PETERSON, CURT M., LIWEI CHEN, ROLAND R DUTE, AND MANDY N. ROGERS.* Department of Biological Sciences, University of Northern Colorado, Greeley, CO 80639; Department of Biological Sciences, Auburn University, AL 36849. - Petiole Abscission in Cotton (Gossypium hirsutum L.): Anatomical and Ultrastructural Observations.
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