National Agriculture and Food Research Organization (NARO) has established an international monitoring network (MINCERnet) of micrometeorology in rice paddy field, which has revealed that heat-induced spikelet sterility (HISS)1 of rice in paddy fields can be evaluated by using the panicle temperature during the flowering time as an indicator, rather than air temperature under various climates. Under hot and humid climates, the risk of HISS was estimated to be high because the panicle temperature tends to be high due to the low evaporative cooling effect2 associated with canopy and panicle transpiration. By utilizing this evaluation method, it will be possible to improve the accuracy of predicting HISS worldwide and to take appropriate adaptation measures against HISS under global warming.
Overview
Rice is a staple food for about half of the world's population, and paddy rice is grown in various climate zones. There is concern about the increase in heat stress due to global warming. In particular, heat-induced spikelet sterility (HISS), which occurs when rice panicles are exposed to high temperatures during flowering, is thought to greatly reduce rice productivity. National Agriculture and Food Research Organization (NARO) has established an international monitoring network (MINCERnet) of micrometeorology in rice paddy field, covering 11 countries and regions in Asia, Africa, and America, and has conducted observations on the thermal environment within the rice canopy and HISS in rice paddy fields in rice growing regions around the world. By analyzing this monitoring data and the heat-balance model to calculate the panicle temperature in paddy fields, we have revealed for the first time in the world that HISS in paddy fields can be evaluated by using the panicle temperature during the flowering period as an indicator, rather than air temperature under various climates.
Panicle temperature depends not only on the air temperature above the canopy, but also on the evaporative cooling effect of canopy and panicle transpiration. In dry climates, the transpiration is stimulated and has a larger cooling effect, resulting in lower panicle temperature than the air temperature above the canopy, while in wet climates, the transpiration is suppressed and has a smaller cooling effect, resulting in higher panicle temperature than the air temperature. As a result, it was found that areas with higher air temperatures did not necessarily coincide with areas with higher panicle temperatures. The sterility rate was highly correlated with panicle temperature rather than with air temperature above the canopy, indicating that HISS can be evaluated under various climatic conditions by using panicle temperature during flowering time as an indicator. It was estimated that the risk of HISS is low in dry climates even in extremely hot climates, while the risk of HISS is high in hot and humid climates.
Thus, the evaluation method using the panicle temperature during flowering time as an indicator is not only useful for identifying areas at high risk of HISS at present, but is also expected to become an important keystone for realizing stable rice production in the world under climate change by improving the prediction accuracy of future rice yield and evaluating the effectiveness of adaptation measures for reducing HISS under global warming.
1 Heat-induced spikelet sterility (HISS)
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. 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.
2 Evaporative cooling effect
Transpiration is the evaporation of water from the plant body as water vapor through leaf stomata. Transpiration lowers plant body temperature because heat of vaporization is lost when water evaporates. This is the same principle as in the case of humans, where evaporation of sweat in dry weather conditions lowers the body surface temperature, making it feel cooler. On the other hand, in very humid weather conditions, where sweat does not evaporate easily and humans are prone to heat stroke, plant body temperature tends to rise.
Relevant Information
Budget: Global Environmental Research Coordination System from Ministry of the Environment of Japan (MAFF1531 and MAFF1842), Japan Society for the Promotion of Science KAKENHI Grant Number JP15H02650.
Reference Information
1. NARO Press Release - Jan 18, 2022
Heat-induced sterility of rice observed in paddy fields in Japan in an extremely hot summer
- Development of a model to predict the rice sterility under current and future weather conditions -
2. Mayumi Yoshimoto, Minehiko Fukuoka, Yasuhiro Tsujimoto, Tsutomu Matsui, Kazuhiro Kobayasi, Kazuki Saito, Pepijn A.J. van Oort, Baba I.Y. Inusah, Chenniappan Vijayalakshmi, Dhashnamurthi Vijayalakshmi, W.M.W. Weerakoon, L.C. Silva, Tin Tin Myint, Zar Chi Phyo, Xiaohai Tian, Huu-Sheng Lur, Chwen-Ming Yang, Lee Tarpley, Norvie L. Manigbas, Toshihiro Hasegawa (2022) Monitoring canopy micrometeorology in diverse climates to improve the prediction of heat-induced spikelet sterility in rice under climate change. Agricultural and Forest Meteorology, 316, 108860. http://doi.org/10.1016/j.agrformet.2022.108860
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