Anions, plants/crops

Soil Nutrient. Molybdenum has been widely used to increase crop productivity in many soils woddwide (see Fertilizers). It is the heaviest element needed for plant productivity and stimulates both nitrogen fixation and nitrate reduction (51,52). The effects are particularly significant in leguminous crops, where symbiotic bacteria responsible for nitrogen fixation provide the principal nitrogen input to the plant. Molybdenum deficiency is usually more prominent in acidic soils, where Mo(VI) is less soluble and more easily reduced to insoluble, and hence unavailable, forms. Above pH 7, the soluble anionic, and hence available, molybdate ion is the principal species. 

The Brassica family produces a wide range of glucosinolate compounds, anionic glycosides produced by the plant as antifeedant protective chemicals. As significant amounts of these compounds are left in the oilseed rape meals that remain after oil extraction, there is currently interest in exploiting these materials as crop-protection products for control of soil-borne diseases (Palmieri, 2003). 

As an essential constituent of several important molybdoenzymes. Mo has specific roles in the metabolism of N and S in crop plants. Further studies are needed to determine the detailed structures of these molybdoenzymes and the involvement of different valence states of Mo in their reaction mechanisms. Furthermore, Mo has a wide range of nonspecific effects involving the regulation of many enzymes other than molybdoenzymes, carbohydrate metabolism, reproductive physiology, anion balance, root exudation, plant water relations, and disease incidence in plants. In the future, attempts must be made to understand the precise roles of Mo in DNA stability, protein synthesis, carbohydrate metabolism, plant stress, and disease resistance. Research is also needed on the effects of Mo nutrition on the quality of food grains and vegetable and fruit crops. 

Application of S, either as gypsum or from other S sources, generally decreases the uptake of Mo by crops. Studies of soybeans have shown that decreases in plant Mo are not limited only to Mo-S interactions in the soil, because foliar-applied Mo has also been shown to decrease Mo concentrations in soybean seeds and leaves in the presence of soil-added S. On low-Mo soils this can lead to induced Mo deficiency, which can decrease yields and crop quality. On soils with high amounts of Mo, application of S can prevent the accumulation of high Mo concentrations and decrease the potential for Mo toxicity to livestock. Competition between sulfate and molybdate anions, competition between bicarbonate and molybdate, and root-zone pH changes have been suggested as explanations for the action of S to reduce Mo uptake. 

The nitrogen in many straight and compound fertilisers is in the ammonium (NH cation) form but, depending on the soil temperature, it is quickly changed by bacteria in the soil to the nitrate (NOj anion) form. Many crop plants, e.g. cereals, take up and respond to the NO3 anions quicker than the NH cations, but other crops, e.g. grass and potatoes, are equally responsive to NH and NO ions. 

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