Vergara - 1976 - Physiological and morphological adaptability of ri

‘ITELD AND YIELD COMPONENTS OF LOWIAND RICE IN THE TROPICS 487
the tltiivering time of rice comes in summer when daily maximum temperature
reaches 33°C or even higher. A ripening period shortened by high temperature.
combined with cloudy weather conditions, is believed to be a major factor in
impaired ripening. Matsushima et al. (1957) demonstrated that a combination
of high temperature and low solar radiation seriously impaired ripening.
Adverse climaticfactor-s. The effect of adverse weather conditions on sterility
or unfilled-grain percentage is straightforward.
Low temperature is a cause of sterility‘ in cool regions. Air temperature as, low
as 15° to 19°C at the meiotic stage of pollen mother-cells (about 10-11 days
before heading) causes very high sterility (Satake. 1969). This occurs in areas at
high latitudes in the temperate region (Ishizuka cl al., 1973) and also at high
altitudes of the tropics (Vergara et al., 1970). The optimum mean daily temperature
for ripening ofjaponica rice has been reported to be about 2O to 22°C
(Matsushima et al.. 1957; Matsushima and Tsunoda, 1958; Aimi et al., 1959;
Tanaka. 1962). Temperatures lower than the optimum may delay ripening and
eventually decrease filled-grain percentage (Matsushima et al.. 1957; Aimi et al..
1959; lanaka, 1962', Wada, 1969). When daily" mean temperature was maintained
at the optimum temperature (21.5°C), a temperature difference as large as
15°C between day and night improved ripening (Moriya and Nara, 1971).
High temperature appears to be a causal factor of sterility and impaired
ripening (Moriya and Nara, 1971', Kusanagi and Washio, 1973; Sato et al.,
1973). In a controlled growth-cabinet experiment, Moriya and Nara (1971)
observed high percentages of sterility and partially‘ filled grains when the rice
plant was exposed, at heading time, to a daily mean temperature of 315°C.
daily maximum temperature being 36°C. and daily minimum. 27°C. Similarly.
Sato et al. (1973) and Kusanagi and Washio (1973) observed in controlledenvironnient
facilities high percentages of sterility and impaired ripening at
daily maximum-minimum temperatures of 35°—3t')°C and of 35°—25°C, respectivelyt
In Cambodia. Sato (1960) observed 100% sterility when rice crops flowered
in April. The sterility was attributed to failure of pollination under high tempera~
tures and low humidities.
1n the central plains of Thailand. (lsada ( 1972) observed high percentages of
empty (sterile) grains in dry season. He attributed the occurrence of empty grains
in this season to high temperatures at flowering time. Varietal difference was very
conspicuous in his observation. The percentage of sterility went up to as high
as 44% for selection BKN 6624—46—2.\vltile it vras less than 10% for variety 1R8.
RDI, and Taichung Native 1. 1n some other countries like Pakistan and Egypt,
ltiglt percentages of sterility occur when temperature is high at lloivering time.
Thus. critical studies of high temperature injury merit more attention.
High percentages of sterility’ may occur at high nitrogen levels combined with
low solar radiation prior to heading, and also under rainy vreather conditions
(Togari and Kashivt-‘akura. 1958).
Strong winds can be a direct cause of sterility at floutering time by desiccating