State true or false. The symptoms of magnesium deficiency appear first in the mature leaves.

Answer

**Step-by-Step Solution:**

1. **Understanding Magnesium's Role**: Magnesium (Mg) is a vital nutrient for plants, primarily because it is a central component of chlorophyll, the green pigment responsible for photosynthesis.

2. **Identifying Symptoms of Deficiency**: When there is a deficiency of magnesium in plants, one of the primary symptoms is chlorosis, which is the yellowing of leaves due to insufficient chlorophyll.

3. **Determining Leaf Age Affected**: Chlorosis due to magnesium deficiency typically appears first in the mature leaves (older leaves) rather than in the younger leaves. This is because plants tend to mobilize nutrients from older tissues to support the growth of new tissues.

4. **Conclusion**: Since the symptoms of magnesium deficiency (chlorosis) appear first in the mature leaves, the statement in the question is true.

**Final Answer**: True.

---