Changes in Endogenous Gibberellin and Auxin Activities during First Internode Elongation in Tulip Flower Stalk

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Abstract
Dark treatment during the most active period of tulip shoot growth induced rapid elongation of the first internode. Endogenous free-form gibberellin and diffusible auxin in the first internode increased while bound-form gibberellin decreased after the dark treatment. Alternating dark and light treatments at 24-h intervals caused increases in elongation of the first internode and the amounts of free-form gibberellin and diffusible auxin in the dark but their decreases in the light. TIBA treatment at the first node inhibited both the elongation and the increase in diffusible auxin, but did not affect the gibberellin amount. Ancymidol application prior to the dark treatment inhibited the increase in both free-form gibberellin and diffusible auxin. Application of gibberellin A3 increased both elongation of the first internode and the amount of diffusible auxin. It also caused recovery from ancymidol-mediated reduction in elongation and diffusible auxin content. Dark-induced elongation of the first internode was inhibited when all organs above the first internode were excised, but endogenous free-form gibberellin increased and bound-form gibberellin decreased. After excision, elongation of the first internode occurred only when both GA3 and IAA were applied exogenously, or when IAA was applied with dark treatment. These results indicate that dark-induced elongation of the first internode of tulip is promoted by auxin, which is transported from the upper organs into the first internode due to stimulation from the dark-induced increase in free-form gibberellin. Free- and bound-form gibberellins changed complementarily with the dark and light treatments. An interconversion system between the two forms in the first internode and its dependence on light conditions are also discussed.