Hydraulic conductance and K⁺ transport into the xylem depend on radial volume flow, rather than on xylem pressure, in roots of intact, transpiring maize seedlings

The investigation of transport processes in roots has so far been hampered by a lack of adequate methods to study water and solute transport simultaneously in intact, transpiring plants. The role of xylem tension in regulating volume flow and nutrient transport could not be addressed properly. In order to overcome limitations of conventional, massive-invasive methods, a gravimetric technique was used to measure water uptake by maize roots while simultaneously recording xylem pressure and xylem K⁺ activity in individual xylem vessels by means of a K⁺-selective xylem probe. This minimal-invasive approach allowed the calculation of the radial K⁺flux into the root xylem and the radial root hydraulic conductance on transpiring seedlings. By changing the light regime or the osmotic pressure of the external solution, radial water and K⁺ flux could be varied in order to study the interaction between water and solute transport. A major finding was that both radial K⁺ transport and hydraulic conductance strongly depended on radial volume flow, whereas xylem pressure had little (if any) effect on these parameters. Results are discussed with respect to relevant membrane transport processes and their regulation by volume flow.