STEM ABSCISSION IN TUMBLEWEEDS OF THE CHENOPODIACEAE: KOCHIA

Anatomical, histochemical, and mechanical studies indicated the presence of a highly modified and weakened stem base in Kochia scoparia L. Schrader. This base, the abscission zone, is the site for stem abscission. In autumn progressive desiccation of the plant is accompanied by the gradual loss of stem flexibility and concomitant increase in rigidity. The tissues of the stem remain relatively tough, but abscission zone tissues become very brash or brittle. When conditioned plants are stressed by winds, the stem acts as a moment arm, and large stresses are generated in the abscission zone. Rupture then occurs across the stem base, often abruptly. Strength tests indicated that breakage occurred with 40% less stress if a soil‐inhabiting fungus ( Rhizoctonia sp.) had degraded the nonlignified cell wall components of the abscission zone. Abscission, therefore, is caused by the wind, an external driving variable, but tissue desiccation, changes in anatomy, and decay are internal, preparatory variables.