In 1955, Osborne [50] showed that diffusates from senescent petioles contained a soluble factor that
accelerated explant abscission. Soon afterward, Van Stevenick [51] demonstrated that developing pods at
the base of a lupin inflorescence stimulated abscission of the flowers above them (Figure 5). He succeeded
in extracting an abscission stimulator from the young pods [51]. A third group headed by Addicott [52]
identified a growth inhibitor that was present in young cotton fruits approaching abscission. The sub-
stance was purified [52], shown to accelerate abscission, and characterized as abscisic acid (ABA). Sub-
sequently, lupin pod extracts were also shown to contain ABA [53]. After a number of correlations were
reported between increasing ABA levels and abscission [54,55], ABA became accepted as a third poten-
tial regulator of the process.
D. IAA and the Control of Abscission
The demonstration that auxin would inhibit leaf absission (Figure 6) was followed by several attempts to
measure its levels in naturally abscising systems. It was found that both the extractable and diffusible
auxin levels dropped rapidly as the leaves yellowed and abscission approached [45,56,57]. Similar cor-
relations were observed between low auxin levels and fruit abscission [58].
A simple model emerged which suggested that if the auxin levels in the AZ remained above a criti-
cal level, abscission was inhibited [58]. Factors that promoted abscission, such as aging, frost damage,
and water stress, were thought to lower the levels of free auxin in the zone. It emerged that the rate of
auxin transport from the distal organ was a major influence on IAA levels in the AZ [59], although rates
of synthesis and degradation are also implicated.
Modifications of the simple auxin concentration theory were necessary when it became clear that the
levels of auxin on the stem or proximal side of the AZ also influenced abscission. Jacobs [60] demon-
strated that the presence of young auxin-producing leaves on the stem seemed to accelerate loss of de-
bladed petioles. Removal of the apical bud and young leaves delayed abscission below it, the influence
of the apical bud being restored if replaced by a supply of IAA [60,61].
As a result of Jacobs’ observations, a number of groups showed that if auxin was applied to
the proximal side of an AZ, it accelerated abscission [62], while if applied to the distal side, it de-
layed weakening. This led Addicott et al. [63] to propose the gradient theory, where the direction of
the auxin gradient across the AZ was important, not the absolute concentration. Auxin approaching the
zone from the distal direction inhibited abscission, whereas that moving from the stem accelerated the
process [45].
The gradient theory was subsequently challenged because very high levels of auxin applied to the
stem side would often inhibit abscission [64]. In 1964, Abeles and Rubinstein [65] reported that auxin
applications would promote the synthesis of ethylene, a potent accelerator of abscission. As a conse-
quence, it is possible to attribute the accelerating effect of proximal auxin to increased ethylene produc-
tion coupled to a failure of proximal IAA to reach the AZ before abscission was under way. Abeles [66]
argued that proximally applied auxin would move to the zone much more slowly than would distal ap-
plications, as its movement by diffusion would be opposed by basipetal auxin transport. Higher auxin
concentrations applied proximally would diffuse to the zone more rapidly, accounting for the inhibition
sometimes observed. Morris [67] has shown that the ethylene synthesis inhibitor aminooxyacetic acid
(AOA) will inhibit the accelerating effect of proximal auxin additions, adding weight to Abeles’ expla-
nation. Morris [67] also proposes that auxin applied proximally induces the synthesis of the ethylene
precursor aminocyclopropane carboxylic acid (ACC), which in turn diffuses to the zone and promotes
ethylene synthesis.
Although ethylene production by proximal auxin applications offers an explanation of the accelera-
tion of abscission, it does not account entirely for the speeding effect of the apex in Jacobs’ [60] experi-
ments, and as a result, the gradient theory still has its advocates.
E. Stage 1 and Stage 2 Responses
In 1963 Rubinstein and Leopold [68] discovered that if auxin was added distally more than 12 hr after de-
blading leaves, it accelerated abscission rather than preventing it. They put forth the view that explants
went through two stages after excision. In stage 1, auxin additions would inhibit abscission and prolong
the stage. If auxin levels fell, stage 2 was entered, when auxin accelerated weakening.
212 SEXTON