A long day plant flowers only when it is exposed to

Options

A. Red light
B. Light more than critical day length
C. Light equal to ciritical day length
D. Light less than critical day length

Answer

To solve the question "A long day plant flowers only when it is exposed to," we need to understand the concept of photoperiodism in plants, particularly focusing on long-day plants.

### Step-by-Step Solution:

1. **Understand Long-Day Plants**:
- Long-day plants are those that require a longer duration of light (day length) to initiate flowering. They typically flower when the day length exceeds a certain critical value.

2. **Identify Critical Day Length**:
- Each long-day plant has a specific critical day length that must be exceeded for flowering to occur. This means that if the day length is shorter than this critical length, the plant will not flower.

3. **Evaluate the Options**:
- **Option 1**: Red light - While red light can influence flowering, it is not the primary factor for long-day plants.
- **Option 2**: Light more than the critical day length - This is the correct condition for long-day plants to flower.
- **Option 3**: Light equal to the critical day length - This may not be sufficient for flowering in long-day plants.
- **Option 4**: Light less than the critical day length - This is characteristic of short-day plants, which require less light to flower.

4. **Conclusion**:
- Based on the understanding that long-day plants require exposure to light that exceeds the critical day length, we conclude that the correct answer is **Option 2: Light more than the critical day length**.

### Final Answer:
A long day plant flowers only when it is exposed to **light more than the critical day length**.

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