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 (29) 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 (12). 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 (19,30). Plants also may be prone to molybdenum deficiency under low temperatures and high nitrogen fertility (31).

Because molybdenum is highly mobile in the xylem and the phloem (32), its deficiency symptoms often appear on the entire plant. This appearance is unlike many of the other essential micronutrients where deficiency symptoms are manifest primarily in younger portions of the plant. Molybdenum deficiency is peculiar in that it often manifests itself as nitrogen deficiency, particularly in legumes. These symptoms are related to the function of molybdenum in nitrogen metabolism, such as its role in N2 fixation and nitrate reduction. However, plants suffering from extreme deficiency often exhibit symptoms that are unique to molybdenum.

Legumes often require more molybdenum than other plants, particularly if they are dependent on N2 as a source of nitrogen (9). Molybdenum-deficient legumes commonly become chlorotic, have stunted growth, and have a restriction in the weight or quantity of root nodules (33,34). In dicotyledonous species, a drastic reduction in leaf size and irregularities in leaf blade formation (whiptail) are the most typical visible symptoms, caused by local necrosis in the tissue and insufficient differentiation of vascular bundles at an early stage of leaf development (35). Marginal and interveinal leaf necrosis is a symptom of extreme molybdenum deficiency, and symptoms are often associated with high nitrate concentrations in the leaf, indicating that nitrate reductase activity is impaired (12).

The whiptail disorder is observed often in molybdenum-deficient cauliflower (Brassica oleracea var. botrytis L.), one of the most sensitive cruciferous crops to low molybdenum nutrition (36). In addition, molybdenum-deficient beans (Phaseolus vulgaris L.) often develop scald, where the leaves are pale with interveinal and marginal chlorosis, followed by burning of the leaf margin (36,37). In molybdenum-deficient tomatoes, lower leaves appear mottled and eventually cup upward and develop marginal necrosis (3). Molybdenum deficiency also decreases tasseling and inhibits anthesis and pollen formation in corn (Zea mays L.) (38). The inhibition of pollen formation with molybdenum deficiency may explain the lack of fruit formation in molybdenum-deficient watermelon (Citrullus vulgaris Schrad.) (9,39).