We conducted a series of experiments in order to analyze the characteristics, persistence and origin of the blast resistance of cv Tsukinohikari and its two related cultivars. Tsukinohikari, a japonica rice cultivar also resistant to rice stripe virus (RSV), was developed at Aichi Prefectural Agriculture Research Center in Japan. The experiments had been carried out in Nagakute and Inabu located in the central part of Japan for 11years from 1984 to 1993 and in 1996. The genotypes of true resistance to blast of Tsukinohikari, Asanohikari and Aichi 67, which are common genotypes in Japan were Pii, Pia. Pii and Pia/+, respectively. The experiments were conducted using race 007 and race 037 of the blast fungus which were the prevalent races in the test fields. These races were virulento all the cultivars and lines used in the tests of blast resistance. Field resistance to leaf blast of Tsukinohikari, Asanohikari and Aichi 67 was comparatively low. Tsukinohikari and Aichi 67 were found to exhibit a moderate to intermediate resistance, and Asanohikari an inetermediate resistance (Tables 1 and 2). On the other hand, in the field, Tsukinohikari and Aichi 67 were estimated to be highly resistant to resistant to panicle blast, while Asanohikari was resistant (Tables 3 and 4). These results showed that the three cultivars expressed a stronger resistance to panicle blast than to leaf blast. Tsukinohikari showed a resistance to panicle blast which persisted during a period of 11years though the experiments consisted of tests in small fields in environments conductive to virulent disease both in Inabu and in Nagakute (Fig.1, Table 5). A genealogical analysis of Tsukinohikari (Fig.2) showed that among its ancestral cultivars, Aoisora, Aichi 6, St.No.1 and indica cv Modane xhibited a high field resistance to panicle blast. All of them were RSV-resistant. Many progeny cultivars of Tsukinohikari or Aoisora, such as Aoinokaze, Akanezora, Aichi 86, Matsuribare, Aichi 89 and Hoshinohikari were found to be highly resistant to panicle blast and they were also resistant to RSV.

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The relationship between the vascular bundle system in a peduncle and the yield components was investigated using 29 temperate-japonica rice varieties, Oryza sativa L., which consisted of 22 Japanese and 7 Chinese improved and native varieties. Nine plants of each variety were cultivated in the field in 1998 with two replications. The number of large vascular bundles (Vb) in a peduncle differed significantly between the varieties ranging from 9.6 to 18.9. According to the ratio of the number of Vb to the number of primary rachis branches, V/R ratio, the 29 varieties were divided into 2 groups; those with high (>1.2) and low (<1.2) V/R ratio. No difference was found in heading date between the varieties with high and low V/R ratio, but the culm length and panicle number were respectively, higher and lower in the varieties with a high V/R ratio than in those with a low V/R ratio. The varieties with a high V/R ratio showed a larger number of spikelets and grains on the secondary rachis branches, but no difference was observed in the grain weight between the 2 groups. It was assumed that the excess of vascular bundles in the varieties with a high V/R ratio indicated that the secondary-branch spikelets had developed and well filled with photosynthetic products. The varieties with a high V/R ratio showed a heavier panicle weight, but lower grain weight per plant because of the lower number of panicles as compared with the varieties with a low V/R ratio. The significance of the vascular bundle system in breeding for a high yielding ability was discussed.

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