ORIGINS OF ELECTRICAL POTENTIAL DIFFERENCE BETWEEN XYLEM SAP OF MAIZE ROOTS AND EXTERNAL SOLUTION

The effects of 2,4-dinitrophenol, of changes in the temperature and concentration of the ambient solution and of variations in salt status on the electrical potential difference between the xylem exudate of maize roots and the ambient solution have been examined. The results are discussed in the light of some of the factors which could give rise to a potential difference between the sap and the solution. The rapid response of the potential difference to dinitrophenol and to changes in temperature suggest that, at least in part, it arises directly from metabolic processes. Rapid changes in the potential difference brought about by addition of salts may be attributed to differential rates of movement of anions and cations in the initial uptake process. Over longer periods the potential difference appears to be dependent on the concentration, but not the composition of the ambient solution, and on the salt status of the roots. The salt status influences the relative rates at which anions and cations are transported to the xylem sap, and a correlation has been found between the potential difference and the ratio of the rates of movement of chloride or sulphate to potassium to the sap. The implications of these findings on the elucidation of the pathways whereby ions are transported to the sap are discussed. © 1969 Oxford University Press.