K+ deprivation induces xylem water and K+ transport in sunflower: evidence for a co-ordinated control.

The effect of K+ deprivation on water and K+ transport in roots was studied in sunflower plants. Deprivation was achieved in two different ways: by removing K+ from the growth medium for varying intervals; and by growing plants permanently in a low-K+ medium. Removal of K+ from the growth medium for a few hours prompted a significant increase in xylem sap exudation, associated with an increase in root hydraulic conductivity; however, it did not give rise to any significant change in plant K+ content, nor did it favour root K+ exudation. By contrast, prolonged K+ deprivation led to a decline in the internal K+ content and stimulated water and K+ transport in roots. Leaf application of K+ (Rb+) in plants grown permanently in a low-K+ medium inhibited the effect of K+ deprivation on root water and K+ transport, without significantly modifying the internal K+ content of the plants. This treatment had no effect on normal-K+ plants. These results suggest the existence of mechanisms enabling perception of plant K+ status and/or K+ availability in the medium, which trigger transduction processes governing the transport of water and K+ from the root to the shoot.