Historical Information

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).

The essential nature of molybdenum for higher plants was first reported by Arnon and Stout in 1939 (3). In earlier experiments, Arnon observed that minute amounts of molybdenum improved the growth of plants in solution culture (4), and that a group of seven heavy metals, including molybdenum, increased the growth of lettuce (Lactuca sativa L.) and asparagus (Asparagus officinalis L.) (5). Prior to these studies (conducted in 1937 and 1938, respectively) only boron, copper, iron, manganese, and zinc were considered to be micronutrients. The observation that plant growth was improved by elements other than these led Arnon to believe that the list of essential elements was incomplete, and prompted him to test whether or not molybdenum was essential for the growth of higher plants (3).

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