Flower induction in perennial fruit trees: Still an enigma?

Flower induction (FI) in adult fruit trees differs considerably from the same process in annual/biennial plants. Whereas in the latter FI is mostly a qualitative, genetically regulated process, in fruit trees, inter-organ (correlative) signals play the predominant role, making FI a more quantitative process. These correlative signals sense many environmental and horticultural cues and then form a message that finally induces an indeterminate vegetative meristem to undergo a developmental switch that either changes it into a generative meristem or keeps it in the vegetative stage. Plant hormones are the only endogenous substances that have consistently shown the ability to affect this developmental process in a positive or negative manner and for this reason are the main focus of this treatise. Particular emphasis was placed on the role of gibberellins and polar auxin transport as an inhibiting signal and of optimal cytokinin concentration as a stimulating signal in the process of flower induction.