D. Mechanical Forces and Separation
Although the walls of living cells in the abscission zone (AZ) are enzymically degraded, mechanical
forces are necessary both to facilitate cell separation and to rupture the xylem [3]. External forces such as
the wind and gravity may be involved, although they are usually not sufficient by themselves. Weisner in
1871 [7] showed that if all the living tissues in a petiole are severed, leaves will often remain attached by
the xylem for long periods, despite these external agencies.
Many mechanisms have evolved to generate the forces required to cause complete separation [3]. In
bean, the growth of cells on the stem side of the abscission zone, coupled with shrinkage on the distal side,
has been implicated in producing stresses at the AZ interface that facilitate rupture [8] (Figure 1). Another
common system involves the rounding up and osmotic expansion of the separating cells stretching and
breaking the xylem [9] (Figure 3). The squirting cucumber provides another rather bizarre example where
internally hydrostatic pressures rupture the abscission zone at the base of the fruit, allowing it to shoot
away like a water-propelled rocket [10].
E. Why Is Weakening Restricted to the Separation Layer?
The positions of abscission zones are a genetically determined characteristic of a given species. For in-
stance, the blackberry (Rubus fruticosus) is shed by an abscission zone across the base of the fruit, leav-
ing the white receptacle or plug within the berry. In the closely related raspberry (R. idaeus), each of the
70 or so drupelets in the berry has an AZ at its base, so when the berry is detached it leaves the recepta-
cle attached to the plant. In hybrids like the loganberry, the blackberry position is dominant [11]. Plant
breeders have also produced varieties that lack the normal AZs, such as the lupin cultivar, which cannot
abscise its leaves [12].
F. Are There Specialized Abscission Cells?
Sections through AZs show that their anatomy is not very different from adjacent regions of the petiole
or pedicel which they bisect [3]. They often have subtle characteristics that allow the general region to be
distinguished [3]. The cells are frequently smaller than those in adjacent tissues, and this close packing
can make AZs rather darkly pigmented when viewed externally. The stele usually divides into separate
bundles before it enters the zone. Abscission zones lack lignification; sclerenchymatous fibers are often
replaced by collenchyma.
These features are all thought to have evolved to facilitate rupture. It is assumed that lignin is reduced
because it makes walls less susceptible to enzymic attack [3]. The close-packed angular AZ cells expand
as the walls are degraded and they round up (Figure 3) [9]. Their enlargement has been implicated in pro-
ducing the forces that rupture the xylem [9]. The branching of the xylem can be explained because under
asymmetrical loading a number of thin, separated strands are more readily ruptured than is one large cen-
tral bundle.
ABSCISSION 207Figure 3 Longitudinal fresh section through an abscising leaf abscission zone. The separation layer that runs
through the center of the micrograph is full of round, separated, turgid cells. The expansion of these cells in the
petiole cortex results in stretching and subsequent rupture of the xylem vessels in the dark vascular trace. (From
Ref. 9.)