WATER TRANSPORT-SYSTEM AND ITS COMPONENTS IN HIGHER-PLANTS .1. METHODS FOR MEASURING CHARACTERISTICS OF WATER EXCHANGE IN PLANT-CELLS AND TISSUES - THE METHODS OF SOFT FLATTENING AND INITIAL FLOWS

Methods have been developed for measuring the volume of aqueous phase (V') and its changes in cylindrical plant samples both in the presence and absence of turgor pressure. In the soft-flattening method, a samp le is placed between two horizontal compressing plates, the distance b etween which (h) is continuously recorded by a small-displacement sens or. In plasmolyzed samples, the small vertical compression prevents th e detachment of plasmalemma from the cell wall, without producing a no ticeable elastic stretching of cell walls in the horizontal plane. In other words, this pressure prevents the development of plasmolysis. Th e water potential (Psi) of a compressed sample was varied by placing a tissue into solutions with specified osmotic pressures. Both for turg escent and nonturgescent cells, the changes in the thickness (h) of un icellular and multicellular samples can be related to their volume V a nd to the volume of the liquid phase (V'). To estimate the kinetic par ameters (the initial hydraulic conductivity of the membrane, L(p)), th e method of ''half-times'' introduced by Philip [9] was replaced by th e method of ''initial flows,'' in which the initial value of L(p)\(t=0 ) is determined from the initial (at the time of test solution applica tion) flow J\(t=0) = 1/S\(t=0) dV/dt\(t=0) = dh/dt\(t=0), where S and V are, respectively, the surface area and the volume enclosed by the c ell membrane. The value of dh/dt\(t=0) is determined by graphically di fferentiating the experimental curve. The previous method was replaced because the volumetric elastic modulus (epsilon) of cell walls is not a constant due to the fact that cells are subject to large relative v olumetric strains (more than 0.2). Since L(p) depends on the extent of stretching of cell walls and membranes, it also depends on cell tumor (P). Hence, the method of initial flows permits estimation of L(p) on ly at the time of test solution application.