Abstract
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.