Correlation between Root-Generated Ionic Currents, pH, Fusicoccin, Indoleacetic Acid, and Growth of the Primary Root of Zea mays

Correlations between root-generated ionic currents, extracellular pH, indoleacetic acid, fusicoccin, and growth were investigated. Current consistently entered the meristematic and elongating tissues of intact growing roots of Zea mays cv Golden Bantam. Mature root regions generated the outward limb of the current loop. Ion-substitution and pH-profile experiments suggested that the bulk of the ionic current was carried by H(+). Calcium ions did not carry current, but calcium may regulate the proton circulation since the proton current density was slightly larger in calcium-depleted media. Increased root elongation at low pH was associated with increased current density and an extended zone of inward current. Conversely decreased elongation at high pH was associated with a reduced current density and a more restricted zone of inward current. The effect of the fungal toxin fusicoccin was to increase the current density of the inward limb of the ion current and to increase root extension. Concentrations of indoleacetic acid that reduced root growth, also reduced the density of the inward current and shortened the inward current zone. The results emphasize the point that roots are electrically contiguous over many millimeters and that the electrophysiology of root growth is best studied in intact root systems.