Nutrient Deficiency Symptoms In Corn

Nutrient Deficienies in Crops Reduce Yields, Quality and Profits

Nutrient deficiencies in crops reduce yields, quality and profits to the farmer. These photos may help in identifying the various symptoms of nutrient deficiencies on major crops grown around the world.

Source : http://www.back-to-basics.net/nds/index.htm#

Copper Deficiency

Iron Deficiency

Magnesium Deficiency

Iron Deficiency

Nitrogen Deficiency

Nitrogen Deficiency

Nitrogen Deficiency

Phosphorus Deficiency

Potassium Deficiency

Potassium Deficiency

Potassium Deficiency

Sulfur Deficiency

Sulfur Deficiency

Zinc Deficiency

Zinc Deficiency

Zinc Deficiency

http://www.back-to-basics.net/nds/index.htm#

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——————————

* Front Cover.
* Table of Contents.
* About the book and the authors.
* Acknowledgements.
* List of scientific names for species mentioned in the text.
* Acronyms, symbols and abbreviations.
* Summary.
* 1. Introduction.
* 2. Micronutrients in soil.
* 3. Micronutrients in plants, animals and humans.
* 4. Benefits of using micronutrient fertilisers.
* 5. Types of micronutrient fertiliser products: advantages and disadvantages of the different types.
* 6. Application strategies.
* 7. Best Management Practices (BMPs) for micronutrients.
* 8. Current research and development trends.
* 9. Micronutrient market.
* 10. Policy and regulatory context of micronutrient use.
* 11. Conclusions and recommendations.
* 12. References.
* 13. Plates on micronutrient defi ciency symptoms.
* Last Page.

3. Micronutrients in plants, animals and
humans
Th e importance of micronutrients in the health of plants (Marschner, 1995), humans and
animals has been well known for many decades (Underwood and Suttle, 1999; McGuire
and Beerman, 2007). However, the extent of micronutrient defi ciencies in humans has
only recently become a priority. Th e only defi ning characteristic of a micronutrient is
its low concentration in most living tissues. However, the nutritional importance of
micronutrients is high and does not refl ect their low abundance. Th e essentiality for
most micronutrients in plants, animals and humans has long been recognised but recent
years have seen increased interest in the function and requirements of micronutrients,
particularly in the fi eld of human nutrition. Micronutrient malnutrition has been
identifi ed as a major underlying cause of numerous human health problems, particularly
in developing countries with estimates of up to 2 billion people being aff ected by Fe
defi ciency alone (Gibson, 2006). It is necessary to understand the diverse range of
functions micronutrients play in plants, animals and humans in order to understand
their importance.
A large part of the functional role of micronutrients that are metals (e.g. Cu, Fe, Mn,
Zn) is their involvement in metalloenzymes. Within enzyme systems, these metals play
either structural or catalytic roles or can be required for enzyme activation as discrete
cofactors. Metals in metalloenzymes are highly specifi c associations where loss of the
metal results in a loss of catalytic activity and, in general, these metals cannot be replaced
by any other. However, individual metalloenzymes are not always the domain of a single
metal. Superoxide dismutase (SOD) for example is involved in the detoxifi cation of
the superoxide free radicle (O2
-) by conversion into hydrogen peroxide (H2O2). Th ree
SOD analogues exist (Mn-, Fe- and Cu-SOD), each using a diff erent metal with similar
properties for its catalytic function. Although they perform a similar function, the
enzymes are not interchangeable as they are each localised to discrete areas of the cell.
Th e metals Fe, Cu, Mn and Mo are capable of valency change under normal
biological conditions. Many enzymes utilize this property for their catalytic activity
(see box below). Whilst not catalysing reactions themselves, other compounds such as
the cytochromes and the Fe-S clusters also utilise the redox properties of these metals.
Th ese proteins usually share their electrons with enzymes that require redox chemistry
thus forming redox chains, such as the role of cytochromes in the reduction of nitrate
(NO3
-) to ammonia (NH3).
Examples of metalloenzymes with more than one valency:
Cu superoxide dismutase (SOD), cytochrome oxidase, polyphenol oxidase
Fe peroxidase, SOD
Mn SOD
Mo nitrate reductase, aldehyde oxidase, xanthine oxidase

