By which of the following method cytokinins leads to delay in leaf senescence ?

Options

A. Nutrient Immobilisation
B. Nutrient Mobilisation
C. By Preventing ABA synthesis
D. By promoting Auxin synthesis

Answer

### Step-by-Step Solution:

1. **Understanding Senescence**:
- Senescence refers to the process of aging in plants, particularly in leaves. As leaves age, they undergo changes that lead to a decline in their physiological functions.

2. **Chloroplast and Leaf Color**:
- Young leaves are green due to the presence of chloroplasts, which contain chlorophyll. As leaves mature and age, chloroplasts begin to degenerate, leading to a loss of green color and the appearance of yellow pigments such as xanthophyll.

3. **Role of Cytokinins**:
- Cytokinins are plant growth regulators (PGRs) that play a crucial role in promoting cell division and delaying the senescence process. By promoting the division of cells, cytokinins help to rejuvenate aging tissues.

4. **Mechanism of Action**:
- When cytokinins are applied to aging leaves, they stimulate the production of new cells, which can lead to the formation of new chloroplasts and chlorophyll. This helps maintain the green color of the leaves and supports continued growth.

5. **Nutrient Mobilization**:
- Cytokinins also facilitate nutrient mobilization within the plant. This is essential for supporting cell division and overall plant health. By ensuring that nutrients are available for new cell formation, cytokinins effectively delay leaf senescence.

6. **Conclusion**:
- Among the options provided, the correct answer to how cytokinins delay leaf senescence is through nutrient mobilization. This process is vital for sustaining growth and preventing the aging of leaves.