Abstract
Humans require over 50 essential elements and molecules in their diet including 17 micronutrients, only 8 of which are also essential for plants (chlorine, iron (Fe), manganese (Mn), zinc (Zn), boron (B), copper (Cu), molybdenum Mo), nickel (Ni)). Historically, micronutrients have received less attention in agriculture than N, P and K but their significance has gained prominence through increasing recognition that low levels in staple foods is a major factor in dietary deficiencies for billions of the world’s population, particularly for Fe, Zn, iodine (I) and selenium (Se).
Studies on micronutrients in agriculture emphasise soil and foliar applications for maximising yield rather than their impacts on grain nutrient content. Differences among crop species and cultivars in their ability to mobilise micronutrients in soil explain their varied adaptation to low micronutrient soils. Variations in internal efficiency also exist but most micronutrients have variable phloem mobility which can be a constraint to loading into grain and foods.
For instance, micronutrients in a large collection of grain of an Australian high-yielding wheat cultivar ranged from 21.3 to 97.2 mg Fe/kg grain, 6.9 to 44.7 mg Zn/kg grain, 5 to 25 μg I/kg grain and 3.8 to 829 mg Se/kg grain. For Fe, Zn, Cu, Mo and B, phloem mobility is variable depending on supply, plant N status, plant part, and plant species. Advances in molecular biology are identifying transporters and channels that regulate uptake, distribution and redistribution of micronutrients within plants, especially to grain.
Among wheat genotypes, there are enormous variations in the capacity of plants to extract micronutrients from the soil so that plant breeding has been identified as a key strategy for biofortification of micronutrients in grain. However, efficiencies in uptake don’t necessarily lead to increases in loading of micronutrients in grain. While foliar applications of micronutrients have been effective and accepted as an important strategy for boosting crop yield on low micronutrient soils, there is increasing evidence that efficiency of cellular uptake and retranslocation of common micronutrient salts and chelates is poor.
There is an opportunity to develop enhanced carriers and transporters of micronutrients. New products with enhanced mobility within the plants may also have a role in boosting root growth, particularly in subsoils that are low in Fe, Mn, Cu, Zn and B.
Another potential spin-off from biofortification of micronutrients in grain is increased seed vigour for crop establishment.