Abscission and thinning of young fruit and thier regulation by plant hormones and bioregulators

Natural abscission of young fruit and its regulation by plant hormones is considered and compared to the generally accepted model of "senescence triggered'' abscission of, for example, leaves or mature fruit. It is concluded that abscission of young fruit cannot be explained by this model. Alternatively, it is suggested that the senescence triggered initial step in the classical abscission model should be replaced by a "correlatively triggered'' step. Polar basipetal IAA transport with its autostimulation and autoinhibition components is the main regulating signal in this correlative acting system and replaces ethylene as the initial driving force from the senescence triggered model. Results supporting this model are presented and tested against existing results from the literature. Finally, this hypothesis is tested as a possible explanation of the "mode of action'' of some thinning chemicals or bioregulators. It is speculated how a thinning chemical should be designed to function in a more reliable way, at least as far as its interference with the endogenous hormone system is concerned.