Analysis of climatic conditions responsible for degradation of rice grain quality in miyagi prefecture, Japan

The percentage of the first grade rice grain is generally lower in Miyagi prefecture than in other prefectures of Tohoku district ; it is particularly low in the Sennan area in southern Miyagi prefecture. However, the reason for the inferior grain quality in the Sennan area has not been clarified. We examined the climatic factors responsible for the regional difference in the degradation of rice grain quality in Miyagi, particularly in the Sennan area. Meteorologic data (temperature, sunshine hours) during specific growth stages (panicle formation stage, 20 days after heading, and 40 days after heading) in the past 28 years were compared among several regions in Miyagi. Regardless of the region, degradation was marked when daily minimum air temperature during panicle formation stage was below 20°C, or when the average of daily mean and minimum temperatures for 20 days after heading exceeded 25°C and 22-23°C, respectively. Even in such years, the first grade grain percentage was significantly lower in the Sennan area than in the other order areas, suggesting that unknown factors other than air temperature are responsible for the degradation of rice quality in this area. When daily minimum temperature during panicle formation stage declined below 20°C, or when the average of daily mean and minimum temperatures for 20 days after heading did not exceed 25°C and 22-23°C, respectively, the degradation of rice quality in the Sennan area appeared to be caused by fewer sunshine hours during the 20 days after heading.