of P. In order to examine these brown rods, the roots were treated with hot H 2 O 2 to
digest the cell walls.
Under the optical microscope, the digested roots showed brown rods, and light
brown-colored or nearly transparent particles with irregular shapes (Fig. 5.19a).
After selecting a dashed square area (Fig. 5.19b), a SEM image (Fig. 5.19c) as
well as element maps of Fe (Fig. 5.19d), Si (Fig. 5.19e), and P (Fig. 5.19f) were
obtained. Integrating these results showed that the brown rods and particles are rich
in Fe and P, suggesting that they are composed of hydrated iron oxide and sorbed
phosphate. Hence, it is probable that the brown rods observed on the root surface
(Fig. 5.18b) are the brown rods observed in Fig. 5.19a. These brown rods were
formed at the interface between oxidizing conditions inside the roots and reducing
conditions outside the roots. The diameter (15 μm) of the large brown rods suggests
that they were formed inside the exodermis cells (Kondo et al. 2000). The longevity
of P-rich hydrated iron oxide inside the sclerenchymatous cells and/or exodermis
cells, and iron plaque, may depend on the intensity of aeration through the
intercellular space, and the reducing conditions in the soil. Nearly transparent
particles observed in Fig. 5.19a, b appear to be phytoliths.
Fig. 5.18 Analysis of a brown thick rice root after rice harvest. (a) Air-dried rice roots after harvest
with an arrow showing the position of the sampled root piece, (b) optical micrograph of the rice root
piece, (c) Fe element map for the dashed square area in (d)
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5 Inorganic Soil Constituents Sensitive to Varying Redox Conditions