the outer parts, which is the site of iron plaque formation. The iron phosphate-like
material is also different from vivianite because it does not show a blue color under
the optical microscope. It was likely formed at the redox interface between the
oxidative intercellular space inside the root and the reductive bulk soil outside the
root. This iron phosphate-like material may be one of the reasons why the vivianite
content of rice roots is as high as 50% even at the end of the paddy field submergence
period (Nanzyo et al. 2013).
Figures 5.16 and 5.17 exhibit the thick iron plaque. However, when the rice roots
are washed out of the soil, the number of brown roots without thick iron plaque is
much greater than those with thick iron plaque. Figure 5.18b shows an optical
microscope photograph of the surface of a thick rice root, which displays a brown
rod-like color pattern. Although this pattern is not observable with SEM, it is
observable with EDX, as shown in the Fe element map (Fig. 5.18c) for the dashed
square area outlined in Fig. 5.18d. The characteristic X-ray used for the EDX
analyses provides information about deeper sites than the secondary electron beam
used for SEM. These observations suggest that the brown rods are located under the
exodermis cell walls. A similar pattern can be obtained for P, but it is less clear than
that for Fe, suggesting that the Fe concentration in the brown rods is higher than that
Fig. 5.17 Iron phosphate material in the exodermis-like cells. (a) magnified SEM image of the
dashed rectangular area in Fig. 5.16c, (b) EDX spectrum obtained from the dashed rectangular area
(b), shown in (a)
5.3 Vivianite
117
material is also different from vivianite because it does not show a blue color under
the optical microscope. It was likely formed at the redox interface between the
oxidative intercellular space inside the root and the reductive bulk soil outside the
root. This iron phosphate-like material may be one of the reasons why the vivianite
content of rice roots is as high as 50% even at the end of the paddy field submergence
period (Nanzyo et al. 2013).
Figures 5.16 and 5.17 exhibit the thick iron plaque. However, when the rice roots
are washed out of the soil, the number of brown roots without thick iron plaque is
much greater than those with thick iron plaque. Figure 5.18b shows an optical
microscope photograph of the surface of a thick rice root, which displays a brown
rod-like color pattern. Although this pattern is not observable with SEM, it is
observable with EDX, as shown in the Fe element map (Fig. 5.18c) for the dashed
square area outlined in Fig. 5.18d. The characteristic X-ray used for the EDX
analyses provides information about deeper sites than the secondary electron beam
used for SEM. These observations suggest that the brown rods are located under the
exodermis cell walls. A similar pattern can be obtained for P, but it is less clear than
that for Fe, suggesting that the Fe concentration in the brown rods is higher than that
Fig. 5.17 Iron phosphate material in the exodermis-like cells. (a) magnified SEM image of the
dashed rectangular area in Fig. 5.16c, (b) EDX spectrum obtained from the dashed rectangular area
(b), shown in (a)
5.3 Vivianite
117