Revision Notes on Mineral Nutrition
(1) Macronutrients (Macroelements or major elements): Nutrients which are required by plants in larger amounts (Generally present in the plant tissues in concentrations of 1 to 10 mg per gram of dry matter).
(2) The macronutrients include carbon, hydrogen, oxygen, nitrogen, phosphorous, sulphur, potassium, calcium, magnesium.
(3) Micronutrients (Microelements or minor elements or trace elements): Nutrients which are required by plants in very small amounts, i.e., in traces (equal to or less than 0.1 mg per gram dry matter).
(4) The micronutrients include iron, manganese, copper, molybdenum, zinc, boron and chlorine. Recent research has shown that some elements, such as cobalt, vanadium and nickel, may be essential for certain plants.
Major Role of Nutrients
Various elements perform the following major role in the plants:
(1) Construction of the plant body: The elements particularly C, H and O construct the plant body by entering into the constitution of cell wall and protoplasm. They are, therefore, referred to as frame work elements. Besides, these (C, H and O) N, P and S also enter in the constitution of protoplasm. They are described as protoplasmic elements.
(2) Maintenance of osmotic pressure: Various minerals present in the cell sap in organic or inorganic form maintain the osmotic pressure of the cell.
(3) Maintenance of permeability of cytomembranes: The minerals, particularly Ca++, K+ and Na+ maintain the permeability of cytomembranes.
(4) Influence the pH of the cell sap: Different cations and anions influence on the pH of the cell sap.
(5) Catalysis of biochemical reaction: Several elements particularly Fe, Ca, Mg, Mn, Zn, Cu, Cl act as metallic catalyst in biochemical reactions.
(6) Toxic effects: Minerals like Cu, As, etc. impart toxic effect on the protoplasm under specific conditions.
(7) Balancing function: Some minerals or their salts act against the harmful effect of the other nutrients, thus balancing each other.
Specific Role of Macronutrients
The role of different elements is described below:
(1) Carbon, hydrogen and oxygen: These three elements though cannot be categorized as mineral elements, are indispensible for plant growth. These are also called ‘framework elements’.
Chlorosis in Leaves(2) Nitrogen: 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.
(i) Deficiency symptoms: The symptoms of nitrogen deficiency are as follows:
(a) Impaired growth
(b) Yellowing of leaves due to loss of chlorophyll, i.e., chlorosis.
(c) Development of anthocyanin pigmentation in veins, sometimes in petioles and stems.
(d) Delayed or complete suppression of flowering and fruiting.
(3) Phosphorus
(a) 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.
(i) Deficiency symptoms
(a) Leaves become dark green or purplish.
(b) Sometimes development of anthocyanin pigmentation occurs in veins which may become necrotic (Necrosis is defined as localised death of cells).
(c) Premature fall of leaves.
(4) Sulphur
(i) Functions
(a) Sulphur is a constituent of amino-acids like cystine, cysteine and methionine; vitamins like biotin and thiamine, and coenzyme A.
(ii) Deficiency symptoms
(b) Leaf tips and margins roll downwards and inwards e.g., tobacco, tea and tomato.
(c) Premature leaf fall.
(d) Delayed flowering and fruiting.
(5) Potassium
(i) Functions
(a) It differs from all other macronutrients in not being a constituent of any metabolically important compound.
(b) It is the only monovalent cation essential for the plants.
(c) It acts as an activator of several enzymes including DNA polymerase.
(ii) Deficiency symptoms
(a) Mottled chlorosis followed by the development of necrotic areas at the tips and margins of the leaves.
