Yoshida - 1981 - Fundamentals of Rice Crop Science

CLIMATIC ENVlRONMENT AND ITS lNFLUENCE 83
2.10. Fertility of the spikelets that flowered at different day temperatures
(Satake and Yoshida 1978).
2.3.7. Interaction between temperature and nutrient supply
a. Number of spikelets per square meter and amount of nitrogen absorbed. Under
most conditions, the major determinant of rice yield is total number of spikelets per
square meter. There is a good correlation between the number of spikelets per
square meter and total nitrogen uptake at heading: as the amount of nitrogen
absorbed by the crop by heading increases, the number of spikelets per square
meter increases (Fig. 2.11). The efficiency of nitrogen use in producing spikelets is
higher in northern than in southern Japan, and data obtained from Los Baños,
Philippines, are similar to those from the latter. That suggests that a cool climate
favors higher nitrogen efficiency.
A minicrop experiment in the phytotron indicates that number of spikelets per
square meter increases as temperature decreases at a given level of nitrogen, and
this effect is most significant at the highest level of nitrogen (Table 2.10).
However, the efficiency was higher at lower temperatures and lower nitrogen
levels. Thus, temperature appears to be a major climatic factor that affects the
efficiency at which nitrogen produces spikelets.
b. Low temperature-induced sterility and nitrogen. Nitrogen fertilization
affects sterility caused by low temperatures at the reduction division stage. When
temperatures are above or far below the critical, nitrogen supply has little effect on
sterility. At moderately low temperatures (16°C), however, the percentage sterility
increases with an increasing nitrogen supply (Fig. 2.12).