Ion Currents and Growth Regulators in Plant Root Development

Factors that promote (low pH and fusicoccin) or reduce (high pH and IAA) the elongation rate of intact primary roots of Zea mays cv. Golden Bantam increased or decreased, respectively, the density of positive current flowing into the meristematic and elongating tissues of the developing root apex. Mature non-growing root regions generated the outward limb of the current loop. Ion-substitution and pH-profile experiments suggested that the bulk of the ion current was carried by H⁺. Calcium ions did not carry a significant portion of the current, but calcium appeared to regulate the proton circulation since the current density was larger in calcium-depleted media. Increased root elongation at low pH was associated with increased current density and a basipetally 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 fungal toxin fusicoccin increased the current density of the inward limb of the ion current and increased root extension. Concentrations of IAA that reduced growth also reduced the density of the inward current and shortened the inward current zone. The effects of both fusicoccin and IAA were dependent on the pH of the bathing medium.