The mode of inheritance of panicle blast (PB) resistance in the rice cultivar, Tsukinohikari and related cultivar Asanohikari, was investigated. Both cultivars are also resistant to the rice stripe virus (RSV). The experiments for PB resistance had been carried out using the F₁, F₃ lines and F₄ lines in environments conductive to the occurrence of severe disease. The percentage of grains infected with Pyricularia grisea in the F₁ plants derived from the cross between the PB-resistant cv Tsukinohikari and the susceptible breeding line Akei-ta 494 was not significantly different from that of cv Tsukinohikari. Therefore, the PB resistance was considered to be a dominant character. The segregation of PB resistance phenotypes in 119 lines of the F₃ generation derived from the cross between the resistant cv Asanohikari and susceptible cv Koshihikari, fitted to the expected ratio of 1: 2: 1, resistant (Asanohikari type), segregating type and susceptible (Koshihikari type). Also, the segregation of the PB resistance phenotypes in 60 lines of the F₄ generation derived from the cross between the resistant cv Tsukinohikari and susceptible cv Koganebare, fitted to the expected ratio of 3: 2: 3, resistant (Tsukinohikari type), segregating type and susceptible (Koganebare type). These results indicate that the PB resistance is controlled by a new dominant major gene. We designated this gene as Pb1. We also performed a linkage analysis between the Pb1 and a RSV resistance gene Stvbi using the F₃ lines derived from the cross between cv Asanohikari (resistant to both PB and RSV) and cv Koshihikari (susceptible to both PB and RSV). The results showed that the two resistance genes are linked to each other with a recombination value of 5.2±1.5%. Linkage analysis using 60 F₄ lines derived from the cross between cv Tsukinohikari (resistant to both PB or RSV) and cv Koganebare (susceptible to both PB or RSV) confirmed the linkage relationship between the two genes. Among the six full related lines in the F7 generation, all derived from the cross between Tsukinohikari and Koshihikari, three with RSV resistance showed a strong resistance to PB, while three without RSV resistance showed a weak resistance to PB. These findings indicated the presence of a linkage relationship between the two resistance genes.

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It is very important to recognize genetic variations in the seed production characteristics of radish (Raphanus sativus L.), because, based on the land area devoted to cultivation and the number of local varieties, radish is one of the most common Japanese vegetables. We used 219 lines, belonging to 23 varietal groups, of cultivated radishes and examined 3 seed productioncharacteristics: the self-incompatibility alleles (S-alleles), the level of self-incompatibility and the number of seeds per pod in bud pollination. Thirty-seven S-alleles, designated as S²⁰¹ to S²³⁷, were identified by cross pollination tests and PCR-RFLP. These varietal groups were classified by principal component analysis, based on the similarity of the S-alleles calculated by quantification method IV. The classification of the varietal groups in these experiments did not correspond to that based on the morphological characteristics or geographical distribution of the local varieties. Thus, the tests revealed that the S-gene is not correlated with the phyletic diversification of Japanese cultivated radishes. The level of self-incompatibility (SI) was evaluated by the rate of self-seed setting, i. e. the number of pods setting seed/the number of flowers artificially self-pollinated in anthesis×100 (%). The level of SI in these lines ranged from 0 to 100 %. However, there was no correlation within these varietal groups in this characteristic as well as in the S-alleles. The number of seeds per pod ranged from 0.9 to 6.2 seeds. No correlation among the above characteristics was detected. In conclusion, we considered the importance of seed production characteristics in the evaluation of radish germplasm collection and suggest that the breeders should pay attention to the selection of these characteristics.

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