Regarding the mineral nutrition of plants, which one of the following statements is correct?

Options

A. The availability of iron to plants decreases with the increase of acidity of the soil
B. Molybdenum is a constituent of nitrate reductase system
C. Sulphur is a constituent of phenylalanine
D. All of the above

Explanation

To determine the correct statement regarding the mineral nutrition of plants, we will analyze each of the provided statements one by one.

### Step-by-Step Solution:

1. **Understanding the first statement**:
- The statement claims that the availability of iron to plants decreases with the increase of acidity of the soil.
- In reality, the availability of iron actually increases in acidic soils (low pH) and decreases in alkaline soils (high pH). Therefore, this statement is **incorrect**.

2. **Analyzing the second statement**:
- This statement asserts that molybdenum is a constituent of the nitrate reductase system.
- Molybdenum is indeed a crucial component of the nitrate reductase enzyme, which is involved in the reduction of nitrate to nitrite in plants. Hence, this statement is **correct**.

3. **Evaluating the third statement**:
- The statement claims that sulfur is a constituent of phenylalanine.
- This is incorrect because phenylalanine is an amino acid that does not contain sulfur. Therefore, this statement is also **incorrect**.

4. **Conclusion**:
- Among the statements provided, only the second statement, "molybdenum is a constituent of nitrate reductase system," is correct.

### Final Answer:
The correct statement regarding the mineral nutrition of plants is: **Molybdenum is a constituent of the nitrate reductase system.**

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