Evaluation of breeding semidwarf rice by induced mutation and hybridization

Summary Semidwarf indica varieties of rice (Oryza sativa L.) were developed by induced mutation. They generally have a yield capacity equal to that of the semidwarf varieties (T(N)1, IR 8 and others) selected from hybridization. Genetic studies showed that semidwarf genes induced in the mutants and spontaneous ones in existing varieties are at the same locus. These semidwarf mutants, selected in Taiwan and from an American long-grain variety, Centura Patna 231, were compared with their parents, the semidwarf variety Dee-goe-woo-gen, T(N)1 and IR 8. The semidwarf mutants average about 35% shorter than their tall parents. The plant height is reduced by shortening five internodes from the top, while panicle length is not affected. The mutants, Dee-geo-woo-gen, and improved short-statured varieties had similar patterns of internode elongation. The mutants derived from varieties sensitive to photoperiod have lost the sensitivity. One more crop can be grown within a year. All induced and hybrid-derived semidwarf varieties were highly susceptible to bacterial leaf blight when Xanthomonas oryzae was inoculated although some of their parents were moderately resistant. The mutation rate from irradiation of this semidwarf gene was estimated to be 0.1% in I-kung-bau, a native variety of Taiwan. This particular type of mutation has not yet been reported in japonica varieties. However, several promising strains with reduced plant height, earlier maturity and more grain yielding were recently obtained from the California japonica variety, Calrose, by X-irradiation. The merits of mutation breeding in rice are discussed.