A near isogenic line carrying the Sdr4 (seed dormancy 4) region from the indica cultivar ‘Kasalath’ on a ‘Koshihikari’ genetic background was developed and named Koshihikari NIL [Sdr4]. Performance tests showed that the main agricultural properties of Koshihikari NIL [Sdr4], including its culm length, heading date, yield, and eating quality, were not significantly different from those of ‘Koshihikari’. Koshihikari NIL [Sdr4] exhibited significantly less pre-harvest sprouting than ‘Koshihikari’, indicating that the Sdr4 gene is effective on the ‘Koshihikari’ genetic background. In addition, Koshihikari NIL [Sdr4] demonstrated significantly lower frequencies of white-back and basal-white kernels than ‘Koshihikari’ in the presence of high temperature conditions during the ripening period. This showed that Koshihikari NIL [Sdr4] is more resistant to heat-induced quality decline than ‘Koshihikari’. Koshihikari NIL [Sdr4] displayed significantly lower kernel expression levels of the α-amylase genes Amy1A and Amy1C than ‘Koshihikari’ at 14 days after flowering. However, the relationship between the downregulation of the expression of these genes and the greater resistance to heat-induced quality decline seen in Koshihikari NIL [Sdr4] remains unclear. Although the seed dormancy of Koshihikari NIL [Sdr4] seemed to be deeper than that of ‘Koshihikari’, the germination rate of preserved Koshihikari NIL [Sdr4] seeds was sufficiently high for them to be used in a seedling nursery at the next spring. In summary, Koshihikari NIL [Sdr4] is a candidate for a new cultivar that exhibits improved pre-harvest sprouting and greater resistance to heat-induced quality decline.

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