Plants absorb molybdenum as molybdate. The form in which molybdenum is translocated is unknown. Molybdenum is located primarily in the phloem and vascular parenchyma and is only moderately mobile in the plant. The requirement of molybdenum in terms of dry matter is usually in the range from 0.1–1.0 ppm. Most plants are very tolerant of excessive amounts of molybdenum in the tissue with levels above 1000 ppm existing without any harmful effects. A unique feature of molybdenum nutrition is the wide variation between the critical deficiency and toxicity levels. These levels may differ by a factor of up to 104 (e.g., 0.1 to 1000 ppm dry weight).
Molybdenum is an essential component of two major enzymes in plants, nitrogenase and nitrate reductase. Nitrate reductase is the most well-studied molybdenum-containing enzyme. It catalyzes the reduction of NO3- to NO2-.