Resistance of Plant Roots to Water Loss¹

In an attempt to resolve questions about the relative magnitudes of root resistance to uptake and exudation, an experiment was conducted in which the root zone of a bermudagrass ( Cynodon dactylon ✕ Cynodon transvaalensis L. Pers.) sod was divided by a barrier in such a manner that water could move from one side to the other only through the lateral stems of the connected plants. A large difference in soil water potential between the two sides was created by watering one side only. Under such circumstances nocturnal transfer of significant amounts of water from one side to the other, through the plant system, was observed repeatedly with a gamma probe. A separately measured resistance to uptake allowed calculations of the exudation resistance, which was found to be 1.2 ✕ 10 7 MPa s− 1 m− 1 , slightly larger, but of the same magnitude as the resistance to uptake. It was concluded that water will move from root to soil if the potential gradient is in that direction, though other factors such as soil hydraulic conductivity may become limiting.