the induction of wall-degrading enzymes. It has also been shown that protein synthesis inhibitors
will stop rapid abscission of petals, removing one of the last objections to the involvement of wall
hydrolases [2].
B. Nature of Cell Wall Breakdown
The evidence that wall breakdown is involved in abscission is almost entirely anatomical [3,5]. There are
very few biochemical analyses of abscission zone cell walls, although Morre [127] reported an 11% loss
of the wall material during weakening and Taylor et al. [128] reported a depolymerization of pectins.
Low-power observations of the fracture surfaces usually show that they are covered in intact rounded
cells [6] (Figures 2 and 3). When washed from the surface, the cells still have their permeability barriers
intact and can be plamolyzed [5]. They are not protoplasts, but retain part of the cell wall, which is still
resilient enough to prevent them bursting when turgid (Figure 10). The burst cells sometimes seen over
limited areas of the fracture surface are probably ruptured by the mechanical forces that facilitate separa-
tion in many abscission systems.
Electron microscope observations show that breakdown of the wall is not restricted to the middle
lamella but involves adjacent areas of the primary wall [5] (Figure 10). Both of these swell during AZ
weakening, leaving a layer of undigested wall around the protoplast. The swollen areas of wall still con-
tain intact cellulose microfibrils, suggesting that the wall matrix and middle lamella are attacked
[3,5,129]. Both x-ray microprobe analysis and autoradiography have shown that Ca^2 is lost from the
wall during cell separation [130]. It is not clear if Ca^2 is lost as a consequence of wall hydrolysis or
whether its active removal contributes to wall weakening [3,5].
The separation layer bisects the petiole or pedicel and therefore crosses many different tissues. In a
study of Impatiensleaf abscission, cell wall breakdown was recorded around cells of the epidermis, col-
lenchyma, cortex, xylem parenchyma, phloem seive tubes, and transfer cells [6]. In situ hybridization
studies [131] indicate that cellulase mRNA is induced in a variety of different cell classes in the separa-
tion layer. Comparisons of AZs in leaves and fruit suggest that there may be differences in the nature and
extent of wall breakdown [129].
218 SEXTONFigure 10 Electron micrograph of the separation layer cells from a weakened leaf abscission zone of Impa-
tiens. Note that the cell walls have degraded, allowing the cells to separate. The inner layer of the wall remains
intact and the cytoplasm is apparently normal. (From Ref. 5.)