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Zinc diff ers from the other micronutrient metals in that it does not undergo redox
transformation in biological systems. Th e catalytic role of Zn in enzymes is instead
performed due to its strong Lewis acid chemistry. Th is, along with its stable structural
properties has resulted in over 300 Zn metalloenzymes, far more than any other
micronutrient.
Micronutrients are also involved in numerous other functions in plants, animals and
humans including storage, transport and regulatory roles. Zinc plays a structural role in
the ‘Zn fi nger’ domains, which act as DNA transcription factors in all eukaryotic cells. It
has been estimated that ‘Zn fi nger’ domains may constitute up to 1 % of all human gene
products. Further roles for trace elements in the regulation of gene expression are likely
to be identifi ed in the future as technology advances.
Compared to the other micronutrients, relatively little is known about the roles of B
in plants, animals and humans. Boron has been recognised as an essential element in
plants for many years, but only recently is it gaining acceptance as an essential element
for animals and humans. So far, B has no known role in enzyme chemistry.
Boron
Functions
Plants Boron is required in the stabilisation of cell walls by forming the boraterhamnogalacturonan
II (RG-II, a complex pectic polysaccharide structurally located
in the primary cell wall) cross-link (Matoh, 1997; O’Neill et al., 2004). Th is primary
function is refl ected in the cessation of growth of young leaves and roots in response to
B defi ciency in plants (Dell and Huang, 1997). Boron also forms cross-links with glycolproteins
in cell membranes and may regulate physical properties such as membrane
fl uidity. Th is may have implications in plant tolerance of high irradiance (Huang et
al., 2002) and low temperature stresses (Huang et al., 2005). Although proposed B
functions in the metabolism of phenols and lignin (Cakmak and Römheld, 1997) may
be secondary eff ects (Cara et al., 2002), B defi ciency can infl uence lumber quality. Boron
is essential for normal development of reproductive tissues and defi ciency results in low
grain set or poor seed quality (Dell et al., 2002). Also, B defi ciency may trigger the early
synthesis of ethylene, leading to the rapid deterioration of fruit quality.
Animals and humans A large volume of information has become available about B
eff ects on the health and nutrition of animals and humans in recent years (Nielsen,
2002; Devirian and Volpe, 2003; Hunt, 2003), but the primary functions of B at the
cellular level remain enigmatic. Primary functions of B in animals and humans could
be related to its binding with cellular membranes and biomolecules involved in various
enzyme activities (Verstraeten et al., 2005). It has been suggested that B may have a
role in immune function (Hunt, 2003). Increasing evidence suggests that B may be
an essential micronutrient in the metabolism of steroid hormones and some mineral
nutrients (calcium (Ca), magnesium (Mg)) and vitamins (Devirian and Volpe, 2003;
Hunt, 2003). It is well documented that dietary B intake improves bone strength in
animals such as rats, chicken and pigs (Armstrong and Spears, 2001; Devirian and

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Volpe, 2003), and that B is required for egg/embryo development in frog and zebrafi sh
(Nielsen, 2002).
Requirements
Plants Plant B requirements have been updated in crop and forest species by using Bbuff
ered solution culture or fl owing solution culture. Critical B concentrations for the
diagnosis of B defi ciency during vegetative growth have been determined for a range of
species (Bell, 1997; Bell et al., 2002) (See box below).
Animals and humans Boron requirements for animal and human health are less
well defi ned, compared to those for plants. Th ere is no established minimal intake
recommendation for humans, but an upper intake limit for a person 19 years or older is
considered to be 20 mg/day (Nielsen, 2002). Low dietary B intake (< 0.3 μg/g) adversely
aff ects bone development, brain function, immune function and insulin secretion in
chicks and rats (Nielsen, 1997). In humans, a daily supplementation of up to 3 mg of B
is considered to help reduce bone loss and arthritis (Schaafsma et al., 2001). A survey
in the United States found that average B intakes for school children (4-8 years old) and
adolescents (14-18 years old) were 0.80 mg/day and 1.02 mg/day, respectively; and for
female and male adults, were 1.00 and 1.28 mg/day (Rainey et al., 2002). Major sources
of B in the human diet are fruits and vegetables (Devirian and Volpe, 2003). Given that
broadleaf crops generally have higher B requirements and uptake than cereals (Bell,
1997), it is not surprising to fi nd that fruits, vegetables, tubers and legumes are much
better dietary B sources than grains of wheat, corn and rice .
Defi ciency symptoms
Plants Boron-defi ciency symptoms in plants have been described in numerous studies
(Bell, 1997; see Chapter 13). In plants subject to mild B defi ciency, leaves appear dark
green and leathery with downward cupping and small size. When B defi ciency becomes
severe, dieback of the shoot tip occurs. Plant symptoms can be greatly infl uenced by
other stress factors, such as high light intensity and low temperature. In reproductive
parts, B defi ciency causes abortion of fl ower buds and/or fl owers (e.g. sterile ears in
wheat), abnormally shaped fruits (e.g. avocado and citrus) and malformed seeds (e.g.
hollow heart in peanut).

