Leaf abscission is promoted by

Options

A. Ethylene
B. Abscisic acid
C. Cytokinin
D. Tryptophan.

Answer

**Step-by-Step Solution:**

1. **Understanding Leaf Abscission**: Leaf abscission refers to the process of leaves detaching from a plant. This is a natural process that occurs in many plants, especially in response to environmental changes.

2. **Identifying Plant Hormones**: Several plant hormones (also known as plant growth regulators) influence various processes in plants, including leaf abscission. The key hormones to consider in this context are ethylene, abscisic acid, cytokinin, and tryptophan.

3. **Role of Ethylene**: Ethylene is a gaseous plant hormone that plays a significant role in promoting leaf abscission. It is known to increase in concentration when leaves are about to fall, leading to the detachment process.

4. **Contradictory Action of Auxin**: Auxin is another plant hormone that generally inhibits leaf abscission. When auxin levels are high, they counteract the effects of ethylene, preventing leaf detachment. Conversely, when ethylene levels rise and auxin levels fall, abscission is promoted.

5. **Enzymatic Action**: Ethylene stimulates the production of enzymes such as cellulase and pectinase. These enzymes break down the components of the cell wall, specifically cellulose and pectin, which leads to the loosening of the cell wall and ultimately results in leaf abscission.

6. **Evaluating Other Options**:
- **Abscisic Acid**: This hormone primarily acts as an inhibitor of growth and is involved in processes like seed dormancy and stress responses. It does not promote leaf abscission.
- **Cytokinin**: This hormone is mainly involved in promoting cell division and growth, not in leaf abscission.
- **Tryptophan**: This is an amino acid and does not function as a plant growth regulator, making it irrelevant to the question.

7. **Conclusion**: Based on the roles of the hormones discussed, the correct answer to the question "Leaf abscission is promoted by" is **ethylene**.

**Final Answer**: Ethylene (Option A) promotes leaf abscission.

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