Each essential element participates in a specific set of metabolic reactions. Gut generally the essential elements function as constituents of compounds, in the activation of enzymes, and in contributing to the osmotic potential of plant cells. However, some other roles include the ability of divalent calcium or magnesium cations to modify permeability of plant membranes and regulation of key enzymes by calcium in the cytosol.
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Because each essential clement perform one or more specific internal roles, inadequate supply of an essential element result in a nutritional disorder, For example, Peppermint plants grown in nitrogen deficient nutrient solutions showed production of very low amount of alcohol-soluble amino acids and amides whereas those grown in sulphur deficient nutrient Solutions have high amounts of these amino acids.
In addition to alterations in metabolic patterns, several deficiencies of essential elements produced a set of characteristic effects in external appearance of leaves, stems, roots, flowers and fruits.
These characteristic symptoms are called deficiency symptoms and include stunted growth, chlorosis (failure of leaves to produce normal amounts of chlorophyll), mottling of leaves, abnormal curling of leaves, development of abnormal leaf discolorations, development of regions of necrosis (blackening and decay of tissue), premature drying and withering of leaves, and premature senescence (maturation) of leaves and flowers.
Soils are more commonly deficient in nitrogen than any other element. Two major ionic forms of nitrogen are absorbed from soils: nitrate (NO1) and ammonium (NH4+).
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Nitrogen is associated with many plant cell components such as amino acids and nucleic acids.
Plants grown with excess of nitrogen usually have dark-green leaves and show abundant foliage, poorly developed root system, therefore a high shoot-to-root ratio, as in potato.
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Phosphorus acts as limiting element in soils, second to nitrogen. It is absorbed primarily as the monovalent phosphate anion (H2P04) and less rapidly as divalent anion (H2P04). The availability of these two forms of ions is controlled by soil PH.
Phosphorus (as phosphate) is an integral component of a number of important compounds present in plant cells, such as sugar-phosphates used in photosynthesis and respiration and phospholipids found in plant membranes. It is also a component of nucleotides used n plant energy metabolism (ATP), NADP and in the molecules of DNA and RNA.
The phosphate accumulate in younger leaves and developing flowers and seeds, therefore deficiency symptoms occur often in mature leaves first.
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Abundant supplies of phosphorus speed up maturity. Thus, a close interaction between phosphorus and nitrogen exists regarding maturity, excess nitrogen delay maturity. The root growth is often increased relative to shoot growth. This is in contrast to effects of excess nitrogen.
After nitrogen and phosphorus, soils are usually most deficient in potassium. That is why commercial fertilizers contain high %age of these elements.
Potassium plays an important role in regulation of osmotic potential of plant cells. It acts an activator for enzymes involve in respiration and photosynthesis. It also activates enzymes required to form starch and proteins. It has very important role in opening and closing of stomata.
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Sulphur is absorbed from sols as divalent sulfate ions (SO42). Sulphur can also be absorbed by leaves through stomata as gaseous sulphur dioxide which is converted into bisulphite (HSO3–) when it combines with water present in the cells. Bisulphite is oxidized further to sulfuric acid (H2SO4) which has toxic effects of acid rains. Enough sulfate is present in the soils, therefore sulphate-deficient plants are uncommon.
Sulphur in plants occurs in proteins specifically in amino acids cysteine and methionine that are building blocks for proteins. Similarly it is an essential component of vitamins thiamine and biotin, and coenzyme A, a compound necessary for respiration, and for synthesis and breakdown of fatty acids.
As the sulphur cannot be redistributed easily from mature tissues in some Species, therefore deficiency symptoms are usually first noted in younger leaves, The deficiency symptoms are similar to those of nitrogen deficiency and include chlorosis, stunted growth and accumulation of anthocyanin. In some species the chlorosis may occur at once in all the leaves or on older leaves first.
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Calcium is absorbed as divalent Ca2+. Most soils contain enough calcium for adequate plant growth. But acidic coils where high rainfall occurs, the soils are deficient of calcium.
The Calcium s used in the synthesis of new cell walls, particularly in middle lamella separating newly divided cells It is also used in formation of mitotic spindle during cell divisions. Plant membranes require calcium for their normal functioning. It is considered that calcium acts as a second messenger for a number of plant responses to both environmental and hormonal signals. It may bind to calmodulin. A protein found in cytosol, to form calcitun-calmodulin complex that is considered to be involved in regulation of many metabolic processes.
The deficiency symptoms are more prominent in young tissues especially in meristematic zones of roots, stems, and leaves; perhaps calcium is required to form middle lamella and mitotic spindle apparatus there.
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Magnesium is absorbed as divalent Mg2+. Magnesium is almost never limiting to the plant growth in soils.
In plant cells, magnesium has a specific role in the activation of enzymes involved in respiration, photosynthesis, and synthesis of DNA and RNA. Magnesium is also a part of the porphyrin component of the chlorophyll. It combines with ATP and allows it to function in many reactions.
The deficiency symptoms appear first in older leaves.
Iron plays an important role as a component of enzymes concerned with the transfer of electrons (redox reactions), such as cytocl3romes. It undergoes alternative oxidation and reduction between the Fe+ and Fe+ states as it acts as electron carrier.
As the iron cannot be readily’ mobilized from older leaves, the deficiency symptoms occur initially on younger leaves.
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Copper is absorbed both as the divalent cupric (Cu) on in aerated soils or as the monovalent cuprous ion in wet soils with little oxygen. Divalent cupric is chelated in various soil compounds. The copper is needed in small amount by the plants.
Like iron, copper is associated with enzymes involved in electron transfer, for example plastocyanin involved in electron transfer during the light reactions of photosynthesis, and cytochrome oxidase, a respiratory enzyme, present in mitochondria.
Boron is absorbed from soils as undissociated boric acid.
Precise function of boron in plant metabolism is unknown; however there is evidence that boron is involved in synthesis of nucleic acids (RNA and DNA), hormone responses and membrane functions. Boron plays an undetermined but essential role in elongation of pollen tubes.
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Plants deficient for boron show characteristic deficiency symptoms. These symptoms are affected by age of the plants and differ from species to species.
Manganese exists in various oxidation states as insoluble oxides in soils, but it is absorbed largely as the divalent manganous cation (Mn2) after reduction of Mn in these oxides as the root surface,
Manganese activate a number of enzymes in plants particularly decarboxylases and dehydrogenases involved in the Krebs cycle. The best defined function of Mn is in the reaction in which oxygen is produced from water during the process of photosynthesis. The manganese plays structural role in the chloroplast membrane system.
The major deficiency symptom of manganese deficiency is interveinal chlorosis associated with the development of small necrotic spots on younger or older leaves depending upon species
Zinc is absorbed as divalent Zn2, probably often from zinc chelates.
Many enzymes (more than eighty) require zinc for their activity. It is also required for chlorophyll synthesis in some plants.
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