Read the following statement regarding the mineral
(i) Both Rhiqobium 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, let's analyze each statement step by step:

### Step 1: Analyze Statement (i)
**Statement:** 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.
- However, they can only fix atmospheric nitrogen when they are in symbiotic relationships with their host plants (e.g., legumes for Rhizobium and actinorhizal plants for Frankia).
- **Conclusion:** This statement is **correct**.

### Step 2: Analyze Statement (ii)
**Statement:** Ammonia synthesis by nitrogenase requires 16 ATP for each ammonia produced.

**Analysis:**
- The process of ammonia synthesis by nitrogenase actually requires 28 ATP molecules for the production of one ammonia molecule, not 16 ATP.
- **Conclusion:** This statement is **incorrect**.

### Step 3: Analyze Statement (iii)
**Statement:** 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:**
- It is true that most plants can utilize both nitrate and ammonium ions.
- Ammonium can be toxic to plants if it accumulates, leading to toxicity issues.
- **Conclusion:** This statement is **correct**.

### Step 4: Analyze Statement (iv)
**Statement:** Along with the transpiration stream, the nodules of some plants export the fixed nitrogen as urides.

**Analysis:**
- Some plants do export fixed nitrogen in the form of ureides (not urides) through the transpiration stream.
- This is a common mechanism in plants like legumes.
- **Conclusion:** This statement is **correct**.

### Final Conclusion:
The correct statements are:
- (i) Correct
- (ii) Incorrect
- (iii) Correct
- (iv) Correct

Thus, the correct options are **(i), (iii), and (iv)**.

---