Anatomy of Ethylene-induced Petal Abscission in Pelargonium × hortorum

When viewed under the light microscope, the abscission zone at the petal base of Pelargonium × hortorum consisted of small cells which, when stained with Toluidine Blue, possessed densely stained cells walls. After treatment with 1 µl l-1 ethylene at 22°C, the force required to separate the petals from the receptacle declined after a lag phase of only 30 min, with separation complete 60-90 min later depending upon the stage of development of the flower. Transmission electron micrographs of the petal abscission zones showed evidence of cell wall degradation, particularly in the middle lamella. These cells also contained extensive rough endoplasmic reticulum and numerous Golgi bodies ribosomes. When abscission was complete, cells at the fracture face showed evidence of breakdown of cellular compartmentalization, often with little sign of an intact tonoplast. Scanning electron micrographs of recently-abscissed surfaces showed that the epidermal cells surrounding the abscisson zone were turgid and rounded, whereas those of the mesophyll cells were partially collapsed. The micrographic evidence is consistent with the hypothesis that ethylene-induced separation is caused by rapid enzymatic of the cell walls.