Yoshida - 1981 - Fundamentals of Rice Crop Science

10 FUNDAMENTALS OF RICE CROP SCIENCE
difficult to relate them to ecological, botanical, or agronomical characteristics.
Geographical distribution of indica and japonica rices suggests that the latter
may germinate better at low temperatures. In fact, some japonica rices germinate
better than indica rices at low temperatures. But it is also true that some other
indica rices germinate better at low temperatures than some other japonica rices.
Thus, it is difficult to generalize a simple relationship between the two rices.
Germination at low temperatures may be complicated by seed dormancy. There
is clear evidence that some indica rices, when the dormancy has been broken, can
germinate at low temperatures as well as or better than japonica rices (Table 1.3).
Local selection pressure, related to either cultural practices or local environments.
may have caused differential selection of varieties better adapted to low temperature.
At 15.6°C, a California variety, Caloro, germinated and grew better than
most Japanese varieties tested (Ormrod and Bunter 1961). In California, rice is
directly sown into cool-water irrigated fields, while in Japan rice seedlings are
grown in the protected nursery beds where high temperatures are maintained. Quite
often, traditional varieties in Japan germinate better than improved varieties at low
temperatures. Before the use of protected nursery beds. those traditional varieties
were raised in unprotected nursery beds and thus were inadvertently selected for
low temperatures (Toriyama 1962).
1.3. SEEDLING
1.3.1. Germination and seedling emergence
Germination, defined as the appearance of the white tip of the coleoptile, and
subsequent growth of the coleoptile are a continuous phase that occurs within a
short time after a seed is placed in soil or water. Seedling emergence is the time
Table 1.3. Germination of indica and japonica varieties at the
fastest germination stage a under low temperatures (9–11°C). b
Average number
Variety Germination of days required
(%) for germination
Kaladumai
IR8
Dharial
Stirpe
Koshihikari
Toyonishiki
Sasanishiki
Honenwase
99
99
96
86
96
66
60
82
Indica
Japonica
7.5
10.6
13.8
9.6
16.6
20.8
20.1
21.3
a The fastest germination stage refers to the time when dormancy
has been broken and consequently germination rate is maximum.
b lkehashi (1973).