Explain the deficiency symptoms of essential elements in plants.

(N/A) $\Rightarrow$ Whenever the supply of an essential element becomes limited,plant growth is retarded.
$\Rightarrow$ The concentration of the essential element below which plant growth is retarded is termed as critical concentration.
$\Rightarrow$ The element is said to be deficient when present below the critical concentration.
$\Rightarrow$ Deficiency symptoms: Each element has one or more specific structural or functional roles in plants. In the absence of any particular element,plants show certain morphological changes.
$\Rightarrow$ These morphological changes are indicative of certain element deficiencies and are called deficiency symptoms. The deficiency symptoms vary from element to element and they disappear when the deficient mineral nutrient is provided to the plant.
$\Rightarrow$ However,if deprivation continues,it may eventually lead to the death of the plant.
$\Rightarrow$ The parts of the plant that show the deficiency symptoms also depend on the mobility of the element in the plant.
$\Rightarrow$ Mobilized elements: For elements that are actively mobilized within the plants and exported to young developing tissues,the deficiency symptoms tend to appear first in the older tissues. In older leaves,biomolecules containing these elements are broken down,making these elements available for mobilizing to younger leaves.
$\Rightarrow$ Immobilized elements: 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 released.
$\Rightarrow$ This aspect of mineral nutrition of plants is of great significance and importance to agriculture and horticulture.