K. Other Potential Regulators
Both gibberellic acid and cytokinins will influence abscission, although they are thought to be less im-
portant than the other plant hormones [55,113]. Cytokinins can delay abscission, probably by indirectly
delaying senescence [114]. Gibberellic acid will accelerate abscission [115,116], there being some debate
as to whether or not the effect is mediated by ethylene [66,117]. Long-chain unsaturated fatty acids such
as linolenic acid also enhance abscission [118]. Experiments on bean abscission zones showed that the
accelerating effect of the C 18 unsaturated fatty acids was mediated by the production of fatty acid hy-
droperoxides and that ethylene was not involved. It is not clear whether these compounds are involved in
the regulation of natural abscission, but they do accumulate in some senescent tissues.
L. Regulation of Abscission: Summary
As far as one can judge from the literature, the consensus is that the regulation of abscission directly in-
volves the concentrations of IAA and ethylene and the sensitivity of AZ tissue to them. Other factors that
influence the process do so through these agencies.
Figure 9 illustrates a balance modelbased on the relative AZ concentrations of auxin on the right and
ethylene on the left. If the left-hand side of the balance goes down, weakening starts, whereas if the right
end is down, the process is inhibited. The concentrations of ethylene and auxin in the AZ are influenced
by a variety of factors, some of which are shown at the top of the diagram. The balance can also be af-
fected by the position of the fulcrum, which can move from the center toward either end. Moving this to
the right represents a decrease in the sensitivity to auxin and an increase in sensitivity to ethylene, and
vice versa. Changing sensitivity probably involves the amount of receptors, and some of the factors that
change it are illustrated.
III. CELL BIOLOGY OF ABSCISSION
A. Mechanism of Abscission: Early Theories
Early botanists believed that abscission was due to the formation of a corky layer on the stem side of the
AZ which cut the supply of sap to the separation layer and caused the cells in it to collapse. The anatomist
Inman in 1848 [119] opposed this idea, suggesting that the process was a vitalone in which the cells of
the separation layer remained plump, fresh, and apparently living. A few years later, Von Mohl [120]
demonstrated that abscission would take place without the formation of a layer of periderm, and as a re-
sult, it became widely accepted that the process involved living cells [120].
Two theories emerged to account for the phenomenon. The turgor theoryproposed that the solute
concentration in the separation zone cells increased as a result of starch degradation. The increased tur-
gor pressure generated in the cells caused them to round up, tearing the wall along the line of the middle
lamella. Kendall in 1918 [121] claimed to disprove this hypothesis when he showed that cell separation
did not always begin at the cell corners and that some separating cells did not round up at all.
One of the first anatomical changes observed to occur after the induction of abscission was increased
rates of cell division in the region of the separation layer [3]. It was assumed that this was an important
part of the weakening process until Gawadi and Avery [122] showed that abscission would occur in the
absence of cell division, as is frequently observed if ethylene is used to accelerate the process.
Beginning in the 1920s, scientists assumed that the newly discovered wall-degrading enzymes
were involved in cell separation, although some researchers believed that wall acidification was also
implicated. Separation zone cells were reported to have very active respiration [117] and protein syn-
thesis [3,123]. These observations fueled speculation that the synthesis and secretion of wall-degrading
enzymes was all-important. The turgor mechanism retained some advocates, as it seemed to be the only
way to account for the abscission of some petals. This took place so rapidly (1 hr) that it was diffi-
cult to believe that protein synthesis, secretion, and wall breakdown could all occur in such a short in-
terval [124].
The finding by Horton and Osborne [125] that cellulase (endo--1,4-glucan 4-glucan hydrolase)
increased in weakening AZs, coupled with the demonstration that both protein and RNA synthesis
inhibitors prevented abscission [3,126], led to the current widespread belief that abscission involve
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