Abscission limits yield and indirectly affects fiber quality in cotton. The purpose of this study was to determine the structural changes contributing to petiole abscission. Ethephon (2-chloroethy1-phosphonic acid) hastened cotyledonary leaf abscission of cotton, Gossypium hirsutum L. Abscission of petioles was induced in explants prepared from 14-day-old seedlings by removal of the leaf blade, and then applying either lanolin or lanolin containing 0.1% ethephon to the petiole stumps. Prominent swelling of collenchyma cells on the adaxial side of the petiole within the abscission zone provided the first morphological indication that abscission had begun. Cell separation occurred distal to this region of swollen cells and was initiated first on the adaxial side of the petiole. Swelling of ground tissue cells also was observed on the abaxial side of the petiole. Tyloses were observed within vessel members proximal to the region where cell separation was observed. Ultrastructural changes in abscission zone cells of the explants treated with ethephon occurred earlier than those of the lanolin treatment. Vesicles appeared within the mid-cell wall regions as cell separation progressed. Free cell wall microfibrils also accumulated at this time. Middle lamella dissolution and partial degradation of the primary cell wall was observed prior to abscission. The plasma membrane of most separation layers cells remained intact following cell wall degradation, and these intact cells contained degraded chloroplasts and mitochondria. Some separation layer cells broke down completely, and organelles within them became embedded in the degraded cell wall matrix. Although ethephon treatment hastens the abscission process, no other differences in structural events leading up to abscission were observed when tissues of ethephon-treated explants were compared to lanolin-treated (control) explants. These anatomical and ultrastructural changes suggest that abscission specific, cell wall hydrolyzing enzymes are being secreted following stimulation of abscission by leaf blade removal.

Key words: abcission, cotton, Gossypium hirsutum, petiole, separation layer, tyloses