Visualisation of xylem sap flow direction in isolated fine lateral roots and estimation of the xylem sap osmotic potential

Abstract

Xylem sap outflow from fine lateral roots (FLRs) isolated from hydroponically grown young maize (Zea mays L.) plants was visualized by local brightening of test solutions contrasted with purified Indian ink particles. Flow into the vessels was indicated by the adsorption of Evans Blue in their walls. The fraction of the FLRs able to exude xylem sap in a mineral medium with 30 mM mannitol decreased with increasing incubation time. This change was strongly retarded, when the FLRs were incubated in a medium containing glucose instead of mannitol. There was a broad range of variation of the osmotic potential of the test solutions (Ψ so ), wherein the fraction of the FLRs showing an initially reversed flow of the xylem sap varied between zero and unity. A median (M) of the osmotic potential of the xylem sap in FLRs (Ψ sx ) was estimated. It represents the value of Ψ so that was lower than Ψ sx in half of the roots of a sample before their transfer to the test solutions (Ψ sxo ). M was dependent on the osmotic potential of the medium used for growth or pre-incubation of the FLRs. Its value was not dependent on the molecular size of the osmolytes used to adjust Ψ so , including dextran 8, which is excluded from cell walls. In all of the studied plants, M was lower than the osmotic potential of the xylem sap collected from the root before isolation of the FLRs. To explain this finding it is assumed that FLRs with Ψ sxo > M had a higher hydraulic conductivity and a larger volume contributed to the exuded sap than those with Ψ sx < M.

Radial transport of salt and water in roots of the common reed (Phragmites australis Trin. ex Steudel)

“…A few of the FLRs attached to the maternal root were decapitated, the root was immersed in an Evans Blue solution (10 g L −1 ) or in an ink particle suspension (Fritz & Ehwald ) both made up with the growth medium, and the tubing connected to a vacuum pump, before photographs of hand slices were taken.…”