Phytochrome is associated with

Options

A. `P_(r)` and `P_("fr")` reactions
B. Absorption of red light of 660 nm
C. Absorption of far-rad light of 740 nm
D. Absorption of blue-light by leaves.

Answer

### Step-by-Step Solution:

1. **Understanding Phytochrome**:
- Phytochromes are photoreceptors in plants that play a crucial role in detecting light. They are sensitive to specific wavelengths of light, particularly red and far-red light.

2. **Forms of Phytochrome**:
- Phytochromes exist in two interconvertible forms:
- **PR (Phytochrome Red)**: This is the inactive form of phytochrome.
- **PFR (Phytochrome Far Red)**: This is the active form of phytochrome.

3. **Light Absorption**:
- When phytochrome absorbs red light (around 660 nanometers), the inactive form (PR) is converted into the active form (PFR).
- Conversely, when it absorbs far-red light (around 740 nanometers), the active form (PFR) can revert back to the inactive form (PR).

4. **Biological Responses**:
- The active form (PFR) is responsible for initiating various biological processes in plants, such as seed germination, flowering, and photoperiodism.

5. **Conclusion**:
- Therefore, phytochrome is primarily associated with the reactions between its two forms, PR and PFR, as well as their respective responses to red and far-red light.

### Final Answer:
Phytochrome is associated with PR and PFR reactions.

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