Lettuce seeds germinate only when

Options

A. Red light is available
B. Blue light is available
C. Green light is available
D. They are sown deep in the soil.

Answer

**Step-by-Step Solution:**

1. **Understanding Seed Germination Requirements:**
- Lettuce seeds have specific light requirements for germination. They need a certain type of light to trigger the germination process.

2. **Identifying the Light Types:**
- The types of light mentioned are red light, blue light, and green light. Each type of light has a different effect on the germination of lettuce seeds.

3. **Role of Phytochromes:**
- Lettuce seeds contain phytochromes, which are photoreceptors that help in detecting light. There are two forms of phytochrome: PR (inactive form) and PFR (active form).

4. **Conversion of PR to PFR:**
- When lettuce seeds are exposed to red light, the inactive form (PR) is converted to the active form (PFR). This conversion is essential for the seeds to germinate.

5. **Effects of Other Light Types:**
- If the seeds are exposed to blue light, green light, or UV light, the PR can still be converted to PFR, but these light types also cause the PFR to revert back to PR. This means that these light types do not support the germination process effectively.

6. **Conclusion:**
- Therefore, for lettuce seeds to germinate, they specifically require red light, as it ensures the accumulation of PFR necessary for germination.

**Final Answer:**
Lettuce seeds germinate only when red light is available.

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