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 p romote flowering while inhibiting stem growth in the long-day grass Lo lium temulentum, were made to micropropagated plants of Metrosideros c ollina cv. Tahiti, a highly ornamental cultivar with an intermittent f lowering pattern. Gibberellin A(4) and 2,2-dimethyl GA(4) stimulated v egetative growth both in elongating shoots, and internodes of shoots d eveloping from buds that were quiescent at the time of GA application. Abscission of the apices of expanding shoots, a feature of mature Met rosideros plants, was inhibited by these GAs, the rejuvenation of micr opropagated 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 dos es 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 respons e to applied GAs is common in many woody angiosperms, and in this inst ance was probably the combined result of abortion of developing floral structures in quiescent buds, and a preferential inhibition of bud br eak 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 sti mulate bud break when applied to vegetative buds close to the expansio n stage. The above findings, in tote, highlight the sensitivity of Met rosideros to both classes of GA in a variety of growth and development processes.