The temporal relations between early responses to indoleacetic acid (IAA), proton secretion, hyperpolarization of the membrane potential, and growth change during the incubation of segments of oat (Avena sativa L.) coleoptiles in a low salt medium. When IAA is added after pretreatment of several hours, proton secretion increases after a latency of 7 minutes and reaches its maximum 10 to 15 minutes later. This timing coincides with both the increase in growth of the segments and the hyperpolarization of the membrane potential of parenchyma cells, consistent with the hypothesis that the change in membrane voltage reflects the activity of an electrogenic proton pump. The extent of IAA-induced hyperpolarization is substantially reduced by elevating [KClI1, most likely because this increases the passive conductance of the membrane. Neither growth nor proton secretion is affected by high [KCJIL (30 millimolar), indicating that neither process is limited by the magnitude of the membrane potential. These results are consistent with the acid growth hypothesis. Following short incubation times, however, IAA-induced hyperpolarization and growth are detected within 10 minutes, while acidification of the medium is delayed for more than 40 minutes. This result is seemingly in conflict with the acid growth hypothesis, but in freshly cut tissue, the pH of the external medium may not reflect the pH of the epidermal cell walls. The temporal coincidence of auxin-induced growth and hyperpolarization suggests that in freshly isolated segments the hyperpolarization is a more sensitive indication of proton secretion than is acidification of the external aqueous environment.through secondary effects on essential transport processes and metabolism, may be more important in the stimulation ofgrowth by auxin than is wall acidification.Since protons are secreted from the cell against both a considerable concentration and electrical gradient, this process requires energy presumably furnished by H+-translocating ATPases in the plasma membrane. Electrophysiological measurements have shown that both IAA and FC3 cause the plasma membrane to hyperpolarize, indicating that the increased proton secretion is electrogenic (1,9). Although a careful comparison has been lacking, the hyperpolarization should be coincident or even prior to measurable acidification of the external medium. The time required to acidify the inner portion of the wall where loosening occurs (26) is not known; however, the onset ofhyperpolarization should be a sensitive indication of the time at which proton secretion and wall acidification actually begins.The purpose of these experiments was to determine the temporal relations between the induction of membrane hyperpolarization, proton secretion, and growth by IAA. Furthermore, by adjusting the membrane potential and the auxin-induced hyperpolarization by varying [KCl]0, we examined the suggestion that a highly negative membrane potential per se is important for the stimulation of growth by auxin. MATERIALS AND METHODS...