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SR‐micro‐XRF is a state‐of‐the‐art imaging technique that enables 2D and 3D imaging. Separate beamlines are usually dedicated to low or high energy X‐ray fluorescence. Low SR energy micro‐XRF is operated in vacuum (e.g. TwinMic, Synchrotron Elettra Trieste (Gianoncelli et al. 2016), ID21, ESRF, Grenoble (Cotte et al. 2017), while hard X‐ray microprobes are operated in air with limited sensitivity for low Z elements (Martínez‐Criado et al. 2016). The detection limits range down to below μg g⁻¹ and the lateral resolution to a few tens of nanometers (Martínez‐Criado et al. 2016). The highest chemical sensitivity can be achieved by tuning the photon beam energy above the absorption edge of the element of interest; however, this limits the number of elements that can be examined during the fingerprinting of the samples. In addition to SR‐micro‐XRF, a bench‐top laboratory XRF instrument with a focused beam and polychromatic excitation (e.g. Tornado, Bruker) can be used, but with two orders of magnitude (or more) lower sensitivity (a few μg g⁻¹) and lower lateral resolution (a few tens of micrometers) (Mantouvalou et al. 2017; Rodrigues et al. 2018).

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.