The influence of day length before and after the start of anthesis on the growth, flowering process, and seed-setting of common buckwheat (Fagopyrum esculentum Moench) was investigated to determine the effect of day length at various growth stages. Exposure to long days (15 h in 2001 and 16 h in 2000) made the node position of the first flower higher, delayed the start of anthesis, decreased the rate of successive flowering, increased the number of nodes and flower clusters on the main stem, and prolonged the main stem elongation period. It increased the number of flowers per flower cluster, but decreased the seed-setting ratio and the number of seeds. The critical day length varied with the cultivar and the growth parameter. ‘Miyazakizairai’ (autumn eco-type) showed significantly greater responses to long days than ‘Shinanonatsusoba’ (summer eco-type). The day length before the start of anthesis significantly influenced on the main stem elongation and flowering process thereafter. This suggests that the day length is a more critical factor for the differentiation than the growth of the flower bud. The seed-setting ratio was influenced both by day lengths before and after the start of anthesis. The 15 h day length before the start of anthesis and around 12 h day length thereafter increased the number of double and multiple clusters in ‘Shinanonatsusoba’ and long clusters in ‘Miyazakizairai’, resulting in an increase in the number of seeds per cluster. Day length did not influence either the form or weight of seeds.

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Pod growth in soybean (Glycine max (L.) Merr.) begins several days after flower opening, compared with more immediate growth in other beans. We investigated the relationship between genotype, raceme order of pod set, assimilate supply or photoperiod and the length of lag period of pod growth (LP, days from flower opening to the time when pod length reaches 10 mm). Soybean (five cultivars) plants were grown in a greenhouse and in the field in 2001. The lengths of pods developed from 20 flowers which opened on the same day and set on the same raceme order, were measured every other day. The length of LP varied with the cultivar from 5 to 16 days and it was longer in late cultivars. The LP in the primary raceme (early flowers) was 15 days but that in the secondary raceme (late flowers) was 8 days. Both late sowing and short photoperiod (10h) after the start of flowering shortened the LP by up to 7 days in Enrei and 5 days in Fukuyutaka. However, neither sink (except the target racemes) removal nor BA application to the target racemes at the start of flowering affected the length of LP, even though these treatments were expected to stimulate pod growth.

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The effects of short-term exposure to a low temperature [two weeks, at 18°C/13°C(day/night)] and long-term exposure to a low temperature [four weeks, at 18°C/13°C(day/night)] from the beginning of flowering on pod setting were examined in one of the most cold-weather tolerant variety cv. Hayahikari and a standard sensitive variety cv. Toyomusume.
The short-term exposure to a low temperature did not decrease the total number of pods even in Toyomusume. However, pod setting ability during the low temperature exposure was higher in Hayahikari than in Toyomusume. After the short-term exposure to a low temperature, pod setting showed recovery from the cold-weather damage by compensative pod setting after the low temperature treatment in both varieties. The long-term exposure to a low temperature significantly decreased the total number of pods in Toyomusume. But Hayahikari showed a greater capacity for pod setting during the low temperature. And the mechanism of tolerance was related to a superior fertilization ability during the low temperature.

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To elucidate the effect of drainage of paddy fields on root lodging tolerance in direct-sown rice, we measured the pushing resistance (R), diameter of hill at the base (Dm), shoot dry weight (Ws) and root dry weight (Wr), in rice varieties grown using several irrigation management schemes that differed in the frequency and length of field drainage during the growing season. Soil hardness was also monitored to investigate the relationship between the variance of soil physical properties caused by different irrigation treatments and root lodging tolerance. Pushing resistance moment (Rh), i.e., product of pushing resistance (R) and height of pushed part of hill (h), showed higher values in rice grown in fields drained more frequently or for longer periods. A similar pattern was found in rice grown in field plots where root penetration to the subsoil layers was prevented by laying an unwoven cloth between the topsoil and subsoil layers. Higher values for pushing resistance efficiency based on root dry weight (Kr : Rh/Wr/Dm) were also found in plots subjected to more frequent or prolonged drainage, irrespective of rice variety. Soil hardness was progressively increased by each field drainage during the growing season, and showed a highly significant relationship with Kr. The above results suggest that field drainage increases the root lodging tolerance in direct-sown rice through improvement of anchoring ability caused by increased soil hardness.

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Genetic studies on the response of plant root to environmental stimuli are important for elucidating the mechanism of the stress tolerance of plants. We isolated and characterized a recessive rice mutant, rrl3, which was highly sensitive to mechanical stimulus and has short roots. No significant difference was observed between the seminal roots of rrl3 mutant and wild type in the mean axial and radial length of mature cortical cells. On the other hand, meristematic zone of the root was smaller and the cortical cell flux in the growing zone of the root was significantly lower in the mutant than in the wild type. In addition, the rrl3 mutant and the wild type did not differ in sensitivity to ethylene, IAA or ABA. These results suggest that the RRL3 gene specifically regulates the cell production process in the root meristematic zone under a mechanically impeded condition and does not regulate the sensitivities to ethylene, IAA and ABA.

