This review paper examines some of the issues concerned with sustainable production of two major cereal crops-wheat and rice. We compare the perceived threats to sustainability in intensive production of rice in Japan and wheat in Europe, and evaluate the contrasting experiences with minimum tillage as a possible technique for greater sustainability. The contrasts which are presented reveal how perceptions about environmental threats vary widely, and how social and economic factors, as well as purely environmental issues, greatly influence the conclusions reached about sustainability. Before drawing these comparisons, however, we present a brief outline of the two crop systems as practised in Japan and Europe, and describe the experiences in both regions of using minimum tillage. The comparison presented has shown that there are no simple solutions to the problem of sustainability. Management techniques which work well in one situation, for example minimum tillage for wheat, may be inappropriate for another crop. Furthermore, the search for sustainability is strongly constrained by social and economic circumstances which ultimately determine which management options are viable. Indeed, although awareness of the need for more sustainable agricultural practices is growing, both in Japan and Europe, short-term economic considerations continue to be the dominant influence affecting agricultural practice in both regions.

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Eleocharis retroflexa(Poir.)Urban ssp.chaetaria(Roem.& Schult.)T.Koyama, an amphibious leafless sedge, grows not only under terrestrial conditions but also under completely submerged aquatic conditions. We investigated the photosynthetic traits and structural features of the culms, which are the photosynthetic organs, in the terrestrial and submerged forms of this species and compared them with those of other amphibious species of Eleocharis which are known to change the photosynthetic modes. The culms of the terrestrial form had Kranz anatomy with well-developed Kranz(bundle sheath)cells and high levels of C₄ enzyme activity typical of the NAD-malic enzyme(NAD-ME)subtype of C₄ metabolism. They also had a δ ¹³C value typical of C₄ plants, indicating that the terrestrial form fixes carbon through the C₄ pathway. The culms of the submerged form had not only a Kranz-like anatomy but also revealed anatomical traits typical of leaves of submerged aquatic plants. The activities of the C₄ enzymes in the submerged form were lower than those in the terrestrial form, but were still in the range typical of C₄ plants. ¹⁴C pulse-¹²C chase experiments with the submerged form indicated that almost all of the fixed ¹⁴C was incorporated into C₄ compounds, and subsequently the raioactivity was transferred into C₃ compounds and sucrose. The submerged form showed no diurnal fluctuation in malate level. These data demonstrate that a C₄ metabolism is operative even in the submerged form. Thes unique amphibious C₄ plant provides an intriguing example of the physiological and ecological adaptability of C₄ plants.

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This study was conducted to reveal the ideotype of wheat(Triticum aestivum L.)canopy structure for high yield and biomass productivity. The assimilation rate and nitrogen contents of the flag, second, third and lower leaves and the ear of the main stem were measured in three spring cultivars with different plant forms(figures), Haruyutaka, Norin 61 and Selpek, under field conditions. The assimilation rate was evaluated based on the amount of water-soluble carbohydrate accumulated during the day(WSC/day). Surface area indexes of the ear and flag leaf in Haruyutaka and Selpek were larger than those in Norin 61. In Haruyutaka and Selpek, the amount of WSC/day was larger in the flag leaf than in the second or third leaf, which had a sufficiently high nitrogen content. On the contrary, in Norin 61, the amount of WSC/day in the flag leaf was similar to that in the second and third leaves during the grain-filling period. The amount of WSC/day in the flag leaf during the last half of the grain-filling period was much higher in Selpek than in Haruyutaka and Norin 61. This may be one of the reasons why Selpek had a heavier total dry weight than Haruyutaka and Norin 61 in 1993 and 1995.

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Four vegetable-type soybean(edamame)cultivars and four grain-type cultivars were cultivated in four prefectures, Iwate, Akita, Yamagata, and Fukushima in Japan. Concentrations of water-soluble N in immature seeds tended to be higher in vegetable-type soybean seeds than in grain-type soybean seeds in all four cultivation areas. However, immature seeds of a grain-type cultivar, Higomusume, which has a small grain size, had higher concentrations of water-soluble N comparable to those in immature seeds of edamame cultivars in all four cultivation areas. This suggests that the concentration of water-soluble N in some grain-type cultivars is determined mainly by genetic characteristics of cultivars. Higomusume may be an important genetic resource for further studies on the synthesis and/or accumulation of storage substances in immature grains.

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We investigated whether drought resistant rice cultivars exhibit higher dry-matter production under wet and dry compacted soil conditions in the vegetative stage and determined the dominant factors governing resistance to soil compaction. Three rice cultivars, a drought-sensitive Nipponbare, and drought-resistant Senshou and Dular, were grown in pots at four soil bulk densities(SBD)ranging from 1300 to 1600 dry soil kg m^-3. Root and shoot dry matter productions was slightly smaller in Nipponbare over the 29 days after sowing under irrigated conditions than in the other cultivars at all SBDs. Senshou and Dular also maintained a higher dry matter production, both in relative and absolute values, than Nipponbare under the condition of withheld irrigation from days 29-39 after sowing. The higher stomatal conductance and leaf water potential of these two cultivars were supported by a larger root system which was mostly accompanied by lower top-root ratios in the irrigated and compacted soils. The higher plant growth rate under the non-irrigated condition might have been a result of both the higher water absorption rate and water use efficiency, which in turn were supported by the larger root biomass. We conclude that the ability of rice to rapidly develop a root system in the early vegetative phase under compacted soils facilitates plant production under subsequent soil desiccated conditions.

