Molybdenum was discovered in 1778 by the Swedish chemist, Carl Wilhelm Scheele. However, its importance in biological systems was not established until 1930 when Bortels discovered that molybdenum was essential for the growth of Azotobacter bacteria in a nutrient medium (1). Subsequently in 1936, Steinberg determined that molybdenum was required for the growth of the fungus Aspergillus niger (2).
In their studies, Arnon and Stout tested the molybdenum requirement of tomato (Lycopersicon esculentum Mill.) by their newly established criteria for essentiality (6). These criteria were (a) a deficiency of the essential element prevents plants from completing their life cycles; (b) the requirement is specific to the element, the deficiency of which cannot be prevented by any other element; and (c) the element is involved directly in the nutrition of plants. Plants grown in purified solution cultures developed deficiency symptoms in the absence of molybdenum, and symptoms were prevented by adding the equivalent of 0.01 mg Mo L-1 to the root medium (6). Normal growth was restored to deficient plants if molybdenum was applied to the foliage, thereby establishing that molybdenum exerted its effect directly on growth and not indirectly by affecting the root environment.
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