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  Section: Plant Nutrition » Micronutrients » Molybdenum
 
 
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Excess

 
     
 
Content
Historical Information
  Determination of Essentiality
  Function in Plants
    - Nitrogenase
    - Nitrate Reductase
    - Xanthine Dehydrogenase
    - Aldehyde Oxidase
    - Sulfite Oxidase
Diagnosis of Molybdenum Status of Plants
  Deficiency
  Excess
  Molybdenum Concentration and Distribution in Plants
  Analytical Techniques for the Determination of Molybdenum in Plants
Assessment of Molybdenum Status of Soils
  Soil Molybdenum Content
  Forms of Molybdenum in Soils
  Interactions with Phosphorus and Sulfur
  Soil Analysis
    - Determination of Total Molybdenum in Soil
    - Determination of Available Molybdenum in Soil
Molybdenum Fertilizers
  Methods of Application
    - Soil Applications
    - Foliar Fertilization
    - Seed Treatment
  Crop Response to Applied Molybdenum
References

Most plants are not particularly sensitive to excessive molybdenum in the nutrient medium, and the critical toxicity concentration of molybdenum in plants varies widely. For instance, molybdenum is toxic to barley (Hordeum vulgare L.) if leaf tissue levels exceed 135 mg Mo kg-1 (40), but crops such as cauliflower and onion (Allium cepa L.) are able to accumulate upwards of 600 mg Mo kg-1 without exhibiting symptoms of toxicity (41). However, tissue concentrations >500 mg Mo kg-1 can lead to a toxic response in many plants (42), which is characterized by malformation of the leaves, a golden-yellow discoloration of the shoot tissues (9), and inhibition of root and shoot growth (43). These symptoms may, in part, be the result of inhibition of iron metabolism by molybdenum in the plant (12).

Toxicity symptoms in plants under field conditions are very rare, whereas toxicity to animals feeding on forages high in this element is well known (44). A narrow span exists between nutritional deficiency for plants and toxicity to ruminants (45). Molybdenum concentrations >10 mg Mo kg-1 (dry mass) in forage crops can cause a nutritional disorder called molybdenosis in grazing ruminants (9). This disorder is a molybdenum-induced copper deficiency that occurs when the consumed molybdate (MoO42-) reacts in the rumen with sulfur to form thiomolybdate complexes, which inhibit copper metabolism (46).



Agricultural practices that can be used to decrease ruminant susceptibility to molybdenosis include field applications of copper and sulfur. The strong depressive effects of SO42- on MoO42- uptake can lower the molybdenum concentration in plants to levels that are nontoxic (47). Increasing the copper content of forages through fertilization may also help to reduce molybdenuminduced copper deficiency in animals (46).
 
     
 
 
     



     
 
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