Yoshida - 1981 - Fundamentals of Rice Crop Science

32 FUNDAMENTALS OF RICE CROP SCIENCE
1.29. Three kinds of rice roots
(Hoshikawa 1975).
1.30. Structural unit of root, leaf, and
tiller (Kawata et a1 1963).
smaller (ranging from 1,000 to 40 µm) as the order of branching increases.
The formation of root hairs is greatly affected by the root environment. Aerobic
conditions in upland soils favor root hair formation (Kawata and Ishihara 1959,
1961, Kawata et a1 1964); the reductive conditions of flooded soils impair it. Rice
root hairs are about 5–10 µm in diameter and about 50–200 µm long. Most rice
varieties reach a maximum rooting depth of 1 m or deeper in soft upland soils. In an
anaerobic environment, such as flooded soils with impaired water percolation, rice
roots seldom exceed a maximum depth of about 40 cm. Actual root depth found in
the field is controlled not only by genetic ability but also by environmental
conditions. Total root length increases as the shoot grows. At heading, it is about
2–3 km/plant when plants are grown singly in a root box. In an upland field, the
total root length ranges from 15 to 34 km/m 2 of soil surface. Specific root length
varies from 10 to 1,000 m/g dry weight. Thick roots are shorter per unit dry weight
than thin roots. The average length of rice roots is about 300 m/g dry weight, which
corresponds to a root with a diameter of 0.2 mm.