Handbook of Plant and Crop Physiology

terial for biochemical studies and regulators can readily be applied directly to the abscising region (Fig-

ure 6).

C. Early Experiments with Regulation

After the demonstration that removal of the leaf blade would cause the abscission of the remaining leaf

stalk, it was proposed that reduction in photosynthate supply from the leaf caused the AZ cells to collapse

and fracture to occur. Kuster in 1916 [42] discovered that a tiny fragment of blade left attached to the peti-

ole was enough to prevent abscission. As it seemed unlikely that this small area of leaf was providing suf-

ficient nutrients, it was suggested that the healthy blade produced a hormonal factor that prevented ab-

scission [42].

The discovery that auxin was produced by young leaf blades led Laibach and his student Mai [43] to

investigate whether Kuster’s inhibitor was auxin. They demonstrated that pollen rich in auxin applied to

the cut end of a debladed petiole delayed abscission. La Rue [44] repeated the experiment with synthetic

indoleacetic acid (IAA) (Figure 6). A few years later, workers using IAA to induce parthenocarpic de-

velopment of fruit noticed that the treatment also delayed fruit abscission [45]. As a result, a general hy-

pothesis was put forward that abscission resulted from a reduction of the amounts of auxin in the abscis-

sion zone caused by a reduced auxin supply from the senescing distal organ (reviewed in Ref. 45).

At the beginning of the 20th century it had been found that traces of the gas used for illumination

would cause the abscission of leaves, petals, and fruits. The active component, identified as ethylene

(ethene), promoted abscission at very low concentrations of 1 to 8 ppm [46] (Figure 6). Some 30 years

later it was shown that ethylene was synthesized by most plants [47,48], and Milbrath et al. [49]

demonstrated that ethylene produced by apples would defoliate roses. However, auxin was in vogue at

the time and it was to be 20 years before the role of ethylene as a natural regulator of abscission was

taken seriously.

ABSCISSION 211

Figure 6 Effects of auxin and ethylene on abscission. The structural integrity (break strength) of bean leaf
abscission zones was measured at various times after deblading (triangles). After a 72-hr lag, the force neces-
sary to rupture the zone slowly decreases. Adding ethylene speeds up this process by reducing the length of the
lag and increasing the rate of weakening (circles). Adding IAA to the end of the petiole inhibits abscission com-
pletely (squares). The corresponding levels of 9.5 cellulase in the absission zones are also shown. (From Ref.
1.)