Mineral Nutrition Test - 7

Microelements (Micronutrients) are those essential which are required by plants in small amounts, equal or less than 0.1 mg/gm of dry matter. Microelements are mostly involved in the functioning of enzymes, as cofactors or metal activators. They are eight in number - Fe, Zn, Mn, B, Cu, Mo, CI and Ni. 

Macroelements (Macronutrients) are those essential elements which are present in easily detectable quantities, 1-10 mg per gram of dry matter. Macroelements are usually involved in the synthesis of organic molecules and development of osmotic potential. They are nine in number — C, H, O, N, P, K, S, Mg and Ca.

Microelements (Micronutrients) are those essential elements which are required by plants in small amounts, equal or less than 10mmole Kg⁻¹ of dry matter. Microelements are mostly involved in the functioning of enzymes, as cofactors or metal activators. They are eight in number — Fe, Zn, Mn, B, Cu, Mo, Cl and Ni.

Essential elements derived from soil are termed as mineral elements. Example: P, K, S, Mg, Ca, Fe, Zn, Mn, B, Cu, Mo, Cl and N. Essential elements got from air or water are known as non-mineral elements. Example: C, H, O. In aquatic habitats as well as in soil solution, mineral elements occur dissolved in water. Nitrogen is included in both mineral and non-mineral elements.

Out of 105 or more elements discovered so far, 60 elements are found in different plants. Out of 105, only 20 elements have been found to be essential for growth and development of the plants.

Three important elements (i.e., N, P, K — nitrogen, Phosphorus and potassium) need to be replenished in crop fields as they are depleted by repeated cultivation. The common sources of N, P and K are - sodium nitrate, amonium sulphate, ammonium nitrate, ammonium chloride, urea, calcium ammonium nitrate, superphosphate, bone meal, rock phosphate and calcium magnesium phosphate. Oxygen is not one of these three elements.

Hydrogen (H) is mainly obtained from soil water by roots. Oxygen (O) is either derived from the air or from the water. Carbon (C) is derived from atmosphere as CO₂. The plants are unable to use atmospheric nitrogen. It is inert and plants cannot make use of it directly. By the process of fixation of atmospheric N₂, plants obtain it from soil in the form of nitrites (NO⁻₂), nitrates (NO⁻₃), ammonium ion (NH⁺₄), etc. These nitrogenous compounds enter plants as nutrients through the roots and are assimilated as organic nitrogen.

An essential element is the one which has a specific structural or physiological role and without which plants cannot complete their life cycle. These essential elements are classified into two broad categories called macronutrients and micronutrients.

Cl^- with Na⁺ and K⁺ helps in determining solute concentration and anion-cation balance in cells.

Fe is an important constituent of proteins like ferredoxin and cytochromes which are involved in transfer of electrons. It is reversibly oxidised from Fe²⁺ to Fe³⁺ during electron transfer. It activates catalase enzyme and is essential for the formation of chlorophyll. Iron is absorbed by the plants from the soil in the form of ferric (Fe³⁺) ions.
Copper helps in formation of starch. It is required for the overall metabolism in plants. It is associated with certain enzymes involved in redox reactions and is reversibly oxidised from Cu⁺ to Cu²⁺. Copper is absorbed by plants as cupric ion (Cu²⁺).

Magnesium (Mg) is found in the chlorophyll. The chlorophyll does not form in its absence, the leaves become chlorotic, and the growth of the plant is retarded. Iron (Fe) also plays an important role in the formation of chlorophyll, though it is not found in the constitution of the chlorophyll. 

• Magnesium is present in phosphoenolpyruvate enzyme.
• Molybdenum is a key element of nitrogenase as Mo-Fe protein.
• Zinc is an activator of alcohol dehydrogenase enzyme.

Potassium (K) helps to determine anion-cation balance in cells, and is involved in protein synthesis, opening and closing of stomata, activation of enzymes, and maintenance of turgidity of cells. It is absorbed as K⁺ ion by plants from the soil.

Sulphur (S) is the constituent of two amino acids, i.e., cysteine and methionine, and hence proteins, several coenzymes, vitamins (i.e., thiamine, biotin), lipoic acid and ferredoxin. Cystine also contains sulphur. It is a dimeric amino acid formed by the oxidation of two cysteine residues that covalently link to make a disulphide bond.

