Mo is absorbed as molybdate (MOO2-4) ion by plants. Its uptake can be competitively reduced by SO2-4. The requirement of plants for Mo is very low. Mo is an essential component of two major enzymes, nitrogenase and nitrate reductase. The functional mechanism of both enzymes probably depends on valency changes of Mo.
The most important function of Mo in plant metabolism is in N assimilation. Mo deficiency resembles N-deficiency. Older leaves becoming chlorotic first, but in contrast to N-deficiency, necrotic symptoms rapidly appear at the leaf margins because of nitrate accumulation. In Cruciferae, in extreme deficiency of Mo, leaf laminae are not formed and only the mid rib is formed. The leaf thus appears like a whip and for this reason the deficiency is called 'Whip tail'.