Yoshida - 1981 - Fundamentals of Rice Crop Science

166 FUNDAMENTALS OF RICE CROP SCIENCE
stresses (Yoshida 1975b).
The application of a silicate slag to degraded paddy soils and peaty soils in Japan
and Korea has been found beneficial to rice yields. Yield increases are usually
around 10% but may exceed 30% when blast disease is severe.
3.14.2. Silicon in soil solution
Silicon dissolved in an aqueous solution exists as orthosilicic acid, Si (OH) 4 .
Because silica sol or silica gel derives from the condensation polymerization of
orthosilicic acids, orthosilicic acid is sometimes called a monomer or a monomeric
form of silica. The term silica refers to SiO 2 .
The concentration of silicon in the soil solution generally increases with flooding
and soils with a high organic matter content give the highest increases
(Ponnamperuma 1965). The concentration of silicon extractable with dilute acids
shows a more rapid increase with flooding than that of silicon in the soil solution.
Readily soluble silicon in the soil is perhaps present as absorbed on or as combined
with amorphous aluminum and ferrous hydroxide (Imaizumi and Yoshida 1958,
McKealgue and Cline 1963). Soils derived from volcanic ash have high levels of
soluble silicon and high silicon-supplying power.
3.14.3. Functions of silicon
The functions of silicon in rice growth are still not clearly understood (Yoshida
1975b).
First, the level of silicon in the plant at which growth is improved by additional
silicates is much higher in the field than in solution culture. In the field, an
application of silicates is considered beneficial when the silica content of straw is
below 11% (Imaizumi and Yoshida 1958). In solution culture, however, the
addition of silicon has little effect on vegetative growth when silica content in the
leaves is 0.07% (Yoshida et al 1959); it has no effect on rice growth and yield when
the silica content of green leaves is above 1.25% (Tanaka and Park 1966). Second,
low-silicon plants are susceptible to various kinds of diseases and attacks by
insects and mites (Yoshida et al 1962). It is difficult to keep such plants free from
these hazards. Third, no direct evidence of the participation of silicon in essential
physiological and biochemical processes has yet been found.
Although the results and interpretation of several studies differ, silicon appears
to have the following functions in rice growth:
• Increased silicon absorption protects plants against infestations of fungi
and insect attacks (Yoshida et al 1962). A thick cuticle-silicon layer (Fig. 3.30)
serves as a barrier against fungi, insects, and mites because of its physical
hardness. Entomologists have found that the mandibles of rice borer larvae fed
on rice plants with a high silicon content were damaged.
• Increased silicon absorption maintains erect leaves (Yoshida et al 1969;
Cock and Yoshida 1970). The importance of leaf angle to photosynthesis by a
crop canopy is well recognized; erect leaves are desirable in a high yielding rice
variety. Although leaf angle is mainly a varietal characteristic, it is affected by