Top rot of tobacco is due to deficiency of

Explain the deficiency symptoms of essential elements in plants.

In a $Citrus$ plantation,all the plants were found to be suffering from the die-back disease,and spraying of fungicides was of no help. This problem was due to the deficiency of:

Deficiency of iron causes

Define the following terms:
$(1)$ Chlorosis
$(2)$ Necrosis

In which part will the deficiency symptoms of nitrogen and potassium first appear?