Specific Role Of Macronutrients

Category : 11th Class

The role of different elements is described below :

(1) Carbon, hydrogen and oxygen : These three elements, though can not be categorised as mineral elements, are indispensible for plant growth. Carbon, hydrogen and oxygen together constitute about 94% of the total dry weight of the plant. Carbon is obtained from the carbon dioxide present in the atmosphere. It is essential for carbohydrate and fat synthesis. Hydrogen and oxygen would be obtained from water which is absorbed by the plants from the soil. Some amount of oxygen is also absorbed from the atmosphere.

(2) Nitrogen

Source : The chief source of nitrogen for green plants is the soil. It is absorbed mainly in the form of nitrate ions The major sources of nitrate for the plants are sodium nitrate, potassium nitrate, ammonium nitrate and calcium nitrate.

Functions : Nitrogen is an essential constituent of proteins, nucleic acids, vitamins and many other organic molecules as chlorophyll. Nitrogen is also present in various hormones, coenzymes and ATP etc. It plays an important role in protein synthesis, respiration, growth and in almost all metabolic reactions.

Deficiency symptoms

(i) Impaired growth

(ii) Yellowing of leaves due to loss of chlorophyll, i.e., chlorosis.

(iii) Development of anthocyanins pigmentation in veins, sometimes in petioles and stems.

(iv) Delayed or complete suppression of flowering and fruiting.

Excessive supply of nitrogen produces following symptoms :

(i) Increased formation of dark green leaves.

(ii) Poor development of root system.

(iii) Delayed flowering and seed formation.

(3) Phosphorus

Source : Phosphorus is present in the soil in two general forms, organic and inorganic. Plants do not absorb organic phosphorus, either from the solid or solution phase of soil. However, organic compounds are decomposed and phosphorus is made available to plants in inorganic form. Soil solution contains phosphorus in inorganic forms as the phosphate ions obtained as  and When pH is low phosphate ions are present in the form of  When pH is high, phosphate ions are represented in

Functions

(i) Phosphorous is present abundantly in the growing and storage organs such as fruits and seeds. It promotes healthy root growth and fruit ripening by helping translocation of carbohydrates.

(ii) It is present in plasma membrane, nucleic acid, nucleotides, many coenzymes and organic molecules as ATP.

(iii) Phosphorus plays an indispensable role in energy metabolism i.e., hydrolysis of pyrophosphate. Thus it is required for all phosphorylation reactions.

Deficiency symptoms

(i) Leaves become dark green or purplish.

(ii) Sometimes development of anthocyanin pigmentation occurs in veins which may become necrotic (Necrosis is defined as localised death of cells).

(iii) Premature fall of leaves.

(iv) Decreased cambial activity resulting in poor development of vascular bundles.

(v) Sickle-leaf disease.

(4) Sulphur

Source : Sulphur is present as sulphate  in mineral fraction of soil. In industrialized areas, atmospheric sulphur dioxide  and sulphur trioxide ; in low concentration) may be important sources of sulphur nutrition.

Functions

(i) Sulphur is a constituent of amino-acids like cystine, cysteine and methionine; vitamins like biotin and thiamine, and coenzyme A.

(ii) It increases the nodule formation in the roots of leguminous plants. It favours soluble organic nitrogen and there is decrease in the quantity of soluble nitrogen with its increase.

(iii) The characteristic smell of mustard, onion and garlic is due to the presence of sulphur in their volatile oils.

(iv) Sulphur in plants is required in stem and root tips and young leaves. It is remobilised during senescence.

Deficiency symptoms

(i) Leaves remain small and turn pale green i.e., symptoms of chlorosis. Chlorosis affects young leaves more because of immobile property of the sulphur. The young leaves develop orange, red or purple pigment.

(ii) Leaf tips and margins roll downwards and inwards e.g., tobacco, tea and tomato.

(iii) Delayed flowering and fruiting.

