Handbook of Plant and Crop Physiology

arsenal of compounds produced in the fracture surface to prevent infection. These protective mechanisms

must be very efficient, as relatively few diseases originate at abscission scars.

II. REGULATION OF ABSCISSION

A. Inductive Stimuli

Normally, the induction of abscission appears to be an integral part of the senescence program

accompanying the yellowing of leaves and ripening of fruit. In most abscission systems the process can

be accelerated and will take place prematurely in the absence of senescence. For instance, pollination

can dramatically accelerate petal abscission [24,25]. In cyclamen, all pollinated flowers shed their

corollas in 5 days, whereas unpollinated flowers retained theirs even after 23 days [24]. Accelerated

floral abscission is thought to have evolved to prevent wasted visits of scarce pollinators to fertilized

flowers.

Leaf loss in temperate species accompanies senescence, which in turn is induced by environmental

factors such as photoperiod changes, low temperatures, and drought. Factors that affect the leaf blade ad-

versely can cause premature shedding. These include frost damage, drought [26], bacterial or fungal at-

tack [27,28], damage by herbivores [29], mineral deficiencies, toxins, excessive shading, darkness [30],

and competition with younger leaves. Leaf fall is not invariably linked to lamina senescence, and water-

stressed ivy plants will shed leaves with the same chlorophyll content as those still attached to normal

healthy plants.

Fruit appear to be abscised at several distinct stages of development [31]. Immature fruit can be shed

in large numbers in what appears to be a natural thinning process. Sometimes, this seems immensely

wasteful, and in species as diverse as oak and avocado less than 10% of potential fruit mature [32,33].

Some young fruit are shed because seed development is defective, although in avocado many embryos in

abscised fruit remain viable [34]. In fruit, water deficits [35], mineral deficiencies, pathogen [36] and her-

bivore attack, and frost damage can also be factors that precipitate premature abscission.

In many cases the reasons why such huge numbers of young fruit are lost are not clear, although com-

petition between developing fruit is certainly an important factor. The chances of a fruit being shed can

be reduced dramatically by removing other fruit from the same plant. In soybean, the fate of fruit that will

be shed (50 to 80%) is probably determined before fertilization takes place, on the day that the flower

reaches anthesis. In the flowers that will be lost, there is a failure for sink intensityto increase, so they do

not accumulate photoassimilate from source leaves [37].

ABSCISSION 209

Figure 4 Adventitious abscission zones on the leaf of ornamental cherry. Three areas of the leaf have been
wounded by heating, and abscission zones have formed in the living tissue around them. The cell separation,
which will result in excision of the damaged area, is evident as a white line. (From R. Sexton, unpublished.)