The enzyme digestibility of steamed rice grains is an important characteristic for sake brewing and is higher when the pasting temperature is lower. On the basis of a low pasting temperature measured by a Rapid Visco Analyzer, we selected ‘Akitasake 44’ from among 41 improved cultivars and lines used in sake brewing. Analysis of its amylopectin chain length distribution shows that it is richer in short-length chains (degree of polymerization 6–12) and poorer in mid-length chains (degree of polymerization 13–24) than the commonly preferred cultivars ‘Akitakomachi’, ‘Gohyakumangoku’, and ‘Okuhomare’. ‘Akitasake 44’ lacked activity of phosphorylase 1, an enzyme related to starch metabolism in the endosperm. Although phosphorylase 1 activity was normal in its ancestry, ‘Akitasake 44’ was bred by gamma-irradiation of ‘58kei3071’. Sequence analysis revealed a single nucleotide substitution in the 12th exon of the phosphorylase 1 gene of ‘Akitasake 44’, which caused an amino acid substitution. This mutation is the likely cause of the lack of phosphorylase 1 activity, and consequently the lower pasting temperature of ‘Akitasake 44’.

Near-isogenic lines (NILs) of “Tohoku 206”, which has medium maturity with high temperature tolerance during the ripening stage, were developed to delay the heading date to 5 to 10 days later than “Tohoku 206” and to clarify the conversion effect of Hd1, Hd16, and Hd18 of “Koshihikari” in Miyagi. NILs were bred by repeated backcrosses between “Tohoku 206” as a recurrent parent and “Koshihikari” as a donor. Substituting a “Koshihikari” chromosomal segment for the Hd1 locus in “Tohoku 206” resulted in a later heading date by 29 days. Similarly, the substitution of a “Koshihikari” chromosomal segment for the Hd16 and Hd18 loci separately in “Tohoku 206” resulted in an earlier heading date by 3 days. The predicted heading date for the gene combination of Hd1 and Hd16 was 5 days later, but the actual heading date was 15 days later. On the other hand, the heading date of the NIL of “Tohoku 206” with the “Koshihikari” Hd1, Hd16, and Hd18 loci was 10 days later than that of “Tohoku 206”. The difference between the predicted and actual heading dates was considered to be due to the mutual interaction of the “Koshihikari” Hd1, Hd16, and Hd18 loci and other genes involved in heading date determination or environmental factors such as temperature and day length. The most promising line with excellent agricultural traits from “Tohoku 206” NILs was with “Koshihikari” alleles at the Hd1, Hd16, and Hd18 loci. This line was named “Tohoku 229”. The heading date of “Tohoku 229” was 9 to 10 days later than that of “Tohoku 206”. It has high temperature tolerance equivalent to “Tohoku 206”. These results indicate that it is possible to rapidly develop a late-maturing and promising line that does not show a decline in grain quality owing to high temperatures during the ripening stage by delaying the heading date with the substitution of “Koshihikari” alleles at the Hd1, Hd16, and Hd18 loci.