The following points highlight the top ten essential elements of Micronutrients. The elements are: 1. Iron 2. Manganese 3. Boron 4. Molybdenum 5. Zinc 6. Copper 7. Chlorine 8. Sodium 9. Cobalt 10. Silicon.
Contents
Micronutrients Element # 1.
Iron:
Iron is mainly absorbed by the plant in the ferrous form, but ferric ion may also be absorbed. Soils are usually not deficient in iron, but they may be deficient in soluble forms of iron. Availability of iron to the plant is controlled by the soil pH. Acid soil favours availability of soluble forms of iron. However, in neutral or alkaline soils, iron is much more insoluble.
Functions of Iron:
Iron performs a number of important functions in the overall plant metabolism. Like other elements, it functions both as a structural component and as a cofactor for enzymatic reactions.
Oxidation-reduction reactions are most commonly associated with iron containing enzymes. Iron is a transition metal, as it exists in more than one oxidation states. That is why, it can accept or donate electrons according to the redox potential of the reactants.
Physiological Role of Iron:
(i) A large portion of iron is found associated with porphyrins in the form of cytochromes, which are necessary for the electron transport system in mitochondria as well as chloroplasts.
(ii) Iron is a component of ferredoxin which is indispensable for the light reactions of photosynthesis and N2-fixation.
(iii) Iron is a constituent of the enzymes catalase and peroxidase, which involve molecular oxygen directly in oxidation-reduction reactions.
(iv) Iron is essential for the synthesis of chlorophyll, but where does iron have its effect on chlorophyll synthesis, is uncertain.
(v) Ferrous form of iron is required for the aconitase reaction in TCA cycle. By coordinate bond formation with the enzyme molecule the ferrous iron helps in substrate recognition and binding.
(vi) Ferric form of iron is required for amylase synthesis in GA-treated barley aleurone layer.
(vii) Iron has also been identified as a component of various flavo-proteins active in biological oxidations.
(viii) Iron is a constituent of leghaemoglobin, which is found in root nodules of leguminous plants. This leghaemoglobin by means of its iron atom protects nitrogenase of bacteroids from oxygen inactivation.
(ix) Iron is a constituent of N2-ase enzyme which is responsible for biological nitrogen fixation both in free living and symbotic bacteria.
Deficiency Symptoms of Iron:
Pronounced interveinal chlorosis occurring first on the youngest leaves, results in iron-deficient plants.
Sometimes interveinal chlorosis is followed by chlorosis of the veins, so the whole leaf then becomes yellow. In severe cases, the young leaves may even become white with necrotic lesions. The symptom may be general or sometimes strictly local. This is due to the fact that iron does not move freely from the older to the younger leaves.
Micronutrients Element # 2.
Manganese:
Manganese exists in the soil in divalent, trivalent, and tetravalent forms, but it is absorbed largely as the divalent manganous cation (Mn2+). Much of the manganese of the soil is present in insoluble compounds in the tri- and tetravalent forms and only a little amount in the bivalent form and thus is largely unavailable to the plant.
In poorly aerated acid soils, the tri- and tetravalent forms are reduced to the bivalent form, thus it becomes available to the plants. Well-aerated alkaline soils, on the other hand, favours the oxidation of manganese to form unavailable manganese oxides, such as Mn2O3 and MnO2.
Functions of Manganese:
(i) Manganese is involved in oxidation-reduction processes together with decarboxylation and hydrolytic reactions.
(ii) Manganese plays an important role in photosynthesis. Photosystem II in chloroplast contains manganese protein. Four Mn2+ ions are bound to one or more proteins and a chloride ion bridges two Mn2+ together. The Mn-protein is a part of the inner side of the thylakoid membrane and is involved directly in H2O oxidation (Barber. 1984) and thus O2 evolution.
(iii) Manganese can replace magnesium in many of the phosphorylating and group-transfer reactions like hexokinase, glucokinase, phosphoglucokinase, phosphoglucomutase, adenosine kinase, etc.
(iv) Manganese is the predominant metal ion of Krebs cycle reactions.
(v) Manganese acts as an activator for the enzymes nitrite reductase and hydroxylamine reductase.
(vi) Manganese is absolutely required by the NAD-malic enzyme system found in leaves of aspartate type C4 plants.
(vii) RNA-polymerase has an absolute requirement for manganese or magnesium.
(viii) Manganese, along with magnesium has been found to be essential for optimal activity of the enzyme responsible for the formation of phosphatidyl inositol.
(ix) There is considerable evidence that manganese influences the level of auxin in plant tissues. It enhances the IAA oxidase activity.
Deficiency Symptoms of Manganese:
(i) Mottled chlorosis with necrotic spots appear in the interveinal areas of the leaf.
