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Studies on Acidification of Media by Avena Stem Segments in the Presence and Absence of Gibberellic Acid ¹

^a Department of Botany, Division of Biological Sciences, University of Michigan, Ann Arbor, Michigan 48109

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 wall-loosening during active growth of the internode.

³ To whom reprint requests should be addressed.

¹ This research was supported by National Science Foundation Grant BMS 75-16359.