Semi-dwarf rice breeding in Japan and genetic study of semi-dwarf rice mutants

In the 15 years from 1958 to 1973, average culm length of Japanese rice varieties showed a rapid reduction from 95 cm to 82 cm, realistically reflecting the postwar breeding trend in Japan with use of radiation induced mutants. Nowadays, it can be said that the improvement of rice towards short culm length has almost reached the goal and the coming objective of breeding will mainly be directed in creating further stiffer-culm varieties with the present culm length. However, some problems remain to be solved. Firstly, the source of semi-dwarf genes are traced back to only a few stocks. Secondly, genetical studies on semi-dwarfness still remain quite undeveloped, although they are considered to be indispensable for enhancing the efficiency of rice breeding. Results of some genetic studies of radiation induced semi-dwarf mutants are presented. Future research plans presented stress identifying and evaluating the useful semi-dwarfing genes in our mutant stocks as cross breeding parents, to contribute to averting the potential risk of genetic vulnerability of semi-dwarf rice