Breakdown of middle lamella pectin by ●OH during rapid abscission in Azolla

Azolla, a small water fern, abscises its roots and branches within 30 min upon treatment with various stresses. This study was conducted to test whether, in the rapid abscission that occurs in Azolla, breakdown of wall components of abscission zone cells by ^●OH is involved. Experimentally generated ^●OH caused the rapid separation of abscission zone cells from detached roots and the rapid shedding of roots from whole plants. Electron microscopic observations revealed that ^●OH rapidly and selectively dissolved a well‐developed middle lamella between abscission zone cells and resultantly caused rapid cell separation and shedding. Treatment of abscission zones of Impatiens leaf petiole with ^●OH also accelerated the separation of abscission zone cells. However, compared with that of Azolla roots, accelerative effects in Impatiens were weak. A large amount of ^●OH was cytochemically detected in abscission zone cells both of Azolla roots and of Impatiens leaf petioles. These results suggest that ^●OH is involved in the cell separation process not only in the rapid abscission in Azolla but also in the abscission of Impatiens. However, for rapid abscission to occur, a well‐developed middle lamella, a unique structure, which is sensitive to the attack of ^●OH, might be needed.