Studies of Root Function in Zea mays: III. Xylem Sap Composition at Maximum Root Pressure Provides Evidence of Active Transport into the Xylem and a Measurement of the Reflection Coefficient of the Root.

The cut ends of excised Zea mays roots were sealed to a pressure transducer and their root pressures recorded. These rose approximately hyperbolically to a maximum value of 4.21 +/- 0.34 bar after 30 to 40 minutes. Xylem exudate could not be collected at this pressure since the flow rate was zero. Samples of exudate were collected at lower applied pressures (DeltaP), however, and Deltapi, the osmotic pressure difference between them and the solution bathing the root, was measured by freezing point depression. A plot of DeltaP/Deltapi against J(v)/Deltapi, where J(v) is the volume flux, proved to be a straight line whose intercept, equal to sigma, the reflection coefficient, was 0.853 +/- 0.016. The maximum xylem concentrations of various chemical species were found by a similar extrapolative method and compared with those in the cell sap. This indicated that (a) Ca(2+), Mg(2+), NO(3) (2-), SO(4) (2-), and most amino acids move from the cells to the xylem down an electrochemical potential gradient; (b) relative to these ions H(+), NH(4) (+), glutamine and asparagine are actively transported into the xylem; and (c) H(2)PO(4) (-), and K(+) are actively retained in the symplasm.