Ultrastructural Localization of Cellulase in Abscission Cells of Tobacco Flower Pedicels

Cortical cells within the abscission zone of Nicotiana tabacum flower pedicels were examined to identify cellulases at the fine structural level. A cytochemical method reported by Bal et al. for the ultrastructural localization of cellulase was modified and applied to study cell wall breakdown during ethylene-induced abscission and abscission where pollination was prevented. The deposition of copper (Cu), a precipitate of the Benedict's solution-cellulose sugars reaction, was utilized as a marker to detect cellulase activity in the abscission zone cells. The Cu precipitate was observed principally in the middle lamella region of the cell wall and was also observed in small amounts in the primary and secondary regions of the cell wall. Time-course studies for both ethylene-treated tissue and tissue where pollination was prevented indicated that cellulase activity was at its highest level during advanced stages of abscission. The break strengths of the abscission zone decreased prior to the increase in cellulase activity for both ethylene-treated tissue and tissue where pollination was prevented. Both "tubular fibril" and "regular fibril" arrangement patterns were observed in cell walls of abscission zone cells. Electron-dense bodies were found in association with cell wall fibrils. Lipid bodies in the cytoplasm near wall regions increased in frequency during the latter stages of abscission.