What are the function of essential elements. Mention the role and deficiency symptom of Nitrogen, Phosphorus and Sulphur?

Functions of essential elements - The following points highlight the eleven important functions of essential elements of plants.

Function # 1. Constituents of Biomolecules:

Carbon and hydrogen are components of all organic substances. Many of them also contain oxygen. Nitrogen is a constituent of all amino acids, proteins, nucleic acid, chlorophyll, auxin, cytokinins and vitamins. Sulphur occurs in two amino acids— methionine and cysteine. They form a number of proteins.  

Phosphorus is present in nucleotides, higher nucleotides (e.g., ADP, ATP, CDP, and CTP), coenzymes like NAD+ and NADP+, RNA, DNA and phospholipids. Fe is found in cytochromes. Mg is a component of chlorophyll. Ca and Mg produce spectates of middle lamella.

Depending upon the organic compound formed, mineral elements are of two types:

(a) Frame-work Elements:  

They produce cell walls and storage products of plants, i.e., С, H, O.

(b) Protoplasmic Elements:

The elements produce protoplasmic constituents like proteins, nucleic acids, chlorophylls,  cytochromes, ferredoxin, i.e., С, H, O, N, S, P, Mg, Fe.

Function # 2. Energy Related Compounds:

Magnesium is a component of chlorophyll that takes part in conversion of light energy into chemical energy. Phosphorus is component of ATP which functions as energy currency of the living systems.  

Function # 3. Osmotic Potential:

Most of the osmotic potential of cell sap is due to inorganic salts. Osmotic potential is required for water absorption and maintenance of cell turgidity

Function # 4. Movements:

Free K+ takes part in stomatal and other turgor movements.  

Function # 5. Buffer Action:

Weak acids and their salts function as a buffer against changes of pH.

Function # 6. Oxidation Reduction System:

Metals with variable valency act as electron carriers, e.g., Iron (Fe2+ and Fe3+) and Copper (Cu+, Cu2+).  

Function # 7. Toxic Effects:

Many elements become toxic in higher concentration than the normal, e.g., Сu, B, Mn, Mo, Zn. Others are toxic even in smaller concentration, e.g., Pb, Ni, Se, Al, Hg.  

Function # 8. Balancing Elements:

Ca2+, Mg2+ and K+ minimise the toxic effect of heavy elements. 

Function # 9. Permeability:

Sodium, Potassium and some other monovalent increase membrane permeability while Calcium and other divalents decrease the same. 

Function # 10. Catalytic Effects

Many enzymes require mineral elements as cofactors. Mg2+ is activator of several enzymes of both respiration (e.g., hexokinase, phosphofructokinase) and photosynthesis (e.g., RuBP carboxylase, PEP carboxylase). 

Zinc is activator of carbonic anhydrase and alcohol dehydrogenase. Molybdenum is required for functioning of di-nitrogenase and nitrate reductase. Mn2+ is involved in photolysis of water. K+ is known to be cofactor of some 40 enzymes.

Function # 11. Phloem Transport:

Boron and Potassium are involved in the translocation of organic substances in the phloem.   

Phosphorus

Regions of requirements: Used mostly in younger tissues. 

Functions:

It is a constituent of phospholipids (membrane lipids), nucleic acids, nucleotides, coenzymes, ATP, metabolic intermediates, sugar phosphates in photosynthesis etc.

Deficiency Symptoms:

(i) Purple or red pigmentation on leaves

(ii) Premature fall of leaves and floral buds

(iii) Delay in seed germination

(iv) Older leaves affected first and become dark brown,

(v) Stunted and slender stem in young plants,

(vi) Accumulation of carbohydrates in Glycine max (Soybean),

(vii) Vascular tissues reduce in tomato plants.  

Nitrogen -

Functions:

(i) It is the major constituent of proteins, purines, pyrimidines, vitamins, cholorophyll and hormones,

(ii) It is also present as a component of coenzymes like FAD, NAD, NADP etc.

(iii) Older leaves when turn yellow, their nitrogen passes to younger parts in the form of amines and amides.  

Deficiency Symptoms:

(i) Stunted growth due to reduced cell division and dormant lateral buds.

(ii) Chlorosis (yellowing of leaves),

(iii) Suppressed or late flowering,

(iv) Increase in starch content but decrease in protein content,  

Sulphur -

Functions:

(i) It is a constituent of amino acids like cysteine, cystine and methionine,

(ii) It is also the main constituents of Coenzyme A, Vitamins (thiamine and biotin), ferridoxin etc.

(iii) It is essential for stabilizing the structure of protein by formation of disulfide bond (S-S) between two cysteine residues to form a cystine,

(iv) Characteristic pungent odour of mustard, onion, garlic etc. is due to presence of sulphur containing volatile oils.

Deficiency Symptoms: The sulphur deficiency symptoms are similar to those of nitrogen deficiency because sulphur and nitrogen are constituents of proteins.  

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