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Let us make an in-depth study of the importance of micronutrients in plant growth.
The early culture solutions were made with salts which were not pure, i.e., the salts contained a lot of traces of other elements as impurities. It was found that as better- purified salts were used for culture solutions, the growth of the plants instead of getting better, definitely got poorer.
This was followed up by direct evidence that other elements are needed for the normal growth of the plant but only in minute amounts. They are just as essential for the life and the growth of the plants as the macroelements and therefore no normal growth was possible in their complete absence.
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But in view of the fantastically small amounts needed, they are usually grouped separately from the macro- nutrients and called trace or micronutrient elements. Iron seems to occupy an intermediate position between the macro- and micronutrients.
It does not enter into the composition of plant food, or in the composition of the plant itself. The amount of iron necessary for normal growth of the plants is very small compared to the other six macronutrients, yet it is hardly as small as to rank as a micronutrient.
Trace elements can be conveniently divided into four groups:
(a) the essential—so far the following six have been conclusively proved to be essential for normal plant growth—B, Zn, Cu, Mn, Mo and Co; (b) the probably essential—-elements like selenium, barium, etc.; (c) the toxic—all essential macro- and micronutrients in high dosages and [d) physiologically inactive elements—arsenic, etc.
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In 1914, Maze, a French scientist, using very highly purified chemicals in water culture solutions found that his very pure salt solutions did not support plant growth satisfactorily. Thus, although this observation was at the time given very little attention, it really laid the foundation of the importance of micronutrients.
Almost 50 years before that, however, Raulin (1869) when studying the role of zinc in the nutrition of Aspergillus niger concluded, with a remarkable insight for his day, that the microelements were required by plants in minute quantity and were present as impurities in the external medium.
The ratio between the amounts of micronutrients and the amounts of macro- nutrients will be roughly between, 1: 1000 and 1: 10,000. In soils, however, quantities are somewhat larger—rate of application in soil, when there is a known micronutrient deficiency, are usually at the rate of a few pounds per acre.
Actual amounts of the trace elements or micronutrients range from as little as 2-3 parts in 100 million parts up to 10-100 parts in 1.00 million. These amounts are so fantastically small that sceptics may be tempted to argue that the whole subject is an exaggeration of science.
Since such small amounts of trace elements are required for plant growth in the soil, it might seem unlikely that soils should ever be unable to provide enough for all crops, yet it is a fact that in the past 15-20 years, increasing number of serious micronutrient deficiencies have been recognised.
In many parts of the world, economic cropping would have probably ceased but for diagnosis of micro-elemental deficiency and subsequent remedial treatment.
Microelement shortage is not always induced by a real absence of the particular element in the top soil. A large supply may be present but it may be locked up as a result of soil condition and thus unable to enter the soil solution and become available to the roots.
Severe Zn deficiency effects on fruit trees were experienced in many countries though the soil contained enough Zn for the whole orchard’s need for hundred years!
Even the addition of Zn salts in many cases to the soil could not correct the deficiency for the added Zn was also quickly made unavailable. The remedy was found in the spraying of a very dilute solution of ZnSO4 on the foliage of the trees.
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Another approach has been the injection of solid salt containing a trace element into the trunk of the tree. Like Zn, manganese deficiency is generally due to non-availability rather than to actual absence of the element from the soil.
On the other hand, boron deficiency is usually due to absence of boron in the soil for borates tend to be easily washed out of the soils, particularly in sandy soil. Here, the remedy of applying boron to the soil is reasonably effective.
Micronutrient elements, although they are as essential as the macronutrients in minute amounts, soon become toxic to the plants if the beneficial rate is exceeded.
Most artificial fertilisers always contain trace elements in any case; for instance, Chilean nitrate contains boron and basic slag contains manganese; superphosphates contain slight amounts of Cu and Zn, derived from the commercial H2SO4 used in the industrial manufacture of superphosphates.
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Why are such minute quantities of some elements needed by plants? What indispensable roles do these elements play in the metabolism of the living cells?
We know that these elements are not direct plant foods. It was supposed also that they may act as catalysts or activators for certain chemical reactions in the plants and for that reason, it was understandable that their requirement will be much smaller, compared to the macronutrients, which directly enter into the composition of plant foods or living cells themselves.
Recent researches have given us the answers to the questions about the functions of the microelement which seemed so perplexing a few years before. There is no doubt whatsoever now, that the essentiality of heavy metals like Cu, Zn, Mn and Fe in minute traces for the normal growth of the plants is due to their forming constituents of the essential enzymes.
The enzyme, tyrosinase, which transfers hydrogen from the amino acid, tyrosine, is active only in the presence of a single atom of copper as a coenzyme (prosthetic group). Other copper-containing enzymes are also known such as Ascorbic acid oxidase which destroys vitamin C.
