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

Dark treatment during the most active period of tulip shootgrowth induced rapid elongation of the first internode. Endogenousfree-form gibberellin and diffusible auxin in the first internodeincreased while bound-form gibberellin decreased after the darktreatment. Alternating dark and light treatments at 24-h intervalscaused increases in elongation of the first internode and theamounts of free-form gibberellin and diffusible auxin in thedark but their decreases in the light. TIBA treatment at thefirst node inhibited both the elongation and the increase indiffusible auxin, but did not affect the gibberellin amount.Ancymidol application prior to the dark treatment inhibitedthe increase in both free-form gibberellin and diffusible auxin.Application of gibberellin A₃ increased both elongation of thefirst internode and the amount of diffusible auxin. It alsocaused recovery from ancymidol-mediated reduction in elongationand diffusible auxin content. Dark-induced elongation of thefirst internode was inhibited when all organs above the firstinternode were excised, but endogenous free-form gibberellinincreased and bound-form gibberellin decreased. After excision,elongation of the first internode occurred only when both GA₃and IAA were applied exogenously, or when IAA was applied withdark treatment. These results indicate that dark-induced elongationof the first internode of tulip is promoted by auxin, whichis transported from the upper organs into the first internodedue to stimulation from the dark-induced increase in free-formgibberellin. Free- and bound-form gibberellins changed complementarilywith the dark and light treatments. An interconversion systembetween the two forms in the first internode and its dependenceon light conditions are also discussed. (Received June 23, 1984; Accepted March 5, 1985)