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

Abscisic acid (ABA) is a plant hormone that plays a crucial role in regulating various physiological processes in plants, including leaf and fruit abscission (falling off).

Here's an explanation of why abscisic acid is responsible for triggering the fall of mature leaves and fruits:

1. Role of Abscisic Acid in Leaf Abscission:
- Abscisic acid is synthesized in the leaves and acts as a signal for leaf senescence and subsequent abscission.
- As the leaves age, the levels of abscisic acid increase, signaling the plant to prepare for leaf abscission.
- Abscisic acid promotes the breakdown of chlorophyll and other leaf pigments, leading to the yellowing and eventual death of the leaf.
- It also stimulates the production of enzymes that degrade the cell walls in the abscission zone, a specialized region at the base of the petiole (leaf stalk).
- The degradation of cell walls weakens the attachment of the leaf to the plant, allowing it to detach and fall off.

2. Role of Abscisic Acid in Fruit Abscission:
- Similarly, abscisic acid also plays a role in fruit abscission.
- As fruits mature and reach their optimum ripeness, the levels of abscisic acid increase in the fruit.
- This increase in abscisic acid triggers a series of events that ultimately lead to fruit abscission.
- It promotes the production of enzymes that break down cell walls in the abscission zone of the fruit stalk, weakening the attachment of the fruit to the plant.
- Additionally, abscisic acid also inhibits the production of auxin, another plant hormone that promotes fruit development, further contributing to fruit abscission.

In summary, abscisic acid is responsible for triggering the fall of mature leaves and fruits from plants. It promotes the degradation of cell walls in the abscission zone, weakening the attachment of the leaves and fruits to the plant.