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

Explanation

Certain mineral nutrients are mobile in the plants while others are not. During deficiency, the mobile elements are mobilised from the mature leaves, and transferred to younger leaves. In such cases, the deficiency symptoms appear first in older leaves, e.g., N, P, Mg. Other nutrients are immobile inside the plants. In such cases, the deficiency symptoms appear first in young leaves, e.g., Fe, Ca, S.