GIBBERELLINS AND BUD BREAK, VEGETATIVE SHOOT GROWTH AND FLOWERING IN METROSIDEROS-COLLINA CV TAHITI

Applications of the growth promotive gibberellins (GAs) GA(4) and 2,2-dimethyl GA(4), and of C-16,17 endo-dihydro GA(5), which is known to promote flowering while inhibiting stem growth in the long-day grass Lolium temulentum, were made to micropropagated plants of Metrosideros collina cv. Tahiti, a highly ornamental cultivar with an intermittent flowering pattern. Gibberellin A(4) and 2,2-dimethyl GA(4) stimulated vegetative growth both in elongating shoots, and internodes of shoots developing from buds that were quiescent at the time of GA application. Abscission of the apices of expanding shoots, a feature of mature Metrosideros plants, was inhibited by these GAs, the rejuvenation of micropropagated plantlets being enhanced. However, C-16,17 endo-dihydro GA(5) differed from GA(4) and 2,2-dimethyl GA(4) by having no promotive effects on vegetative growth, and no inhibition of apical abscission. Notwithstanding this contrasting effect on vegetative growth, high doses of GA(4) or C-16,17 endo-dihydro GA(5) similarly reduced flowering on shoots to which either GA was applied. Reduced flowering in response to applied GAs is common in many woody angiosperms, and in this instance was probably the combined result of abortion of developing floral structures in quiescent buds, and a preferential inhibition of bud break for floral buds relative to vegetative buds, particularly by GA(4). Finally, both C-16,17 endo-dihydro GAs and GA(4) strongly inhibited bud break in this woody angiosperm, although GA(4) could initially stimulate bud break when applied to vegetative buds close to the expansion stage. The above findings, in tote, highlight the sensitivity of Metrosideros to both classes of GA in a variety of growth and development processes.