Yoshida - 1981 - Fundamentals of Rice Crop Science

114 FUNDAMENTALS OF RICE CROP SCIENCE
Table 3.2. Development of air space in the culm of variety IR36
grown under upland and lowland conditions. a
-
Number
Mean diameter (mm)
3rd b 4th 5th 3rd 4th 5th
Upland 0 4 21 – 0.09 0.10
Lowland 5 26 28 0.15 0.22 0.23
a IRRI (1979). b lnternode position from the top.
Air enters the rice plant through the leaf blade and leaf sheath stomates and
moves downward to the nodes at the plant base. Oxygen is supplied to tissues
along the air path and moves further downward into the roots where it is used for
respiration. Finally, the air diffuses outward from the plant roots into the surrounding
soil.
When oxygen moves downward from shoot to root and from the basal part of the
root to the root tip, a certain amount of' oxygen may leak out laterally or may be
consumed by tissues along the pathway. Thus, partial oxygen pressure in the root
tissue is expected to decrease with an increase in the distance between the root tip
and the root base. At about 40 cm from the root base, partial oxygen pressure drops
to about one-tenth the partial pressure at the root base (Fig. 3.2). Because oxygen
is essential for cell division and enlargement at the root tip, root elongation slows
or ceases when the partial oxygen pressure drops considerably. Thus, in an
anaerobic environment where root elongation is totally dependent on oxygen from
3.2. Relative partial pressure of oxygen at the root tip as a function
of root length (Jensen et al 1967).