This study was conducted to evaluate the suitability of rice varieties for nature farming by analyzing the relationship between the function of nitrogen-fixing rhizobacteria and the growth of rice plants cultivate in rice fields under nature and conventional (control) farming conditions.
In 1994, four rice varieties (Koshihikari, Nihonmasari, J195 and J235) and two rice varieties (Koshihikari and J195) were grown in fields under nature and conventional farming conditions in Shimada City and Yaizu City, respectively, Shizuoka Prefecture. The number of nitrogen-fixing rhizobacteria was found to change with the growth stages of rice varieties in relation to the changes in dry matter weight and nitrogen content in the plants both under nature and conventional farming conditions. The varieties with a higher number of nitrogenfixing rhizobacteria at the early growth stages (from planting to the maximum tiller stage) showed a higher increase of the ratio of dry matter weight and nitrogen content in plants at the early growth stages. On the other hand, the varieties with a higher number of nitrogenfixing rhizobacteria at the late growth stages (from heading to the ripening stage) showed a higher increase of the ratio of dry matter weight and nitrogen content in plants of the late growth stages. The number of nitrogen-fixing rhizobacteria under nature farming was found to be higher than that in the conventional farming. It is noted that, under nature farming, the varieties with a higher number of nitrogen-fixing rhizobacteria at the late growth stages showed high rough rice yields compared with those under conventional farming. It is suggested that selection of the varieties with higher number of nitrogen-fixing rhizobacteria at the late growth stages might be effective for the breeding of high-yielding varieties under nature farming. Japanese native J195 line used in this experiment may be a suitable material for the breeding of high-yielding varieties under nature farming or for sustainable agriculture, because this line showed a higher number of nitrogen-fixing rhizobacteria at the late growth stages and higher yield under nature farming.
The growth stage when the number of nitrogen-fixing rhizobacteria was high under nature farming differed depending on the locations (Yaizu and Shimada) where the experiments were conducted. The stage was early in Yaizu, whereas late in Shimada. The Differences in the experimental results with the locations might be attributed to the kind of organic manure used. Namely, rice bran applied in Shimada had a high C/N ratio compared to Chinese milk vetch applied in Yaizu, resulting in a slower effect of organic manure on the growth of the plants. The type of organic manure applied should be considered in the breeding process for developing rice varieties suitable for the nature farming or sustainable agriculture.

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Genetic analysis of low-amylose content in a rice variety, ‘Milky Queen’, which is the N-methyl-N-nitrosourea (MNU)-induced mutant of the wild type variety, ‘Koshihikari’, was conducted. The F₁ progenies derived from the reciprocal cross between Milky Queen and Koshihikari, showed an amylose content intermediate between that of the parents. However, the amylose content in the F₁ (Koshihikari/Milky Queen) was higher than that in the F₁ (Milky Queen/Koshihikari). Thus, gene dosage effect on the mutant gene was observed. Among the F₂ seeds derived from Milky Queen/Koshihikari, high-amylose content (Koshihikari type) and low-amylose content (mutant type) seeds were segregated according to a 3: 1 ratio. Also, the segregation ratio in the BC₁F₁ (Milky Queen/Koshihikarii/Milky Queen) seeds fitted to the expected 1: 1 ratio. These results indicated that the mutation was controlled by a single recessive gene. An allelism test showed that the mutant gene in Milky Queen was not allelic to the known five du loci, but allelic to the wx locus.

