Question

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.

Answer 1

When any essential element in plants is present below the level of critical concentration, the plant shows different symptoms of deficiency for that particular element.

Complete answer:
1.Each essential element has a specific structural or functional role in the growth of the plant, Hence all the essential elements must be present in sufficient quantity.
2.Whenever the plants do not get a sufficient amount of essential elements different morphological changes occur in plants that are the symptoms of element deficiency.
3.Now whether the deficiency symptoms appear first in the younger plant parts or the mature parts depends on the mobility of a particular element.
4.Some elements such as Magnesium, potassium, and nitrogen are very actively transported within the plant and this mobility leads to the transfer of these elements to younger plant parts which are still not fully developed due to which the older and mature plant parts get deprived of these elements and that is why the symptoms of these elements are observed first in the mature organs.
5.Whereas some elements like calcium and sulfur have very little mobility due to which these elements are not actively transported to the younger plant parts and the deficiency symptoms appear first in the younger developing plant parts.

Note: The deficiency symptoms shown by plants include yellowing of leaves due to loss of chlorophyll known as chlorosis which mainly occurs due to deficiency of sodium, potassium, zinc, iron, etc. Whereas deficiency of calcium, magnesium, copper causes the death of plant tissues (Necrosis). Some elements show multiple deficiency symptoms in plants.