Yoshida - 1981 - Fundamentals of Rice Crop Science

CLIMATlC ENVIRONMENT AND ITS INFLUENCE 75
2.6. Effect of temperature on respiratory rate of the rice plant
(average of 4 varieties, 30-day-old plants grown in culture solution)
(adapted from Tanaka et al 1966).
The mean optimum temperature for ripening of japonica rice in Japan is reported
as about 20°–22°C (Aimi et al 1959, Matsushima et al 1957, Matsushima and
Tsunoda 1958). This optimum temperature agrees with the results of a statistical
analysis of the effects of climatic factors on rice yield (see equation 6.1). The
temperature during ripening appears to affect the weight per grain. The 1,000-
grain weight of the same variety is almost constant under different environments
and cultural practices. However, the 1,000-grain weight of the same variety varied
from about 24 g at a mean temperature of 22°C in the 3-week period after heading
to 21 g at a mean temperature of 28°C in Kyushu, southern Japan (Murata 1976).
In the tropics, a daily mean temperature as high as 29°C is not detrimental to
ripening when solar radiation is high. Yields of 8–10 t/ha are obtained at Los
Baños, Philippines, when rice crops ripen in April and May, 2 months characterized
by high temperatures and high solar radiation. This suggests that indica
varieties are better adapted to high temperatures, while japonica varieties require
low temperature for better ripening. A controlled-temperature experiment showed
that the optimum daily mean temperatures for grain filling range from 19° to 25°C
for IR20, an indica rice, and from 16° to 22°C for Fujisaka 5, a japonica rice (Fig.
2.7). Note that there is no sharp optimum temperature for grain filling, but there is
a range of optimum temperatures.
As discussed in Chapter 1, the length of ripening is inversely correlated with
daily mean temperature. Thus, persistent cloudy weather conditions will be more
detrimental to grain filling under high temperatures because of a shorter ripening
period. In fact, a combination of high temperatures and low light can seriously
impair ripening (Table 2.5). Since grain weight ( W ) and percentage of filled
spikelets ( F ) are both affected by light and temperature, the overall effect of the
light-temperature interaction on grain filling can be assessed by computing the
ripening grade ( W × F ).