11th Class Biology Mineral Nutrition In Plants Specific Role Of Micronutrietns

Specific Role Of Micronutrietns

Category : 11th Class

(1) Iron

Source : It is present in the form of oxides in the soil. It is absorbed by the plants in ferric as well as ferrous state but metabolically it is active in ferrous state. Its requirement is intermediate between macro and micro-nutrients.

Functions

(i) Iron is a structural component of ferredoxin, flavoproteins, iron prophyrin proteins (Cytochromes, peroxidases, catalases, etc.)

(ii) It plays important roles in energy conversion reactions of photosynthesis (phosphorylation) and respiration.

(iii) It acts as activator of nitrate reductase and aconitase.

(iv) It is essential for the synthesis of chlorophyll.

Deficiency symptoms

(i) Chlorosis particularly in younger leaves, the mature leaves remain unaffected.

(ii) It inhibits chloroplast formation due to inhibition of protein synthesis.

(iii) Stalks remain short and slender.

(iv) Extensive interveinal white chlorosis in leaves.

(v) It may develop necrosis aerobic respiration severely affected.

(vi) In extreme deficiency scorching of leaf margins and tips may occur.

(2) Manganese

Source : Like iron, the oxide forms of manganese are common in soil. However, manganese dioxide (highly oxidised form) is not easily available to plants. It is absorbed from the soil in bivalent form \[(M{{n}^{++}}).\] Increased acidity leads to increase in solubility of manganese. In strong acidic soils, manganese may be present in toxic concentrations. Oxidising bacteria in soils render manganese unavailable to plants at pH ranging from 6.5 to 7.8.

Functions

(i) It acts as activator of enzymes of respiration (malic dehydrogenase and oxalosuccinic decarboxylase) and nitrogen metabolism (nitrite reductase).

(ii) It is essential for the synthesis of chlorophyll.

(iii) It is required in photosynthesis during photolysis of water.

(iv) It decreases the solubility of iron by oxidation. Hence, abundance of manganese can lead to iron deficiency in plants.

Deficiency symptoms

(i) Chlorosis (interveinal) and necrosis of leaves.

(ii) Chloroplasts lose chlorophyll, turn yellow green, vacuolated and finally perish.

(iii) 'Grey spot disease' in oat appears due to the deficiency of manganese, which leads to total failure of crop.

(iv) 'Marsh spot's in seeds of pea.

(v) Deficiency symptoms develop in older leaves.

(3) Copper

Source : Copper occurs in almost every type of soil in the form of complex organic compounds. A very small amount of copper is found dissolved in the soil solution. It is found in natural deposits of chalcopyrite (CuFeS2).

Functions  

(i) It activates many enzymes and is a component of phenolases, ascorbic acid oxidase, tyrosinase, cytochrome oxidase.

(ii) Copper is a constituent of plastocyanin, hence plays a role in photophosphorylation.

(iii) It also maintains carbohydrate nitrogen balance.

Deficiency symptoms

(i) Both vegetative and reproductive growth are reduced.

(ii) The most common symptoms of copper deficiency include a disease of fruit trees called 'exanthema' in which trees start yielding gums on bark and 'reclamation of crop plants', found in cereals and legumes.

(iii) It also causes necrosis of the tip of the young leaves (e.g., Citrus). The disease is called 'die back'.

(iv) Carbon dioxide absorption is decreased in copper deficient trees.

(v) Wilting of entire plant occurs under acute shortage.

(vi) Grain formation is more severely restricted than vegetative growth.

(4) Molybdenum

Source : It is available to the plants mostly as molybdate ions. It is required in extremely small quantities by plants. It is found relatively in higher concentration in mineral oil and coal ashes.

Functions 

(i) Its most important function is in nitrogen fixation because it is an activator of nitrate reductase.

(ii) It is required for the synthesis of ascorbic acid.

(iii) It acts as activator of some dehydrogenases and phosphatases.

Deficiency symptoms

(i) Mottled chlorosis is caused in the older leaves as in nitrogen deficiency, but unlike nitrogen-deficient plants, the cotyledons stay healthy and green.

(ii) It is also known to inhibit flowering, if they develop, they fall before fruit setting.

