Effects of external cations and respiratory inhibitors on electrical potential of the xylem exudate of excised corn roots.

Glass capillary microelectrodes were used to study the electrical potential difference (PD) between the xylem exudate of excised corn roots, Zea mays L. Golden Bantam hybrid, and the external solution. A survey of the effects of various ions on the PD was made. With 1 mm single salt solutions, the PD was between 25 and 50 mv, exudate negative. The PD responded to concentration differences in single salt solutions of K(+), Na(+), and Ca(2+) in a manner suggestive of cation selectivity and cation diffusion potentials. With Ca(2+) present, the PD was insensitive to concentration changes of other cations. Substitution of NO(3) (-) for Cl(-) in K(+) solutions increased the PD by 2 to 5 mv, although in general the PD showed little response to anion concentration changes. The PD was partially abolished by cyanide. The remaining fraction of the PD was sensitive to concentration changes in external K(+), and we postulate that the PD is the result of both a diffusion potential and an electrogenic pump.