Studies on the hormone mechanism of apical dominance in Vicia faba

Abstract

The dominance exerted by apical buds over the growth of laterals Is believed to be a phenomenon mediated by auxin. In Vicia faba one of the actions of auxin in causing bud inhibition, seems to be on the conducting strands of the stem at the base of the lateral buds. Sieve-tube development is hampered by auxin, transported from the apical buds. This leads to a restriction of supply of cotyledonary nutrients to the buds which then retards development. Auxin Inhibition action is largely on the cell division phase of bud growth.It Is primarily controlled by auxin and is not due to lack of substances like kinetin or gibberellic acid which normally participate in cell division phenomenon. Maleic hydrazide Is found to break 'apical dominance' by negating auxin action In plants, possibly by causing its oxidation.The polar transport and the effect of IAA In the main stem presumably at the base of the lateral buds has been confirmed by blocking the auxin movement by TIBA. IAA transported normally In the stem inhibits the growth of the lateral buds, whether or not its entry into the lateral buds is blocked by TIBA. However when IAA is prevented from reaching the stem at bud region, the inhibition of bud growth is invariably released.By applying IAA labelled with C 14 to the decapitated/decapitated plants, the presence of radioactivity In the inhibited buds can be detected. Extraction of radioactive substance from the plant parts and Its identification by paper chromatography suggested that the radioactive substance Is not IAA. Its growth promoting activities are low In bioassay with the Avena mesocotyl. These observations positively support the theory that the primary auxin action is in the stem at the base of the lateral buds and that the effect on the lateral buds is a secondary one.