Yoshida - 1981 - Fundamentals of Rice Crop Science

GROWTH AND DEVELOPMENT OF THE RICE PLANT 47
1.39. The relationship between
temperature and days to heading
for 4 photoperid-insensitive varieties
grown in temperaturecontrolled
glasshouses (IRRI
1975).
become commercially available and are being used for many such studies.
The responses of four photoperiod-insensitive varieties to constant temperatures
are shown in Figure 1.39. There are clear varietal differences in the number of days
to heading for each temperature in terms of temperature summation or heat
requirement. Yukara, a variety from northern Japan, has a much lower temperature
summation value than IR8 and IR26, both tropical varieties. The shape of the
response curves is basically the same and typical for many other crops. Within a
daily mean temperature range of 21°-30°C. the number of days to heading is not
linearly related to temperature. When temperature drops from 24° to 21°C, there is
a sharp increase in days to heading. For example, the number of days to heading
for IR26 increases from 96 days at 24°C to 134 days at 21°C. In other words, a
temperature drop by 1°C causes a 13-day delay in heading. When the temperature
is increased above 24°C, however, days to heading decreases to 91 days at 27°C
and to 86 days at 30°C. In other words, a temperature rise of 1°C above 24°C
shortens the number of days to heading by less than 2 days, These results appear to
indicate that temperatures above 24°C are much less effective than those below
24°C, suggesting the existence of a ceiling temperature. This idling effect of high
temperature on growth duration is similar to that observed in the field. When
IR747B2-6 is grown in the field throughout the year at Los Baños, Philippines,
where monthly mean temperatures change from 25.3°C in January to 29.4°C in
May, it matures in 95 days regardless of planting time.
A generalized relationship between temperature and length of time required to
complete development is shown in Figure 1.40. The curve indicates a temperature
below which the plant will not grow. Also, there is an intermediate optimum
temperature that permits the most rapid development. Above the optimum,