Which one of the following statements can best explain the term critical concentration of an essential element?

Options

A. Essential element concentration below which plant growth is retarded
B. essential element concentration below which plant growth becomes enhanced
C. Essential element concentration below which plant remains in the vegetative phase
D. none of the above

Explanation

To answer the question regarding the term "critical concentration of an essential element," we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Concept of Critical Concentration**:
- Critical concentration refers to the specific level of an essential nutrient in plant tissues. It is the threshold below which the nutrient concentration is insufficient for optimal plant growth.

2. **Identify the Importance of Critical Concentration**:
- When the concentration of an essential element falls below this critical level, it can lead to negative effects on plant growth, such as retardation or stunted growth.

3. **Analyze the Given Options**:
- Option A: "Essential element concentration below which the plant growth is retarded."
- Option B: "Essential element concentration below which the plant growth becomes enhanced."
- Option C: "Essential element concentration below which plant remains in the vegetative phase."
- Option D: "None of the above."

4. **Evaluate Each Option**:
- Option A correctly describes that if the concentration of an essential element is below the critical level, plant growth is retarded.
- Option B is incorrect because a deficiency does not enhance growth.
- Option C is misleading as it does not directly relate to the concept of critical concentration.
- Option D is not applicable since Option A is correct.

5. **Conclusion**:
- The best explanation for the term "critical concentration of an essential element" is given by Option A: "Essential element concentration below which the plant growth is retarded."

### Final Answer:
**The correct option is A: Essential element concentration below which the plant growth is retarded.**

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