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Abscission - Effects of Plant Growth Regulators
Abscission of leaves and fruit is one of the more obvious characteristics of senescence. Leaves do not fall simply because they are dead. A zone of cell division, the abscission zone, develops near the base of the leaf so that numerous cross walls form at right angles to the long axis of the petiole. Then pectinases and cellulases are induced in these cells of the abscission zone. These dissolve the lamellae of the cross walls of these cells, so that the petiole breaks off, The vascular connections break and usually become plugged by the formation of tyloses (deposits of gummy substances) and layers of corky cells.
Thus, at least two important events are involved in abscission: cell division and the induction of hydrolases. Both of these are processes of active metabolism and must therefore, be a programmed part of the development of the plant. Causes of abscission involve several interlocking events. It seems possible that certain growth-inhibiting substances may be involved. ABA clearly stimulates abscission in cotton petiole explants, as does GA. Addition of auxin to the petiole or blade of a senescing leaf prevents the formation of the abscission layer, and thus inhibits abscission.
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