4. A newly induced semidwarfing gene with agronomic potentiality

The general use of the Dee-Geo-Woo-Gen semidwarfing gene (sd-1) in the breeding of improved cultivars has been criticized as narrowing the genetic basis of rice cultivars. Although a number of useful semidwarf mutants were induced in different Indica and Japonica cultivars, all those genes were found to have the same locus as of sd-1 (Hu 1973; Kikuchi and Ikehashi 1984; Rutger 1983, 1984). Most of semidwarfing genes non-allelic to sd-1 expressed agronomically poor characters (Chang et al. 1984).

The author selected two semidwarf mutants with economic potential from the gamma-rayed progeny of a tall long-grained (Indica) cultivar, California Belle (developed by the author, Hu 1983). When crossed with the maternal cultivar, both showed a 3 tall: 1 semidwarf F2 ratio. One of them, R- 16, was found to have an sd-1 gene. The other, R-34, had a semidwarfing gene independent of sd-1. When crossed with R- 16, the F1 was a tall type and the F2 segregated into 51 tall: 34 semidwarf: 7 double-dwarf (below 65 cm), giving a good fit to the 9: 6 : 1 ratio.

So far, four induced semidwarfing genes, symbolized sd-2, sd-3, sd-4 (Mackill and Rutger 1979; Rutger 1983, 1984) and sd-5 (McKenzie and Rutger 1986) have been reported in California. All these mutants were inferior to the sd-1 carrier in agronomic characters. As the new mutant line, R-34, showed different characters from these, it was considered to have a new gene, symbolized sd-6(t).

The semidwarf line R-34 shows an open-stemmed plant type and a larger number of spikelets per panicle (Table 1; Fig. 1). It has erect leaves and is highly resistant to lodging as is the sd-1 carriers. It gave a somewhat lower grain yield than California Belle at a low nitrogen level (100 kg/ha) but a higher grain yield at a high nitrogen level (200 kg/ha), Although hybridization breeding with this new gene is still under way, this gene must have an economic potentiality.

The sd-1 gene is known to express a plant type favorable for high-yielding potential, composed of increased tillering, erectly standing leaves and reduced internode elongation conferring lodging-resistance. However, this plant type is

not ideal from the viewpoint of reactions to diseases and insect pests, as the densely standing stems reduce ventilation within the plants. The sd-1 carriers were more susceptible to the bacterial leaf blight (Xanthomonas oryzae) than their tall parents (Hu 1973). Furthermore, when the semidwarf cultivars were planted at a high density and high nitrogen level, the canopy would provide a favorable environment for the brown planthoppers (Nilaparvata lugens) to propagate, resulting in serious damages (Chang 1979; Chung and Heu 1980). The author has observed in Taichung that even in F2 populations segregating into tall and semi-dwarf plants, the semidwarf segregants attracted more brown planthoppers (unpubl.). In this relation, the sd-6(t) carrier showing an open-stemmed plant type is expected to provide a better situation.

Chang, T. T., 1979. Crop genetic resources. In Plant Breeding Perspectives. p. 83-103. Gent. Agric. Publ., Wageningen.

Chang, T. T., C. Zuno, A. Marciano-Remena and G. C. Loresto, 1984. Semidwarfing genes in rice germplasm collection. RGN 1: 94-95.

Chung, G. S. and M. H. Heu, 1980. Statue of Japonica-Indica hybridization in Korea. In Innovative Approaches to Rice Breeding, p. 135-152, IRRI, Manila.

Hu, C. H., 1973. Evaluation of breeding semidwarf rice by induced mutation and hybridization. Euphytica 22: 562-574.

_____, 1983. Breeding of California long-grain and pearl rice varieties with high yields and good grain quality. Jpn. J. Breed. 33: 195-207.

Kikuchi, F. and H. Ikehashi, 1984. Semidwarfing genes of high-yielding rice varieties in Japan. RGN 1: 93-94.

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

McKenzie, K. S. and J. N. Rutger, 1986. A new semidwarf mutant in a long-grain rice cultivar. Crop Sci. 26: 81-84.

Rutger, J. N., 1983.Application of induced and spontaneous mutation in rice breeding and genetics. Advances in Agronomy -36: 383-413.