4. A newly induced semidwarfing gene with agronomic potentiality

Chao-Hwa HU

N. F. Davis Drier & Elevator Inc., Firebaugh, CA 93622, U.S.A.

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.

Table 1. Agronomic characters and grain yield of California 
         Belle and its two induced mutants
Character                           California   R-16    R-34
                                       Belle    (sd-1)  (sd-6)

Plant height (cm)                      113        92      85
Panicle length (cm)                     25        22      23
No. of spikelets per panicle           145       171     271
Seed setting (%)                        67        82      71
Seedling vigor (5=best)                  4.1       4.0     4.5
Days to 50% heading                     92        95      98
Lodging (%)                             43         5       1
Grain yield (t/ha)* at N level,
  100 kg/ha                              9.6       9.0     8.0
  200 kg/ha                             10.0      10.6    10.6
*LSD(1%)=O.56 t/ha

Fig. 1. Two semidwarf breeding lines derived from California Belle. The left with erect stem carries sd-1 from R-16, and the right with open stems carries sd-6(t) from R-34.


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