2 USEFUL SEMIDWARFING GENES IN A SHORT-CULM MUTANT LINE HS90 OF RICE (ORYZA-SATIVA L)

A short-culm mutant line HS90, which was induced from a Japanese rice variety Gimbozu by gamma-ray irradiation, was analyzed for the genetic factors controlling its short culmness. Compared with the original va riety, HS90 has about 35 % reduced culm length and flowers later by a few days. The F-1, F-2 and F-3 generations of four crosses, Gimbozu X HS90, Jukkoku X Shiranrii, HS90 X Jukkoku and Gimbozu X Jukkoku, were grown in a paddy field for genetic analysis. A variety Shiranui has al ready been proved to harbor a semidwarfing gene sd-1, while Jukkoku so far has been assumed to harbor sd-1 because of the cross parent of th e former. In each cross, several F-1 plants, about 300 F-2 plants, and 50 to 90 Fs lines which were raised from randomly selected F-2 plants were esamined for culm length and heading date. It was clarified that the shortening of HS90 is caused by an induced mutant semidwarfing ge ne (tentatively sd-1(h)), an incomplete recessive gene identical with or allelic to sd-1. This mutant semidwarfing gene causes remarkable re duction of culm length (ca. 35 %) : its genotypic value in the heteroz ygous state seems almost equal to that of the midparent. In addition, HS90 was proved to harbor another incomplete recessive semidwarfing ge ne (tentatively sd(t)) non-allelic to sd-1, which was derived from the original variety Gimbozu. This semidwarfing gene reduces culm length less than sd-1(h) (ca. 20 % ) : its genotypic value in the heterozygou s state seemed somewhat larger than that of the midparent. As Gimbozu and its derivative varieties have often been used as a cross parent in Japanese rice breeding, the semidwarfing gene sd(t) seemd to be widel y distributed among Japanese rice varieties. The F-1 plants of HS90 (s d-1(h) sd-1(h) sd(t) sd(t)) X Gimbozu(Sd-1 Sd-1 sd(t) sd(t)) exhibited a culm length coming within the category of semidwarfism optimum for the present cultivation methods; hence, the genotype of the F-1 plants , Sd-1 sd-1(h) sd(t) sd(t), was considered favorable for F-1 hybrid ri ce varieties. In this study, it was also verified that the genotype of major-genes controlling the culm length of Jukkoku is identical with that of Shiranui, which indicates that Shiranui inherited sd-1 from Ju kkoku.