Read the following statement regarding the mineral
(i) Both Rhizobium and Frankia are free living in soil, but as symbionts, can fix atmospheric nitrogen.
(ii) Ammonia synthesis by nitrogenase requires 16 ATP for each ammonia produced.
(iii) While most of plants can assimilate nitrate as well as ammonium ions, the latter is quite toxic to plnats and hence cannot accumulate in them.
(iv) Along with the transpiration stream, the nodules of some plants export the fixed nitrogen as urides.
Which of these are correct

Options

A. (i), (iii) & (iv)
B. (ii), (iii) & (iv)
C. (iii) & (iv)
D. All are correct

Explanation

To determine which statements regarding mineral nutrition are correct, we will analyze each statement one by one.

### Step-by-Step Solution:

1. **Statement (i)**: "Both Rhizobium and Frankia are free living in soil, but as symbionts, can fix atmospheric nitrogen."
- **Analysis**: Rhizobium and Frankia are indeed free-living nitrogen-fixing bacteria found in the soil. They form symbiotic relationships with certain plants (like legumes for Rhizobium and actinorhizal plants for Frankia) and are capable of fixing atmospheric nitrogen into a form that plants can utilize.
- **Conclusion**: This statement is **correct**.

2. **Statement (ii)**: "Ammonia synthesis by nitrogenase requires 16 ATP for each ammonia produced."
- **Analysis**: The enzyme nitrogenase is responsible for the conversion of atmospheric nitrogen (N₂) into ammonia (NH₃). It is well-established that the process requires 8 ATP molecules for the synthesis of one molecule of ammonia, not 16.
- **Conclusion**: This statement is **incorrect**.

3. **Statement (iii)**: "While most plants can assimilate nitrate as well as ammonium ions, the latter is quite toxic to plants and hence cannot accumulate in them."
- **Analysis**: Most plants can take up both nitrate (NO₃⁻) and ammonium (NH₄⁺). However, ammonium can be toxic to plants if it accumulates in high concentrations, leading to toxicity and potentially harming the plant. Therefore, plants typically regulate ammonium levels to prevent toxicity.
- **Conclusion**: This statement is **correct**.

4. **Statement (iv)**: "Along with the transpiration stream, the nodules of some plants export the fixed nitrogen as urides."
- **Analysis**: In plants, particularly legumes with root nodules, fixed nitrogen is often exported in the form of ureides (such as allantoin and allantoic acid) through the transpiration stream. This is a common method of nitrogen transport in these plants.
- **Conclusion**: This statement is **correct**.

### Final Conclusion:
Based on the analysis:
- **Correct Statements**: (i), (iii), and (iv).
- **Incorrect Statement**: (ii).

### Answer:
The correct statements are (i), (iii), and (iv).

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