Additional evidence for two separable responses to auxin is pre-sented. The averge of 24 control experiments indicated lag times of 12.4 and 35.4 min, and maxium rates of 0.57 and 0.54 mm-hr-', for the first and second response, respectively. The auxin analog 4-azido-2-chlorophenoxyacetic add increased the lag time of the second response (but not the first), resulting in the temporal separation of the two responses. Plots of elongation rates against time, taken from the literature, alowed the characterization of the two responses in monocotyls and dicotyls. Study of published rate-time elongation curves showed that the maimum rate of the first response is frequently greater than the maximum rate of the second response; however, the maximum rate of the second response has not yet been shown to exceed the maximum rate of the first response.The short lag time between auxin application and increased elongation rate was first described by Yamaki (27) and Kohler (9). The possible implications of this work received considerable attention when Evans and Ray (3), using a unique growth apparatus that continuously measured and recorded growth, continued the earlier work of Ray and Ruesink (22). The rapid response of stem, hypocotyl, and coleoptile cells to auxin has since been cited frequently as evidence that auxin-induced cell elongation is not mediated by gene activation (21, and references therein), as first suggested by Skoog and co-workers (23,24), and later proposed for elongating cells by Nooden and Thimann (11-13) and Key and co-workers (6-8).The cytokinin, isopentenyladenine, inhibited auxin-induced elongation in long term (6-8 hr) experiments (26, and references therein). This naturally occurring hormone, however, did not inhibit the rapid response of elongating soybean hypocotyl cells to auxin. The study of the auxin-cytokinin interaction in elongating cells has produced evidence that there are separable responses to auxin (25). This possibility has been previously con- ' This research was supported by grants from the National Science Foundation (GB-36586, BMS72-02496) (6,8,21), it was the important conclusion from this work (25) that neither is it disproven by experiments which describe the fast response to auxin.The experiments described herein further characterize the separable responses to auxin, and present additional evidence that two elongation reactions to auxin do indeed occur in soybean hypocotyl cells. MATERIALS AND METHODSSoybean seedlings (Glycine max L. Merr. var. Wayne) were germinated in the dark and the elongating segment of the hypocotyl was excised as described (26), except that all procedures were performed under green light (460-590 nm) at 30 C.Hypocotyl extension was measured continuously with a linear transducer. The apparatus was modified after that reported by Green and Cummins (4). To facilitate clamping of the segment in the growth chamber, a 2-cm section, which included the 1-cm elongating section directly below the hypocotyl hook plus a centimeter of tissue basal to it, w...