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Animals and humans Despite a range of physiological eff ects of B dietary intake in
animals and human, no specifi c B-defi ciency symptoms have been observed.
Copper
Functions
Plants Copper plays important structural and functional roles in oxidative enzymes
(such as SOD, cytochrome oxidase, ascorbate oxidase, polyphenol oxidase and diamine
oxidase) and electron-transfer proteins (such as plastocyanin in chloroplasts). Due
to these functions of Cu in plants, physiological consequences of Cu defi ciency may
include inhibition of photosynthesis and low availability of soluble carbohydrates in
leaves (Marschner, 1995). Another distinct function of Cu is the role of several Cumetalloenzymes
in the lignifi cation of cell walls. Lignifi ed walls are required for support,
water transport and release of pollen.
Animals and humans Copper is essential for the immune system, the nervous system,
skeletal health, for Fe metabolism and for formation of red blood cells (Johnson, 1998).
As in plants, Cu is involved in redox reactions and the scavenging of free radicals. It is a
component of more than 12 metalloenzymes and a few genes are known to be regulated
by Cu-dependent transcription factors (Uauy et al., 1998). Th e mitochondrial enzyme,
cytochrome c oxidase, plays a critical role in cellular energy production by reducing
oxygen to water, which generates an electrical gradient allowing the formation of the
energy-storing molecule, adenosine triphosphate (ATP).
Ceruloplasmin and Cu-SOD function as antioxidants that scavenge for free radicals.
Lysyl oxidase is essential for the cross-linking of collagen and elastin and helps maintain
the integrity of connective tissue in the heart and blood vessels, and skeletal system.
Ferroxidase II catalyses the oxidation of Fe(II) to Fe(III), which facilitates transport
to sites of red blood cell formation. Several cuproenzymes are important for normal
function of the central nervous system including dopamine beta-hydroxylase and
monamine oxidase. Tyrosinase is required for the formation of the pigment melanin.
Requirements
Plants In general, critical Cu levels in vegetative growth lie in the range of 1-5 mg/kg
dry matter (Marschner, 1995), but diff erent species may have diff erent requirements
(Reuter and Robinson, 1997; Dell et al., 2001). Plant reproduction may have higher Cu
requirements than vegetative growth.
Animals and humans Th e diet of cattle (pasture, range, hay, etc.) should contain about
4-10 mg Cu/kg to supply the needs of cattle. Less than this amount may result in Cu
defi ciency. Th e absorption of Cu is sensitive to the presence of dietary antagonists,
particularly Fe, Mo and sulphur (S). Excessive Mo can induce secondary Cu defi ciency
by combining with Cu and S in the rumen to form the insoluble product, copper
thiomolybdate. Concentrations as low as 0.5 mg Mo/kg dry matter (DM) in pasture

