Yoshida - 1981 - Fundamentals of Rice Crop Science

MINERAL NUTRITION OF RICE 115
the shoots, the longest roots are much shorter than those in an aerobic environment.
A quantitative measurement of oxygen diffused from root surfaces can be made
by polarography. Along the root axis, the maximum rate of oxygen diffusion is
observed within about 1 cm of the apical region (Armstrong 1964, 1967).
Recently, large varietal differences in the amount of oxygen released from the
roots have been found. These differences are closely related to differences in the
tolerance for straighthead disease, which is attributed to hydrogen sulfide toxicity
(Joshi et al 1975).
Generally, oxygen transport from shoot to root is considered a physical diffusion
of air through the air-passage system within the tissues. Mitsui (1965),
however, proposed an enzymatic excretion of oxygen in rice roots. He suggested
that rice roots possess a glycolic acid pathway in which glycolic acid is successively
oxidized to carbon dioxide. The energy liberated during this oxidation can
not be stored as adenosine triphosphate (ATP) and is transferred to the formation of
hydrogen peroxide. The hydrogen peroxide thus produced is decomposed by
catalase into molecular oxygen and water. Whether a physical diffusion or an
enzymatic excretion operates in the rice plant needs further examination.
3.2.2. Root oxidizing power
Oxygen diffusion from rice roots constitutes an important part of the roots’
oxidizing power. For example, ferrous iron and hydrogen sulfide can be oxidized
by molecular oxygen diffused from root surfaces. In addition, rice roots are able to
oxidize certain pigments, such as a -naphthylamine and o-dianisidine. These
pigments are only slowly oxidized by molecular oxygen but are easily oxidized by
peroxidase in the presence of hydrogen peroxide (Matsunaka 1960; Kawata and
Ishihara 1965). Young roots oxidize a -naphthylamine better than old roots.
Along the root axis, the maximum oxidation is observed about 4–5 cm from the
root tip, except in the apical region 0.5 cm from the root tip (Nomoto and Ishikawa
1950).
The oxidation of a -naphthylamine is controlled by the amount of hydrogen
peroxide rather than by peroxidase activity per se. The a -naphthylamine test
measures the ability of plant tissues to produce hydrogen peroxide. Old roots may
have higher peroxidase activity than young roots but a lower oxidizing power
because a smaller amount of hydrogen peroxide is produced, In some bog species,
the roots’ oxidizing activity is nine times greater than can be accounted for by
oxygen diffusion from the roots. Thus, enzymatic oxidation is the principal
mechanism for the high oxidizing activity (Armstrong 1967).
The a -naphthylamine oxidizing power of rice roots is well correlated with the
respiratory rate (Fig. 3.3); therefore, it is used as a quick test to diagnose the
metabolic activity of rice roots (Yamada et al 1961).
3.2.3. Reduction of the rhizosphere and development of special roots
The rhizosphere of rice plants becomes reductive from about panicle initiation