Yoshida - 1981 - Fundamentals of Rice Crop Science

GROWTH AND DEVELOPMENT OF THE RICE PLANT 7
about 21% of the total grain weight (Table 1.1). For convenience, the proportion
of hull to grain is considered 20%. Conversion factors of 0.8 and 1.25 are usually
used to obtain brown-rice weight from grain weight, and grain weight from
brown-rice weight, respectively.
1.2.2. Dormancy
Most indica rices have a grain dormancy of several weeks or longer, a desirable
varietal trait in tropical areas where rains frequently occur during the harvesting
period and temperature stays at optimum for germination all the year round.
Nondormant or weakly dormant grain often germinates in situ under such conditions
and especially when the crop lodges into standing water.
Several methods to break rice dormancy are available. The simplest is to heat
the seed at about 50°C in an open system for 4-5 days. Seed exposed directly to
heat is not injured, but seed sealed to prevent loss of moisture may be killed when
heating is prolonged. Varieties differ markedly in their response to the heat
treatment. As shown in Table 1.2, the dormancy of most varieties was broken by
the 4-day heat treatment at 49°C. Five varieties, however, had under 50%
germination even after the treatment. The dormancy of variety Seraup 27 was only
slightly broken, but increased to 84% after a 10-day heat treatment at 50°C. Within
a range of 50°–65°C, the treatment's duration was more effective in breaking
dormancy than the temperature.
1.2.3. Germination
Germination starts when seed dormancy has been broken, the seed absorbs
adequate water, and is exposed to a temperature ranging from about l0° to 40°C.
a. Water absorption. The germination process may be subdivided into
imbibition, activation, and postgermination growth stages (Fig. 1.5). When
soaked, a seed rapidly absorbs water for the first 18 hours. This water intake,
caused by imbibition forces, increases the seed’s water content to between 25 and
35%. Although the duration of this stage is almost independent of temperature,
temperature influences water content (Fig. 1.6). Water absorption is not affected
by respiratory inhibitors, and the Q 10 value is much lower than 2.
Water absorption at the activation stage is negligible compared with that at the
imbibition stage. The duration is dependent on temperature and increases at lower
temperatures. Germination occurs as a consequence of very active metabolic
changes during the activation stage (Takahashi, N. 1965). The water content of the
seed at the onset of germination varies with temperature; it ranges from about 30 to
40%.
At the end of the activation stage, the white tip of the coleoptile emerges from the
ruptured seed coat. This stage is called germination. Subsequently, the coleoptile
grows and the first leaf appears.
b. Effect of temperature. Temperature has a profound influence on germination
by affecting the activation stage and postgermination growth.
The effects of temperature on germination can be examined in three aspects:
temperature, time, and germination percentage.