(iii) It leads to drop in concentration of ascorbic acid.

(iv) Its deficiency causes 'whiptail disease' in cauliflower and cabbage. The leaves first show an interveinal mottling and the leaf margins may become gray and flaccid and finally brown.

(5) Zinc

Source : Zinc occurs in the soil in the form of ferromagnesian minerals like magnetite, biotite and hornblende. Increase in soil pH decreases the availability of zinc.

Bivalent form of zinc \[(Z{{n}^{++}})\] is exchangeable and is readily available in the soil. Plants require this mineral only in traces and its higher concentrations are highly toxic.

Functions

(i) It is required for the synthesis of tryptophan which is a precursor of indole acetic acid-an auxin.

(ii) It is a constituent of enzymes like carbonic anhydrase, hexokinase, alcohol dehydroge-nase, lactic dehydrogenase and carboxypeptidase.

(iii) It is required for metabolism of phosphorus and carbohydrates.

(iv) Zinc also appears to play an important role in protein synthesis because in its absence there is substantial increase in soluble nitrogenous compounds.

Deficiency symptoms

(i) The first symptom appears in the form of interveinal chlorosis of the older leaves, starting at the tips and the margins.

(ii) Growth becomes stunted due to formation of smaller leaves and shortened internodes. Reduced stem growth is due to less synthesis of auxin.

(iii) The leaves become distorted and sickle shaped and get clustered to form rosettes. This effect is known as 'little leaf disease'.

(iv) In maize, zinc deficiency produces 'white bud disease' which leads to greatly reduced flowering and fruiting as well as poorly differentiated root growth.

(v) Its deficiency causes khaira disease of rice and mottled leaf of apple, Citrus and walnut.

(6) Boron

Source : Boron is present in the soil in very small amounts. It appears in exchangeable soluble and nonexchangeable forms in the soil \[BO_{3}^{3-}\] or \[{{B}_{4}}O_{7}^{2}.\] It is absorbed from the soil as boric acid \[({{H}_{3}}B{{O}_{3}})\] and tetraborate anions.  Its calcium and magnesium salts are soluble. Its availability to plant decreases with increase in pH.

Functions

(i) It facilitates the translocation of sugars.

(ii) It is involved in the formation of pectin.

(iii) It is also required for flowering, fruiting, photosynthesis and nitrogen metabolism.

(iv) Boron is required for uptake and utilisation of \[C{{a}^{2+}},\]pollen germination, seed germination and cell differentiation.

(v) It regulates cellular differentiation and development.

Deficiency symptoms

(i) The first major symptom of boron deficiency is the death of shoot tip because boron is needed for DNA synthesis.

(ii) Generally flowers are not formed and the root growth is stunted.

(iii) The leaves develop a thick coppery texture, they curve and become brittle.

(iv) Some of the physiological diseases caused due to boron deficiency are internal cork of apple, top rot of tobacco, cracked stem of celery, browning of cauliflower, water core of turnip, hard fruit of Citrus and heart rot of sugar beets and marigold. These diseases can be cured by application of small doses of sodium tetraborate in the soil.

(v) Its deficiency checks the cells division of cambium but continues cell elongation.

(7) Chlorine

Source : It is absorbed from the soil as chloride ions. Hence, it is rarely supplied as fertilizer.

Functions

(i) It is required for photolysis of water during photosynthesis in photosystem-II.

(ii) In tobacco, it increases water volume inside the cell and also regulates carbohydrate metabolism.

(iii) With \[N{{a}^{+}}\]and \[{{K}^{+}},\]chlorine helps in determining solute concentration and anion cation balance in the cells.

(iv) It is essential for oxygen evolution in photosynthesis.

Deficiency symptoms

(i) The deficiency symptoms of chlorine consist of wilted leaves which later become chlorotic and finally attain a bronze colour.

(ii) Roots become stunted or thickened and club shaped and fruiting is reduced.

(iii) Photosynthesis is also inhibited.

Critical elements : Macroelements which become commonly deficient in the soils are called critical elements. They are in number–N, P and K most fertilisers contain critical elements. They are called complete fertilisers.



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