Excess

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 (MoO₄^2-) 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 SO₄^2- on MoO₄^2- 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).