Effect of Indole-3-acetic Acid on Membrane Potentials of Oat Coleoptile Cells.

This paper reports an effect of indole-3-acetic acid on cell membrane potentials. Solutions with increasing IAA concentrations (from 109 to 10-7 M) make membrane potentials of coleop-tile cells more negative. This effect of IAA, which occurs at concentrations that also stimulate tissue elongation, was found inadvertently in the course of an experiment on the effect of IAA on cell wall potentials. In the latter case, no significant effects were found. MATERIALS AND METHODS Microelectrode techniques (1-3) were used for measuring membrane potentials of coleoptile cells immersed in solutions containing different concentrations of IAA. Growth rates were ascertained on the same tissues used for electrical potential measurements. Coleoptiles were obtained from dark-grown oat (Avena sativa cv. Victory) seedlings. Coleoptile tips (5 mm in length) were removed , and the next 7 mm were excised and secured (apical end up) in a Lucite chamber. Coleoptile segments were immersed in a treatment solution the composition of which was maintained constant for the entire experimental period. Membrane potentials were measured after the segment had been exposed to a treatment solution for 2 to 8 hr. Segment lengths were ascertained immediately after excision and at the end of the experiment. The pH was 5.3. Solutions containing IAA were always freshly prepared. A solution with the above concentrations of inorganic ions was also used to moisten the sand on which the seedlings were grown. Membrane potentials of the parenchyma cells exposed on the cut surface of the apical end of the coleoptile were measured by slowly inserting a microelectrode into each cell and recording the change in potential (between the microelectrode and a reference electrode in the external solution) as the microelectrode passed through the cell wall and cytoplasm and into the vacuole. Equipment used for measuring potentials was similar to that described previously (1, 2). It included a motor-driven micro-manipulator which was mounted on the stage of a compound microscope. Lucite chambers containing coleoptile segments were also attached to the stage of the microscope. A treatment solution flowed constantly through the chamber. Insertion of a microelec-trode into cells was observed through a 40X long working distance objective. Ling and Gerard type microelectrodes (3) and similar