravitational potential. As the gravitational component of the total water potential is only 0.01 MPa m- 1 (0.1 MPa = 1 bar), it can be neglected, except in very tall trees (Conner et al., 1977). For cells in equilibrium with their surroundings the total water potential is the same throughout the system, i.e. in the wall, cytoplasm, organelles and vacuole. However, the components of the total water potential may be quite different: in the vacuole the total water potential arises largely from osmotic and turgor forces, whereas in the wall, it arises largely from matric forces and to a small degree from osmotic forces. Thus the total water potential of a plant cell is given