(ii) In manganese deficiency, chloroplast is markedly affected. The chloroplasts lose chlorophyll and starch grains, become yellowish, vacuolated and granular, and finally disintegrate.
Micronutrients Element # 3.
Boron:
Boron is absorbed by the plants from the soil as un-dissociated boric acid (H3BO3). Boron is also present in the soil as calcium or manganese borates. The dissolved boron content is very low in the soil solution. The amount of boron is higher in acid soils. With the increase in pH of the soil, boron becomes less available to the plants.
Functions of Boron:
It is evident that essentiality of boron for all green plants is not absolute. It is required by higher green plants and diatoms but has not been shown to be essential for all species of green algae.
In higher plants boron performs the following important functions:
(i) Boron is essential for sugar transport. It complexes with sugar. Owing to their negative charge, the sugar borate complexes might pass through a negatively charged membrane more readily than neutral sugar molecules. Boron reacts with membrane constituents to form borate loci or borate reaction centres on membranes. These loci or centres might facilitate the passage of sugar through membranes.
It was suggested that boron might perform this role either:
(a) Through formation of sugar borate complexes which traverse membranes more readily than non-borated sugar molecules, or
(b) Through the formation of boron loci on membranes which facilitate the passage of sugars. The second suggestion is favoured as the mode of action of boron.
(ii) Boron is essential for the formation of pectin from uridine diphosphate D-glucose. Thus, boron is concerned with the cell wall metabolism specially involved in cell wall bonding.
(iii) Boron is involved in the polymerization of lignin precursors.
(iv) Dugger (1957) postulated that boron inhibits conversion of sugar to starch. Boron inhibits starch formation by forming an unreactive glucose-1 -phosphate-boron complex.
(v) Boron helps in germination and growth of pollen grains.
(vi) Boron along with GA3 influences the α-amylase activity in germinating seeds. Boron apparently has a regulatory role in synthesis of GA3.
(vii) Boron is essential for DNA synthesis.
The biochemical and physiological functions proposed for boron were reviewed by Dugger (1983), Philbeam and Kirkby (1983) and Lovatt and Dugger (1984). No specific function is yet certain, but evidence favours special involvement of boron in nucleic acid synthesis and some unclear functions in membranes.
Deficiency Symptoms of Boron:
(i) Boron deficiency results in ‘heart rot’ of beets, ‘water core’ of turnip, ‘stem crack’ of celery and ‘drought spot’ of apples.
(ii) Root tip elongation is inhibited.
(iii) Shoot apices die.
(iv) Nodule formation in legumes does not occur.
(v) The branches at the ends of twig form a rosette.
Micronutrients Element # 4.
Molybdenum:
Molybdenum is present in the soil as dissolved molybdate ions, in an exchangeable form adsorbed to soil particles, and in a non-exchangeable form. It is absorbed by the plants as molybdate ions (MoO42-).
Functions of Molybdenum:
(i) Initially molybdenum was shown to be essential for nitrogen fixation by Azotobacter chroococcum.
(ii) Later it was also found to be essential in nitrogen fixation by legumes.
(iii) Molybdenum is a component of the enzyme nitrate reductase, which is a sulphydryl, metallo-FAD-protein containing Mo.
(iv) Molybdenum is a component of nitrogenase found in nitrogen fixing organisms. The enzyme consists of two components or fractions known as component I or fraction I containing molybdenum and iron, and component II or fraction II containing iron.
Deficiency Symptoms of Molybdenum:
(i) Molybdenum deficiency causes chlorotic interveinal mottling of the leaves with marginal necrosis and in-folding of leaves.
(ii) In crucifers molybdenum deficiency causes ‘whitetail’ disease, the partial or complete withering of leaf lamina except the midrib.
(iii) In severe deficiency the mottled areas become necrotic and may cause the leaf wilt.
(iv) The flowers abscise before fruit setting.
Micronutrients Element # 5.
Zinc:
Zinc is absorbed as divalent Zn2+ ion. The sources of zinc in the soil are ferromagnesian minerals, magnetite and biotite. Zinc is released from these minerals. Soil pH is the main factor for the availability of zinc.
Functions of Zinc:
Zinc is an essential microelement performing the following important functions:
(i) Zinc is associated most commonly with the biosynthesis of indole-3-acetic acid (an auxin). Skoog suggested that zinc prevented oxidation of IAA. Zinc is essential for the biosynthesis of tryptophan which is the precursor of auxin. So, through the synthesis of tryptophan zinc affects auxin levels.
(ii) Zinc is essential for the activity of many enzymes like pyridine nucleotide dehydrogenases, alcohol dehydrogenase, glucose-6-phosphate and triose phosphate dehydrogenases. Zn2 + is involved in binding NAD to the enzyme protein.
