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This study aimed to determine the appropriate steam treatment conditions, using a steam nursery cabinet, to break the dormancy of Japanese rice cultivar seeds exhibiting various dormancy levels. The influence of the temperature and duration of the steam treatments on the germination percentage and germination rate was investigated. In highly dormant ‘Takanari’ seeds, the steam treatment at 40 °C for 7 d increased the germination percentage and decreased the 50% germination time (T50S; based on seed number); this treatment was as effective as the dry heat treatment at 50 °C for 7 d. For the medium dormant ‘Moeminori’ and ‘Hitomebore’ seeds, the steam treatment at 40 °C for 5 d decreased T50S sufficiently and more effectively than did the dry heat treatment at 50 °C for 7 d. For the slightly dormant ‘Moeminori’ seeds, the steam treatment at temperatures ranging from 24 °C to 40 °C for 7 d decreased T50S without a corresponding decrease in germination percentage to

Quantitative trait loci (QTL) affecting the germination rate, coleoptile length, and shoot dry weight were analyzed under submerged and low temperature conditions using inbred lines derived from crossing the rice cultivars Ouu 365 and Arroz da Terra. The QTLs that increased the germination rate and shoot dry weight by Arroz da Terra allele were detected on the same region of chromosome 3, where the low temperature germinability gene, qLTG3-1, localized, suggesting that greater germinability might lead to increased shoot growth in paddy fields. The QTLs that increased the coleoptile length by Ouu 365 and Arroz da Terra alleles were detected on chromosome 8 and 11, respectively. The analysis of the known germinability genes suggested that functional allele of qLTG3-1 increased germination rates of the inbred lines. However, Rc which induced red pigmentation in pericarp declined the germination rates of the lines with functional qLTG3-1.

The F1 plants of crosses between indica and japonica rice cultivars often have black hulls during ripening, even though both of the parental cultivars have yellow hulls. Complementary genes are suggested to be necessary for the black hull phenotype, and one of them is predicted to be Phr1, which encodes polyphenol oxidase. On the other hand, Bh4, which encodes a tyrosine transporter, is known to cause the black hull phenotype in wild rice species, Oryza rufipogon. However, the relationship between Bh4 and Phr1 in the black hull phenotype has not been elucidated. In this study, a genotype analysis of the segregating populations from the cross between an indica cultivar, Habataki, and a japonica cultivar, Arroz da Terra, indicated that only those plants that had both functional genes, Bh4 and Phr1, showed the black hull phenotype, suggesting that a complementarity of Bh4 and Phr1 was necessary for the black hull phenotype.

Excess ferrous iron in lowland soil is known to inhibit the growth of rice. A quantitative trait locus (QTL) analysis for susceptibility to ferrous iron was performed using chromosomal segments substitution lines (CSSLs). Kasalath, an indica rice cultivar, is known to be susceptible to ferrous iron and accumulate excess iron in shoots. The shoot iron concentration was examined in 39 CSSLs carrying Kasalath chromosomal segments in a background of Koshihikari, a japonica cultivar. Kasalath grown in a hydroponic culture solution containing excess ferrous iron, had a higher shoot iron concentration than Koshihikari. Of the CSSLs, SL208, which carries the Kasalath chromosomal segment on chromosome 3, had a significantly higher shoot iron concentration than Koshihikari, and none of the CSSLs had a shoot iron concentration significantly lower than Koshihikari. This finding suggests that the putative QTL affecting the shoot iron concentration is between the markers R663 and S1571 on chromosome 3.

