Spatial (3D) image construction from imaging plate (IP) images and the development of the microautography (MAR) method were presented. A rice grain was sliced every 5 μm, IP images were taken for successive slices, and the series of 2D images acquired by an IP were used to construct 3D images. In the case of 109Cd and 137Cs, the spatial distributions in the grain showed that the concentrations increased at the surface of the grain during the maturing process.
3D image 109Cd 137Cs Rice Grain Distribution Accumulation IP image
Seeds of rice plants (Oryza sativa L. var. Nipponbare) grown in water culture were employed for 3D image construction. During the maturing process of the grain, after flowering, 109Cd (1 MBq) was supplied to the water culture. After 24 h of treatment, the brown rice was harvested and subsequently embedded in resin under freezing conditions. The rice grain was sliced into sequential sections 5 μm in thickness (Fig. 6.1), and the sliced sections were removed successively every 100 μm and stored at −20 °C. The height of the whole grain was approximately 6 mm; therefore, the number of slices was approximately 1200 sheets when sliced every 5 μm. Out of these 1200 sheets, successive sheets were removed every 100 μm, and these 60 sheets were used to construct the 3D image.
Since the ionic form of nuclides can move inside the plant during the sectioning process, such as during fixation and dehydration, sample preparation and radiography were performed under frozen conditions to diminish radionuclide movement. After the samples were placed on an IP, the image produced in the IP was read by a scanner to obtain the radiographs. Then, the successive 137Cs distribution images acquired from the IP were used to construct the 3D image employing ImageJ software.
Figure 6.2a and c show the sliced grain picture and the radiation images of 2 sliced grains taken by an IP, respectively. In each unit (red square), two horizontal and two vertical images of the two grains are shown. In each unit, the two images on the left side and right side are the grains 20 and 15 days after flowering, respectively, showing a horizontal (small round image) and a vertical (longer image) pair of images for each grain. When the picture and the radiograph image are superposed (Fig. 6.2b), it was clear that there was hardly any 109Cd observed in the left side grains in the unit, i.e., the grains 20 days after flowering.
The grains after 5, 10, 15, and 20 days were sliced in the same way, and from the series of approximately 60 images, 3D images were constructed. The volume of the endosperm increased from day 5 to 20 after flowering, along with the development of the grain. When 3D images were constructed, the decrease in 109Cd accumulation at the endosperm was more clearly shown. However, the 109Cd image of the grain 20 days after flowering was hardly observed. Figure 6.3 shows the 3D images of the grain after 5, 10, and 15 days. 109Cd did not accumulate in the middle of the endosperm in any stage of the grains, and the accumulation amount of 109Cs decreased during seed development.
The crystallization of the rice grain develops during the maturing process. Therefore, the relation between the 109Cd distribution and crystallization was investigated. It was found that along with the formation of crystallization in the grain (Fig. 6.4), the accumulation of 109Cd in the endosperm decreased, and 109Cd was confined at the surface of the grain. Although the image shows the distribution profile of 109Cd and does not reflect the route of the movement of the tracer, the imbalance in the 109Cd decrease on one side of the grain suggested that the transferring function of the grain was lost from this site.
6.2 3D Image of 137Cs in a Rice Grain
In the case of 137Cs, the rice plant was grown in water culture solution including 137Cs (200 Bq/mL), and the grains were harvested 3, 5, 7, 9, 12, and 15 days after flowering. Then, each grain was sliced to 5 μm in thickness, and every 100 μm, successive slices were selected, in the same way as in the case of 109Cd. Figure 6.5 shows the 3D image of the rice grain harvested 15 days after flowering. As shown in the figure, 137Cs accumulated in the embryo and the outer bran layer. Before serving the grain as food, the periphery of endosperm is removed as bran through threshing. Therefore, the processed grain was estimated to contain a minimal amount of 137Cs.
To compare the accumulation profile of 137Cs with those of other elements in the grain, the distribution of K and Mg in the grain was analyzed by SEM-EDX (scanning electron microscopy/energy dispersive X-ray spectroscopy). Figure 6.6 shows the distribution of 137Cs as well as of K and Mg in the horizontal plane of the grain. 137Cs accumulated from an early stage of the maturing process of the grain, and the distribution was uniform throughout the grain. Distinct accumulation sites, including embryos, were not observed until the middle of the maturing process. Then, the accumulation of Cs at the grain surface and embryo began to increase, along with a decrease in the Cs amount in the middle of the endosperm. On day 15 after flowering, the accumulation of Cs at the surface and embryo was completed. In the case of K, the distribution was uniform, similar to that of Cs, until the middle of the maturing process, and then the accumulation of K in the grain at the surface and embryo was observed. Since K and Cs are alkaline elements located in the same group in the periodic table, their chemical behavior was estimated to be similar. Magnesium was transferred to the grain later than K and Cs during the maturing process; however, the accumulation of Mg on the surface started from a rather early stage of maturation. The distribution of 137Cs in the grain will be described in more detail in the next section.
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Authors and Affiliations
Tomoko M. Nakanishi
1.Graduate School of Agricultural and Life SciencesThe University of TokyoBunkyo-kuJapan