Transient and permanent changes of xylem sap exudation by root systems of Zea mays after application of hydrostatic and osmotic forces

Abstract

Effects of relatively small changes of hydrostatic and osmotic pressure on root exudation were studied with maize (Zea mays L.) plants grown in hydroculture to estimate the root reflection coefficient for the applied osmolyte (PEG 600). During the first seconds after a change in hydrostatic pressure, the exudation rate measured with a microflow sensor was instantaneously and strongly changed due to elastic deformation of the metaxylem vessels in the branched part of the main root axis. In osmotic experiments, a time of 10–20 s was required before the maximum change of the exudation rate was recorded. This retardation can be explained by diffusive saturation of the non-agitated root surface film and radial turgor propagation. A new standing osmotic gradient was reached within 4 min after a change of the water potential difference (osmotic, hydrostatic). The steady-state exudation rate J was altered by osmotic and hydrostatic forces with nearly equal efficiencies when branch roots were not injured. Hence, the reflection coefficient of the intact root for PEG 600 was close to unity. The results are in accord with nearly ideal reverse osmosis at high rates of water uptake by roots and confirm the absence of a significant hydraulic bypath circumventing the protoplasts.