Gibberellins and bud break, vegetative shoot growth and flowering in Metrosideros collina cv. Tahiti

Abstract

Applications of the growth promotive gibberellins (GAs) GA4 and 2,2-dimethyl GA4, and of C-16,17 endo-dihydro GA5, 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 A4 and 2,2-dimethyl GA4 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 GA5 differed from GA4 and 2,2-dimethyl GA4 by having no promotive effects on vegetative growth, and no inhibition of apical abscission. Notwithstanding this contrasting effect on vegetative growth, high doses of GA4 or C-16,17 endo-dihydro GA5 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 GA4. Finally, both C-16,17 endo-dihydro GA5 and GA4 strongly inhibited bud break in this woody angiosperm, although GA4 could initially stimulate bud break when applied to vegetative buds close to the expansion stage. The above findings, in toto, highlight the sensitivity of Metrosideros to both classes of GA in a variety of growth and development processes.