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Biofortification of staple food crops with micronutrients by either breeding for higher uptake efficiency or fertilization can be an effective strategy to address widespread dietary deficiency in human populations. Selenium and iodine deficiencies affect a large proportion of the population in countries targeted for biofortification of staple crops with Zn, Fe, and vitamin A, and inclusion of Se and I would be likely to enhance the success of these programs. Interactions between Se and I in the thyroid gland are well established. Moreover, Se appears to have a normalizing effect on certain nutrients in the body. For example, it increases the concentration of Zn and Fe at key sites such as erythrocytes when these elements are deficient, and reduces potentially harmful high Fe concentration in the liver during infection. An important mechanism in Se/Zn interaction is selenoenzyme regulation of Zn delivery from metallothionein to Zn enzymes. More research is needed to determine whether sufficient genetic variability exists within staple crops to enable selection for Se and I uptake efficiency. In addition, bioavailability trials with animals and humans are needed, using varying dietary concentrations of Se, I, Zn, Fe, and vitamin A to elucidate important interactions in order to optimize delivery in biofortification programs. Key words: biofortification, crops, selenium, iodine, zinc, iron, vitamin A

There are similarities between mammals and plants in the absorption and transport of trace elements. The chemistry of trace element uptake from food sources in both cases is based on the thermodynamics of adsorption on charged solid surfaces embedded in a solution phase of charged ions and metal-binding ligands together with redox systems in the case of iron and some other elements. Constitutive absorption systems function in nutrient uptake during normal conditions, and inducible 'turbo' systems increase the supply of a particular nutrient during deficiency. Iron uptake is the most studied of the micronutrients, and divides the plant kingdom into two groups: dicotyledonous plants have a turbo system that is an upregulated version of the constitutive system, which consists of a membrane-bound reductase and an ATP-driven hydrogen ion extrusion pump; and monocotyledonous plants have a constitutive system similar to that of the dicots, but with an inducible system remarkably different that uses the mugeneic acid class of phytosiderophores (PS). The PS system may in fact be an important port of entry for iron from an iron-rich but exceedingly iron-insoluble lithosphere into the iron-starved biosphere. Absorption of trace metals in these graminaceous plants is normally via divalent ion channels after reduction in the plasma membrane. Once absorbed, iron can be stored in plants as phytoferritin or transported to active sites by transport-specific ligands. The transport of iron and zinc into seeds is dominated by the phloem sap system, which has a high pH that requires chelation of heavy metals. Loading into grains involves three or four genes each that control chelation, membrane transport and deposition as phytate. KEY WORDS: * micronutrients * iron * zinc * absorption * transport * plants * animals * genetics

In field studies in 1992 and 1993, biosolid applications of 2 t/ha with supplements of mineral N and P were compared with a standard mineral fertiliser application (20 kg N/ha, 20 kg P/ha, 1.8-2.8 kg Zn/ha, 1.4-1.9 kg Cu/ha, 0.5-4kg Mn/ha) on 4 soil types. Biosolid rates from 2 to 10 t/ha were applied in 1993. Shoot dry matter production at different stages of plant development (9 and 15 weeks after sowing) and grain production of Triticum durum and Hordeum vulgate, as well as nutrient concentration in shoots and grain, were taken as indicators for comparing nutrient availability of the 2 sources. A 2-t biosolid application was found to enhance dry-matter production and yield to the same extent as a mineral fertiliser application of 20 kg N/ha, 20 kg P/ha, and 1.8-2.8 kg Zn/ha. In some crop rotations and on sites with a high yield potential, additional nitrogen with 2 t biosolids/ha would be necessary to achieve such yields. However, development of durum wheat fertilised with 2 t biosolids was slower and P uptake lower than with mineral fertiliser until late booting. Durum wheat fertilised with 4 t biosolids showed similar P-uptake values as plants fertilised with the mineral fertiliser. The Zn uptake of plants was positively correlated with biosolid application rates, showing a linear relationship. A 2-t biosolid application alleviated micronutrient deficiency to the same extent as the same rate of Zn (and Cu) given in mineral form. Additional keywords: sewage sludge, fertiliser, micronutrient., Introduction Since 1993 when dumping of biosolids into the sea was officially terminated in Australia, a fully land-based disposal scheme was required by cities and states. The application of digested [...]