Essential micronutrient

Essential macronutrients Essential micronutrients Beneficial Essentiality not demonstrated... 

Micronutrients. Attention to meeting the micronutrient needs of crops has greatiy increased as evidenced in an analysis undertaken by TVA and the Soil Science Society in 1972 (99). The micronutrient elements most often found wanting in soil—crop situations are boron, copper, iron, manganese, molybdenum, and zinc. Some of these essential micronutrients can be harmful to plants when used in excess. 

In 1956 selenium was identified (123) as an essential micronutrient iu nutrition. In conjunction with vitamin E, selenium is effective iu the prevention of muscular dystrophy iu animals. Sodium selenite is adrninistered to prevent exudative diathesis iu chicks, a condition iu which fluid leaks out of the tissues white muscle disease iu sheep and infertility iu ewes (see Eeed ADDITIVES). Selenium lessens the iacidence of pneumonia iu lambs and of premature, weak, and stillborn calves controls hepatosis dietetica iu pigs and decreases muscular inflammation iu horses. White muscle disease, widespread iu sheep and cattle of the selenium-deficient areas of New Zealand and the United States, is insignificant iu high selenium soil areas. The supplementation of animal feeds with selenium was approved by the U.S. EDA iu 1974 (see Eeed additives). Much of selenium s metaboHc activity results from its involvement iu the selenoproteia enzyme, glutathione peroxidase. 

Boron in the form of borate is an essential micronutrient for the healthy growth of plants and is present in the normal daily human diet at an estimated level of 3—40 mg as boron. It is not a proven essential micronutrient for animals (119). 

Although it is toxic in large doses, selenium is an essential micronutrient in all known forms of life. It is a component of the unusual amino acids selenocys-teine and selenomethionine. In humans, selenium is a trace element nutrient. 

Copper is an essential micronutrient required in the growth of both plants and animals. In humans, it helps in the production of blood haemoglobin. In plants, copper is an important component of proteins found in the enzymes that regulate the rate of many biochemical reactions in plants. Plants would not grow without the presence of these specific enzymes. Research projects show that copper promotes seed production and formation, plays an essential role in chlorophyll formation and is essential for proper enzyme activity, disease resistance and regulation of water in plants (Rehm and Schmitt, 2002). 

Nickel is an essential micronutrient for maintaining health in certain species of plants and animals. Its deficiency effects from dietary deprivation have been induced experimentally in many species of birds and mammals. To prevent nickel deficiency in rats and chickens, diets should contain at least 50 pg Ni/kg ration, while cows and goats require more than 100 pg Ni/kg rations, perhaps reflecting the increased use by rumen bacteria. Nickel deficiency is not a public health concern for humans because daily oral intake is sufficient to prevent deficiency effects. 

Molybdenum (Mo) is present in all plant, human, and animal tissues, and is considered an essential micronutrient for most life forms (Schroeder et al. 1970 Underwood 1971 Chappell and Peterson 1976 Chappell et al. 1979 Goyer 1986). The first indication of an essential role for molybdenum in animal nutrition came in 1953 when it was discovered that a flavoprotein enzyme, xanthine oxidase, was dependent on molybdenum for its activity (Underwood 1971). It was later determined that molybdenum is essential in the diet of lambs, chicks, and turkey poults (Underwood 1971). Molybdenum compounds are now routinely added to soils, plants, and waters to achieve various enrichment or balance effects (Friberg et al. 1975 Friberg and Lener 1986). 

Because of its common occurrence and biological importance, it is an essential micronutrient for most organisms. Thus, a number of analytical procedures for the analysis of iron species have been developed and typically concentrate on biologically available species [9],... 

Manganese is an important element in the aquatic environment. It is an essential micronutrient U 2) and is the subject of much interest because its oxides scavenge other heavy metals (3). Of particular interest are ferromanganese nodules, which are abundant in the aquatic environment. These nodules contain high concentrations of cobalt, nickel, copper and other heavy metals (4). 

Figure 5.1 The structure of a glycerophospholipid. A simple diagram showing the charges on the head group. In this struction, palmitic and oleic acids, provide the hydrophobic component of the phospholipids and choline (and four bases) and the phosphate group provide the hydrophilic head. The unsaturated fatty acid, oleic acid, provides a kink in the structure and therefore some flexibility in the membrane structure which allows for fluidity. The more unsaturated the fatty acid, the larger is the kink and hence more fluidity in the membrane. Cholesterol molecules can fill the gaps left by the kink and hence reduce flexibility. Hydroxyl groups on the bases marked are those that form phosphoester links. Choline and inositol may sometimes be deficient in the diet so that they are, possibly, essential micronutrients (Chapter 15).

Iron is an essential micronutrient for all organisms. It is a required element in cytochromes and the Fe-S centers of redox proteins involved in key metabolic processes such as photosynthesis, respiration, and the reduction of nitrate. Given the importance of Fe in these major metabolic enzymes, microorganisms under Fe-deficient conditions reduce their cellular Fe quotas (Fe C content) (Sunda et al., 1991) and have reduced photosynthetic (Raven, 1988 Green et al., 1994) and bacterial growth (Tortell et al., 1996) efficiencies. Thus ecosystem C metabolism is regulated in part by the availability of Fe. 

Heavy metals. The most common heavy-metal pollutants are arsenic, cadmium, chromium, copper, nickel, lead, and mercury. Some metals, such as manganese, iron, copper, and zinc, are essential micronutrients. Each type of heavy metal in its own way affects water ecosystem biochemistry and can accumulate in bottom deposits and in the biomass of living elements. 

Kromhout D. Essential micronutrients in relation to carcinogenesis, Am J Clin Nutr I 987 45 1361-1376,... 

There is no reason to suppose that all the essential micronutrients have been identified and careful experimentation will undoubtedly prove the essentiality of other elements in due course. The agricultural significance of the most recently discovered plant micronutrients, sodium and nickel, is uncertain. There is no evidence to suggest that they limit crop production anywhere since they are required in such tiny amounts that most soils can probably satisfy a plant s normal requirements. 

It has not been possible so far to establish that Cr is an essential element required by plants, however, addition of Cr to soils deficient in the element has been shown to increase growth rates and yields of potatoes, maize, rye, wheat or oats (Scharrer and Schropp, 1935 Huffman and Allaway, 1973 Bertrand and De Wolf, 1986). Nickel appears to be an essential element for plants (Farago and Cole, 1988). Zerner and coworkers (Dixon et al., 1975) demonstrated that urease isolated from jack bean (Canavalia ensiformis) was a nickel enzyme. Eskew et al. (1983) have shown that Ni is an essential micronutrient for legumes. Most plants contain nickel in the range 1 - 6 mg kg-1 (Vanselow, 1966 Hutchinson, 1981). The uptake of Ni is enhanced by low pH values, and available nickel increases at pH less than 6.5 as a consequence of the breakdown of Ni complexes in the soil with Fe and Mn oxides. Uptake of nickel by plants and questions of toxicity and tolerance have been reviewed by Farago and Cole (1988). Nickel toxicity toward plants has been reviewed by Vanselow (1966) and Hutchinson (1981). 

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