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C₄ plants have many attractive traits for crops, but their structural and functional relationships are complicated. C₄ plants are different in bundle sheath cell (BSC) chloroplast location (centrifugal or centripetal) among species. The effects of light intensity on the centrifugal location of BSC chloroplasts were investigated in four grass species of NADP malic enzyme (NADP-ME) type (Zea mays, Echinochloa utilis, Sorghum bicolor and Eriachne aristidea) by light and electron microscopy. Furthermore, the degree of granal development was examined to investigate the relation between BSC chloroplast location and dependence of BSC chloroplasts on the reducing power of mesophyll cells. We investigated BSC chloroplast location grown under high intensity light (HL) (600 μmol m^-2s^-1), low intensity light (LL) (2.5 μmol m^-2s^-1) and dark conditions and counted the number of granal thylakoids per granum. Although BSC chloroplasts of maize maintained the centrifugal position under all light conditions, the centrifugal location of other species was disturbed under LL and in the dark. Granal development in BSC chloroplasts in the plants grown under HL was suppressed, although the suppression in Z. mays and S. bicolor was more prominent than in other two species. These findings indicate that there is a difference in the strictness of centrifugal location of BSC chloroplasts among NADP-ME type C₄ grass species and the strictness had no relation to the degree of granal development in BSC chloroplasts.

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To investigate whether increased sucrose-phosphate synthase (SPS) activity alters photosynthetic activity and/or the export of carbon from leaves under elevated CO₂ partial pressure ([CO₂]), we raised two lines of transgenic rice (H54-9 and H69-7), each overexpressing a maize SPS gene, and wild-type rice under ambient [CO₂] (35 Pa) and elevated [CO₂] (100 Pa). Under ambient [CO₂], no significant difference was observed between the transgenic and wild-type plants in the levels of sucrose or starch in leaves or the photosynthetic activity; but the carbon export rate was higher in H69-7 than in the wild-type. Under elevated [CO₂], photosynthetic activity increased in all plants, but the accumulation of starch was significantly repressed in H54-9, whose SPS activity was about 12.5 times higher than that of the wild-type. The carbon export rate was higher in both transgenic lines than the wild-type. We considered that increased SPS activity in rice plants would promote the export of carbon from leaves and, as a result, starch accumulation in the leaves would be suppressed and/or photosynthetic activity would be promoted under elevated [CO₂].

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In Japan, there are two taxa of the genus Salicornia plants; S. europaea L. distributed in Hokkaido and S. herbacea L. distributed on the coast of Inland Sea of Seto. To estimate the polymorphism of the Salicornia plants, we statistically analyzed the morphological features and random amplified polymorphic DNA (RAPD) of five groups from three populations found at Lake Tofutsu and Lake Notori in Hokkaido and Okayama Prefecture on the coast of Inland Sea of Seto. The morphological features, such as plant length, segment number, length and number of branches, and incidence of the secondary branches showed variations among locations. The morphological plasticity of Salicornia plants was also observed at different plant densities. Thereby these features were difficult to use for identifying the populations. On the other hand, the genotype based on the RAPD markers implied five groups : two groups from the Notori population, two groups from the Tofutsu population and one group from the Okayama population. Additionally the Notori and Tofutsu populations were identified as genotypically related, and different from the Okayama population. The RAPD method, which is one of the simplest and fastest molecular techniques, was found useful for identifying the type of Salicornia plant.

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Spatial variability of wheat and maize biomass was studied under two tillage systems, minimum tillage (MT) and conventional moldboard plowing (CT). The above-ground biomass data were collected from 10 locations separated at 5 m intervals along the crop rows. The range of semivariogram, estimated by fitting a linear model to observed semivariances, was used to investigate spatial dependence. A range of >5 m was regarded as the existence of spatial dependence and that of <5 m as the non-existence of spatial dependence. The frequencies of existence of spatial dependence were 46% and 63% under MT and CT, respectively, in wheat and 13% and 42%, respectively, in maize. Spatial dependence appeared more frequently under CT than under MT. Wheat showed spatial dependence more frequently and had a longer range than maize. Spatial dependence was unstable and variable with the year even on the same sampling lines. The origin of tillage-induced spatial dependence and its possible role in precision farming are discussed.

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To estimate the site-specific optimal amount of nitrogen (N) to be applied as top-dressing (N top-dressing) at panicle initiation, we examined the effects of the amount of N accumulated in the aboveground biomass at the panicle initiation stage (N₁), the total N content of soil and the amount of mineralizable soil N (Nm) on the amount of soil N taken up by the plants during the period from panicle initiation to heading (Ns) and the recovery ratio of applied chemical N during this period (J). We analyzed the data obtained in paddy fields with a similar soil type and weather condition in the growth period from 1999 to 2001. The growth of rice plants was regulated by the amounts of N applied before transplanting and at panicle initiation. The relative N uptake rate during the period from panicle initiation to heading (RNR) in the plots without N top-dressing was explained about 89% by multiple regression of N₁, the total N of soil and Nm. Ns in the plots with N top-dressing that was estimated using RNR showed a significantly negative partial correlation with N₁, the total N of soil and Nm. J in the plots with N top-dressing showed a significantly negative partial correlation with N₁, but a significantly positive partial correlation with Nm. These results suggest that the fertilizer N should be applied considering the contribution of N₁, the total N content of soil and Nm to Ns and J to fit the site-specific N demand of rice.

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