Abstract
The unusually cool summer of 1980 caused the severe sterility of rice plant grown in Tohoku District (the northeastern part of Honshu), due to the low temperature at the booting stage. Although the maximum and minimum temperatures were essentially similar between Takizawa on the Pacific Ocean side and Kuroishi on the Japan Sea side (Table 1), the sunshine duration during the booting stage at Takizawa was much shorter than that at Kuroishi, due to topographical characteristics. Spikelet sterility of rice plants at the former was much higher than that at the latter, suggesting the decisive role of the sunshine duration on the microclimate of a rice canopy. A simulation model for the prediction of canopy climate (Fig 1) proposed by INouE was applied to elucidate the main causes of differences in spikelet sterility at two locations. Numerical experiment by this model were conducted during the period from July 16 to 18 in 1980. During this time the Okhotsk Sea-anticyclone spread southwestward covering the northern part of Japan and had a strong influence on the weather conditions and rice growth in this area. The results of the numerical experiments were summerized as follows: 1. Using the observed meteorological conditions (Fig. 3), the effect of solar radiation on the temperature of the air, leaf and flood water in the rice canopy was simulated by our model (Fig. 4 and 5). Although the degree of chilliness above the rice canopies defined by Σ (20-Ta) was approximately the same at both locations, the air and leaf temperatures at the height of the young panicles in the rice canopy were lower at Takizawa than those at Kuroishi. Therefore the degree of chilliness of young panicles was estimated to be about twice greater at Takizawa than at Kuroishi (Table 2). This correlation suggests that the difference in spikelet sterility was mainly due to the difference in the temperatures of young panicles in the rice canopy. 2. The simulation results indicate that irrigation with a 20 cm deep flood improves canopy climate in cool weather conditions (Fig. 6 and 7). Rice plants grown in most parts of Japan can be protected from severe sterility due to low temperature at the booting stage, if the paddy fields are irrigated with a 20 cm deep flood during the cool period at the booting stage of the rice plants. 3. The net photosynthesis at Kuroishi was estimated at about 2.5 time as large as that at Takizawa (Table 3) reflecting the difference of solar radiation. 4. The model for the prediction of canopy climate was proved to be applicable for the precise evaluation of effect or micrometeorological conditions in the rice canopy.