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may have a signifi cant eff ect on Cu absorption, much lower than earlier estimates (Lee
et al., 1999).
Th e Recommended Dietary Allowance (RDA) established by the US Food and
Nutrition Board (USFNB, 2001) for children aged 4-8 is 440 μg Cu/day, and 900 μg
Cu/day for males and females 19 years and older. Dietary factors known to inhibit Cu
absorption in humans include ascorbic acid, phytates, sucrose, fructose, Zn and Fe, but
the eff ects are only seen at very high intake levels and are thought to be of little practical
signifi cance in normal diets. Th e possibility exists however, that some factors may aff ect
Cu absorption from atypical diets e.g. in countries where the diets have a high phytate
component or when taking Zn supplementation (USFNB, 2001).
Defi ciency symptoms
Plants Th e most typical anatomical change induced by Cu defi ciency is the distortion
of young leaves, stem bending and twisting and lodging (in cereals) (Marschner, 1995).
Leaf chlorosis can occur at the onset of Cu defi ciency followed by twisting and cupping
in young leaves and leaf death (Grundon et al., 1997). In fruit trees, an early sign of Cu
defi ciency is the pendulous habit of lateral branches and during reproduction, increased
production of sterile pollen (Grundon et al., 1997). In timber trees, snake-shaped trunks
and sparse canopies are common early signs of Cu defi ciency (Dell et al., 2001).
Animals and humans Copper defi ciency is well known in farm animals. Symptoms
include loss of coordination of the hind limbs in lambs due to impaired development
of the central nervous system, sudden death in cattle due to heart failure, and depigmentation
of the hair around eyes in cattle and of black wool in sheep (McDowell,
2003). Poorly crimped wool in sheep, poor weight gains, diarrhoea and fragile bones
occur but are not specifi c to Cu defi ciency. Copper defi ciency lowers the immune
response and makes animals more susceptible to disease. Liver Cu concentration is one
of the best indicators of Cu status and blood serum levels are generally only useful
where defi ciency is severe.
Copper defi ciency in humans is rare (Turnlund, 1999) and most cases have been
described in malnourished children. Th e most frequent clinical manifestations of Cu
defi ciency are anemia, neutropenia (low white blood cell count) and bone abnormalities.
Less common symptoms can include abnormal glucose tolerance, loss of pigmentation
and neurological problems (Uauy et al., 1998). Cow’s milk is relatively low in Cu, and
cases of Cu defi ciency have been reported in high-risk infants and children fed only
cow’s milk formula.
Iron
Functions
Plants Iron is essential for many biochemical and physiological processes in plants
including the utilization of N and S, the production of the plant hormone ethylene,
the biosynthesis of chlorophyll and the composition of some cell walls. Iron(II) is an
electron donor and hence is a key constituent of electron-transport chains. Iron is stored

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as Fe(III), mostly as phytoferritin, and is reduced to Fe(II) for physiological function in
the cell. Iron is generally incorporated into heme and non-heme proteins. Th e most
well-known heme proteins are the cytochromes, which contain a heme Fe-porphyrin
complex (Marschner, 1995).
In legume nodules, a related class of compounds, the leghemoglobins, regulate
the supply of oxygen to the bacteroids responsible for fi xation of N. Th e bacterial
nitrogenase enzyme, which reduces N2 to NH3, consists of two metalloproteins, the
Fe protein (dinitrogenase reductase) and the FeMo protein (dinitrogenase) (Bosch and
Imperial, 2000).
Other heme proteins are involved in the formation of lignin and suberin (peroxidases),
and the breakdown of H2O2 to water and oxygen (catalases). Th e most common nonheme
proteins contain Fe-S clusters, which serve as cofactors for redox, catalytic and
regulatory functions. Examples of Fe-S proteins are ferredoxin and Fe-SOD. Ferredoxin
is an electron carrier assisting enzymes involved in the reduction of nitrite (NO2
-) and
sulphite (SO3
2-), and biological N2 fi xation, and is an essential component of the electron
transport pathway in photosynthesis.
Animals and humans Iron is essential for humans and animals. It plays a central role
in metabolic processes involving oxygen transport and storage as well as oxidative
metabolism and cellular growth. About 85 % of body Fe is a constituent of two heme
proteins: hemoglobin, essential for transferring oxygen in the blood from the lungs
to tissues; and myoglobin, the oxygen store in muscles. A number of other heme
proteins are enzymes and include the cytochromes involved in energy production
in the mitochondria, and peroxidases that degrade reactive by-products of oxygen
metabolism.
Non-heme proteins can store and transport Fe (ferritin, transferritin) or function as
enzymes (metallofl avoproteins, Fe-S proteins, ribonucleotide reductase). Examples of
Fe-S proteins are NADH dehydrogenase and succinate dehydrogenase that play roles in
energy metabolism (Yip and Dallman, 1996). In adult men, about one third of the total
body Fe is stored Fe, whereas, in women, storage accounts for about one eighth of total
body Fe (Yip and Dallman, 1996). Dietary Fe overload can cause acute Fe poisoning as
the body has no adjustable Fe excretory mechanism (Lynch, 2003a).
Requirements
Plants Critical defi ciency concentrations for Fe in leaves typically range from 50–150
mg Fe/kg although levels may be marginally greater in C4 plants (Marschner, 1995). Iron
defi ciency is common on calcareous and high pH soils due to impaired Fe acquisition
commonly known as ‘lime-induced chlorosis’. Diagnosis of Fe defi ciency by plant
analysis is limited, since there is oft en no relationship between total leaf Fe content and
defi ciency symptoms. Moreover, Fe concentrations in chlorotic leaves are frequently
greater than those in healthy green leaves. Th is phenomenon, known as the ‘chlorosis
paradox’, is caused by inactivation of Fe in the leaf.