A two-year field experiment was conducted with japonica type high-yielding variety “Fukuhibiki,” indica type high-yielding variety “Takanari,” large grain type high-yielding variety “Bekoaoba” and conventional japonica type varieties “Akitakomachi” and “Hitomebore” in the Tohoku region (colder area of Japan). The panicles of Takanari and Bekoaoba located lower in the canopy, and the flag leaf of those varieties intercept more solar radiation, because the flag leaf was long and the first internode of panicle did not emerge above the lamina joint of the flag leaf in Takanari and the panicle hung lower in Bekoaoba. The three high-yielding varieties produced a large sink size, due to the large potential sink size in Takanari, the very large grain size in Bekoaoba and the rather large number of spikelets per panicle and somewhat large grain size in Fukuhibiki. Based on dry matter production, the three high-yielding varieties had a high harvest index, although there were little varietal differences in top dry weight at maturity. Unhulled rice weight of the three high-yielding varieties was heavier than that of other varieties at 30 days after heading (30 DAH). However, there were no varietal differences in the increase in unhulled rice weight from 30 DAH to maturity (later than 50 DAH), probably due to the continual growth of the grains located lower until the late ripening period in all varieties. In conclusion, the traits related to high yield varied greatly among varieties. However, the three high-yielding varieties had a large sink size, heavy unhulled rice weight at 30 DAH and high harvest index.

To achieve super high yield of more than 10 t ha-1 in the Tohoku region (colder area of Japan), we conducted a two-year field experiment using a large grain type high-yielding rice variety Bekoaoba. Although high nitrogen application (HN) increased the top dry weight at the full heading stage only slightly, it increased the sink size (single grain weight x the number of spikelets per area), leaf area index and nitrogen content at the full heading stage and accordingly the dry matter production during the ripening period. As a result, the gross hulled rice yield was higher under HN than under standard nitrogen application (SN). Under HN, early topdressing increased not only the number of differentiated spikelets but also the number of degenerated spikelets. As a result, the sink size and gross hulled rice yield of the plants were not changed by the early topdressing. Under HN, the higher planting density increased the number of panicles per area but decreased the number of spikelets per panicle. As a result, the sink size and gross hulled rice yield were not changed with the planting density. In conclusion, HN produced the gross hulled rice yield of more than 9 t ha-1. However, the gross hulled rice yield could not be increased more than 10 t ha-1 by regulation of the timing of topdressing and /or the planting density. We discussed the strategies for super high-yielding rice in the Tohoku region.

Sucrose synthase, which catalyzes uridine diphosphate (UDP)-dependent cleavage of sucrose into fructose and UDP-glucose, is induced by oxygen deficiency in rice seedlings and is considered to play an important role in energy production under hypoxic conditions. In this study, we analyzed the relationship between coleoptile elongation and sucrose synthase activity in rice (Oryza sativa L.) cultivars under submerged conditions. We also analyzed the activity of α-amylase, which digests starch reserves in the endosperm and is considered to be important for energy production in young seedlings. The results indicated that different rice cultivars had different sucrose synthase and α-amylase activities under submerged conditions. Moreover, sucrose synthase activity in whole seedlings was significantly correlated with coleoptile length under submerged conditions, whereas the correlation between α-amylase activity and coleoptile length was low. Sugar content of shoots differed with the cultivar. Correlation analysis demonstrated that sucrose content was highly correlated with coleoptile length and sucrose synthase activity, but not with α-amylase activity.

Preharvest sprouting of buckwheat (Fagopyrum esculentum Moench) is an important problem, but cultivar differences in preharvest sprouting and their causes have not been investigated. We detected cultivar differences under natural field conditions. Preharvest sprouting of three cultivars was significantly lower than that of the current main cultivar. Seeds collected before rainfall were threshed and incubated on a wet filter paper in a petri dish for 10 days at 10, 20, 30 and 40ºC in the dark, or at an alternating light and temperature condition of 8 h light at 30ºC and 16 h darkness at 20ºC. Germination was promoted by a higher temperature except for 40ºC, suggesting that the risk of preharvest sprouting in buckwheat is higher at a relatively higher temperature. The risk of preharvest sprouting in the field was highly correlated with germination at 20ºC (r = 0.98***) and 30ºC (r = 0.99***) in the dark, suggesting that germination test can be used to predict preharvest sprouting in the field. Preharvest sprouting was significantly correlated (r = −0.77**) with main stem length, suggesting that ecotype is partly responsible for this phenomenon.

