Yoshida - 1981 - Fundamentals of Rice Crop Science

GROWTH AND DEVELOPMENT OF THE RICE PLANT 15
1.12. The effect of temperature on the elongation of the
radicle (Yamakawa and Kishikawa 1957).
ture for shoot elongation ranges from 7° to 16°C and that for root elongation from
12° to 16°C (Nishiyama 1977). Hence, about 10°C may be considered as the critical
minimum for elongation of both shoot and root. These critical temperatures may
vary with variety, seed history, and cultural management.
The elongation of tissue results from two components of cell growth: cell
division and cell enlargement. The optimum temperature for cell division of the
radicle tip is 25°C, and that for cell enlargement, 30°C (Fig. 1.11). The elongation
of the radicle as a whole, however, is optimum at 30°C, indicating that cell
enlargement dominates the elongation. Elongation of the radicle stops below 15°C
and above 40°C (Fig. 1.12).
b. Light. As mentioned earlier, darkness induces elongation of the coleoptile,
mesocotyl, and first and second leaves.
c. Oxygen. Rice seed can germinate under anaerobic conditions but subsequent
growth is greatly affected by oxygen supply. In flooded soil where oxygen is
limited, germination occurs normally, but the coleoptile becomes unusually long
and the first leaf, radicle, and nodal roots may not grow at all or their growth may
be highly impaired. As shown in Fig. 1.13, the elongation of the plumule was
accelerated and that of the radicle was highly impaired in water. When rice seed
was germinated in sand with adequate moisture and oxygen, the radicle grew faster
than the plumule. The radicle and plumule were both retarded under inadequate
moisture. The oxygen concentration required for the appearance of the radicle is 2
ppm; for root elongation, 4ppm; for active root elongation and development, more
than 5 ppm; and for growth of the first leaf, 5–6 ppm (Mitsuishi 1975). Hence, 5–6
ppm oxygen is necessary for seedling growth after the appearance of the coleoptile.
In rice cultivation, appearance of the coleoptile itself is not meaningful unless
it is followed by the roots and the first and second leaves.