The decline in the ratio of first-grade quality rice grains has been more serious issue with the recent progressive global warming. Meanwhile National Agriculture and Food Research Organization(NARO) quantitatively assessed the superior grain appearance of 'Niji-no-kirameki' to that of the Japanese leading cultivar 'Koshihikari' as the result of a four-year survey in the heat-vulnerable regions of Japan. In addition, it was clarified that 'Niji-no-kirameki' has a "heat avoidance" mechanism in which the panicle temperature can be maintained cooler than 'Koshihikari' under high temperature conditions during the ripening period.
Overview
In recent years, the deterioration of the appearance quality of brown rice due to the high temperature during the ripening period along with the global warming has become a problem. For example, in 'Koshihikari', the leading rice cultivar in Japan, the occurrence of chalky grains increases when the daily average temperature (which has a great effect on the appearance quality of brown rice) for 20 days after heading reaches 27℃ or higher, and the perfect grains decreases significantly. Therefore, in the midst of recent global warming, it is difficult to secure perfect grain percentage of about 70%, which is a threshold to be the first-grade quality rice.
In this regard, with the cooperation of public research institutes and producers in Niigata, Gunma, and Gifu prefectures where the 'Niji-no-kirameki' is widely distributed, a four-year survey of the temperature and ratio of perfect grain of 'Niji-no-kirameki' was conducted. As a result, it was clarified that perfect grain percentage can be maintained at about 70% even when the average daily temperature for 20 days after heading was even at a high temperature of 28℃. In addition, it was clarified for the first time in the world that there is "heat avoidance" in 'Niji-no-kirameki' in which the panicle temperatures can be maintained cooler than 'Koshihikari' under high temperature conditions. "Heat avoidance" mechanism is hypothesized to be because panicles are hidden in the leaves and hence (i) the amount of solar radiation to the ears is reduced, (ii) the leaf around the ear is protected by transpiration cooling. From these factors, it is inferred that the appearance quality of brown rice of 'Niji-no-kirameki' can be maintained at the high level even in the hot climates.
The results of this research outcome provide valuable basic information on formulating both cultivation and breeding strategies of the new rice genotypes with upgraded heat-resilience during the ripening period. Simultaneously, it contributes to high-level stabilization of brown rice appearance quality in the midst of worsening global warming.
Relevant information
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Publication
Tsutomu ISHIMARU, Masaki OKAMURA, Ichiro NAGAOKA, Hiromichi YAMAGUCHI, Mayumi YOSHIMOTO, Youichi OHDAIRA (2022) Quantitative assessment on the grain appearance of a new Japanese rice cultivar 'Niji-no-kirameki' with a novel heat-avoidance mechanism during ripening. Plant Stress, 4, 100074. doi.org/10.1016/j.stress.2022.100074
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