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The non-protein prosthetic groups or coenzymes in these cases, consist only of a single metal atom of copper and nothing else. Terminal cytochrome oxidases (Cy a and Cy a3) whose functions are to transfer electrons or hydrogen ions to molecular oxygen in the respiratory chain of oxidative phosphorylation Vitamin C, always contain either one or two atoms of copper per molecule of heme. Plastocyanin discovered by Katoh (1960) is a natural one-electron-transfer copper protein which occurs in chloroplastids in concentration of 1: 500 chlorophyll.
It may be a normal electron carrier between the two photochemical reaction. Thus it is only too true to suggest that copper serves a direct functional role in photosynthesis.
Atomic Zn is an activator component for a host of enzymes, e.g., carbonic anhydrase (catalyses the reaction CO2 + H2O → H++ HCO–3), alcohol dehydrogenase, lactic dehydrogenase, glutamic dehydrogenase, triosephosphate dehydrogenase, aldolase (can be replaced by Co or Fe), etc.
It has been known for quite a long time that Zn is essential for the formation of the most important plant hormone, indole acetic acid. More recently it was shown that Zn in minute traces is indispensable for the formation of the amino acid, tryptophan, the generally accepted precursor of indole acetic acid and is not directly concerned with the synthesis of the auxin.
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The catabolic breakdown of tryptophan to, the indole nucleus of the auxin is accomplished by the enzyme tryptophanase. At the present moment, however, it is clear that the essentiality of Zn for the formation and breakdown of tryptophan is certainly due to its acting as an activator component of enzime tryptophanase.
Manganese forms activator components of several enzymes, such as some dehydrogenases, decarboxylases, kinases, oxidases and peroxidases, specifically and also non-specifically by other divalent cation-activated enzymes.
Manganese is also reportedly required by nitrite and hydroxylamine reductases in lower non-green plants but not in higher plants. One of the fundamental discoveries of the last few years is that a manganese enzyme is specifically required for photosynthetic evolution of oxygen.
Iron is an activator constituent of many enzymes such as peroxidase, catalase and of cytochromes. In the case of iron, however, the iron atom does not form the coenzyme by itself, but forms a more complex molecule with a porphyrin (tetrapyrrole structure).
And since the iron-porphyrin proteins are the primary catalysts in respiration of living cells, it is not surprising that minute traces of iron should be essential for the maintenance of life in animals as well as in plants.
Iron is also a constituent of non-heme iron proteins, such as, ferredoxin, which is involved in photosynthesis as the primary electron acceptor, as well as in nitrogen fixation and, also of respiratory-linked flavoprotein dehydrogenases.
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It is now known definitely that molybdenum plays an important role in nitrate assimilation in plants and the fixation of atmospheric nitrogen by micro-organisms can occur only in the presence of minute traces of Mo. But in spite of great amount of work for almost 50 years on boron, its exact functions are as yet unknown.
Boron may be necessary in order that the plant can obtain calcium in an efficient way. According to the more recent prevalent idea, boron forms a complex with sugar in the plant cells that can penetrate through the living cell walls, more rapidly than free sugars and is, therefore, more readily translocated to the growing meristematic cells where the carbohydrates are most needed.
Cobalt and iodine are essential elements for animal nutrition, but all attempts to prove that these elements are essential micronutrients for plants have so far failed though iodine is accumulated in sea-weeds in large quantities.
Very recently, however, the importance of cobalt in promoting auxin formation in plants is being recognised— cobalt ion is known to depress the specific oxidase enzymes which destroy auxins. Likewise boron has been found to be essential to plants but there has been no definite proof that it has any function in animal nutrition.
This might very well be our ignorance; just as the gaps in Mendeleef’s periodic table were for a long time no more than evidences of our inability to isolate the missing elements.
So this strange anomaly in what seems to be an otherwise admirably planned natural arrangement (most of the element needed for plant nutrition as Ca, P, Fe, Mg, K and S are also essential for animal nutrition) will, it is hoped, in near future be satisfactorily explained.
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Some investigators have produced evidence for the essentiality of a few other macroelements, not mentioned before, such as aluminium, silicon and selenium, at least for certain plants, but other workers have failed to confirm these results.
It is undoubtedly true for silicon (SiO2) which occurs in large quantities in Equisetum and grasses, but silicon does not seem to be indispensable to such plants—Equisetum and grasses can get on quite as well without silicon.
As a matter of fact all soil-grown plants contain silicon and in many Gramineae (in rice it is about 10-15%) in addition to grasses and also other monocots, considerable silica is deposited in the form of opal, hydrated amorphous silica (SiO2.nH2O).