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In order to analyze the reproductive systems of brown mustard (Brassica juncea), variations in automatic self-pollination ability (ASPA) and degree of self-compatibility (DSC) within twenty-one cultivars of brown mustard were investigated in 1990, based on the percentage of seed set after automatic self-pollination or artificial self-pollination. The percentage of seed set after automatic self-pollination is an index of ASPA (Fig. 2), and the percentage of seed set after artificial self-pollination with a large number of pollen grains is an index of DSC. Relationships among ASPA, DSC and floral traits in eight cultivars selected from twenty-one cultivars of brown mustard were also investigated in 1991. The percentage of seed set after artificial self-pollination as an index of DSC ranged from 51.4% to 88.4%, and the percentage of seed set after automatic self-pollination as an index of ASPA ranged from 32.4% to 70.8%, showing variations in both traits within twenty-one cultivars. The percentage of seed set after automatic self-pollination in most of the cultivars was lower than that after artificial self-pollination (Table 1). These results suggest that a pollinator is indispensable for obtaining a sufficient number of seeds in most of the cultivars. Automatic self-pollination started when the anthers came into contact with the stigma or the time when the anthers dehisced after flowering, and continued for a few hours even though the anthers became separated from the stigma with time (Table 2, Fig. 3). Among eight cultivars, the number of pollen grains on the stigma after automatic self-pollination depended on the number of anthers in contact with the stigma, and the position of the anthers and the stigma (Table 6). Based on the time course of automatic self-pollination, in various cultivars the percentage of seed set increased by automatic self-pollination after flowering or after automatic self-pollination over a long period of time after flowering (Fig. 3). These results indicated that there were variations in the process of automatic self-pollination within the eight cultivars. The cultivars with higher ASPA and DSC values also showed smaller variations in several floral traits within a cultivar than those with low ASPA and DSC values (Table s 1 and 3). These results demonstrate that brown mustard displays variations within cultivars in self-compatibility, automatic self-pollination ability in relation to floral traits like contact between, or position of anther and stigma. These variations in the reproductive systems within cultivars suggest that there are wide variations in the need for pollinators in brown mustard cultivars which could be categorized as selfcompatible but incomplete selfers.

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Amphidiploid plants between Brassica campestris (AA) and B. oleracea (CC) were produced by ovule culture, and the pollen and selfed seed fertility of the amphidiploid plants obtained was investigated. For ovule culture, the optimum time was 15-20days after pollination, and the optimum medium was B5 medium supplemented with coconut milk 150ml/l, kinetin 2.5mg/l and NAA 2.5mg/l. In cruciferous leaf vegetables, plants were obtained from 29 cultured ovules in 12 crosses from 28 cross combinations, while, in cruciferous leaf-stem vegetables, plants were obtained from 119 cultured ovules in 15 crosses of 24 cross combinations. In the F₁ generation of amphidiploids, the frequency of plants with a high pollen fertility was high while that of plants with a high seed fertility was low. In the F₂ generation selected from F₁ plants, the frequency of plants with a high pollen fertility as well as with a high seed fertility was higher than that in the F₁ generation. Moreover, plants with more than 10 seeds in one pod were obtained, indicating that the fertility in the F₂ generation was improved by selection. Although the correlation between pollen fertility and seed fertility was not significant, it was difficult to obtain seeds from the plants with a pollen fertility below 70% in leaf vegetables, and below 60% in leaf-stem vegetables. It is suggested that plants with a pollen fertility above 80% are required for obtaining plants with a high seed fertility in both leaf and leaf-stem vegetables.

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The tea cultivar Sayamakaori bred for the production of Japanese green tea is resistant to Kanzawa spider mites and mulberry scales and is tolerant to low-temperature and frost stress. Thus, Sayamakaori is a very important breeding material for resistance and tolerance in the breeding of tea plants. This cultivar was selected at Saitama Prefectural Tea Experiment Station from natural crossing of seeds harvested from a tea cultivar Yabukita at Shizuoka Tea Experiment Station in 1947. Sayamakaori has very specific traits that are absent in Japanese original teas, for example extremely upright leaves, dark green leaves and so on. It has been reconfirmed that each individual in the F1 progeny had inherited these traits from Sayamakaori, which indicates that the traits of Sayamakaori are controlled by genes with dominant effects. Since Yabukita does not have the traits specific to Sayamakaori, it was assumed that the pollen parent should exhibit these traits. Therefore, 78 clones were screened as putative pollen parents of Sayamakaori in the field of Shizuoka Tea Experiment Station. All the 78 putative pollen parents were introduced from abroad, mostly from China, not including the Japanese original clone. The parentage test using DNA markers was performed between Yabukita and the putative pollen parents, and no clone was found to be the pollen parent of Sayamakaori. Therefore, the pollen parent of Sayamakaori may not exist presently.

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