Why is that in certain plants deficiency symptoms appear first in younger parts of the plant while in others they do so in mature organs?

Explanation

The areas of the plants that show the deficiency symptoms depend on the mobility of the element in the plants. Mobility of an element determines whether the deficiency symptom appears in younger parts or in older parts. Where elements are actively mobilised within the plants and exported to young developing tissues, the deficiency symptoms tend to appear first in the older tissues. For example, the deficiency symptoms of nitrogen, potassium and magnesium are visible first in the senescent leaves. In the older leaves, bimolecules containing these elements are broken down, making these elements available for mobilising to younger leaves. The deficiency symptoms tend to appear first in the young tissues whenever the elements are relatively immobile and are not transported out of the mature organs, for example, elements like sulphur and calcium are a part of the structural component of the cell and hence are not easily modified.