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Changes in Endogenous Gibberellins and the Metabolism of [³H]GA₄ after Geostimulation in Shoots of the Oat Plant (Avena sativa) ¹

Peter B. Kaufman, Najati S. Ghosheh, J. Donald Lacroix and Keith Heller

Department of Biology, University of Calgary, Calgary, Alberta T2N 1N4, Canada, Department of Botany, University of Michigan, Ann Arbor, Michigan 48109, Division of Biological Sciences, University of Michigan, Ann Arbor, Michigan 48109

The recovery from "lodging," or bending over, by shoots of 42-day-old Avena sativa plants is controlled primarily by a negatively geotropic differential growth of the lower halves of the p-1 node-pulvinus and the base of the p-1 internode, relative to the upper halves. Although geostimulation causes a significant reduction in p-1 internode length, dry matter accumulation in the p-1 node-pulvinus is increased, apparently at the expense of the sheath. Recovery to an angle of 30° is associated with changes in endogenous gibberellin-like substances (GAs), and in differential metabolism of applied [³H]GA₄ (1.4 Curie per millimole). Although geostimulation depressed total GAs (relative to upright plant parts) to 0.40 and 0.13 for node-pulvini and sheaths, respectively, it increased them 2-fold for internodes. Within the plant part geostimulation increased GAs (relative to upper halves) 29- and 7-fold in lower halves of node-pulvini and internodes, respectively, but reduced GAs to 0.3 in lower halves of sheaths. At age 42 days a GA_4/7-like (nonpolar) substance predominates, with lesser amounts of a GA₃-like (polar) substance. Native GAs of Avena include GA₃, GA₄, and GA₇. Geostimulation enhanced the ratio of nonpolar to polar GAs for both halves of internodes, but tended to depress it for sheaths and nodepulvini.

The disposition and metabolism of applied [³H]GA₄ confirmed the above trends for endogenous GAs regarding localization (e.g. up to 2-fold increases in [³H]GA₄ and acidic ³H-metabolites in the lower halves, relative to upper halves). Also, metabolism into highly water-soluble ³H-metabolites (biologically inactive conjugates?) was greater (up to 1.8-fold) in upper than in lower halves. The end result of such metabolic trends would be to reduce acidic (biologically active?) GAs in the upper half, while retaining them for a longer time in the lower half.

Geotropically stimulated Avena shoots thus increase, within 24 hours, the levels of acidic GAs in the lower halves of the p-1 node-pulvinus and p-1 internode, the two plant parts responsible for the geostimulated growth.

³ Present address: Japan Tobacco and Salt Public Corp., 6-2 Umegaoka, Midori-Ku, Yokohama, Kanagawa 227, Japan.

¹ This work was supported by Grants NAGW-34 from the National Aeronautics and Space Administration to P. B. K. and A-2585 from the National Research Council of Canada to R. P. P.

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