Solute concentrations in xylem sap along vessels of maize primary roots at high root pressure

The elemental composition of xylem sap has been determined by cryo-analytical microscopy in situ along vessels in the roots of maize plants frozen intact while root pressure was high. The only chemical element (including carbon) present in significant concentrations in the vessels was potassium at ∼11 mM and ∼15 mM in the late (LMX) and early (EMX) metaxylem, respectively. There was no gradient of [K] along the vessels, which each run the length of the mature proximal end of the roots. At the distal end of each vessel, in the oldest still living vessel elements, there was a sharp rise in [K] to 110 mM and 130 mM in the LMX and EMX, respectively.