Root nodules of pulse plant are the sites of nitrogen fixation. The nitrogen fixation is done by the enzyme nitrogenate →an Mo-Fe protein). Other requirements for this process are electrons, protons and ATP molecules. Nitrogen is bound to the surface of the enzyme and it is reduced to ammonia in a stepwise reaction with the help of reducing agents and hydrolysis of ATP. When eight electrons →and 8H⁺) are accepted by N₂, 2 NH₃ are released from the enzyme. The biochemical pathway can be summarized by the reaction below:

N₂ + 8 e^- + 8 H⁺+ 16 ATP 2 NH₃ + 12 ADP + 12 Pi

The end product of nitrogen fixation is ammonia.

The ammonia is protonated at physiological pH to form NH₄⁺ ion which is further used to synthesize amino acids in plants. The formation of amino acids takes place in two ways – transamination and reductive amination.

OR

Since each element has one or more specific structural or functional role in plants, in the absence of any particular element, plants show certain morphological changes. These morphological changes are indicative of certain element deficiencies and are called deficiency symptoms.

a. It is actively mobilized

b. Highly mobile – Nitrogen, Magnesium

Relatively immobile – Sulphur, Calcium

c. For elements that are actively mobilised within the plants and exported to young developing tissues, the deficiency symptoms tend to appear first in the older tissues.

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.

This idea can be used in horticulture and agriculture to get a broad idea of deficiency elements in plants.