(iii) Zn2+ is required by the enzyme phosphodiesterase.
(iv) The enzyme carbonic anhydrase requires Zn2 + for maximal activity. Zinc acts as a metal activator of this enzyme.
(v) Zn2+ induces de novo synthesis of cytochrome.
(vi) Zn2 + participates in chlorophyll formation and prevents chlorophyll destruction.
Deficiency Symptoms of Zinc:
Zinc deficiency shows the following symptoms:
(i) Interveinal chlorosis of the older leaves starts at the tips and margins.
(ii) Growth is stunted in severe zinc deficiency.
(iii) The leaves become smaller and the internodes shorten to give a rosette form.
Micronutrients Element # 6.
Copper:
Copper is sufficiently available in nearly all soils. It is absorbed as both divalent and monovalent cations. Copper source in the soil is chalcopyrite (CuFeS2).
Functions of Copper:
Copper is required by plants in very minute amounts and copper performs the following important functions:
(i) Copper is a component of cytochrome a, which is further a component of cytochrome c oxidase complex. This copper atom alternates between a+ 2 oxidized form and a + 1 reduced form as it transfers electrons from cytochrome a3 to molecular oxygen (Nason and McElroy, 1963).
(ii) In the thylakoid membrane, there is an electron carrier which is a small copper containing protein named plastocyanin. It remains bound loosely to the inside of thylakoid membrane. When the copper atom of plastocyanin becomes reduced from Cu2 + to Cu1 + by PS II, it can move along the membrane carrying an electron to PS I where it is re-oxidized to the Cu2+ form.
(iii) Copper is found in a group of enzymes in which oxygen is used directly in the oxidation of substrate. These enzymes are tyrosinase, laccase, and ascorbic acid oxidase.
The general reaction is:
Here also copper mediates the enzyme transformations by undergoing cyclic oxidation and reduction: 
(Price, 1970).
(iv) Copper has an indirect effect on nodule formation (Cartwright and Halls-worth, 1970). Copper deficiency reduces cytochrome oxidase activity, which in turn increases oxygen – levels in the nodule, thus restricting nitrogen fixation.
Deficiency Symptoms of Copper:
Plants are rarely deficient in copper, mainly because it is required in very minute amount.
Still there are certain copper deficiency symptoms as follows:
(i) Copper deficiency causes yellow leaf tips or reclamation disease in cereals accompanied by failure to set seeds.
(ii) Copper deficiency results in exanthema, a disease of fruit trees that is characterized by gummosis, accompanied by dieback and glossy brownish blotches on leaves and fruits.
(iii) Necrosis of the young leaf tips proceeds along the margin and gives it a withered appearance. In severe cases, the leaves fall and the whole plant tends to wilt.
Micronutrients Element # 7.
Chlorine:
No chloride-containing compound has been found in higher plants. It appears that it is not so essential in most higher plants.
Still it performs some important functions as follows:
(i) Chloride ion is involved in the primary process of oxygen evolution. PS II contains one or more proteins containing manganese, called manganese protein which is involved directly in the first step of H2O oxidation. It is thought that a chloride ion bridges two Mn2 + together.
(ii) During stomatal opening in light there is an influx of both potassium and chloride ions into the guard cells from subsidiary cells, giving osmotic potentiality to the guard cells. But the exact role of CI– ion in stomatal opening is not clear.
Micronutrients Element # 8.
Sodium:
Sodium is not essential for many plants, but its essentiality is restricted to certain species normally found in high saline environments. Sodium has been found to be essential for Artiplex vesicaria, Halogeton glomeratus, etc. Sodium can partially replace potassium in many of the reactions known to require potassium. Sodium is apparently necessary for C4 carbon fixation in certain plant species.
Aeluropus litoralis, a halophyte, has been found to fix carbon through C3 pathway when depleted of sodium. If a C3 plant is grown in presence of sodium, the photosynthetic pathway is shifted to C4 mode. It was also demonstrated that sodium influences the balance between PEP- carboxylase and RuBisCO in maize.
Certain CAM plants show a requirement for Na for the expression of Crassulacean acid metabolic pathway. If the plants are treated with NaCl, CO2 uptake is increased in the dark with the increase in malate content in the leaves. Sodium plays a role in maintaining a favourable water balance in plants.
Micronutrients Element # 9.
Cobalt:
Cobalt is required by the symbiotic organisms for nitrogen fixation. It is also involved in leghaemoglobin metabolism. In Rhizobium ribonucleotide reductase requires cobalt for maximum activity.
Micronutrients Element # 10.
Silicon:
Silicon is essential for the formation of silicified walls of diatoms. Growth of the diatom cells is directly proportional to the concentration of the silicon in the medium. Silicon reduces toxicity of other elements.