The Movement of Ions to the Xylem Exudate of Maize Roots

Abstract

The electrical potential difference (Eex) and the electrochemical potential differences for potassium and chloride in the xylem exudate of excised maize roots were determined and the values obtained indicated active transport of both ions to the xylem exudate. The potential differences were depolarized by increases in the external KC1 concentration ([KC1]0), and Eex was a linear function of log[KCl]0. The corresponding data for the cell vacuoles of the same roots were also determined and trends similar to those found for the exudate were obtained. The electrochemical potential differences between the cell vacuoles and the external solution for potassium (A/ZTOC> K) and chloride (ApvaCt ci) indicated that both ions were actively transported to the vacuoles. Both Apvac.K an<? ^P-vac, ci decreased in a linear manner as [KCl]o was increased. However, in spite of the similar trends in both sets of data, the values of the electrical potential and chloride electrochemical potential differences were always markedly larger for the vacuoles than for the exudate. For potassium, there were no signifi cant differences. It is concluded that the discrepancy between the data for the vacuoles and the whole roots does not support the idea that the exuding root system behaves like a single cell, as has been suggested by some workers.