Yoshida - 1981 - Fundamentals of Rice Crop Science

46 FUNDAMENTALS OF RICE CROP SCIENCE
The temperature summation in equation 1.13 is computed only for days when
daily mean temperature is greater than the predetermined threshold temperature.
Different threshold temperatures (ranging between 5° and 15°C) are used for
different crops. The temperature summation is also called heat units, heat sum, or
degree-days (Chang, J.H. 1968, Lowry 1967).
The concept of temperature summation presupposes that the growth or
development of a plant is linearly related to temperature or the total amount of heat
to which it is exposed. For example, if it takes 100 days from germination to
flowering at an overall daily mean temperature of 20°C, temperature summation is
2,000 degree-days (20°C × 100 days). It follows then that the same plant will take
80 days to flower at 25°C (2,000 degree-days/25°C). It must be kept in mind that a
linear relationship between developmental rate and temperature is an essential
empirical basis for the use of temperature summation.
For more than 200 years, plant ecologists, climatologists, and crop scientists
have used temperature summation to relate temperature to crop duration and
productivity. For example, temperature summation was used to examine whether
rice should be introduced into Hokkaido, northern Japan, where summer is short
and, therefore, temperature determines the crop species to be grown (Nagai 1962).
Often temperature and crop productivity are well correlated because there is a
close association between temperature and solar radiation. Solar radiation, however,
is the primary climatic determinant for crop productivity. Since it is more
direct and rational to examine the relationship between solar radiation and crop
productivity, several refined crop productivity models based on photosynthesis
and respiration have been proposed (Monteith 1972).
b. Response of rice varieties to temperature. Temperature summation varies
with the maturity of a variety; early-maturing varieties have smaller temperature
summations than late-maturing ones. Temperature summation from sowing to
heading for selected Japanese varieties ranges from about 1,000 to 3,000 degreedays
depending on varieties, latitudes, and planting seasons (Toriyama et al
1969). Ripening requires an additional 700–800 degree-days (Ishizuka et al 1973,
Komoda 1958). Consequently, from sowing to maturity, a rice crop requires about
2,000–4,000 degree-days, which corresponds to 80–160 days when grown at a
mean temperature of 25°C.
Generally, a crop planted in early spring in the temperate region requires a larger
temperature summation because early-spring temperatures are too low to be
effective. Similarly, a variety adapted to a warm climate, when planted in cool
regions, requires a larger temperature summation because it has a high threshold
temperature. The estimated threshold temperatures for some Japanese varieties
range from 9º to 18°C (Asakuma 1958). If the threshold temperature is subtracted
from the daily mean temperature, the temperature summation for the sowing-toheading
period ranges from about 600 to 1,000 degree-days, depending on variety.
The relationship between temperature and growth duration can be examined
experimentally by growing photoperiod-insensitive varieties in temperaturecontrolled
facilities. In recent years, growth chambers and the phytotron have