Molybdenum (Mo) is a trace element encountered in the soil and is essential for the growth and metabolism of many biological organisms, including plants and animals. Mo is a transition element that can exist in several oxidation states from 0 to +6, where +6 is the most common form in most agricultural soils. The mineral form of Mo is commonly found in rocks like molybdenite, wulfenite, and ferrimolybdenite, which weather and leach Mo. Once dissolved, Mo becomes available to plants via anion adsorption and desorption reactions, in the soluble anionic form of molybdate (MoO4-). Availability for plant uptake from the soil depends on soil pH, the concentration of adsorbing oxides, water drainage, and other organic soil colloid compounds. As the pH of the soil increases, molybdate solubility will increase as the adsorption of metal oxides decreases. Another factor affecting Mo uptake is soil moisture, where poor soil drainage allows high concentrations of molybdate to build up in the soil, often leading to molybdenosis.

Arnon and Stout demonstrated the importance of Mo in plant development in 1939. They found that tomato plants grown in a Mo-deficient hydroponic environment developed mottling lesions on the leaves and changes in leaf morphology causing the involution of the lamella, also known as “whiptail.” Mo was found to be the only element to mediate the whiptail phenotype. The first documented report of a Mo deficient crop was in Australia. Well-irrigated pastures were reported as significant failures by pastoralists growing subterranean clover (Trifolium subterranean). The pastures began to show improvement after adding lime, and they later discovered that Mo was the most abundant trace element present in the lime. Additionally, Mo becomes more soluble in alkaline soils.

Like many metals required for plant growth, Mo is utilized by plant enzymes which undergo reduction and oxidative reactions. Once Mo has entered the plant, the Mo co-factor (Moco) binds to molybdenum-requiring enzymes known as molydo-ezymes. There is evidence that Mo is required for symbiotic nitrogen fixation. Azotobacter chroococcum is a symbiotic nitrogen-fixing bacterium in the rhizosphere which benefits from Mo in the plant as it fixes nitrogen from the air. As the plant takes up Mo, Moco must travel via the interconnected nodule membranes, including the bacteroid outer and inner membranes, to reach the bacterial nitrogenase complexes known as modABC. There is very little information on the mechanism controlling molybdate transport to nodules across the peribacteroid membrane. However, it is known that Mo availability is closely related to nodule development regarding Mo and legume nitrogen fixation.

Plants have a higher threshold for molybdenum than any other micronutrient. There are vast numbers of plants that accumulate up to 900 times the concentration they need before showing any symptoms of molybdenum toxicity. Although Mo toxicity in plants is rare, higher concentrations may cause leaves to accumulate anthocyanins, causing leaves to turn purple in color. Some plants, like legumes, will present with yellowing leaves.

Photo of whiptail caused by Mo deficiency in cauliflower leaves. Image from Wikipedia.

Molybdenum deficiency affects plant metabolism and is primarily associated with poor nitrogen health. The effects are strongly linked to the Mo requirements of the molybdoenzymes present in the plant. Phenotypic Mo deficiencies have been reported in many plant species and range in severity and appearance. Some of the deficiencies are highly pronounced and reproducible. Younger plants display leaf cupping, grey tinting, flaccid leaves, and dwarfed dying seedlings; mature plants present symptoms on new leafs displaying loss of proper lamina development (whip-tail), leathery leaves, and meristem necrosis. Mo-deficient cereal and legume crops express necrotic areas at leaf margins with decreased plant growth and metabolism. Oat and wheat deficiency tends to develop necrotic regions on the leaf blades, and seeds are poorly developed and shriveled. Maize develops shortened internodes under Mo deficiency, decreased leaf size, and development of chlorotic leaves.

Furthermore, the reproductive tissue in maize shows altered phenotypes in flower development, delays in tassel emergence, dwarfed anthers, underdeveloped stamens, and reduction of pollen grain development leading to poor germination rates. Millerandage (“hen and chicken”) is a developmental disorder from Mo deficiency in grapevines that causes bunches to develop unevenly. The matured grapes are present and bunched with small shriveled fertilized and unfertilized grapes. This phenotype is also present in Merlot grapes and Cabernet Sauvignon alongside phenotypes showing zigzag-shaped internodes, pale-green leaves, increased cupped and flaccid leaves, and marginal leaf necrosis.