These silica deposits (natural crystalline silica SiO2) have also been called opal phytoliths and their distribution in several species of Gramineae is known, without any indication, whatsoever, about the significance of their presence in these plants.
On the other hand, silicon seems to be essential for diatoms, the cell walls of which are almost entirely composed of silicon acid and which gives permanence and rigidity to the diatom cells.
Aluminium, while found in small quantities in almost all higher plants, is accumulated in any considerable amounts in only a few plants, such as Lycopodium. An interesting relation between large accumulation of aluminium in the cells and the development of blue-coloured flowers has been established recently in many blue-flowered or blue- fruited plants. Aluminium may also be more necessary to the water plants than to the land plants.
Elements such as aluminium and silicon have been called ballast elements by investigators who refuse to attribute any useful metabolic function to these elements because they are usually present in large amounts in plant tissues though the plant can be grown perfectly normally without them.
As ballast (heavy material) in a ship’s hold gives stability to the ship and maintains its equilibrium, so can the large quantities of aluminium and silicon in the living cells be used for maintaining the stability of ionic potential and thus preventing the cell system from ‘capsizing’.
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Some plants are accumulators, i.e., they concentrate in their cells large quantities of certain elements, as for example, iodine in sea-weeds, silicon in grasses, sodium in halophytes, selenium by some species of Astragalus (about 5%), molybdenum by clover, etc.
An interesting point:
Geological distribution of Selenium points to an accumulation of the element by plants, possibly as long ago as the paleozoic era (Rosenfield & Beath, 1964). It has even been suggested that during the mesozoic, herbivorous dinosaurs and other prehistoric giants ate themselves into oblivion with massive ingestion of seleniferous vegetation.
The distribution of the species of Astragali was perhaps global at that time and much more numerous, which could accumulate selenium to such an extent as to render the herbage extremely lethal to the herbivores. Selenium replaces sulphur in several organic molecules and may become toxic. Deficiency symptoms due to macro- and micronutrients
We know that abnormalities in structural and physiological make-up of plants are brought about by deficiencies of essential elements inside the plants. The symptoms of such deficiency are more or less specific for each element, but it is sometimes very difficult to distinguish the differences.
Furthermore,, different plants may show somewhat different symptoms. In some cases, the deficiency symptoms occur first on the oldest leaves (as for example, N, K, P and Mg) while in others the first symptoms are visible in the youngest leaves (such as S, Ca, Fe, Mn, B, Cu and Zn).
The symptoms associated with deficiency of particular elements occur in the oldest leaves for the first group of elements because the elements of the first group are mobile and are transferred from the older inactive leaves to the younger leaves during the first stage of deficiency.
The younger leaves can get a supply of element as long as there is any in the older leaves and as a result the older leaves show the symptoms before the newly formed young leaves.
The elements of the second group are immobile—they remain in the old leaves and hence the younger leaves are deprived of supply of these elements when deficiency of a particular element occurs in the plants. Hence the younger leaves are starved and show the deficiency symptoms, though the older leaves do not.
The commonest type of deficiency symptoms are chlorosis and necrosis, reduction in the amount of green pigment chlorophyll in the leaf. But chlorosis can occur as a result of deficiency of any one of the several elements such as Fe, Mg, N, Mn and even S. 32 [one]
However, chlorosis, due to deficiency of each one of them, is different from the other. In case of nitrogen deficiency, the chlorosis of the leaf blade is uniform whereas in case of iron deficiency, chlorosis though uniform is more pronounced between the veins. Calcium deficiency results in increased chromosome fragility.
In many plants excesses of several metals including Mn, Cr, Cu, Zn, Co, Ni, Cd, may induce symptoms apparently identical with those of iron deficiency.
There are also deficiency diseases associated with microelements. Dry rot of sugarbeet and internal browning of cauliflower due to boron deficiency, little leaf of fruit trees due to zinc deficiency, yellow spots of pea seeds, etc., are usual symptoms of trace-element deficiency.
The deficiency of a -particular element found in the plant may not correspond with actual deficiency in the soil. Thus definite iron deficiency symptoms of chlorosis may actually be due to non-availability of iron to a plant due to high pH conditions and not really to an actual absence; addition of iron in such soils does not improve the position to a great extent because the added iron is also soon made unavailable by precipitation.
A recent method which has been successfully used in citrus orchards to overcome this difficulty is to apply iron combined with an organic compound to the soil in such a way that it can ionise and therefore cannot be precipitated. Sometimes the ratio of the quantities of elements in the soil may be more important in determining deficiency of particular elements in the soil than the absolute amounts (e.g., Fe: Mn and Ca: B).