1. Induced semidwarf mutants

U.S.D.A. Agric. Research Service and Agronomy and Range Science Department, University of California, Davis, CA 95616, U.S.A.

In the last 14 years several semidwarf mutants have been induced in rice cultivars in California. The most useful mutant was released in 1976 as the semidwarf cultivar Calrose 76. It originated as a single gene semidwarf mutant from the very well-adapted tall cultivar Calrose. Genetic studies showed that Calrose 76 possessed a single recessive gene for semidwarfism, designated sd\1\. The sd\1\ gene reduces plant height about 25% through approximately proportional reductions in lengths of the top five internodes; panicle length remains essentially unchanged.

In practice Calrose 76 has been more important as an adapted semidwarf donor than as a cultivar per se. It has been the source of semidwarfism either directly or indirectly for five additional cultivars released from the cooperative industry-state-federal breeding program in California: M7, M-101, S-201, M-301 and M-302. Height of the California semidwarfs is generally about 90 cm, compared to 120-130 cm for the previous tall cultivars. The cumulative evidence indicates that the semidwarfing gene increases rice yields 15%, and when the semidwarf cultivars are used with intensified cultural practices, farm yields increase about 25%.

Genetic studies have shown that the induced mutant gene sdi is allellic to the major semidwarfing gene in Dee-geo-woo-gen (DGWG) and the widely grown Green Revolution cultivars derived from DGWG. Thus, in F\2\ generations of crosses between sd\1\ and DGWG types, no truly tall recombinants have been recovered, although considerable variation exists in height of the F\2\ semidwarfs (Foster and Rutger 1978; Mackill and Rutger 1979).

Allelism tests have shown that at least three independent, recessively inherited semidwarf genes were induced in the tall cultivar Calrose: the sd\1\ locus present in Calrose 76, the sd\2\ locus in CI11033, and the sd\4\ locus in CI11034. However, neither the sd\2\ nor the sd\4\ source has been as agronomically useful as the sd\1\ source. The sd\4\ source reduces height only 15 cm and has an additional pleiotropic effect for a 20% reduction in seed size.

After the three independent semidwarfing genes sd\1\, sd\2\, and sd\4\ were identified, subsequent genetic studies concentrated only on determining if new mutants were allelic to sd\1\. To date, seven additional semidwarfs have been found to be non-allelic to sd\1\. Again, none has been as agronomically useful as the sd\1\ source. Overall, semidwarf mutants have been induced in ten different rice cultivars in U.S.A.

Foster, K. W. and J. N. Rutger, 1978. Inheritance of semidwarfism in rice, Oryza sativa L. Genetics 88: 559-574.

Mackill, D. J. and J. N. Rutger, 1979. The inheritance of induced-mutant semidwarfing genes inrice. J. Hered. 70: 335-341.