Yoshida - 1981 - Fundamentals of Rice Crop Science

72 FUNDAMENTALS OF RICE CROP SCIENCE
where temperature is in degrees centigrade, and h is the altitude above sea level in
hectometers. For example, if a monthly mean temperature at sea level is 30°C, the
corresponding temperature at 1,000 m above sea level would be 24°C. While
the above equation gives an easy way to estimate temperature at a given elevation
under most conditions, it must be used with caution because the local variations of
the temperature drop with elevation are sometimes quite large.
2.3.2. Critical low and high temperatures
Extreme temperatures are destructive to plant growth and, hence, define the
environment under which the life cycle of the rice plant can be completed.
The critically low and high temperatures, normally below 20°C and above 30°C,
vary from one growth stage to another (Table 2.4). These critical temperatures
differ according to variety, duration of critical temperature, diurnal changes, and
physiological status of the plant.
Subjecting the rice plant to temperatures below 20°C at about the reduction
division stage of the pollen mother cells usually induces a high percentage of
spikelet sterility (Satake 1969). Differences among rice varieties in response to
low temperatures at this stage have been clearly demonstrated. When Norin 20, a
susceptible variety, was held at 15°C for 4 days, 51% of the spikelets were sterile.
Hayayuki, a tolerant variety, under the same conditions produced only 5% sterile
spikelets.
Temperatures as low as 12°C will not induce sterility if they last for only 2 days,
but will induce about 100% sterility if they last for 6 days.
Low temperature-induced sterility is normally attributed to low night temperatures.
High day temperatures, however, appear to alleviate the effects of low night
temperatures (see Table 2.7). When the plant was subjected to a constant night
Table 2.4. Response of the rice plant to varying temperatures
at different growth stages. a
Growth stage
Critical temperature b (°C)
Low High Optimum
Germination
Seedling emergence
and establishment
Rooting
Leaf elongation
Tillering
Initiation of panicle
primordia
Panicle differentiation
Anthesis
Ripening
10
12–13
16
7–12
9–16
15
15–20
22
12–18
45
35
35
45
33
–
38
35
30
20–35
25–30
25–28
31
25–31
–
–
30–33
20–25
a Adapted and modified from Yoshida (1977a). b Refers to daily
mean temperature except for germination.