The finding that auxin induced ethylene formation was to provide an explanation of the opposing ef-
fects of auxin in stages 1 and 2 [65]. It was proposed that in stage 1 the AZ cells were sensitive to auxin
but insensitive to ethylene and that in stage 2 the reverse applied. If auxin was added in stage 1, it pro-
longed the length of the ethylene-insensitive condition. If the auxin levels fell below a critical threshold,
stage 2 was entered, when auxin no longer had any effect and the additional ethylene accelerated weak-
ening. Some support for this hypothesis has come from experiments that reduced the accelerating effect
of auxin in stage 2 by removing ethylene or inhibiting its production [66].
It is widely assumed that a loss of responsiveness to auxin in stage 2 is due to a loss of auxin recep-
tors. Jaffe and Goren [69] have reported that during stage 2 there is not only a reduction in IAA’s ability
to retard abscission but also in its power to evoke an Hion efflux. They speculate that these two diverse
processes may become ineffective because a common component of the response machinery (such as a
receptor) is lost. Another possible explanation is that auxin is simply not reaching the separation zone
cells if additions are delayed because of a decline in auxin transport. When, after abscission, auxin is
added to the cells of the fracture surface, it still inhibits production of the wall-degrading enzymes, sug-
gesting that at least one system is still auxin sensitive [70,71].
The role of auxin in the regulation of abscission has been eclipsed by work on ethylene. However, it
should be remembered that IAA additions will completely prevent any effect of ethylene for extended pe-
riods of time and that IAA should therefore be included in any model of abscission control.
F. Ethylene Accelerates Abscission
The ethylene-induced acceleration of abscission is probably the most consistently demonstrated of all
plant growth regulator responses (Figure 6). It has been shown to induce shedding of a wide variety of or-
gans and in a huge range of plant species (see the lists in Ref. 72). The threshold concentrations neces-
sary to induce the response are between 0.1 and 5 L/L [72]. Other unsaturated hydrocarbons, such as
propene, acetylene, and butene, will act as ethylene substitutes, but they are much less effective [73].
Some analogues, such as 2,5-norbornadiene, are competitive inhibitors [74,75].
There can be dramatic changes in the responsiveness of AZs to ethylene. We have already seen how
distal auxin additions to fruit and leaf AZs makes them insensitive to ethylene. There are also well-doc-
umented changes in natural sensitivity. For instance, Halevy et al. [24] showed that unfertilized cyclamen
flowers will not shed their corollas in ethylene, whereas fertilized flowers will. Similarly, styles of orange
would not abscise in the presence of ethylene until fertilization had occurred [76].
It is assumed that the presence of both the gas and its receptors are required for ethylene action to oc-
cur.
C 2 H 4 receptor→ethylene-receptor complex →abscissionFactors that increase sensitivity, such as water deficit, aging, and ethylene itself, are thought to increase
the levels of the receptor, while auxin reduces it [72].
G. Is Ethylene a Natural Regulator of Abscission?
The early observations that ethylene accelerated abscission were treated as a curious artifact, and
even after it was shown that plants naturally evolve ethylene, this gas was not widely envisaged as a
natural regulator. Part of the problem was that young leaves seemed to produce more ethylene than
older ones [78]. After the demonstration that auxin inhibited the accelerating effects of ethylene, Bar-
low [79] proposed that it was the auxin/ethylene balance in the tissue that was important. In young
leaves there was sufficient auxin to inhibit ethylene’s abscission accelerating effect, while in old leaves
there was not.
In 1962, a review by Burg [80] argued that the concentrations of ethylene in plants were such that
they could easily control abscission. The first claims that the gas was a natural regulator of abscission
were based on demonstrations that increased ethylene production rates were correlated with abscission.
Such parallels were shown in a wide variety of leaves, fruit, and flowers, although a few authors found
no simple correlation (reviewed in Ref. 72).
Of course, correlations do not constitute proof of involvement. To implicate ethylene firmly in ab-
scission, it is necessary to show that endogenous ethylene concentrations increase above the threshold
ABSCISSION 213