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: Corolla expansion in Ipomoea nil appears to be triggered by changes in gibberellin concentration and ethylene production during development . We investigated the role of responsiveness to GA and ethylene in corolla expansion . The effects of growth regulators applied in vitro were measured as a change in area of corolla segments from younger (15)(16)(17) and older (18-20 mm) whole corollas . Applied gibberellic acid (GA 3) significantly (p < 0 .05) promoted growth in the younger segments but was less effective in the older segments . Moreover, applications of the GA biosynthesis inhibitors, PP333 (paclobutrazol) AM0 1618 (2-isopropyl-4-dinlethylamino-5-methylphenyl-l-piperidinecarboxylate methyl chloride), chlorocholine chloride, and tetcyclasis had little effect on younger segments but inhibited growth of older segments . The older corollas have apparently synthesized and accumulated enough GA-like substances to become less responsive to additional applied GA3 . The amount of growth induced by applied or endogenous GA depended on the amount of ethYlene simultaneously produced in the tissue . The younger corollas rapidly Produced ethylene from endogenous l-aminocyclopropane-l -carboxylic acid (ACC) and did not respond to applied ACC whereas the older corollas naturally produced much less ethylene and were significantly (p < 0 .05) inhibited by applied ACC . When ethylene production was inhibited by apPlying aminoethoxyvinylglycine (AVG), growth was promoted in all segments . However, only the growth of the younger segments was further stimulated by simultaneously applied AVG and GA 3 over the GA3 control . Thus the differential responses of segments from 15-to 20-mm long corollas to applied growth regulators reflect developmental, changes in re-C Sent address :

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