Regulation of plant water potential by membranes of the endodermis in young roots

Abstract

Abstract The relationship between the flow of water through roots in young plants and the associated difference in water potential has often been found to be nonlinear. In this paper it is shown how the Casparian strip forces water, moving from the soil to the xylem of a young root, to pass through the cells of the endodermis, flushing solutes from them and reducing their turgor. It is suggested that the membranes of these cells like those of giant algal cells, respond to change in turgor by changing their hydraulic conductivity. These effects, when combined, are shown to produce a feedback system with a steady state behaviour which could account for some or all of the non‐linear relations, between rate of water uptake by young plants and differences between water potentials of the xylem and rooting medium, observed by workers such as Stoker & Weatherley (1971). Some non‐steady state responses of the system are explored, and it is suggested that the system also provides the link between the diurnal rhythms of solute concentration in the xylem sap, observed by Vaadia (1960), and of resistance to uptake of water by roots, reported by Parsons & Kramer (1974) amongst others.