Tona Sanchez-Palacios

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.

Agronomic interventions such as foliar application of fertilisers can increase the mineral content of grains, consequently improving wheat flour for human consumption. We established a field trial at Wongan Hills in Western Australia's moderate rainfall zone (325-450 mm) during the winter season of 2021. Wongan Hills' soil is deficient in available Zn in subsoil layers containing 0.2 ± 0.1 mg Zn-DTPA kg-1.

The topsoil layer, a pale-yellow sandy clay, is Zn adequate with 0.84 ± 0.2 mg Zn-DTPA kg-1. First, we evaluated the efficiency of Zn foliar applications on wheat plants in producing Zn-enriched grains. Second, we used synchrotron radiation techniques to determine the localisation of Zn in grains. Third, we conducted grain quality studies to determine levels of mineral nutrition adequacy in food products. In the field, we tested two Zn forms including ZnSO4 and Zn-EDTA, with or without soluble nitrogen (0.4% N) in the formulation.
Foliar treatments were applied four times from anthesis to grain-filling developmental stages. Foliar-control treatments with and without N produced wheat grains with 13.6 ± 0.4 mg Zn kg-1 and 12.9 ± 0.8 mg Zn kg-1, respectively. Foliar ZnSO4 with and without N resulted in a 2.1- and 1.7-fold increase in Zn concentration relative to controls.

Foliar Zn-EDTA with and without N resulted in a moderate 0.6- and 0.4-fold Zn increase relative to controls. Nitrogen in the formulation did not affect significantly grain yield or concentrations of Zn, iron and phosphorous in grains. X-ray fluorescence microscopy (XFM) studies revealed that Zn was accumulated in the embryo primarily, followed by aleurone layers and to a lesser extent in the crease region in control grains.

Grains of treated plants with Zn foliar fertilisers showed the same distribution pattern with a slight enrichment of Zn in the crease region. Flour milling and bread-making studies showed that Zn-enriched grains derived from ZnSO4 treatment retained twice as much Zn in white bread products with 9.1 ± 0.1 mg Zn kg-1 Zn compared to the control white bread with 3.9 mg Zn kg-1. A moderate increase of Zn relative to controls was observed in white bread produced from the Zn-EDTA foliar treatment at 5.7 ± 0.4 mg Zn kg-1. Similarly, raw noodles from grains of the ZnSO4 treatment retained 6.5 ± 0.1 mg Zn kg-1 which is higher than the control with 2.4 ± 0.05 mg Zn kg-1.

Zinc concentrations in cooked noodles decreased compared to uncooked noodles but still the cooked noodles from flour of Zn enriched grains remained higher with 3.2 mg Zn kg-1 compared to the control at 1.5 mg Zn kg-1.

In conclusion, applying Zn via foliar sprays on wheat resulted in enhanced grain products with dietary Zn advantage, making foliar biofortification a worthwhile agronomic method for agricultural systems with low Zn availability.