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Animals and humans Hemoglobin accounts for approximately 60 % of body Fe. For
this reason, the Fe requirements of livestock vary with age and level of activity. Iron
requirements decline with age because as animals grow, increase in red-cell mass
constitutes a progressively smaller component of weight gain. Supplementation of prestarter
and starter diets is widely used in the livestock industry as milk alone is oft en
not suffi cient to supply Fe. Non-working animals may not be adversely aff ected by mild
anaemia, however, it is recommended that Fe levels be maintained within adequate
limits as defi ciency may lead to increased absorption of potentially harmful elements
such as Cd and Pb (Underwood and Suttle, 1999).
For humans, RDAs have been estimated based on the need to maintain a normal,
functional Fe concentration, but only a minimal store. Th e USFNB (2001) gives a RDA
of 10 mg Fe/day for children (4-8 years), 8 mg/day for adult (31-50 years) males and 18
mg/day for females. RDAs are based on an estimated Fe bioavailability of 18 % in the
average North American diet.
Iron bioavailability is largely determined by the two main dietary Fe forms: heme
and non-heme. Heme Fe is readily bioavailable and dietary factors have little eff ect on
absorption. Non-heme Fe on the other hand is absorbed through a separate pathway,
which requires the reduction of the predominantly Fe(III) to the Fe(II) form. Reduction
and subsequent absorption can be greatly increased by the presence of ascorbic acid
(Vitamin C) in the diet. Conversely, phytates and phenolic compounds can inhibit
absorption through the formation of stable ferric complexes. Th e main sources of heme
Fe are haemoglobin and myoglobin in red and white meats and fi sh. Non-heme Fe is
present in plant and dairy products.
Defi ciency symptoms
Plants Iron defi cient leaves typically develop interveinal chlorosis and, when defi ciency
is acute, the whole leaf takes on a bleached appearance and necrotic lesions may develop
(see Chapter 13). Because of restricted phloem mobility of Fe, symptoms usually progress
from young to mature leaves. Examples of symptoms are illustrated in Bergmann (1992)
and Dell et al. (2001). Iron defi ciency is particularly severe on calcareous soils.
Animals and humans Iron defi ciency seldom occurs in farm animals as there is usually
adequate Fe intake in the diet (Underwood and Suttle, 1999). However, Fe defi ciency can
occur in animals reared in confi nement primarily on milk or due to severe loss of blood
caused by parasitic infestations, injury or disease. Symptoms of Fe defi ciency include
anemia (inadequate number of red blood cells), poor growth, listlessness, laboured
breathing aft er mild exercise, rough hair coat and paleness of mucous membranes
(McDowell, 2003).
A lack of Fe is the most common nutritional disorder in humans, with estimates
ranging from 500 million (Lynch, 2003a) to 2 billion (Stolzfus and Dreyfuss, 1998)
people aff ected worldwide. Iron defi ciency is most prevalent in the developing world.
According to EVM (2002), people most vulnerable to Fe defi ciency are: infants greater
than six months, toddlers, adolescents and pregnant women due to high requirements;
the elderly and people consuming foods high in inhibitors of Fe absorption; and

