Phenology is controlled by

Options

A. Humidity
B. Photoperiodism
C. PAR
D. Hydrological cycle.

Answer

**Step-by-Step Solution:**

1. **Understanding Phenology**: Phenology refers to the study of cyclic and seasonal natural phenomena, particularly in relation to climate and plant and animal life. It involves events such as flowering, leafing, and migration.

2. **Identifying the Factors**: The question asks what controls phenology. The options provided are:
- Humidity
- Photoperiodism
- PAR (Photosynthetically Active Radiation)
- Hydrological Cycle

3. **Analyzing Each Option**:
- **Humidity**: While humidity can affect plant growth, it does not primarily dictate the timing of phenological events.
- **Photoperiodism**: This is the response of organisms to the length of day or night. It is a key factor in controlling the timing of phenological events such as flowering and leaf shedding.
- **PAR (Photosynthetically Active Radiation)**: This refers to the light wavelengths that plants use for photosynthesis. While important for growth, it does not control the timing of phenological events.
- **Hydrological Cycle**: This cycle involves the movement of water through the environment. It influences plant health but does not directly control phenological timing.

4. **Conclusion**: Based on the analysis, the correct answer is **Photoperiodism**. It is the primary factor that controls phenology by determining the timing of seasonal events in plants.

**Final Answer**: Photoperiodism

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