(iv) Apical growth is retarded whereas premature development of lateral buds starts.

(v) The tea yellow disease is caused in tea plants.

(vi) Decrease in stroma lamellae and increase in grana stacking.

(vii) Increase in starch and sucrose accumulation, and decrease in reducing sugars.

(5) Potassium

Source : Source of  to the plants is inorganic compounds like potassium sulphate, potassium nitrate, etc. Potassium is usually present in sufficient amount in clay soils. It contains approximately 0.3 to 6.0 percent of whole plant. In seeds, it is found in less amount.

Functions

(i) It differs from all other macronutrients in not being a constituent of any metabolically important compound.

(ii) It is the only monovalent cation essential for the plants.

(iii) It acts as an activator of several enzymes including DNA polymerase.

(iv) It is essential for the translocation of photosynthates, opening and closing of stomata, phosphorylation, synthesis of nucleic acid and chlorophyll.

It takes part in the formation of cell membrane and it is also responsible for maintenance of turgidity of cells.

Deficiency symptoms

(i) Mottled chlorosis followed by the development of necrotic areas at the tips and margins of the leaves.

(ii)  deficiency inhibits proteins synthesis and photosynthesis. At the same time, it increases the rate of respiration.

(iii) The internodes become shorter and root system is adversely affected.

(iv) The colour of leaves may turn bluish green.

(v) Widespread blackening or scorching of leaves may occur as a result of increased tyrosinase activity.

(vi) Rosette or bushy habit of growth may be seen in plants.

Destruction of pith cells of tomato and increased differentiation of phloem elements.

(6) Calcium

Source : It is absorbed by the plants in the form of  from calcium carbonate etc. It occurs abundantly in a non-exchangeable form such as anorthite . Much of the exchangeable calcium of the soil is absorbed on to the surface of clay micelle.

Functions

(i) It is necessary for formation of middle lamella of plants where it occurs as calcium pectate.

(ii) It is necessary for the growth of apical meristem and root hair formation.

(iii) It acts as activator of several enzymes, e.g., ATPase, succinic dehydrogenase, adenylate kinase, etc.

(iv) Along with  and  it maintains the permeability of plasma membrane.

(v) It is involved in the organisation of spindle fibres during mitosis.

(vi) It antagonises the toxic effects of and

It is essential for fat metabolism, carbohydrate metabolism, nitrate assimilation and binding of nucleic acids with proteins.

Deficiency symptoms

(i) Ultimate death of meristems which are found in shoot, leaf and root tips.

(ii) Chlorosis along the margins of young leaves, later on they become necrotic.

(iii) Distortion in leaf shape.

(iv) Roots poorly developed or may become gelatinous.

(v) Young leaves show malformation and leaf tips becomes hooked.

(vi) Its deficiency checks flowering and causes the flowers to fall early.

(vii) Potato tubers become small and malformed.

(7) Magnesium

Source : Magnesium occurs in the soil in the form of magnesite  dolomite  magnesium sulphate  and as silicates. It is absorbed from the soil in the form of (Exchangeable cation) ions  It is easily leached and thus become deficient in sandy soils during rainy season.

Functions 

(i) It is an important constituent of chlorophyll.

(ii) It is present in the middle lamella in the form of magnesium pectate.

(iii) It plays an important role in the metabolism of carbohydrates, lipids and phosphorus.

(iv) It acts as activator of several enzymes.

(v) It is required for binding the larger and smaller subunits of ribosomes during protein synthesis.

Deficiency symptoms

(i) Interveinal chlorosis followed by anthocyanin pigmentation, eventually necrotic spots appear on the leaves. As magnesium is easily transported within the plant body, the deficiency symptoms first appear in the mature leaves followed by the younger leaves at a later stage.

(ii) Stems become hard and woody, and turn yellowish green.

(iii) Depression of internal phloem and extensive development of chlorenchyma.



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