Figure 2.5 Quantitative mineral‐element distribution maps of a Tartary buckwheat grain cross‐section, comprising centrally‐positioned cotyledons surrounding the endosperm and the pericarp as obtained by micro‐PIXE and described previously (Pongrac et al. 2013b). The color scales for Mg, P, S, K, and Ca are in weight %, while for Mn, Fe and Zn they are in mg kg⁻¹ dry weight.

      Synchrotron micro‐X‐ray fluorescence was, for example, used to determine the in vivo mineral distribution patterns in rice (Oryza sativa) grains and shifts in these distribution patterns during progressive germination stages. The results of bulk analyses of hulled, brown and polished rice showed that half of the total Zn, two thirds of the total Fe and most of the total K, Ca, and Mn were removed by the milling process when the hull and bran were thoroughly polished. The concentrations of all elements were high in the regions of the embryo, though local distributions within the embryo varied between the elements. The mobilization of minerals from certain seed locations during germination was also element specific. A high mobilization of K and Ca from the grains to the growing roots and leaf primordia was observed; the flow of Zn to these expanding tissues was slightly lower than that of K and Ca; the mobilization of Mn or Fe was relatively low, at least during the first days of germination (Lu et al. 2013).

      Understanding the spatial distribution of inorganic nutrients within edible parts of plant products helps biofortification efforts to identify and focus on specific uptake pathways and storage mechanisms. Thus, the distribution of inorganic nutrients was studied in maize and sweetcorn. The results show that localization of elements is largely similar between maize and sweetcorn, but defer markedly depending upon the maturity stage after further embryonic development. The micronutrients Zn, Fe, and Mn accumulated primarily in the scutellum of the embryo during early kernel development, while trace amounts of these were found in the aleurone layer at the mature stage. Though P accumulated in the scutellum, there was no direct relationship between the concentrations of P and those of the micronutrients, compared to the linear trend between Zn and Fe concentrations (Cheah et al. 2019).

Localization of elements in Khorasan wheat (Triticum turgidum ssp. turanicum) with SR‐micro‐XRF showed an increased Fe accumulation in scutellum together with Zn, Mn and Ca, while in T. aestivum aleurone was the most Fe dense tissue (Figure 2.6).

      In a more detailed analysis of wheat aleurone, synchrotron radiation soft X‐ray full‐field imaging mode (FFIM) provided detailed images of globoids covered by oleosomes. Low‐energy X‐ray fluorescence (LEXRF) spectro‐microscopy showed that these structural features were connected to subcellular distribution of elements (Zn, Fe, Na, Mg, Al, Si, and P)(Regvar et al. 2011). This evidence suggest that membranous globular structures provide a basic structural scaffold for deposition on mineral elements within the aleurone cells and most likely affect their bioavailability.

2.3.3 Laser Ablation‐Inductively Coupled Plasma Mass Spectrometry

LA‐ICPMS is characterized by its superior chemical sensitivity down to the μg kg⁻¹ level, and is, therefore, suitable for imaging trace element distributions. Further, it is a benchtop laboratory technique with significantly lower operating costs than accelerator‐ or synchrotron‐based techniques. The lateral resolution reaches down to 1 μm thanks to laser ablation cells with a fast washout and sensitive ICPMS detectors (Gundlach‐Graham and Günther 2016; Van Elteren et al. 2016; Van Malderen et al. 2016). In contrast to micro‐PIXE and micro‐XRF techniques, LA‐ICPMS belongs to the destructive methods, because biological samples are usually completely ablated during the measurement (Wu and Becker 2012) and after LA‐ICPMS no other analytical technique can be applied.

Figure 2.6 Element (K, Ca, Fe, Mn, and Zn) localization in Khorasan wheat (Triticum turgidum ssp. turanicum) by micro‐XRF, Synchrotron Light Research Institute, Thailand, lateral resolution 50 μm, step size 50 μm, polychromatic excitation, data fitted by PyMCA (Solé et al. 2007). Cps, counts per second.

      LA‐ICPMS was used to map the Zn localization in durum wheat at different nitrogen supply (Persson et al. 2016). Elemental distribution imaging was combined with the analysis of Zn‐binding proteins by liquid chromatography ICPMS and orbitrap MS. The increase in Zn and N supply had a major impact on the Zn concentration in the endosperm and reached concentrations higher than current breeding targets. The sulfur concentration also increased, but S was only partially co‐localized with Zn. The mutual accumulation of Zn and S was reflected in much more Zn bound to small cysteine‐rich proteins (apparent size, 10–30 kDa), while the response of larger proteins (apparent size, 50 kDa) was only moderate. Most Zn‐reactive proteins were associated with redox and stress processes.

In assessing quality parameters in grain, it is also important to track potentially hazardous metals like Cd. Cd is one of the most mobile hazardous elements in plants, therefore, it readily accumulates in grains of rice, wheat and other cereals, posing threats to consumers. Besides essential macro in micronutrients, LA‐ICPMS was used to image the distribution of hazardous Cd in durum wheat (Yan et al. 2020). Wheat grains accumulate extremely low concentrations of Cd (<1 μg g⁻¹), so it is impossible to probe the distribution using X‐ray based techniques. Cd is a high energy element with a Cd─K line of 23.17 keV and a Cd‐L₃ line of 3.13 keV. The K line is hard to excite and the L₃ line is hidden in the tail of the macronutrient potassium K‐line, raising the limit of detection to tens of μg g⁻¹ (Vogel‐Mikus et al. 2008). In durum wheat, Cd accumulated mostly in the crease region, and inside the crease, in the pigment strand. In the vascular bundle, Cd was mainly associated to Sulfur ligands, and a relatively high concentration was therefore found in endosperm, the Sulfur‐containing protein (gluten) fraction.

Figure 2.7 LA‐ICPMS images of the Ca and K distribution in the maize (Zea mays L.) kernel. The lateral resolution is 50 μm.

Localization of the major elements Ca and K in maize (Zea mays) (Figure 2.7) shows similar patterns as in other monocotyledonous species. Ca is mainly localized in the seed coat, while K is found in high concentrations in embryonic tissues.

2.3.4 Nano Secondary Ion Mass Spectrometry

      Nano Secondary Ion MS (nano SIMS) works based on a coaxial optical design of the ion gun and secondary ion extraction, and on an original magnetic sector mass spectrometer with multicollection. Nano SIMS fills a unique niche as an MS tool for biological analysis by providing unmatched lateral resolution of elemental and isotopic distributions in samples of interest (Nuñez et al. 2018).

Cereals are an important source of Se to humans; many people suffer from Se deficiency

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