Electrical Potential Measurements, Ion Pumps and Root Exudation—a Comment and a Model Explaining Cation Selectivity by the Root

S ummary The value of measurements of the electrical potential difference between the external solution and the xylem sap for determining the presence or absence of an active ion flux in a root is discussed. It can be shown that the presence of an ion pump within the cells of the root is by no means incompatible with electrochemical equilibrium between the exuding sap and the external solution. A model is proposed for an exuding root of which the important features are a sodium pump at the tonoplast and the absence of any role for the endodermis. This model is shown to explain many experimental facts of which detailed consideration is given to the selective effect of the root on the cation composition of the exudate and the effect of an induced water flow through the root in reducing this selectivity.