Q.Why is that in certain plants deficiency symptoms appear first in younger leaves while in others they do so in mature organs ?

Whether deficiency symptoms appear first in old or young leaves depends on the mobility of that mineral within the plant: mobile elements are withdrawn from old leaves to feed young ones (so old leaves suffer first), while immobile elements cannot be moved once deposited (so young leaves suffer first).

Essential mineral elements differ in how easily they can be remobilised (retranslocated) from mature, older plant parts to younger, actively growing regions when the overall supply becomes limited:

Mobile elements: Elements such as nitrogen (N), phosphorus (P), potassium (K), and magnesium (Mg) are relatively mobile within the plant. When these elements are in short supply, the plant preferentially withdraws them from older, mature leaves (where they are less urgently needed) and translocates them via the phloem to the younger, still-growing leaves and shoot tips, which have a greater metabolic priority. As a result, the older leaves are effectively 'sacrificed' and deficiency symptoms (such as chlorosis or necrosis) appear FIRST in the older, mature leaves, while the young leaves remain healthy for longer.

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