Solute osmotic potentials (psi(x)) in the vessels of hydroponically grown maize roots were measured to assess the osmotic-xylem-sap mechanism for generating root pressure (indicated by guttation). Solutes in vessels were measured in situ by X-ray microanalysis of plants frozen intact while guttating. Osmotic potentials outside the roots (psi(o)) were changed by adding polyethylene glycol to the nutrient solution. Guttation rate fell when psi(o) was decreased, but recovered towards the control value during 3-5 days when psi(o), was greater than or equal to -0.3 MPa, but not when psi(o) was equal to -0.4 MPa. In roots stressed to psi(o) = -0.3 MPa, psi(x) was always more positive than psi(o) and psi(x) changed only slightly (ca. 0.05 MPa). Thus the adjustment in the roots which increased root pressure cannot be ascribed to psi(x), contradicting the osmotic-xylem-sap mechanism. An alternative driving force was sought in the osmotic potentials of the vacuoles of the living cells (psi(nu)), which were analysed by microanalysis and estimated by plasmolysis. psi(nu) showed larger responses to osmotic stress (0.1 MPa). Some plants were pretreated with abundant KNO3 in the nutrient solution. These plants showed very large adjustments in psi(nu) (0.4 MPa) but little change in psi(x), (0.08 MPa). They guttated by 4 h after psi(o) was lowered to -0.4 MPa. It is argued that turgor pressure of the living cells is a likely alternative source of root pressure. Published evidence for high solute concentrations in the xylem sap is critically assessed.