Conventional seedling mats for rice transplanters in Japan are heavy to carry, and much labor is required to collect, wash and store the nursery boxes. In addition, seeding time overlaps with the labor peak in spring. To reduce such labor, we developed a “seed-mat” consisting of hardened rice seeds (Oryza sativa L.) glued onto a molded rice-hull mat with a cover of soil glued on. Seed hardening, which is done by soaking seeds in water at 15ºC for 5 d followed by drying, and heating of the seeds at 50ºC for 5 or 7 d, reduced the time to 50% germination (T50). In dormant seeds, the heating before the hardening enhanced the effect of the hardening more than the heating after the hardening. Seed hardening increased the dry weight of the shoots from the seed-mats. Covering the mat with soil also increased the percentage of seedling emergence, shoot length, number of leaves and shoot dry weight of the seed-mats. The hardened seeds maintained short T50 and 95% or higher germination for 120 d at room temperature. In the seed-mat, the hardened seeds maintained 95% or higher seedling emergence for 208 d. In conclusion, superior seed-mats could be produced using the procedure involving both seed hardening and cover of soil. The seed-mats can be prepared in winter and stored until seedling-raising period in spring. By the seed-mats, seedling mats can be prepared for the rice transplanter without using nursery boxes, which take up storage space and require much labor.

The objectives of this study were to identify the growth parameters involved in determining the number of spikelets on an individual tiller (ST) and to elucidate how ST is determined in rice (Oryza sativa L.). We examined the correlation of ST with the dry weight (DWT), leaf area (LAT), and the amount of nitrogen (NT) at heading for individual tillers grown under different conditions that were expected to affect spikelet production. In 1999, the japonica rice cultivars “Mineasahi” (short-duration), “Hinohikari” (intermediate-duration), and “Akebono” (long-duration) were transplanted in a paddy field on two different dates (EARLY and LATE). In 2000, Hinohikari was grown under three different treatments (gibberellic acid application, nitrogen topdressing, and thinning of hills at panicle initiation) and without treatment (control). Covariance and partial correlation analyses indicated that ST was positively and essentially correlated with DWT rather than with LAT and NT. The regression of ST on DWT was stable within each cultivar regardless of the treatment, year, tiller order, and number of differentiated spikelets. The slope of the regression was the steepest in the short-duration cultivar. These results suggest that the steep slope is desirable for high-yielding cultivars with large panicles. We conclude that ST is mostly determined by dry matter production of an individual tiller regardless of the number of differentiated spikelets. We present a diagram showing the relationship between dry matter production and the number of differentiated, surviving, and degenerated spikelets on an individual tiller.

The objective of this study was to identify rice cultivars with high emergence ability under low soil-moisture conditions using a large number of rice cultivars, and to clarify the differences between the rates of emergence in lowland and upland, white- and red-kerneled, and non-glutinous and glutinous cultivars. First, 30 cultivars with a high emergence rate at 14 days after sowing (DAS) were selected at a soil water potential of -1.17 to -0.89 MPa from 382 cultivars through fourscreening experiments. In these experiments, the emergence rate was significantly higher in the upland cultivars than in the lowland cultivars. The red-kerneled cultivars also had a significantly higher emergence rate than the white-kerneled cultivars. However, no difference in emergence rate was observed between the non-glutinous and glutinous cultivars. Second, the emergence rates of various cultivars were examined at a soil water potential of -1.62, -1.23, and -1.07 MPa. The higher the soil water potential, the higher the emergence rate at 28 DAS and the shorter the time to the emergence. The order of cultivars in their emergence rate at 13 DAS at -1.07 MPa was similar to that at 28 DAS at -1.62 MPa. Therefore, the dataat 13 DAS at -1.07 MPa were used to compare the selected 30 cultivars for their emergence ability under low soil-moisture conditions. Moulla Topa showed the highest emergence rate (78.1%) and Gaiya Rate Bhasunamathe the shortest time to 50% emergence (11.1 d) under these conditions.