96 The Global Food System
conditions. Many studies have examined the germination pattern of rice seeds
under submerged conditions (Yokoi, 1898; Sasaki, 1926; Takei, 1941; Takahashi,
1978). These researchers have concluded that low oxygen tension in water caused
elongation of coleoptiles and the prevention of radicle growth at initial stages.
When Takahashi (1978) looked at the growth pattern of seedlings under different
levels of moisture content in the media (Figure 4.1), he found that the elongated
growth of the mesocotyl took preference over that of the coleoptile until near pF
2.4 (the forces necessary for plants to extract water from the soil for their use);
then, as the pF was further lowered, coleoptiles growth occurred preferentially. No
growth of the mesocotyl occurred under saturated condition. Takahashi (1978)
reported that at lower oxygen tension in the water (as with hypoxic condition)
compared to the atmosphere, several metabolites including carbon dioxide are
accumulated in the surrounding root media and adversely affect seedling growth.
Note: Where M = mesocotyl, C = coleoptile, R = root, L = leaf.
Equivalent values are shown between pF and percent scale: e.g. 2.69 is equivalent to 50%
moisture content in the medium (vermiculite).
Source: Takahashi, 1978.
Figure 4.1 Relationship between seedling growth and moisture content in a culture
medium