It is also an integral part of the chlorophyll molecule and thus plays a key role in photosynthesis. An adequate supply of nitrogen is associated with vigorous vegetative growth and a dark-green color of the plant.
Nitrogen deficiency is characterized by reduced plant growth and a pale green or yellow color.
This yellowing generally begins at the tip of the leaf and goes down the middle of the leaf. If the deficiency is severe, the affected area of the leaf eventually turns brown and dies. Since nitrogen is mobile in the plant, older leaves show the first symptoms of nitrogen deficiency. The Agronomy Guide, 1989-1990, page 9. Where trade names are used, no discrimination is intended and no endorsement is implied.