Morphological and physiological aspects of the early phases of flower bud formation of apple

For consistent yields in apple fruit production, knowledge of the factors affecting flower bud formation is required. The aim of this study was to gain more insight in the role of endogenous factors in flower bud formation of apple. The effects of temperature, applied gibberellin (GA 4+7 ), the presence of fruits, defoliation and bending on the plastochron (the time interval between formation of successive primordia by the meristern) and the number of appendages per bud at the start of floral differentiation were studied. Although most treatments distinctly affected flower bud formation, the plastochron was only affected by temperature and defoliation. Considerable variation was observed in the number of appendages at the start of floral differentiation, which varied between cultivars, between experiments with the same cultivar and between bud positions. It is concluded that the length of the plastochron and the number of appendages are not critical to the occurrence of flower bud formation.Defoliation inhibited flower bud formation and caused an immediate reduction in the soluble sugar content of the shoot. Bending slightly enhanced flower bud formation, or had no effect, and did not affect the soluble sugar level in the shoot, but increased starch content. The poor correlation between the assimilate level of the shoot and the number of flower buds formed suggests that it is not a main regulatory factor in flower bud formation.It was further investigated whether ammonium affects flower bud formation of apple through affecting endogenous levels of polyamines (PAs). Ammonium and applied PAs have previously been shown to stimulate flower formation of apple. However, due to a lack of effect of ammonium on flower bud formation in the present work, no conclusive evidence could be given. PA levels of buds did not greatly respond to treatments affecting flower bud formation (applied GA 4+7 and shoot bending) despite the fact that both treatments did affect flower bud formation. PA levels did not show distinct changes during the supposed time of floral induction and floral differentiation. It is tentatively suggested that PAs are not major inductive stimuli of flower bud formation of apple.