Leaf fall occurs in a tree when there is increase in concentration of

Options

A. abscissic acid
B. auxin
C. gibberellins
D. cytokinins

Answer

**Step-by-Step Solution:**

1. **Understanding Leaf Fall**: Leaf fall, also known as abscission, is a natural process that occurs in trees, particularly during the autumn season. This process involves the shedding of old leaves to make way for new growth.

2. **Identifying the Hormone Involved**: The primary plant hormone responsible for leaf fall is abscisic acid (ABA). This hormone plays a crucial role in regulating various physiological processes in plants, including leaf senescence (aging) and dormancy.

3. **Role of Abscisic Acid**: Abscisic acid promotes the aging of leaves, leading to senescence. As the concentration of abscisic acid increases, it triggers the processes that result in the leaves detaching from the tree.

4. **Comparing Other Hormones**:
- Auxin: This hormone is involved in promoting growth and development, particularly in elongating stems and leaves. It does not promote leaf fall.
- Gibberellin: This hormone is primarily associated with stem elongation and fruit development, not leaf abscission.
- Cytokinins: These hormones help in cell division and growth, and they work to maintain leaf health by mobilizing nutrients, rather than promoting leaf fall.

5. **Conclusion**: Based on the role of abscisic acid in leaf senescence and the comparison with other hormones, we conclude that leaf fall occurs when there is an increase in the concentration of abscisic acid.

**Final Answer**: Leaf fall occurs in a tree when there is an increase in the concentration of **abscisic acid**.

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