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III. Physiological Role of Molybdenum in Plants

III. Physiological Role of Molybdenum in Plants

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tion of NO,- and in the fixation of N,, respectively. This section will include a

discussion of these two enzymes (molybdoproteins) as they function in the plant


The first molybdoprotein, nitrate reductase, is known to require Mo and flavin

for its activity and in the reduction of NO,- to No,- as follows:

Reduced N A D

+ NO,-


+ NO,- + H,O


where NAD is nicotinamide adenine dinucleotide. The reduction mechanism

from NO,- to NOz- has been proposed by Nicholas (1975) as


+ 2H' + 2 e - + NO,- + H,O

+ H+ + 2e- -+ NO,- + OH-

Reaction (2) is based on the acidic half reaction, whereas reaction (3) allows for

OH- participation at physiological pH.

Nitrate reductase is found in most plant species as well as fungi and bacteria

(Price et al., 1972). The increased Mo requirement of most plants grown on

No,--N compared with NH,+-N can be almost completely accounted for by the

Mo in nitrate reductase (Evans, 1956).

The other major known molybdoprotein of plants, nitrogenase, fixes elemental

nitrogen in the form of NH,, which is then assimilated by the plant (Koch ef al.,

1967). The role of Mo in the fixation of N, has been reviewed in detail by Chatt

(1974). The unique role of Mo in biological systems is exemplified by nitrogenase, the enzyme that converts N, into NH, at room temperature and normal

pressure (Schrauzer, 1976). The nitrogenase is an enzyme complex composed of

two distinct components that combine to reduce N, to NH, [reaction (4)] or

acetylene to ethylene [reaction (5)] (Nicholas, 1975):

+ 6H+ + 6 e C,H, + 2H+ + 2eN,







Nitrogenases have been isolated from a variety of different sources, for example, from Azotobacter vinelandii, Rhizobium japonicum, Azotobacter chroococcum, and Klebsiella pneumonianum (Schrauzer, 1976).

Recent studies by Agarwala et al. (1978) have shown that in addition to

reduced nitrate reductase activity, Mo deficiency in corn resulted in significantly

lower activities of catalase, aldolase, and alanine aminotransferase and higher

activities of peroxidase, P-glycerophosphatase, and ribonuclease.

In addition to the involvement of Mo in the fixation of N, and nitrate reduction, Mo is associated with other processes in plants. However, many of these

processes are interrelated with the two main functions. For example, experiments

of Malonosova (1 968) showed that addition of Mo to the soil resulted in better

development of lupine (Lupinus spp.) and increased weight of its roots and

nodules although the N, was not fixed. In this review of Russian work it was



reported that on soils containing little Mo, plants will develop many nodules on

roots, but N, is not fixed.

Merkel et al. (1975) showed that Mo deficiency in tomatoes decreased organic

nitrogen content of the leaves to the same degree that it decreased the organic

anion content of the leaves. This change was mainly in the contents of malate and

citrate. Studies by Anderson and Spencer (1950) showed that deficiency of Mo

decreased both the protein nitrogen and the nonprotein nitrogen percentages in

the clover. The percentage crude protein in a number of plant species has been

found to increase with optimum rates of Mo applied to the soil (Reddy, 1964).

Hagstrom and Berger (1965) found that applications of Mo increased the nodulation and nitrogen content of peas and soybeans. The effect of Mo in plants is to

increase the content of proteins and to create favorable conditions for the biosynthesis of nucleic acids (Peyve, 1969).

Inden (1975) stated that deficiency of Mo can cause plant chlorosis, which is

due to the inability of the plant to form chlorophyll. Further, since the deficiency

of Mo reduces the rate of NO3- reduction in plants, photosynthesis decreases

because the end products are not removed by combination with nitrogenous

compounds. Das Gupta and Basuchaudhuri (1977) found that Mo applications

enhanced the nitrate reductase activity in the rice (Oryza sativa L.) plant, particularly under the high-nitrogen nutrition. This ultimately led to greater concentration of reduced nitrogen and thereby created a concentration gradient for the

uptake and subsequently greater assimilation of nitrogen in the tissues as

suggested by Wardlaw (1968). This suggests that there exists a close functional

relationship between the nitrate reductase activity and chlorophyll content as

observed by Das Gupta and Basuchaudhuri (1977). The chlorophyll content of

corn has been found to decrease due to a deficiency of Mo (Agarwala et al.,

1978). Molybdenum was also considered to be associated with the metabolism of

Fe and phosphoric acid (Inden, 1975).

Premature sprouting of maize grain on the cob was shown to be controlled in

glasshouse and field experiments by the Mo concentration of grain (Tanner,

1978). It was found that when the Mo concentration of grain fell below 0.05

ppm, sprouting occurred, the severity of which was enhanced by heavy, late

sidedress of nitrogenous fertilizer. The explanation offered was that Mo-deficient

plants have a decreased ability to reduce NO3- and consequent accumulation of

inorganic NO3- promotes sprouting.

Under Mo deficiency, plants accumulate low-molecular-weight nitrogen compounds and may have defective vascular bundles in the midrib in young leaves.

There is a deposition of a brown substance on tissue surface and in intercellular

spaces and cells (Busslar, 1970).

