Long day plants show flowering when

Options

A. the light period is greater than their critical photoperiod.
B. the light period is lesser than the critical photoperiod.
C. the day length is longer than 15 hours but interrupted by short dark period.
D. the day length is shorter than a critical period.

Answer

**Step-by-Step Solution:**

1. **Understanding 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 light period exceeds a certain critical length.

2. **Identifying the Critical Photo Period**: The critical photo period is the minimum amount of light that long day plants need to flower. This period is specific to each species.

3. **Conditions for Flowering**: Long day plants will show flowering when the light period is greater than their critical photo period. This usually occurs during the summer months when days are longer.

4. **Evaluating the Options**:
- **Option 1**: The light period is greater than their critical photo period. This is correct.
- **Option 2**: The light period is lesser than the critical photo period. This is incorrect for long day plants.
- **Option 3**: The day length is longer than 15 hours but interrupted by a short day period. This is incorrect as long day plants need uninterrupted long light periods.
- **Option 4**: The day length is shorter than the critical period. This is also incorrect for long day plants.

5. **Conclusion**: The only correct condition for long day plants to flower is when the light period is greater than their critical photo period.

**Final Answer**: Long day plants show flowering when the light period is greater than their critical photo period.

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