Which of the following plant hormone substtutes for long photoperiods in flowering plants

Options

A. Ethylene
B. Auxin
C. Gibberellin
D. Cytokinin.

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks which plant hormone can substitute for long photoperiods in flowering plants. This indicates that we need to identify a hormone that plays a role in promoting flowering, especially in long-day plants.

2. **Identifying the Options**: The options provided are:
- Ethylene
- Auxin
- Gibberellin
- Cytokinin

3. **Analyzing Each Hormone**:
- **Ethylene**: This hormone is primarily involved in the ripening of fruits and does not play a significant role in flowering related to photoperiods.
- **Auxin**: Auxins are mainly responsible for promoting cell elongation and are not directly involved in the flowering process related to photoperiods.
- **Cytokinin**: Cytokinins promote cell division and are involved in various growth processes, but they do not substitute for long photoperiods in flowering.
- **Gibberellin**: Gibberellins are known to stimulate flowering in long-day plants. They can also substitute for low temperatures required by biennial plants to initiate flowering.

4. **Conclusion**: Based on the analysis, gibberellin is the hormone that can substitute for long photoperiods in flowering plants, particularly in long-day plants.

5. **Final Answer**: The correct option is **gibberellin**.