Handbook of Plant and Crop Physiology

10

Abscission

Roy Sexton

Stirling University, Stirling, Scotland

205

I. GENERAL FEATURES OF ABSCISSION

A. Definitions

During its life a plant will shed many of its organs, such as leaves, fruit, petals, buds, bud scales, and bark.
Two distinct processes contribute to this loss:

1. General attritionis responsible for the detachment of dead or dying tissues such as bark, old
   branches, and roots. In these cases, large mechanical forces such as the wind or differential
   growth of the stem rupture an inherently weak region of tissue, usually producing an irregular
   tear.


2. Abscissionis involved in the loss of leaves, fruit, flowers, and floral parts. It is an active
   metabolic process resulting in the weakening of anything from 1 to 20 rows of cells in geneti-
   cally determined abscission zones. Cell wall breakdown is an important element in the loss of
   structural integrity. Unlike attrition, abscission is under very precise internal hormonal control.

This chapter is concerned with the mechanism, regulation, and agricultural importance of abscission.

B. Abscission Zones

The shedding of leaves, flowers, fruit, and so on, occurs as a result of the weakening of abscission zones
(AZs). These bisect the base of leaf petioles, leaflets, petals, styles, flower buds, axillary buds, young fruit,
and the nodes of very young stems. Abscission zones are not inherently weak and a 0.5-kg weight can be
hung on a bean leaf AZ without rupturing it. After abscission has been induced, the same AZ will weaken
so that it will break at the slightest touch. This progressive loss of structural integrity occurs over 72 hr af-
ter an initial 18-hr lag [1]. Although leaf and fruit abscission usually takes up to 3 days, the process can be
extremely rapid and some petals [2] and flower buds [3,4] are shed 1 to 4 hr after the inductive treatment.

C. Weakening Process

The scanning electron micrograph in Figure 1 shows a fracturing abscission zone at the base of a bean
leaflet. The discrete nature of the fracture line reveals that loss of structural integrity is restricted to just