X-ray fluorescence microscopy of zinc localization in wheat grains biofortified through foliar zinc applications at different growth stages under field conditions
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Biofortification of wheat with zinc (Zn) through foliar Zn application has been proposed as an agronomic strategy to increase grain Zn concentration, which could serve as a nutritional intervention in regions with dietary Zn deficiency.
Bread wheat (Triticum aestivum L.) was biofortified through foliar Zn applications at different growth stages. The concentration of Zn and associated micronutrient in harvested whole grains was determined by ICP-OES. Synchrotron-based X-ray fluorescence microscopy (XFM) was then used to investigate the localization of Zn and associated micronutrients in cross sections of these grains.
The concentration of Zn and other micronutrients (Mn, Fe, and Cu) was higher in grains treated with foliar Zn during grain-filling (early milk/dough) than those treated at stem elongation. The increase in Zn concentration of wheat grain with foliar application during grain-filling can be attributed to the intense localization of Zn in the aleurone layer, modified aleurone, crease tissue, vascular bundle, and endosperm cavity, and to a modest localization in endosperm, which is the most dominant grain tissue. These tissues and the Zn they contain are presumed to remain after milling and can potentially increase the Zn concentration in wheat flour.
By using XFM, it was shown that foliar Zn spray represents an important agronomic tool for a substantial Zn enrichment of different fractions of wheat grain, especially the endosperm. Further investigation of the chemical speciation of Zn in the endosperm is recommended to assess Zn bioavailability in harvested whole grain of wheat that has been biofortified through different timing of foliar Zn application.
KeywordsBiofortification Foliar application Wheat grain Zn localization Zn fertilizer X-ray fluorescence microscopy Aleurone layer Endosperm Endosperm cavity
X-ray fluorescence microscopy
Inductively coupled plasma optical emission spectroscopy
Laser ablation inductively coupled plasma mass spectroscopy
Secondary ion mass spectroscopy
Photon induced X-ray emission
X-ray absorption near edge structure spectroscopy
This research was undertaken on the X-ray Fluorescence Microscopy (XFM) beamline at the Australian Synchrotron, Victoria Australia. The sectioning of the grains with the vibratome was carried out at the Australian Centre for Plant Functional Genomics (ACPFG), which was facilitated by Dr Gwen Mayo. Ms Casey Doolette helped in preparing the transverse sections. This study was partially supported by the HarvestPlus program (www.harvestplus.org). Financial support by The Mosaic Co to the Fertilizer Technology Research Centre is duly acknowledged. The authors will like to thank Enzo Lombi for his useful comments during the draft manuscript review.
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