Handbook of Plant and Crop Physiology

is a competitive inhibitor and also interferes with natural abscission [74,75]. Sisler [77] has shown that

diazocyclopentadiene inhibits ethylene responses, probably by binding irreversibly to the receptor in the

light. This substance inhibits sweet pea abscission (Figure 8).

As a result of this wealth of data, there is widespread acceptance that ethylene is involved in natural

abscission. Whether it acts directly or indirectly is a more contentious question. There is evidence that the

movement of the ethylene precursor ACC in the xylem from water-stressed roots to the leaves may be an

important mechanism for inducing abscission [97].

H. Is Ethylene Having a Direct or an Indirect Effect?

It has been shown that ethylene lowers auxin concentrations in the abscission zone by reducing synthesis

[98] and transport [99] of the hormone and increasing loss by conjugation and breakdown [100]. As a re-

sult, it was proposed that ethylene speeds abscission indirectly by lowering auxin levels [72].

In a series of simple but elegant experiments, Beyer [101] showed that ethylene was actually involved

in abscission both directly and indirectly. A system was developed whereby the leaf blade and abscission

zone could be exposed to ethylene independently. If the zone or leaf blade was treated alone, abscission

did not occur; however, if both were treated, the lamina was shed. Beyer showed that treatment of the leaf

blade with ethylene reduced auxin transport down the petiole by over 80%. The effect could be mimicked

by auxin transport inhibitors and could be reversed if the supply of auxin from the blade was augmented.

This suggested that ethylene served indirectly to reduce the levels of auxin in the AZ by interfering with

auxin movement from the blade. Abscission would not occur, however, if auxin transport alone was im-

peded: ethylene also had to be present at the zone, suggesting a second direct role in abscission induction.

I. Does Ethylene Induce Abscission or Merely Accelerate It?

The question of whether ethylene induces abscission or just acts as an accelerator has not been resolved.

If ethylene is the inducer, removing it or interfering with ethylene action should not just slow the process

down but should stop it entirely. In the majority of experiments of this type, either abscission does even-

tually occur or observations are not continued long enough to distinguish between stopping and slowing.

J. Abscisic Acid

After Addicott’s group had isolated ABA from young cotton fruit, a number of correlations were reported

in which increases in ABA seemed to be associated with abscission [54,55,57,102–104]. In contrast, there

ABSCISSION 215

Figure 8 Inhibition of sweetpea floral abscission by the ethylene antagonist DACP [77]. Sweetpea buds ex-
posed to DACP for 16 h (far left) are just beginning to abscise after 144 hr in a vase; the untreated controls (sec-
ond left) shed their buds completely 48 hr earlier. Opened flowers (far right) had abscised all their petals by 144
hr while those exposed to DACP (middle right) were still firmly attached. Note that the unopened buds are shed
at an abscission zone at the base of the pedicel, in contrast to older flowers, where this zone is inactive and the
petals alone are lost, leaving the remainder of the flower attached. (From R. Sexton, unpublished.)