Water Uptake and Release by Alfalfa Roots1

Abstract

AbstractWater release to dry upper soil by alfalfa roots amply supplied with water in the bottom of the rootzone could not be measured by gamma‐ray attenuation. Variations in the CO2and O2concentrations of the soil water had no measurable effect on water release. After having been nearly inactive with respect to water uptake in dry soil for more than 50 days, the upper roots took up water at a high rate immediately after rewetting. If metabolic influences of the plants are negligible, such polarity between water uptake and release must depend on the location and relative magnitudes of the main resistances in the two types of flow. Comparison of leaf water and soil water total potentials indicated little resistance in the xylem and major resistance in the radial pathway. Water uptake across large radial resistances causes low water potentials inside the xylem. For this water to be released to drier soil elsewhere, water potentials there must be even lower than in the xylem. Associated large soil‐root interface resistances are likely to prevent water release of any significance. Such resistances are suggested as an explanation for the generally observed polarity between water uptake and release.