Conversion effect of Hd1, Hd16, and Hd18 on the heading date of the rice line “Tohoku 206”, which has high temperature tolerance during the ripening stage

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.