During the growth phase of a fruit, abscission seems to be inhibited by the presence of the develop-
ing seeds. Mature fruit are usually induced to abscise as one of the terminal events in the ripening pro-
gram. In some species, abscission is not complete but serves just to loosen the fruit so that birds and small
animals can detach them. The manipulation of fruit abscission is very important in developing mechani-
cal harvesting.
Flower bud loss is a serious problem in some crops and decorative plants. In lupins, the development
of young fruit at the base of the flower spike seems to induce the loss of buds at the apex [38] (Figure 5).
Removing some flower buds usually decreases the likelihood of abscission in those that remain. Disease,
water stress, waterlogging, mechanical shaking, and frost damage are also reported to enhance bud ab-
scission.
It is not widely recognized that the cessation of stem growth in several tree species involves abscis-
sion of the growing apex. In Tilia[39] and Salix[40] photoperiod seems to be a primary determinant of
when abscission occurs, but position of the branch in the canopy, its orientation, conditions for root
growth, competition from other apices, and other climatic factors provide modifying influences [21].
There is very little literature concerned with the inductive conditions that lead to the loss of bud scales,
stigmas, anthers, and sepals.
B. Experimental Induction of Abscission
Experiments early in the 20th century showed that removal of the leaf blade resulted in rapid abscission
of the subtending petiole (Figure 6). This was a conveniently reproducible system to study abscission, and
seedlings of bean, cotton, and Coleuswere commonly used. The need for faster synchronized abscission
led to the explanttechnique. Here, the abscission region was removed by cuts 1 to 2 cm on either side of
the zone. The isolated piece of tissue was kept in a sealed container often over 2% agar until it abscised
2 to 3 days later [41]. This explant system has become extremely popular because it provides a lot of ma-
210 SEXTONFigure 5 Influence of maturing pods on floral abscission. Lupin flower spike buds open from the base up-
ward (right). If the basal flowers are fertilized, the apical buds abscise, leaving bare stem (center). If the basal
flowers are removed, the apical buds remain and develop into pods (left). (From R. Sexton, unpublished.)