For elements that are actively mobilised within the plants and exported to young developing tissues, the deficiency symptoms tend to appear first in the older tissues e.g., the deficiency symptoms of nitrogen (N), potassium (K) and magnesium (Mg) are visible first in the senescent leaves. In the older leaves, biomolecules containing these elements are broken down, making these elements available for mobilising to younger leaves. On the other hand, the deficiency symptoms tend to appear first in the young tissues whenever the elements are relatively immobile and are not transported out of the mature organs, for example, elements like sulphur (S) and calcium (Ca) are a part of the structural components of the cell and hence are not easily released. 

Manganese (Mn²⁺) and chlorine (Cl⁻ both are micro elements, responsible for photo production of oxygen during photolysis of water in photosynthesis.

Phosphorus (P) is a constituent of cell membranes, certain proteins, all nucleic acids and nucleotides and is required for all phosphorylation reactions. It is usually found in nudeoproteins and protoplasm.

Major function of magnesium (Mg²⁺) is formation of chlorophyll, carotenoids and nucleic, acids, growth, metabolism and nodule formation in legumes Ca²⁺ is involved in the organisation of mitotic spindle.

Chlorosis is non-development or loss of chlorophyll, that leads to yellowing of leaves. Chlorosis is caused due to deficiency of N, K, Mg, S, Fe, Mn, Zn and Mo.

Nacrosis is browning or blackening of tissue due to the death of a cell or group of cells, while still a part of the living plant. It occurs due to the deficiency of calcium (Ca²⁺) magnesium (Mg²⁺).

Nitrogen deficiency causes yellowing of older leaves (chlorosis). Deficiency of nitrogen also causes dormancy of lateral buds, late flowering, purple colouration and wrinkling of cereal grains. Molybdenum deficiency causes chlorotic interveinal mottling of the older leaves; flower formation is inhibited and also causes whip tail disease in cauliflower plants. Sulphur deficiency causes yellowing of leaves and delay in flowering.

Manganese toxicity (brown spots surrounded by chlorotic veins) is due to
(i) Reduction in uptake of iron and magnesium.
(ii) Inhibition of binding of magnesium to specific enzymes.
(iii) Inhibition of calcium translocation into shoot apex.
Therefore, excess of manganese causes deficiency of iron, magnesium and calcium.

Potassium (K+) is usually found in the growing regions of plant. It is one of the constituents of the protoplasm. In the scarcity of potassium, the normal edges of the leaves become yellowish, and the stem becomes somewhat thin. The plants take potassium from the soil in the form of potassium nitrate and potassium chloride, etc.

Carbon is a macronutrient, and is a major component of all organic compounds but is not obtained from soil Carbon is derived from atmosphere as carbon dioxide (CO₂).

Etiolation is the abnormal form of growth observed when plantws grow in darkness or severely reduced lights. Such plants characteristically have branched leaves and shoots, excessively long shoots, and reduced leaves and roots systems.

Magnesium activates the enzymes of respiration, photosynthesis and is involved in the synthesis of DNA and RNA. Manganese activates many enzymes which are involved in photosynthesis, respiration and nitrogen metabolism.

Many members of the genus Glomus form mycorrhiza associations with roots of higher plants. The fungal symbiont i.e., Glomus absorbs phosphorus from soil and passes it to the plant.

Phosphorus (P) deficiency causes premature leaf fall. Dead necrotic areas develop on leaves or fruits, and leaves turn dark to blue-green in colour. It also causes delay in seed germination.

In higher doses, micronutrients become toxic. Mineral ion concentration in the tissues which reduces dry weight of tissues by 10% is called toxic concentration. Critical toxic concentration is different for different micronutrients as well as different plants. For example, Mn²⁺ is toxic beyond 600μg g⁻¹ for soybean and beyond 5300μg g⁻¹ for sunflower. Toxic effects may be due to an excess of the micronutrient or its interference in the absorption and functioning of other nutrients.

Soil seves as a reservoir of essential elements. More than sixty essential elements are found in soil. It is the upper weathered humus containing part of Earth's surface which sustains terrestrial plant life. Weathering or breaking of rock into fine powder can occur due to atmospheric changes, mechanical forces, chemical changes and biological breakdown over the years.