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menstruating women or individuals with pathological blood loss due to high Fe losses.
Iron defi ciency results in anemia (Lynch, 2003b).
Th e main clinical symptoms include pallor, fatigue, weakness, dizziness, reduced
intellectual performance and reduced maximal work capacity (Yip and Dallman, 1996;
Lynch, 2003a). Iron-defi ciency anemia is associated with impaired mental development
and physical coordination in children under the age of 2 years (Beard and Connor,
2003; Lynch, 2003a). Behavioural changes include reduced attention span and reduced
emotional responsiveness (Hulthen, 2003).
Manganese
Functions
Plants Manganese is essential for photosynthesis in all plants as the Mn4Ca cluster
(Rutherford and Boussac, 2004) is part of the catalytic centre for the light-driven water
oxidation in photosystem II, commonly known as the Hill reaction. Th e main enzyme
known to contain Mn is SOD. Manganese is an activator of a large number of enzymes
that catalyse oxidation-reduction, decarboxylation and hydrolytic reactions. Th rough
these enzymes, Mn plays a role in the production of lignin, fl avonoids, fatty acids, indole
acetic acid and other pathways. It can also aff ect N metabolism. In C4 plants (e.g. maize)
and plants that fi x their carbon at night, Mn is essential for CO2 assimilation because it
activates the enzyme phosphoenylpyruvate carboxylase (Mengel and Kirkby, 2001).
Animals and humans Like in plants, Mn functions as a constituent of metalloenzymes
and as an enzyme activator in animals and humans (Nielsen, 1999; Keen and
Zidenberg-Cherr, 1999). Th e metalloenzymes include: Mn-SOD; pyruvate carboxylase,
which is required for gluconeogenesis (carbohydrate synthesis from pyruvate); and
arginase, which is required by the liver for urea formation. Th ere are a large number of
enzymes, which are activated by Mn, either by the Mn binding to the protein or to the
substrate: glycosyltransferases are required for the synthesis of proteoglycans in bone
and cartilage; phosphoenolpyruvate carboxykinase is required for gluconeogenesis;
glutamine synthetase is required for the assimilation of NH3 and in N metabolism; and
prolidase produces proline for collagen in skin.
Requirements
Plants Th e critical defi ciency concentrations for Mn vary little among plant species.
Typical values range between 10 to 20 mg/kg in fully expanded leaves (Marschner, 1995),
with the exception of narrow-leaf lupin, which may be up to double this (Hannam and
Ohki, 1988). Defi ciencies commonly occur on soils derived from parent materials low
in Mn, highly leached tropical soils and alkaline soils, particularly when combined with
high levels of organic matter.
Animals and humans Manganese requirements of sheep and cattle are quite low, and
pastures rarely fail to meet requirements. Manganese defi ciency can be a problem in
domestic animals such as pigs, poultry and non-grazing ruminants due to protein

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supplements being low in Mn. Inorganic supplements such as MnSO4, MnO and
MnCO3 are commonly used for the treatment and prevention of defi ciency.
Th e human requirement for Mn is very low, and even during prolonged total
parenteral nutrition (TPN) no clear evidence of defi ciency has emerged. However,
because of the potential importance of Mn, additives containing this element have been
included in TPN regimens.
Widespread Mn defi ciency has not been shown to occur in humans eating natural
diets. USFNB has set an adequate intake level based on average dietary Mn intakes,
which in the USA range from 2.1-2.3 mg/day for men and 1.6-1.8 mg/day for women.
Rich sources of Mn include whole grains, nuts, leafy vegetables and teas. Absorption of
Mn is reduced by the presence of phytate, oxalate and tannins.
Defi ciency symptoms
Plants Th ough symptoms of Mn defi ciency vary among crop species, the most
common are interveinal chlorosis and discoloured spots (Bergmann, 1992; see Chapter
13). Symptoms progress from young to mature foliage. Reduced lignifi cation of the
xylem can lead to wilting of leaves and impaired wood development. Other symptoms
observed include split seed in lupins, impaired pollen development, delayed maturity
and reduced yields. Defi cient plants may show increased susceptibility to disease
(Graham and Webb, 1991).
Animals and humans Manganese defi ciency has not been reported for humans. In
experimental animals, dietary Mn defi ciency can result in numerous biochemical and
structural abnormalities. Defi cient animals can be characterized by impaired insulin
production, alterations in lipoprotein metabolism, an impaired oxidant defence system,
and perturbations in growth factor metabolism. If the defi ciency occurs during early
development, there can be pronounced skeletal abnormalities and abnormal inner ear
development (Keen et al., 1999). Th e most common symptoms observed in livestock are
impaired reproductive performance, skeletal deformities and shortened tendons in the
new born (McDowell, 2003).
Molybdenum
Functions
Plants Molybdenum, when inserted as part of a prosthetic group known as the Mocofactor,
is required for the function of a few enzymes involved in redox processes
(Mendel and Haensch, 2002; Sauer and Frebort, 2003). Nitrate reductase is essential
for the reduction of NO3 before it can be assimilated in plants. Aldehyde oxidase is
involved in the synthesis of the hormones indole-3-acetic acid and abscisic acid.
Xanthine dehydrogenase is required for purine catabolism and the synthesis of ureides
in soybean and cowpea (Marschner, 1995). Molybdenum is also required by bacteria in
root nodules as a component of the Mo-Fe sub-unit of nitrogenase, which is essential
for biological N2 fi xation.