Yoshida - 1981 - Fundamentals of Rice Crop Science

40 FUNDAMENTALS OF RICE CROP SCIENCE
When shallow-rooted lowland and deep-rooted upland varieties are grown in the
field, there is not much difference between their root densities down to 30 cm
beneath the soil surface. The differences are greater in the soil horizons below 30
cm (Table 1.10). This justifies the use of deep root-shoot ratios in the root box
technique.
In the field, rice roots grow and extend vertically and laterally. IR20, a
shallow-rooted, high-tillering variety, has a very high root density around the plant
base but the root’s growth, both vertically and laterally, is limited (Fig. 1.37). On
the other hand, OS4, a deep-rooted, low-tillering variety, has a well-spaced lateral
root distribution and its roots penetrate deeper than those of IR20. Such differences
may in part account for the differences in drought resistance between the two
varieties.
1.8. PHASIC DEVELOPMENT FROM VEGETATIVE TO
REPRODUCTIVE GROWTH STAGES
The rice plant initiates panicle primordia when certain physiological and environmental
conditions are met. Panicle primordia on top of the culm can be seen under
a microscope about 30 days before heading. Although the duration from panicle
1.37. Iso-root density diagram of rice varieties OS4 and IR20 grown in the field (IRRI 1978).