Quantitative assessment on the grain appearance of a new Japanese rice cultivar ‘Niji-no-kirameki’ with a novel heat-avoidance mechanism during ripening

Abstract

There is an increased demand for developing a heat-resilient cultivar with high palatability and acceptable grain yield to cope with the impacts of progressive changes in global climate conditions. This study attempted to quantify the heat resilience level and explore the heat resilient mechanism of ‘Niji-no-kirameki’, a new Japanese cultivar with heat resilience during ripening, and compare it with the leading Japanese cultivar ‘Koshihikari’. Multi-environmental testing on grain appearance under diverse high temperature field conditions across three prefectures revealed superior performance of ‘Niji-no-kirameki’ compared to ‘Koshihikari’, especially when the average of daily mean temperatures during the first 20 days after heading was over 27 °C, a general threshold temperature previously reported to decrease grain appearance in ‘Koshihikari’. Starch properties such as amylose content and amylopectin chain length distribution were similar between cultivars across seasons. Severely chalked grains by high temperature stress had lower protein content and 13 kDa prolamin content in the grains, suggesting that inter-relationships between starch and storage protein biosynthesis affect the formation of the chalky phenotype irrespective of cultivars with different heat resilience levels. As a possible heat resilience mechanism, a novel biological response of ‘Niji-no-kirameki’ for preventing an increase in panicle temperature to rising temperatures during ripening was found. It is assumed that the erect plant phenotype until maturity concealing panicles inside the canopy could create a cooler environment for panicles of ‘Niji-no-kirameki’ during ripening. We conclude that ‘Niji-no-kirameki’ is a promising cultivar for the improvement of grain appearance in the heat-vulnerable regions of Japan.