On-line measurements of K+ activity in the tensile water of the xylem conduit of higher plants

In higher plants the xylem is the main pathway for anti-gravitational, long-distance transport of nutrients and water from the root through the shoot to the upper leaves. In the xylem conduit water is in a metastable state if tension larger than 0.1 MPa (i.e. negative pressure) is developed. While diurnal changes in negative pressure of individual xylem vessels can quite accurately be recorded by the minimal-invasive xylem pressure probe technique and water flow by non-invasive NMR techniques, the problem of continuous monitoring of solute flow remains a hitherto unresolved challenge. As shown here, integration of a K+ selective and a potential measuring microelectrode into the xylem pressure probe allowed on-line measurements of the K+ activity in individual xylem vessels of maize roots together with pressure and trans-root potential, the potential difference between the xylem and the external medium (i.e. the overall driving force of ions through the root tissue). When light irradiation was increased from 10 micro mol m(-2) s(-1) to 300 micro mol m(-2) s(-1) and negative pressure developed in the vessel, xylem K+ activity dropped from 3.6 +/- 2.6 mm to 0.9 +/- 0.7 mm (n = 16), whereas the trans-root potential depolarized from -2 +/- 11 mV to + 12 +/- 11 mV (n = 11), i.e. by + 14 +/- 7 mV. The effect of light on all three parameters was reversible. Exposure of the root to various K+ activities in the bath ranging from 0.1 to 43 mm revealed that the K+ activity of the xylem sap was shielded against short-term fluctuations in K+ supply to a large extent. In contrast, control experiments in which the root was cut 1 cm below the probe insertion point, allowing direct entry of external K+ into the xylem vessels, demonstrated that the xylem equilibrated rapidly with external K+. This was taken simultaneously as a proof for the correct reading of the probe.