Agriculture Reference
In-Depth Information
The abscission process
Typically flower and fruit shed occur in four waves (Murneek,
). The
first is of flowers with no fertilized ovules and the second, often overlapping
with it, is of fruitlets with
endosperm nuclei. Most shed occurs at these
stages. The third wave is of fruits with endosperm and embryos with up to
-
cells, and the fourth is of fruits with completely developed endosperm and
embryos of various sizes, but usually fewer or smaller than those of retained
fruits. The fruitlets shed in the third and fourth waves are relatively large and
conspicuous and these waves, usually occurring in late May and early June in
the northern hemisphere, are collectively known as the June drop. Subsequent
shed is numerically slight up to the time of fruit maturity and abscission. Stages
of fruit and seed development are shown in Figure
.
The flowers and fruitlets, and later the mature fruits, show a constriction at
the base of the pedicel (the individual flower stalk) and apexof the peduncle
(inflorescence stalk) to which it is joined. In this constricted zone the cells of
all tissues, except those of the pith, are smaller than adjacent ones (McCown,
.
cells in width in the cortical tissue, less wide
in the xylem and phloem. Flowers and immature fruits abscise following the
differentiation of an abscission layer in the basal portion of the pedicel as a
result of cell division. This abscission layer is usually differentiated within the
limits of the constriction zone but it may also appear in the pedicel distal to
the constriction zone. Cells in all tissues undergo division and the resulting
abscission layer is sixto eight cells deep. A slight swelling of the middle lamel-
lae and primary walls, followed by disintegration of their pectic compounds,
precedes cell separation. The vessels are ruptured, apparently as a result of
mechanical force. If all flowers or immature fruits shed, then abscission layers
form so that the entire peduncle is shed. Following the abscising of the pedicel
or the peduncle, cork is formed by a cambium which is initiated a few cells
below the surface of the scar.
Two contrasting assumptions underlie attempts to explain why flowers,
fruitlets and fruits shed. One is that it is the natural tendency for the fruit
to remain attached in the absence of nutritional or other stresses leading to
abscission. The other is that fruitlets have a tendency to abscise and are only
prevented from doing so by specific hormonal stimuli originating in the fruit
itself (Luckwill,
). The zone is usually
-
).
It is now generally accepted that flowers are pre-programmed to shed after
anthesis unless they receive a new stimulus to trigger continued growth. This
stimulus is commonly given by pollination and fertilization, with the result that
resources are subsequently allocated only to those fruitlets likely to give fruits
with viable seeds. Sometimes pollination alone, or the application of plant
growth substances during or shortly after flowering, can provide the trigger
