GIBBERELLIN STRUCTURE-ACTIVITY EFFECTS ON FLOWER INITIATION IN MATURE TREES AND ON SHOOT GROWTH IN MATURE AND JUVENILE PRUNUS-AVIUM

When applied to spurs of mature Prunus avium before floral initiation, gibberellins GA1, GA4 and GA5 inhibited floral initiation by 9-17%, GA7 by 43%, GA3 by 65-71% and 2,2-dimethyl GA4 by 78%. GA9 and GA20 were inactive. Thus activity only of the GAs with a C-3 hydroxyl was increased markedly by a double bond in the C-1,2 or C-2,3 position, and activity increased with increasing hydroxylation. None of the GAs affected the total number of buds (vegetative and floral) surviving in the spur. Measured by the threshold dose required for activity, seedling shoot growth responses to GA3, GA7, GA1 or GA4 resembled those of floral initiation, but di-methylation of GA4 at C-2 had no effect, and GA9 was as active as GA7. Mature shoots, including those on rooted cuttings, were less responsive to GA treatment than were juvenile shoots, with terminal shoots on mature trees more responsive than spur shoots. Spur shoot growth on mature trees responded to GA3, and to a lesser extent GA7, but not to GA1 or GA4. However, all these GAs promoted the growth of terminal shoots on mature trees to similar extents, whereas 2,2-dimethyl GA4 was less active than GA4. The differences between juvenile and mature shoot growth in sensitivity to a C- 1,2 or C-2,3 double bond, and between mature shoot growth and floral initiation in GA-structure requirements, indicate that phase change alters the GA complement and/or GA receptor/transduction mechanisms of P. avium. The difference in sensitivity to 2,2-dimethyl GA4 indicates that floral initiation and growth have different requirements for GA transport and/or action.