- Development of a model to predict the rice sterility under current and future weather conditions -
National Agriculture and Food Research Organization (NARO) has conducted field survey in paddy rice in 2018, when high temperature lasted over a wide area in Japan, and found that a higher-than-normal percentage of sterility; heat-induced spikelet sterility (HISS)1 occurred in paddies that encountered high temperatures during the flowering period. By investigating the relationship between the sterility rate and meteorological factors, a simulation model was developed to estimate the sterility rate in paddy fields based on the panicle temperature during the flowering period. The model estimation showed that HISS may be occurring in paddy fields in Japan, depending on the timing of rice flowering and the frequent heat waves in recent years. The results can improve the prediction accuracy of rice yields and provide basic data for taking appropriate adaptation measures against HISS under climate change.
Global warming will likely reduce rice yields due to heat stresses. One of the major concerns in heat stresses in rice is heat-induced spikelet sterility (HISS) at flowering, in which the panicles are exposed to high temperatures during flowering, which inhibits pollination and results in sterile grains(empty grains). There have been reports of reduced rice yields due to HISS in tropical regions and the Yangtze River basin in China. While in Japan, there are no reports of the actual situation of HISS in paddy fields, except for a survey conducted during the unusually hot summer in 2007. There was also a lack of knowledge on the conditions for the occurrence of HISS in the paddy fields.
In recent years, extremely high temperatures in summer have frequently occurred in Japan, and in 2018, high temperatures lasted over a wide area in the Kanto, Tokai, and Kinki regions from mid-July to early August, which included flowering period for many rice varieties, raising concerns about the occurrence of HISS in paddy fields. Therefore, researchers at NARO, in cooperation with public experiment stations and farmers in eight prefectures (Ibaraki, Chiba, Gunma, Saitama, Gifu, Aichi, Mie, and Kyoto), conducted a field survey on the occurrence of HISS in paddy fields of Koshihikari, a major variety of rice in Japan.
A higher-than-normal percentage of sterility occurred in paddies that encountered high temperatures during the flowering period. The sterility rate was highly correlated with the daytime panicle temperature during the flowering period, which began to increase when the panicle temperature exceeded about 33℃. Based on the results, a simulation model was developed to estimate the sterility rate in paddy fields using the panicle temperature as a predictor. The model was applied to the weather conditions during the entire summer season in the surveyed paddy fields in eight prefectures, which showed that the sterility rate was high at several times and places, other than the actual survey of high sterility rate.
Although it was previously predicted that HISS in Japan would become apparent around the time when global warming progressed to the current temperature range in the tropics, the results indicate that HISS may occur in paddy fields in Japan even under the current climatic conditions, depending on the timing of rice flowering and the heat waves. However, the increase in sterility does not necessarily result in yield reduction, because the final yield is determined not only by the sterility rate, but also by other factors such as the total number of grains and the amount of photosynthates. The results can improve the prediction accuracy of rice yields and provide basic data for taking appropriate adaptation measures against HISS under climate change.
1   Heat-induced spikelet sterility (HISS)
Sterility is a phenomenon in which spikelets do not grow due to failure in pollination or fertilization. Rice originally includes physiologically sterile spikelets about a several percentages even under normal environment, but sterility can be increased by many environmental stresses, such as high/low temperatures, drought, and salinity, and can result in yield reduction. As global warming progresses, there is concern about heat-induced spikelet sterility (HISS) due to high temperatures. When the panicles are exposed to high temperatures during flowering, pollination fails because anther dehiscence is inhibited or, even if anther dehisces, pollens do not shed from anthers onto the stigma. In laboratory experiments, it has been reported that HISS begins to occur when the temperature exceeds about 35℃, and that a 1℃ increase results in a 16% increase in sterility rate. However, the actual occurrence of sterility in paddy fields had not been clarified, because the thermal environment in the paddy fields is different from the environment of the laboratory experiments.
Mayumi YOSHIMOTO, Hidemitsu SAKAI, Yasushi ISHIGOOKA, Tsuneo KUWAGATA, Tsutomu ISHIMARU, Hiroshi NAKAGAWA, Atsushi MARUYAMA, Hitoshi OGIWARA, Kenji NAGATA (2021) Field survey on rice spikelet sterility in an extremely hot summer of 2018 in Japan. Journal of Agricultural Meteorology, 77 (4), 262-269, doi:10.2480/agrmet.D-21-00024