Molybdenum deficiency (plant disorder) on:
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Molybdenum (Mo) deficiency occurs when plant growth is limited because the plant cannot take up sufficient quantities of this essential micronutrient from its growing medium. For crops growing in soil, this may be a result of low concentrations of Mo in the soil as a whole (i.e. the parent material of the soil is low in Mo), or because the soil Mo is held in forms that are not available to plants – sorption of Mo is strongest in 
acid soils
Soil pH is a measure of the acidity or basicity (alkalinity) of a soil. Soil pH is a key characteristic that can be used to make informative analysis both qualitative and quantitatively regarding soil characteristics. pH is defined as the neg ...
.
Functions
In plants, the primary functions of Mo are related to the valence changes that it undergoes a component ofenzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
s that catalyze diverse redox
Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate (chemistry), substrate change. Oxidation is the loss of Electron, electrons or an increase in the oxidation state, while reduction ...
reactions. This is because the element can exist in a variety of oxidation states; oxidation states IV, V, and VI are important in biological systems. A large number of Mo-deficiency disorders are associated with the disruption of the normal activity of several enzymes involved in nitrogen metabolism
The nitrogen cycle is the biogeochemical cycle by which nitrogen is converted into multiple chemical forms as it circulates among atmospheric, terrestrial, and marine ecosystems. The conversion of nitrogen can be carried out through both biologi ...
.
These enzymes include:
* Nitrogenase, which is required for biological fixation by both asymbiotic and symbiotic nitrogen-fixing bacteria
Nitrogen fixation is a chemical process by which molecular nitrogen (), with a strong triple covalent bond, in the air is converted into ammonia () or related nitrogenous compounds, typically in soil or aquatic systems but also in industry. Atmos ...
;
* Nitrate reductase, which is required for the reduction of nitrate – this is necessary for the incorporation of in proteins; and
* Xanthine dehydrogenase, which is involved in the synthesis of uric acid from purines.
Symptoms
Molybdenum deficiency symptoms in most plants are associated with a build-up of nitrate in the affected plant part. This is a result of poornitrate reductase
Nitrate reductases are molybdoenzymes that reduce nitrate (NO) to nitrite (NO). This reaction is critical for the production of protein in most crop plants, as nitrate is the predominant source of nitrogen in fertilized soils.
Types
Euka ...
activity. Symptoms include:
* pale leaves with interveinal and marginal chlorosis
In botany, chlorosis is a condition in which leaves produce insufficient chlorophyll. As chlorophyll is responsible for the green color of leaves, chlorotic leaves are pale, yellow, or yellow-white. The affected plant has little or no ability to ...
(yellowing) and necrosis
Necrosis () is a form of cell injury which results in the premature death of cells in living tissue by autolysis. Necrosis is caused by factors external to the cell or tissue, such as infection, or trauma which result in the unregulated dige ...
(scald);
* the whiptail disorder in ''Brassica
''Brassica'' () is a genus of plants in the cabbage and mustard family (Brassicaceae). The members of the genus are informally known as cruciferous vegetables, cabbages, or mustard plants. Crops from this genus are sometimes called ''cole cr ...
'' crops (especially cauliflower);
* decreased tasselling in maize;
* premature germination of maize grain.
In legumes, inhibition of N2 fixation may lead to pale, yellowing, nitrogen-deficient plants. The size and number of root nodules are often reduced.
Soil conditions
Molybdenum deficiency is common in many different types of soil; some soils have low total Mo concentrations, and others have low plant-available Mo due to strong Mo sorption. Symptoms are most common where both conditions apply, such as in acid sandy soils. Molybdenum may be strongly sorbed in ironstone soils. Liming of soils frequently relieves Mo deficiency by decreasing Mosorption
Sorption is a physical and chemical process by which one substance becomes attached to another. Specific cases of sorption are treated in the following articles:
; Absorption: "the incorporation of a substance in one state into another of a dif ...
.
Molybdenum requirements
Molybdenum is an essential micronutrient which means it is essential for plant growth and development, but is required in very small quantities. Although Mo requirements vary among crops, Mo leaf concentrations (on a dry matter basis) in the range are adequate for most crops.Treatment
Raising the soil pH by liming frequently relieves Mo deficiency. However, there are many situations where a soil-, seed- or foliar application of a Mo fertilizer is far more cost-effective than the use of lime to increase Mo availability. Compounds used as fertilizers include (in order of decreasing solubility): sodium molybdate, ammonium molybdate, molybdic acid, molybdenum trioxide, and molybdenum sulfide. Typical soil and foliar application rates are ; recommended rates for seed treatment range from .References
{{DEFAULTSORT:Molybdenum Deficiency (Plant Disorder) Physiological plant disorders Molybdenum