|Function and Deficiency symptoms of Nitrogen
Function of Nitrogen
i. Nitrogen is a major structural part of the cell. Cytoplasm and the particulate fractions of the cell organelles contain nitrogen in varying amounts which exist in combination with C, H, 0, P and S. Primary cells are found to have about 5% of nitrogen.
ii. Nitrogen is essential to cell division, expansion, and therefore growth.
iii. It imparts the dark green colour of the leaves, promotes vegetative growth, and improves the quality of produce including fodder, leafy vegetables and food crops.
iv. It is essential for carbohydrate use within plants and stimulates root growth and development as well as uptake of other nutrients.
v. It helps in harvesting solar energy through chlorophyll, in energy transformation through phosphorylated compounds, in transfer of genetic information through nucleic acids (DNA and RNA).
vi. It plays a vital role in various metabolic activities of plants and is a constituent of amino acids, proteins, nucleic acids, porphyrins, flavins, purine and pyrimidine, nucleotides, enzymes, co-enzymes and alkaloids.
vii. Besides, it is essential in cellular and protein metabolism and acts as biological catalyst.
Deficiency symptoms of Nitrogen
i. The most easily observed symptom of nitrogen deficiency is the yellowing (chlorosis) of leaves due to a drop in chlorophyll content. This symptom is usually noticed first in the more mature leaves and last in the upper, more actively growing leaves. The yellowing starts always from apex or outer margins of leaf blade of older leaves and spreads gradually towards the base of the leaves. These symptoms slowly spread towards younger leaves. The nitrogen deficiency symptoms appear last in the younger leaves because of the high mobility of nitrogen in the plant. The younger leaves retain their nitrogen and, in addition, obtain nitrogen trans-located from older leaves.
ii. Nitrogen deficient plants remain stunted with erect stem, spindly growth, lateral buds are fully suppressed and foliage is severely restricted having pale or yellowish-green colour due to absence of chlorophyll.
iii. As nitrogen is an essential constituent of protein, low nitrogen availability must cause a decrease in protein synthesis, which subsequently causes a decrease in cell size and especially cell division. A decrease in leaf epidermal cell size due to nitrogen deficiency in millet and buckwheat has been noted.
iv. Under severe yellowing or pailing, the affected tissues become dry and necrotic referring to as firing.
v. In cereals, restricted tillering, dry fibrous and bronzed stems with poor root growth are observed.
vi. Under severe conditions of nitrogen deficiency, the lowermost leaves on plants such as tobacco or bean will be dry and yellow and, in many cases, abscise. Under these conditions, the topmost leaves are generally pale green in colour.
vii. In fruit trees, the fruits remain smaller and majority of them drop even before maturity. Poor starch and protein synthesis result in retarded growth of plants with lateral buds.
viii. One interesting characteristic of nitrogen deficiency found in many plants is the production of pigments other than chlorophyll when nitrogen is lacking. For example, in tomato plants a purple colouring of the leaf petioles and veins caused by anthocyanin formation may be observed. This response to nitrogen deficiency may also be observed on the stems of many plants.
Excess application/over supply/toxicity
i. If a plant is supplied high concentrations of nitrogen, there is a tendency to increase in leaf cell number and cell size with an overall increase in leaf production.
ii. It delays ripening by encouraging more vegetative growth. The leaves acquire a dark green colour, become thick and leathery and in some cases crinkled. They also become soft and sappy. The plants become more liable to attack of certain fungi and its resistance to disease is lowered.
iii. In case of cereal crops, the straw becomes weak, and the crop very often lodges and straw and grain ratio is increased.
iv. Excess nitrogen deteriorates the quality of some crops such as potato, barley and sugarcane.
v. It delays reproductive growth and may adversely affect fruit and grain quality.
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