With regard to the bilogical nitrogen fixation by Rhizobium in association with soy bean, which one of the following statement/statements does not hold true?

Options

A. Nitrogenase may require oxygen for its functioning.
B. Nitrogenase is Mo-Fe protein
C. Leg-haemoglobin is a pink coloured pigment.
D. Nitrogenase helps to convert `N_(2)` gas into two molecules of ammonia.

Explanation

To determine which statement regarding biological nitrogen fixation by Rhizobium in association with soybean does not hold true, let's analyze each option step by step.

### Step-by-Step Solution:

1. **Understanding Nitrogenase Function**:
- Nitrogenase is the enzyme responsible for the conversion of atmospheric nitrogen (N2) into ammonia (NH3).
- It operates in anaerobic conditions, meaning it does not require oxygen for its activity.

2. **Evaluating the Statements**:
- **Statement 1**: "Nitrogenase may require oxygen for its functioning."
- This statement is **not true** because nitrogenase is inhibited by oxygen. It functions best in low-oxygen environments.

- **Statement 2**: "Nitrogenase is an MoFe protein."
- This statement is **true**. Nitrogenase is indeed a molybdenum-iron (MoFe) protein complex.

- **Statement 3**: "Leghaemoglobin is a pink-colored pigment."
- This statement is **true**. Leghaemoglobin, which is found in root nodules of legumes, is an iron-containing pigment that appears pink.

- **Statement 4**: "Nitrogenase has to convert N2 gas into two molecules of ammonia."
- This statement is **true**. The primary function of nitrogenase is to convert nitrogen gas (N2) into ammonia (NH3), typically producing two molecules of ammonia from one molecule of nitrogen.

3. **Conclusion**:
- The only statement that does not hold true is **Statement 1**: "Nitrogenase may require oxygen for its functioning." Therefore, this is the correct answer to the question.

### Final Answer:
The statement that does not hold true is: **"Nitrogenase may require oxygen for its functioning."**