Yoshida - 1981 - Fundamentals of Rice Crop Science

MINERAL NUTRITION OF RICE 167
3.30. Schematic representation of the rice leaf epidermal cell (Yoshida 1965).
plant nutrition. In general, nitrogen tends to make rice leaves droopy while
silicon keeps them erect. A simulation method estimates that total photosynthesis
varies by as much as 36% within a range of leaf angle from 40 to 70°. In
other words, the maintenance of erect leaves as the result of silicate application
can easily account for a 10% increase in the photosynthesis of the canopy and
consequently a similar increase in yields.
• Increased silicon absorption decreases transpiration losses, perhaps
through the cuticle, and increases the plant’s tolerance for decreased osmotic
potential in the rooting medium (Yoshida 1965).
• Increased silicon absorption increases the oxidizing power of rice roots
and decreases the excessive uptake of iron and manganese (Okuda and
Takahashi 1965; Tadano 1976).
3.14.4. Interrelationship between silicon and nitrogen nutrition
When the amount of nitrogen application is increased, the rice plant becomes more
susceptible to diseases and insects and the leaves tend to droop. The silicon content
of the rice plant is very much affected by nitrogen applications and, hence, by
increased growth (Table 3.33). Increasing the amount of nitrogen application
increases grain yield and the total uptake of nitrogen and silicon but decreases the
silicon content in the straw. This relationship is more pronounced in pot experiments
than in the fields. When the silicon-supplying power of the soil is low and
when application of nitrogen is increased to achieve high yields, a similar decrease
in silicon content in the rice plant may occur even in the field.
3.14.5. Criteria for determining the need for silicate application
The silica contents of straw in Japan range from 4 to 20% and average 11%.
Applications of silicates are normally considered beneficial when the silica content