Hardness of glutinous rice cake is an important characteristic for the processing of rice crackers in Japan. We have examined a trait for high hardness of rice cake in an upland glutinous rice line "Kantomochi172" based on peak and pasting temperatures using a rapid visco-analyzer (RVA). In this report, we identified a new gene for high hardness of rice cake in Kantomochi172, based on the segregation of both temperatures in a progeny of a cross between Kantomochi172 and a lowland rice variety "Mangetsumochi" with low hardness. Two hundred twenty F₂ plants were divided into three groups; Kantomochi172 (K) type with higher peak and pasting temperatures, F₁ (F) type with higher peak and intermediate pasting temperatures and Mangetsumochi (M) type with lower temperatures. The segregation frequency of the K, F and M-types fitted to the expected ratio of 1:2:1, suggesting that the differences in the peak and pasting temperatures are controlled by a single gene. F₃ populations harvested from each of 20 F₂ plants, which were randomly selected from either K, F or M-type were also analyzed. The peak and pasting temperatures of the F₃ populations derived from F₂ plants of the K or M-type corresponded to those of the parental plants, while, the F-type was not observed in either populations. The F₃ population derived from F₂ plants of the F-type segregated into K, F and M-types. Sixty five BC₁F₁ plants (Mangetsumochi∗2/ Kantomochi172) segregated into F and M-types at a ratio of 36:29. Therefore, it was concluded that the K and M-types were homozygous, and, that the F-type was heterozygous for both temperatures. The segregation frequencies in the F₂, F₃ and BC₁F₁ generations fitted to the expected ratio due to single genic control, suggesting that the allele for the peak temperature of Kantomochi172 was dominant and that for the pasting temperature was incompletely dominant. We tentatively designated this gene locus as hhg(t). The influence of hhg(t) on the hardness of rice cake was examined using F₅ lines of K and M-types that were classified by their peak and pasting temperatures. The F₅ lines of the K-type harboring hhg-k(t) showed a high hardness of rice cake similar to that of Kantomochi172. On the other hand, the hardness of the rice cake of the F₅ lines of the M-type harboring hhg-m(t) was lower. Therefore, it is considered that the high hardness of the rice cake of Kantomochi172 is controlled by a dominant allele at the hhg(t) locus.

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Heritability of white-core (shinpaku) expression was estimated based on F₂ variance and selection response in the F₂ for high and low frequency using reciprocal crosses of a sake-brewing rice (Oryza sativa L.) cultivar Yamadanishiki and an eating-rice cultivar Reihou. The mean white-core frequency in the F₁s and the frequency distribution in the F₂s suggested that two or more genes were involved, which were dominant for the reduction of the white-core frequency. A broad-sense heritability estimate based on the F₂ variance was high (0.825 - 0.869), while the heritability estimate based on selection for both high and low frequency was much lower (0.309 - 0.447). It was noted, however, that the heritability estimate based on the selection for a higher white-core frequency (0.588 - 0.897) was much higher than that for a lower frequency (−0.037 - −0.093). This observation suggests that the selection for a higher white-core frequency is effective and suitable for the early-generation selection process. On the other hand, only a very few F₂ plants showed a white-core frequency as high as that of Yamadanishiki, suggesting that a larger population and/or additional backcrosses to Yamadanishiki are required for the early-generation selection of individuals in which effective alleles are accumulated. Since no adverse effects of the semi-dwarf gene (sd-1) on the white-core frequency were detected, it is suggested that the gene could be useful for breeding programs of sake-brewing rice.

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