Final answer:
Ethylene stimulates the abscission zone of a leaf, which leads to the fall of the leaf. The abscission zone is where cell degradation occurs, allowing the leaf to detach. Abscisic acid, contrary to past beliefs, is not significantly involved in this process.
Step-by-step explanation:
The region of the leaf that responds to ethylene and triggers the abscission, or the fall of leaves, is the abscission zone.
This zone is specifically designed to facilitate the separation of the leaf from the plant when it is time for the leaf to fall. Ethylene is a plant hormone that plays a vital role in the process of leaf abscission. It induces the cells in the abscission zone to degrade and to produce enzymes which weaken the cell walls, thus allowing the leaf to detach from the plant.
It is interesting to note that abscisic acid, which was once mistakenly believed to be involved in this process, actually has different functions, such as closing stomata and maintaining dormancy under stress conditions, but it does not play a major role in the abscission of leaves.