Leaf senescence is delayed by

Options

A. ethylene
B. cytokinin
C. ABA
D. auxin

Answer

To solve the question "Leaf senescence is delayed by," we need to analyze the roles of the mentioned plant hormones: ethylene, cytokinin, ABA (abscisic acid), and auxin.

### Step-by-Step Solution:

1. **Understand Leaf Senescence**:
- Leaf senescence refers to the process of aging in leaves, which leads to their eventual death. It is a natural part of a plant's life cycle.

2. **Evaluate Ethylene**:
- Ethylene is a gaseous plant hormone primarily known for promoting fruit ripening and senescence. Therefore, it does not delay leaf senescence but rather promotes it.
- **Conclusion**: Ethylene does not delay leaf senescence.

3. **Evaluate Cytokinin**:
- Cytokinins are hormones that promote cell division and growth. They also play a role in delaying senescence by promoting nutrient mobilization and stimulating growth processes.
- **Conclusion**: Cytokinin is known to delay leaf senescence.

4. **Evaluate ABA (Abscisic Acid)**:
- ABA is often referred to as the stress hormone. It is involved in processes such as dormancy and closing of stomata. It does not play a role in delaying senescence.
- **Conclusion**: ABA does not delay leaf senescence.

5. **Evaluate Auxin**:
- Auxins are involved in various growth processes, including promoting parthenocarpy and maintaining apical dominance. They do not have a direct role in delaying leaf senescence.
- **Conclusion**: Auxin does not delay leaf senescence.

6. **Final Conclusion**:
- Based on the evaluations, the hormone that delays leaf senescence is **cytokinin**.

### Answer:
Leaf senescence is delayed by **cytokinin**.