Which phytohormone promotes falling of old leaves whereas inhibits falling of young leaves?

Options

A. ABA
B. Cytokinin
C. GA
D. Auxin

Answer

**Step-by-Step Solution:**

1. **Identify the Phytohormones:**
The question asks about a specific phytohormone that has two distinct effects: promoting the falling of old leaves and inhibiting the falling of young leaves. The phytohormones to consider are abscisic acid, cytokinin, gibberellic acid, and auxin.

2. **Analyze Abscisic Acid (ABA):**
Abscisic acid is known to promote abscission, which is the process of leaf and fruit detachment. It is also associated with the aging process (senescence) of plant parts. However, it does not specifically inhibit the falling of young leaves.

3. **Evaluate Cytokinin:**
Cytokinins primarily promote cell division and are involved in various growth processes. They do not play a significant role in leaf abscission, particularly in the context of promoting the falling of old leaves or inhibiting young leaves.

4. **Consider Gibberellic Acid:**
Gibberellic acid is involved in promoting stem elongation and seed germination. It does not have a direct role in the regulation of leaf abscission, so it can be ruled out.

5. **Examine Auxin:**
Auxin is a key hormone that regulates various growth processes, including apical dominance and leaf abscission. It specifically inhibits the falling of young leaves and promotes the abscission of older leaves. This makes auxin the correct answer to the question.

6. **Conclusion:**
Based on the analysis of the functions of each phytohormone, the correct answer is auxin, as it promotes the falling of old leaves while inhibiting the falling of young leaves.

**Final Answer: Auxin**

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