The cell separation process does not usually involve the entire AZ but only a narrow layer of cells
across it. The cells actually involved are known as the separation layer. One might expect that the sepa-
ration layer would contain a distinct specialized class of abscission cellsthat degrade their walls as a re-
sult of the abscission signal. However, the cells that will be involved in separation cannot be picked out
from their neighbors by simple microscopic examination [3].
One of the enigmas of abscission is why these separation layer cells degrade their walls when ap-
parently identical cells on either side do not. It has been suggested that separation layer cells are bio-
chemically distinct without there being any structural manifestations of the difference [3,12]. An alterna-
tive hypothesis envisages that potentially all cells can degrade their walls but that the abscission triggering
signal is restricted to just a few rows [3]. McManus and Osborne [13] have used immunological methods
to demonstrate specific proteins in the AZ prior to separation. This observation gives credibility to the hy-
pothesis that there are discrete abscission cells.
Abscission zones can be formed very early in the development of some organs; for instance, the
minute leaves and flowers inside dormant buds can already have responsive AZs. Why AZs develop at
specific positions is not understood. The smaller cells found in them are created by more persistent cell
division in the region [14]. Interspecific chimeras between two species of tomato with different AZ posi-
tions have been used to study the differentiation of the AZ. It seems that the position of the AZ is dictated
by the genetic status of the inner cell lineages, the outer cells responding to signals produced by them [15].
G. Loss of Abscission Zone Responsiveness
Abscission zones are not necessarily active throughout the entire life of the organ. An unfertilized orange
flower can be shed by activating the AZ in the pedicel, but after fruit development starts, this abscission
zone becomes inactive and will not respond to identical inductive stimuli [16] (also Figure 8). In peach
flowers there are three AZs [17], each of which is active at different phases of flower and fruit develop-
ment.
Two explanations have been put forward to account for the loss of AZ responsiveness. The first en-
visages that there is a loss of some vital component of the response machinery, such as a hormone recep-
tor. The second proposes that the response still occurs but that the cell walls of the separation layer are
modified by substances such as lignin or suberin that make them resistant to attack by wall hydrolases. In
orange pedicels, evidence seems to support the latter hypothesis [18].
H. Adventitious Abscission Zones
Although virtually all abscission takes place at precisely predictable sites, there are a few interesting cases
where it occurs rather randomly. An example of this type of adventitiousabscission is the shot hole ef-
fect, where diseased or damaged areas of the leaf blades of Prunusspecies are abscised, leaving holes [19]
(Figure 4). The leaves appear as though a shotgun has been fired through them—hence the name. Ad-
ventitious AZs also form in internodes of stems of Impatiens[20] and mulberry [21], where fracture can
occur at variable positions along the internode.
At first sight the ability to induce abscission at random positions seems to contradict the hypothesis
that there are discrete preprogrammed abscission cells at genetically defined positions. Since cell division
precedes fracture in adventitious zones, it is possible to argue that these new cells differentiate into an AZ,
which is then induced to abscise. This need for differentiation of an AZ before abscission can take place
might explain why adventitious stem abscission in Impatienstakes so much longer (5 to 14 days) [20]
than normal leaf abscission (22 to 36 hr) at preformed zones [5]. In a very interesting series of experi-
ments, Warren Wilson et al. [20] have shown that the position of adventitious AZs in Impatienscan be
modified in a predictable manner by manipulating auxin gradients in the tissue.
I. Protection of the Fracture Surface
After fracture has occurred, cells in what remains of the AZ on the plant divide and suberize to form the
scar that protects the wound [3,22]. The broken xylem vessels become blocked with gums or tyloses and
the phloem sieve plates are callosed over to prevent pathogen entry [3]. The antimicrobial enzymes chiti-
nase and 1–3 glucanase are produced in bean abscission zones [23] and are likely to be one part of an
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