Radial transport of water across cortical sleeves of excised roots ofZea mays L.

Abstract

The stationary radial volume flows across maize (Zea mays L.) root segments without steles (“sleeves”) were measured under isobaric conditions. The driving force of the volume flow is an osmotic difference between the internal and external compartment of the root preparations. It is generated by differences in the concentrations of sucrose, raffinose or polyethylene glycol. The flows are linear functions of the corresponding osmotic differences (Δ π) up to osmotic values which cause plasmolysis. The straight lines obtained pass through the origin. No asymmetry of the osmotic barrier could be detected within the range of driving forces applied (δ π=±0.5 MPa), corresponding to volume-flow densities of j_{v, s}=±7·10⁻⁸ m·s⁻¹. Using the literature values for the reflection coefficients of sucrose and polyethylene glycol in intact roots (E. Steudle et al. (1987) Plant Physiol.84, 1220–1234), values for the sleeve hydraulic conductivity of about 1·10⁻⁷ m·s⁻¹ MPa⁻¹ were calculated. They are of the same order of magnitude as those reported in the literature for the hydraulic conductivity of intact root segments when hydrostatic pressure is applied.