The substance that triggers the fall of mature leaves and fruits from plants is abscisic acid.

Abscisic acid (ABA) is a plant hormone that is involved in several physiological processes, including the regulation of leaf and fruit abscission. Abscission is the natural process by which plants shed or drop their leaves or fruits when they are mature or damaged.

Role of Abscisic Acid in Leaf Abscission:
1. Promotion of Senescence: Abscisic acid promotes the senescence (aging) of plant tissues, including leaves. As leaves age, the levels of ABA increase, triggering various metabolic changes that lead to the breakdown of chlorophyll and other pigments. This results in the yellowing and eventual shedding of leaves.

2. Stimulation of Abscission Zone Formation: The abscission zone is a specialized region at the base of a leaf petiole where the separation of the leaf from the plant occurs. Abscisic acid stimulates the formation of this abscission zone by signaling the activation of specific genes. These genes produce enzymes that break down the cell walls in the abscission zone, allowing the leaf to detach from the plant.

3. Inhibition of Auxin Transport: Auxin is another plant hormone that plays a crucial role in leaf retention. Abscisic acid inhibits the transport of auxin from the leaf to the stem, preventing the maintenance of leaf attachment. This disruption in auxin transport further promotes the abscission process.

Role of Abscisic Acid in Fruit Abscission:
1. Regulation of Ethylene Production: Ethylene is a gaseous plant hormone that is involved in several physiological processes, including fruit ripening and abscission. Abscisic acid regulates the production of ethylene in fruit tissues, triggering the breakdown of cell walls and promoting fruit abscission.

2. Induction of Abscission Layer Formation: Similar to leaf abscission, fruit abscission also involves the formation of an abscission layer or zone. Abscisic acid induces the formation of this layer by activating specific genes that produce enzymes responsible for cell wall degradation. This allows the fruit to detach from the plant.

In conclusion, abscisic acid is the substance that triggers the fall of mature leaves and fruits from plants. It promotes leaf senescence, stimulates the formation of abscission zones, inhibits auxin transport, regulates ethylene production, and induces the formation of abscission layers, ultimately leading to leaf and fruit abscission.