High concentration of which of the following causes leaf curling, leaf drop and reduced leaf size in plants?

Options

A. `SO_2`
B. CO
C. `NO_2`
D. None of these

Explanation

**Step-by-Step Solution:**

1. **Understanding the Question:**
The question asks which substance, when present in high concentrations, causes leaf curling, leaf drop, and reduced leaf size in plants.

2. **Identifying the Substance:**
The key to answering this question lies in understanding the effects of various gases on plant health. One significant gas that affects plants negatively is carbon monoxide.

3. **Explaining Carbon Monoxide:**
Carbon monoxide (CO) is produced through the incomplete combustion of carbon-containing fuels. Common sources include cigarette smoke, automobile exhaust, and burning fossil fuels in homes.

4. **Effects of High Concentration of Carbon Monoxide:**
- **Leaf Curling:** High levels of carbon monoxide can disrupt normal plant physiological processes, leading to abnormal leaf shapes.
- **Leaf Drop:** The toxicity of carbon monoxide can cause stress in plants, resulting in leaves dropping prematurely.
- **Reduced Leaf Size:** The gas can hinder cellular respiration, which is vital for growth, leading to smaller leaves.

5. **Conclusion:**
Therefore, the high concentration of carbon monoxide is responsible for causing leaf curling, leaf drop, and reduced leaf size in plants.

**Final Answer:**
High concentration of carbon monoxide causes leaf curling, leaf drop, and reduced leaf size in plants.

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