Hewitt (195 1 ) pointed out that a low concentration of ascorbic acid in tissues is

a characteristic of Mo deficiency in a number of species. Agarwala (1952) also

demonstrated that cauliflower plants grown with various nitrogen sources, in-



cluding urea and ammonium sulfate, developed characteristic Mo deficiency

symptoms known as “whiptail” and contained reduced concentrations of ascorbic acid. Subsequent studies by Agarwala and Hewitt (1955) showed that Mo

deficiency decreased the total and reducing sugars in the leaves of young cauliflower plants.

In nutrient culture studies in flax (Linurn usitatissirnurn L.), Mo has been

found to be closely associated with the regulation of the deleterious effect of Mn,

Zn, Cu, Ni, or Co on the physiological availability of Fe to the plant (Millikan,




Thiocyanate and dithiol are the two most commonly used reagents employed

in the colorimetric determination of Mo from biological materials. The dithiol

method used by Piper and Beckwith (1948), Clark and Axley (1955), and

Bingley (1963) for plants and soils has been found to be more sensitive and

precise than the thiocyanate method because the green-colored complex formed

between Mo and dithiol was stable for at least 24 hours (Gupta and MacKay,

1965a). Later, Fuge (1970) developed a rapid and simple method in which a

Technicon AutoAnalyzer is used for the determination of Mo on the basis of its

catalytic action on the potassium iodide-hydrogen peroxide reaction. Fernandez

et al. (1978) used 2,2-dihydroxybenzophenone reagent and considered it to be

superior to the most commonly used Mo-thiocyanate complex method because

an extra step is not necessary and the results are more reproducible. Molybdenum

has also been determined spectrochemically after chemical concentration, using

the cathode-layer arc technique (Mitchell, 1974) and polarography (Dekhkankhodzhayeva and Kruglova, 1972). Trace quantities of Mo have been determined

by atomic absorption spectroscopy (AAS), both flameless (Henning and Jackson,

1973; Jarrel and Dawson, 1978) and flame (Khan et al., 1979). Little and

Kemdge (1978) used a carbon rod analyzer for determining very low levels of

Mo. The high temperature required to atomize Mo in this procedure makes it

easy to remove matrix materials during the ashing phase.

The colorimetric method using dithiol and the most recently used AAS are

probably the most commonly used techniques for determining Mo in soil and

plant materials. The detection limits for determination of Mo by AAS using

flame and graphite furnace have been found to be 10 and 2 ng/ml, respectively

(Khan et al., 1979). Using the dithiol colorimetric method (Gupta and MacKay,

1965a), the satisfactory detection limit is about 20 nglml. The recovery of Mo

added to the plant material as determined by these two methods has been found to

range from 92 to 95%.






The most common method for extracting Mo from soils is by perchioric acid

digestion (Reisenauer, 1965). Dry ashing of soil and the extraction of ash using

concentrated acids was employed for determining total Mo in soils by Perrin

(1946) and Grigg (1953a). Total Mo has also been extracted by Na&O, fusion of

soil (Purvis and Peterson, 1956). Unpublished data of the first author of this

article showed that such extracts contained large quantities of interfering materials and required purification, which is time consuming. Little and Kerridge

(1978) used HF-HC104 digestion for determining total Mo in soils.

As for other plant nutrients, total Mo content of soils, except for very low

levels, is generally not a good indicator of plant Mo availability (Little and

Kemdge, 1978; Williams, 1971). Available Mo content has not been found to

be closely related to the total Mo content of soils (Stone and Jencks, 1963).

However, soil with a total Mo content of more than 20 ppm may be regarded as

potentially “teart” (producing Cu deficiency in animals) in Scotland (Williams,

1971). Soils with low total Mo and neutral to alkaline pH may be depleted by

many years of intensive cropping (Davies, 1956). Liming can correct Mo deficiency; therefore an estimate of total Mo content may provide some indication of

the Mo supplying power of acid soils. Details of the effect of liming on Mo

availability will be dealt with in Section VI,B.

Little information exists on the levels of Mo in various soils but, in general,

contents of 0.5-5 ppm are normal (Robinson and Alexander, 1953; Williams,

1971) and in agreement with the relative abundance of Mo in the lithosphere (2.3

ppm), whereas figures of 0.5 pprn or less would be considered particularly low

(Williams, 1971). The Mo content of a few soils selected from areas of Canada

close to industrial plants ranged from 1 .O to 11.3 ppm (Warren, 1973). MacLean

and Langille (1973) reported that the Mo content of Nova Scotia (Canada) podzol

soils ranged from 0.05 to 12.1 ppm.




The presence of extremely small quantities of Mo in the soil, the influence of

chemical characteristics of soils (Karimian and Cox, 1979), the importance of

seed reserves (Gurley and Giddens, 1969), and the possibility that seed reserves

may mask a deficiency in the soil make the problem of determining Mo

availability more difficult than for the other micronutrients. The first report on

the available Mo in soils, which related extracted Mo to plant uptake, was by

Grigg (1953b) in New Zealand. This involved an acid oxalate extractant buffered

at pH 3.3. The responses and lack of responses as related to Mo extracted by

Grigg’s reagent for a number of crops have been summarized by Reisenauer

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