phytochrome becomes active in

Options

A. Red light
B. Green light
C. Blue light
D. None of these

Answer

**Step-by-Step Solution:**

1. **Understanding Phytochrome**: Phytochrome is a photoreceptor in plants that plays a crucial role in regulating various physiological processes, including flowering. It exists in two forms: Pr (inactive form) and Pfr (active form).

2. **Role of Light in Phytochrome Activation**: Phytochrome becomes active when it absorbs specific wavelengths of light. The two critical wavelengths are red light (around 660 nm) and far-red light (around 730 nm).

3. **Activation Process**: When phytochrome absorbs red light, it converts from the Pr form to the Pfr form, which is the active form. This conversion is irreversible under normal conditions.

4. **Photoperiodic Response**: The activation of phytochrome is essential for the photoperiodic response in plants, which determines their flowering time based on the length of day and night.

5. **Critical Duration**: Plants can be categorized based on their light requirements:
- Long-day plants require more light than a critical duration to flower.
- Short-day plants require less light than a critical duration to flower.
- Day-neutral plants are not affected by light duration.

6. **Conclusion**: Phytochrome becomes active in response to red light, which is crucial for the flowering process in many plants.

**Final Answer**: Phytochrome becomes active in red light.

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