FUNCTION IN PLANTS
The transition element molybdenum is essential for most organisms and occurs in more than 60 enzymes catalyzing diverse oxidation-reduction reactions . Although the element is capable of existing in oxidation states from 0 to VI, only the higher oxidation states of IV, V, and VI are important in biological systems. The functions of molybdenum in plants and other organisms are related to the valence changes that it undergoes as a metallic component of enzymes .
 With the exception of bacterial nitrogenase, molybdenum-containing enzymes in almost all organisms share a similar molybdopterin compound at their catalytic sites . This pterin is a molybdenum cofactor (Moco) that is responsible for the correct anchoring and positioning of the molybdenum center within the enzyme so that molybdenum can interact with other components of the electron-transport chain in which the enzyme participates . Molybdenum itself is thought to be biologically inactive until complexed with the cofactor, Moco.
 Several molybdoenzymes including nitrogenase, nitrate reductase, xanthine dehydrogenase, aldehyde oxidase, and possibly sulfite oxidase are of significance to plants. Because of its involvement in the processes of N2  fixation, nitrate reduction, and the transport of nitrogen compounds in plants, molybdenum plays a crucial role in nitrogen metabolism of plants

DIAGNOSIS OF MOLYBDENUM STATUS OF PLANTS

DEFICIENCY
The discovery of molybdenum as a plant nutrient led to the diagnosis of the deficiency in a number of crop plants, with the first report of molybdenum deficiency in the field being made by Anderson  for subterranean clover (Trifolium subterraneum L.). The critical deficiency concentration in most crop plants is quite low, normally between 0.1 and 1.0 mg Mo kg  1  in the dry tissue . Symptoms of molybdenum deficiency are common among plants grown on acid mineral soils that have low concentrations of available molybdenum, but plants may occasionally become deficient in peat soils due to the retention of molybdenum on humic acids . Plants also may be prone to molybdenum deficiency under low temperatures and high nitrogen fertility

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  , 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 . However, tissue concentrations 500 mg Mo kg  1 can lead to a toxic response in many plants , which is characterized by malformation of the leaves, a golden-yellow discoloration of the shoot tissues , and inhibition of root and shoot growth . These symptoms may, in part, be the result of inhibition of iron metabolism by molybdenum in the plant .