plaque. Figure 5.16b shows a SEM image of the rice root iron plaque. At the center,
there is a thin rice root, of which only the exodermis-like layer and stele are likely to
remain. Element maps of Si (Fig. 5.16d) and Fe (Fig. 5.16e) show that the iron
plaque developed in the bulk soil around the rice root. These characteristics are
similar to those shown in Fig. 5.7d, e. However, a difference observed in the Fe
element map (Fig. 5.16e) is that the iron concentration in the exodermis-like layer is
higher than that in the iron plaque developed outside the exodermis-like layer.
Examining the P element map (Fig. 5.16f) shows that a P distribution pattern closely
related to that for Fe was obtained. Hence, the iron plaque is the site of P accumulation in the plow layer soil, and these Fe and P accumulations may be converted to
vivianite if the iron plaque is reduced. Remarkably, both Fe and P are highly
accumulated in the exodermis-like layer in this example. Therefore, an area of the
exodermis-like layer (dashed square in Fig. 5.16c) was magnified in Fig. 5.17a.
The EDX spectrum of the material inside the exodermis-like cells shows that it is
similar to iron phosphate (Fig. 5.17b, Nanzyo et al. 2004). Since the material in each
cell is localized toward the inner part of the root, the oxidizing power originated from
within the root, possibly through the intercellular space. Based on the cell diameter,
10–17 μm, these cells are exodermis rather than sclerenchyma (Kondo et al. 2000).
The intracellular material also contains a small amount of Ca, but almost no Si. Thus,
the inner parts of the exodermis-like cells may be under a different environment from
Fig. 5.16 Distribution of P in root iron plaque from the plow layer soil of a paddy field. (a) Rice
roots washed out of the soil after harvest, (b) SEM image of polished cross-section of a rice root
containing iron plaque, (c) dashed square area for Fig. 5.17, (d, e, and f) element maps for Si, Fe,
and P, respectively
116
5 Inorganic Soil Constituents Sensitive to Varying Redox Conditions
there is a thin rice root, of which only the exodermis-like layer and stele are likely to
remain. Element maps of Si (Fig. 5.16d) and Fe (Fig. 5.16e) show that the iron
plaque developed in the bulk soil around the rice root. These characteristics are
similar to those shown in Fig. 5.7d, e. However, a difference observed in the Fe
element map (Fig. 5.16e) is that the iron concentration in the exodermis-like layer is
higher than that in the iron plaque developed outside the exodermis-like layer.
Examining the P element map (Fig. 5.16f) shows that a P distribution pattern closely
related to that for Fe was obtained. Hence, the iron plaque is the site of P accumulation in the plow layer soil, and these Fe and P accumulations may be converted to
vivianite if the iron plaque is reduced. Remarkably, both Fe and P are highly
accumulated in the exodermis-like layer in this example. Therefore, an area of the
exodermis-like layer (dashed square in Fig. 5.16c) was magnified in Fig. 5.17a.
The EDX spectrum of the material inside the exodermis-like cells shows that it is
similar to iron phosphate (Fig. 5.17b, Nanzyo et al. 2004). Since the material in each
cell is localized toward the inner part of the root, the oxidizing power originated from
within the root, possibly through the intercellular space. Based on the cell diameter,
10–17 μm, these cells are exodermis rather than sclerenchyma (Kondo et al. 2000).
The intracellular material also contains a small amount of Ca, but almost no Si. Thus,
the inner parts of the exodermis-like cells may be under a different environment from
Fig. 5.16 Distribution of P in root iron plaque from the plow layer soil of a paddy field. (a) Rice
roots washed out of the soil after harvest, (b) SEM image of polished cross-section of a rice root
containing iron plaque, (c) dashed square area for Fig. 5.17, (d, e, and f) element maps for Si, Fe,
and P, respectively
116
5 Inorganic Soil Constituents Sensitive to Varying Redox Conditions