Yoshida und Hara - 1977 - Effects of air temperature and light on grain fill

Temperature <strong>and</strong> Light on Rice Grain Filling 103
nation <strong>of</strong> high <strong>light</strong> intensity <strong>and</strong> a low <strong>temperature</strong> gave the maximum ripening grade
for both rice varieties (Table 4). Taking the maximum value as 100 for each variety,
the ripening grade was reduced by 36 percent in IR20 <strong>and</strong> 18 percent in Fujisaka 5
when the <strong>light</strong> intensity was low <strong>and</strong> the <strong>temperature</strong> was high. Apparently, grain
filling, <strong>and</strong> hence, the yield <strong>of</strong> IR20 is more likely to be reduced by adverse climatic
environments than are those <strong>of</strong> Fujisaka 5 rice.
DISCUSSION
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The length <strong>of</strong> the grain filling period markedly affects the yield <strong>of</strong> rice because it
affects the maximum amount <strong>of</strong> solar energy available for grain filling. Additionally,
it constitutes the last part <strong>of</strong> the rice plant's life cycle; this knowledge about the grain
filling period can contribute to the overall <strong>und</strong>erst<strong>and</strong>ing <strong>of</strong> the total growth duration
<strong>of</strong> rice varieties.
Existing information on the effect <strong>of</strong> environmental factors <strong>and</strong> (or) plant physiological
factors on the length <strong>of</strong> the grain filling period <strong>of</strong> rice is scarce. An.n et al. (1)
reported that while the grain filling <strong>of</strong> a japonica rice was completed in about 25 days
after anthesis at a daily mean <strong>temperature</strong> <strong>of</strong> 25°C, it took much longer when the daily
mean <strong>temperature</strong> was 21 °C, <strong>and</strong> was not complete even after 75 days from anthesis
when the daily mean <strong>temperature</strong> was 17°C. Their experiment was conducted in
<strong>temperature</strong>-controlled glasshouse rooms where day <strong>and</strong> night <strong>temperature</strong>s were held
constant. NAGATO <strong>and</strong> EBATA (11, 12) also measured the rate <strong>of</strong> grain filling <strong>und</strong>er
different <strong>temperature</strong> regimes. When YAMAKAWA (24) planted 12 rice varieties in
the field in Japan at different times so that these rice crops would be exposed to different
daily mean <strong>temperature</strong>s during the ripening period, he showed that japonica rice
ripened over 30 days when the daily mean <strong>temperature</strong> was about 29°C, while ripening
took 53 days when the daily mean <strong>temperature</strong> was 18°e. The time for maturity was
determined by visual examination in this experiment.
In the present study, rice grains were fo<strong>und</strong> to mature faster than was expected,
only 13 days were required for the upper grains <strong>of</strong> IR20 rice to reach the maximum
weight when the daily mean <strong>temperature</strong> was 28°C, <strong>and</strong> 18 days for Fujisaka 5 rice.
When the daily mean <strong>temperature</strong> was lower, 16°C, the upper grains <strong>of</strong> IR20 rice
achieved maximum weight in 33 days, whereas those <strong>of</strong> Fujisaka 5 rice required 43 days.
The critical low <strong>temperature</strong> for photosynthesis has been reported as IS.5°C for
japonica rice (23) <strong>and</strong> between 20° <strong>and</strong> 25°e for indica rice (14). In the experiment
<strong>of</strong> Aimi et al., the day <strong>and</strong> night <strong>temperature</strong>s were both 17°0 which is a subnormal
<strong>temperature</strong> for photosynthesis <strong>of</strong> japonica rice, In our experiment, however, at a
daily mean <strong>temperature</strong> <strong>of</strong> 16°0, the day <strong>temperature</strong> was held at 20°C, which is above
the critical low <strong>temperature</strong> for photosynthesis <strong>of</strong> japonica rice. This difference in the
<strong>temperature</strong> regimes may explain the variation in the results obtained in the present
study <strong>and</strong> those <strong>of</strong> Aimi et aI,
It is normally assumed that <strong>light</strong> intensity has little or no effect on the length <strong>of</strong> the