Which does not cause stomatal opening?

Options

A. Light
B. Circadian rhythm
C. Influx of `K^(+)` lons
D. High `CO_2` concentration

Answer

**Step-by-Step Solution:**

1. **Understanding Stomatal Opening**: Stomata are small openings on the surfaces of leaves that allow for gas exchange. The opening and closing of stomata are influenced by various factors, including light, potassium ions, and carbon dioxide levels.

2. **Evaluating the Options**:
- **Light**: Light is known to cause stomatal opening. During the day, increased light intensity leads to higher transpiration rates, prompting stomata to open to facilitate gas exchange.
- **Circadian Rhythm**: This refers to the internal biological clock of plants. While it helps plants respond to environmental changes, it does not directly cause stomatal opening or closing.
- **Influx of Potassium Ions**: The movement of potassium ions into guard cells causes them to swell with water, leading to stomatal opening. Thus, this factor does contribute to stomatal opening.
- **Higher Concentration of CO2**: When carbon dioxide levels are high, stomata tend to close because the plant has sufficient CO2 for photosynthesis, which means this factor does not cause stomatal opening.

3. **Conclusion**: Among the options provided, the **circadian rhythm** does not cause stomatal opening. It plays a role in the plant's overall response to environmental changes but does not directly influence the opening of stomata.

**Final Answer**: The factor that does not cause stomatal opening is the **circadian rhythm**.

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