Abscission of flowers and floral parts

The abscission of inflorescences, flowers, petals, sepals, styles, and stamens is discussed, with emphasis on the anatomy and ultrastructure of the abscission zones, and the role of cell wall degrading enzymes and hormonal control. Shedding of these parts is usually due to cell wall dissolution, but abscission of petals, stamens, and styles in some species occurs due to the forces generated by the growing fruit. Flower abscission is clearly regulated by ethylene, whilst auxins apparently decrease the sensitivity to ethylene. Petal, style and stamen abscission also seems to be controlled by endogenous ethylene. Auxin is apparently involved in abscission of styles and stamens, but in petals its role is at yet unclear. The ultrastructural data indicate high protein synthesis and high secretory activity of material toward cell walls of abscission zone cells. The physiological evidence indicates a role of both polygalacturonase and cellulase in cell wall dissolution, whilst the role of other cell wall degrading enzymes is still unknown. The physiological processes occurring in the walls of the separating cells should be distinguished from those relating to defence against microbial intrusion, such as deposition of lignin and suberin and tylose formation. Experimentation using mutants and transgenic plants may aid in separating these processes. Sequencing of the isoenzymes specific for the abscission zone and a search for abscission zone-specific promoters seems a requirement for the successful evaluation of the enzymes involved in cell wall degradation.