Yoshida - 1981 - Fundamentals of Rice Crop Science

GROWTH AND DEVELOPMENT OF THE RICE PLANT 59
1.49. Changes in the water content at successive stages of growth of 2 varieties of
rice under 3 temperature regimes (Yoshida and Hara 1977).
decrease in water content. The rate of grain growth is faster and the grain filling
period is shorter at higher temperatures (Fig. 1.48). The duration of grain filling,
defined as the number of days required to reach maximum weight, is 13 days at a
mean temperature of 28°C, and 33 days at 16°C for IR20, an indica rice. Fujisaka
5, a japonica rice, takes a little longer to ripen: 18 days at a mean temperature of
28°C and 43 days at 16°C. Comparable figures for a field crop are much greater than
those for a single grain because time of heading and anthesis vary among panicles
and among spikelets within the same panicle. Hence, the crop, as a whole, takes
a longer time to ripen than does a single grain. The final grain weights attained at
high and low temperatures are about the same for IR20, suggesting that IR20 is
well adapted to high temperatures during ripening. In Fujisaka 5, however, the
final grain weight at 28°C is about 15% less than that at 16°C. Thus, high
temperatures appear to have some detrimental effect on Fujisaka 5. Their detrimental
effect is much more conspicuous in wheat (Sofield et al 1974).
During the initial phases of grain filling, the water content of the grains is about
58% and it declines to about 20% or less at maturity (Fig. 1.49). As temperatures
increase, water content decreases faster.
1.11.3. Grain growth of a field crop
Grain growth of a field crop is initially slow, enters a linear phase where the growth
rate is fast, and then slows down toward maturity (Fig. 1.50).
Agronomically, the duration of ripening is from the date of heading to the time
when the maximum grain weight is attained. The time of harvest is usually
determined by past experience, grain color, and leaf senescence. The length of
ripening, predominantly affected by temperature, ranges from about 30 days in the
tropics to 65 days in cool temperate regions such as Hokkaido, Japan, and New
South Wales, Australia (Tanaka and Vergara 1967).
In crop physiology, the effective grain-filling period (T) or the period of
physiological maturity is defined as shown in Figure 1.50. The length of T is
considered more meaningful for grain growth than the length of the agronomic