Deficiency symptom of mobile elements (e.g. N, Mg, K, P) first appear in-

Answer

### Step-by-Step Solution:

1. **Understanding Mobile Elements**: Mobile elements in plants refer to nutrients that can move from older tissues to younger tissues. Examples include nitrogen (N), magnesium (Mg), potassium (K), and phosphorus (P).

2. **Deficiency Symptoms**: When there is a deficiency of these mobile elements, the plant will first exhibit symptoms in the older parts of the plant. This is because the plant reallocates these nutrients to support the growth of new tissues.

3. **Mechanism of Mobility**: In response to the deficiency, the plant breaks down biomolecules that contain these elements in older tissues. This breakdown releases the nutrients, which are then transported to the younger parts of the plant where they are needed for growth.

4. **Conclusion**: Therefore, the deficiency symptoms of mobile elements (N, Mg, K, P) first appear in the older parts of the plant, as the nutrients are moved to the younger parts.

### Final Answer:
Deficiency symptoms of mobile elements (e.g., N, Mg, K, P) first appear in the older parts of the plant.

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