The rate of acidification of media by Avena stem segments was studied with a titrimeter. GA3 increased this rate by an average of 17% if supplied to the segments 90 min prior to measurement. GA3 inhibited the rate by 15% if supplied 10 min prior to measurement. After 90 min incubation, stimulation of elongation had started; at 10 min, GA3 had not yet started to stimulate elongation in the segments.The acidification rates of the nodes (including the sheath-pulvinus), leaf sheath bases, and the internode bases of the stem segments were determined for plus and minus GA3-treated segments. The internode fraction contributes most to modification of the acidification rate, the node-pulvinus fraction less so, and the nongrowing sheath not at all.Acidification rates were measured for segments in different stages of elongation (lag, log, and plateau phases of growth). Segments in these growth stages were obtained from intact plants and from segments preincubated in sucrose and sucrose + GA3. Segments from all sources which are in the log phase of growth have the highest rates, those in the plateau phase the lowest. For lag and log growth phases, segments preincubated in sucrose + GA3 show the highest rates, those preincubated in sucrose the lowest rates. The opposite occurs for segments in the plateau phase of growth.Segments stimulated to grow by GA3 cause the pH of their incubation media to drop to pH 5.15 from an initial pH of 6.5. Nonstimulated segments cause a drop to pH 5.6. Long term growth of the segments is maximal in media buffered to pH 5 in the presence and absence of GA3.Our results support the idea that GA3 stimulates an active acidification process in Avena stem segments just after GA3 starts to stimulate growth in the segments, and that such an acidification process could play an important role in wail-loosening during active growth of the internode.Evans (6), Cleland and Rayle (5), and Marre et al. (12) have shown that IAA causes a significant increase in proton efflux from Avena coleoptile and Pisum epicotyl sections. Evans and Schmitt (7) have indicated that such hydrogen ion efflux can also be evoked in oat coleoptile sections by cutting them. These investigators have suggested that IAA-stimulated proton efflux could play a significant role in the initiation of hormone-promoted growth. In contrast to these systems, Fisher and Albersheim (8) 24061.3 To whom reprint requests should be addressed.Very little information is available on the effects of gibberellins (GAs) on hydrogen ion efflux, or any other acidification process during the relatively short times just before and after GA begins to stimulate growth in Avena stem segments. In this paper, we report on acidification induced in the medium by Avena stem segments in the presence and absence of gibberellic acid (GA3). MATERIALS AND METHODS Preparation and Incubation of Stem Segments for Growth andTitrimeter Experiments. For the growth experiments 2-cm stem segments, containing next-to-last internodes (p-li) were excised from Avena shoots ac...