Revealing the spatial characteristics of rice heat exposure in Japan through panicle temperature analysis

Abstract

　Elevated temperatures during the flowing stage can induce spikelet sterility in rice, posing a major threat to production under climate change. Projecting the impacts and developing effective strategies are critical, but our understanding of regional, seasonal, and long-term trends in rice heat exposure remains limited. Previous studies on spikelet sterility revealed that panicle temperature, estimated using a micrometeorological model and common meteorological factors, serves as a reliable indicator of rice heat exposure. In this study, we employed this model to identify the temporal and spatial variation of panicle temperatures, the differences between panicle and air temperatures (DPAT), and their causes over the past 45 years in Japan. A gridded daily meteorological dataset covering Japan was interpolated at an hourly time step and used as input data of the micrometeorology model for estimating panicle temperatures. Before 2000, estimated panicle temperatures around the heading stage, the most susceptible stage to heat, had rarely exceeded the critical threshold of 33°C, but the heat exposure frequency above this threshold (HEF) has increased almost exponentially in multiple regions since then, increasing the risk of heat-induced sterility. In the unprecedentedly hot summer of 2023, the mean HEF in the Kanto and Chubu regions exceeded 30%. In most regions in Japan, panicles were estimated to be warmer than air, indicating the inadequacy of relying solely on air temperature to gauge rice heat stress. DPAT values showed substantial inter-regional variations in both mean values (from －0.5°C to 3.0°C) and seasonality. Through machine learning and statistical methods, the relationship between DPAT and meteorological factors was characterized, delineating the effects of the meteorological factors on regional and seasonal DPAT variations. Focusing on major high-risk regions, we show that mitigation strategies should be adapted to consider regional characteristics and avoid high panicle temperatures and DPAT conditions during rice heading periods.