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The effects of nitrogen-free basal dressing(BNo)and top-dressing on the growth and yield of two rice varieties, Akitakomachi and Hitomebore, planted at a standard density(22.2 hills m^-2)and sparse density(16.7 hills m^-2)were examined. The maximum number of stems and the number of panicles per unit area were lower in both BNo plots with a standard planting density(BNo22)and sparse planting density(BNo17)than in the control plot with standard nitrogen dressing and standard planting density(CONT)in both varieties. The number of panicles and spikelets per unit area was lower in both BNo22 and BNo17 than in CONT, but the number of spikelets per panicle in BNo17 was significantly higher than that in CONT, although that in BNo22 was not. In BNo22, the percentage of ripened grains and 1, 000 grain weight were significantly higher than those in CONT, but in BNo17, 1, 000 grain weight was similar to that in CONT, although the percentage of ripened grains was higher than that in CONT. Grain yield in both BNo22 and BNo17 was not significantly different from that in CONT, and that in BNo17 was 660 and 710g m^-2 in Akitakomachi and Hitomebore, respectively. The leaf area index was lower and crop growth rate was higher in both BNo22 and BNo17 than in CONT at the grain-filling stage. The net assimilation rate and specific leaf weight were significantly higher in BNo22 and BNo17 than in CONT at the panicle formation and grain-filling stages, respectively. Nitrogen top-dressing at the neck node differentiation stage was found to be very useful for the cultivation of these rice varieties under BNo with sparse planting density in the Tohoku district.

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The changes in growth, dry matter productivity and yielding ability in the course of domestication from O.breviligulata to African rice(O.glaberrima)were examined. Between the two rice species, differences were hardly detected in plant height, culm length and final leaf number, although O.breviligulata had a wider variation of these characteristics as compared with African rice. The shorter strains tended to produce more tillers at the maximum tiller number stage, and the high tillering capacity was correlated with a low percentage of productive tillers, although the strains having a small number of grains per head showed a high percentage of productive tillers, in both species. No significant difference was detected between the two species in straw weight at maturity, and the differences in straw weight among strains were attributed to the differences in days required from transplanting to heading and to plant height. No significant difference between the two species was observed in top and total weights. The grain number per head was conversely related with panicle number in both species. Although the percentage of ripened grain tended to be slightly lower in O.breviligulata than in African rice, this may be attributed partly to the difference in grain number per hill. Single grain weight in O.breviligulata was nearly equal to that in African rice. These results suggest that agronomic traits such as growth, dry matter productivity and yielding ability have been changed only slightly in the course of domestication from O.breviligulata to African rice.

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This paper describes the effects of auxin added to the culture medium on main and branch root formation of banana(Musa spp.)shoots and growth characters of the plantlet rooted on the medium with and without auxin. Banana shoots cultured in vitro on Murashige and Skoog medium supplemented with 2μM 1-naphthylacetic acid(NAA), rooted earlier and also had more adventitious roots than those cultured on the medium without NAA. However, the adventitious roots formed on the medium without NAA showed more lateral branching. Plant height and number of leaves per plantlet in in vitro culture were not influenced by the addition of NAA but under nursery conditions, plantlets rooted without NAA showed better growth in terms of days to the appearance of new leaf, plant height and number of leaves per plant. This might be due to the presence of abundant lateral roots. Even though auxings are generally known to promote rooting, NAA inhibited the formation of lateral roots in Banana plants.

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Anthers at various maturation stages were collected from seven genotypes of Sorghum bicolor, and cultured on MS medium supplemented with 3 mg L^-1 2, 4-D and 2.0 mg L^-1 kinetin. The rate of callus induction was high in young anthers, and decreased with maturation of the anther. Young anthers were collected from the plants grown in the greenhouse(greenhouse anthers), in the field(field anthers), and in the chambers kept at 20, 25 or 30 °C. Then they were cultured on MS medium containing 2.0 mg L^-1 kinetin, 1.0 mg L^-1 IAA and 2.5 mg L^-1 2, 4-D, on which the rate of callus induction was the highest. The average callus-induction percentage was 6.4% in the field anthers, and 3.7% in the greenhouse anthers. Callus induction was low in the anthers collected from the plants grown at 30°C. Among the genotypes examined, TX403 produced calli at the highest frequency. The rate of plant regeneration from anther-calli was the highest on the medium containing 2.5 mg L^-1 kinetin and 3.0 mg L^-1 IAA. The regeneration rate of green plantlets on this medium was 27% on the average, and was 46.2 and 28.6% in PP290 and TX403, respectively. The chromosome number in root-tip cells was unstable, but was 20 in most of the regenerated plantlets.

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The present study was conducted to establish an efficient protocol of plantlet regeneration through somatic embryogenesis in garlic(Allium sativum L.). Root tips measuring 2 to 3 mm were excised and cultured on agar-solidified MS medium containing various concentrations of 2, 4-dichlorophenoxyacetic acid(2, 4-D)for callus and embryo formation. The optimum concentration of 2, 4-D was 0.5 μM. At a concentration higher than 1.0 μM, 2, 4-D had an inhibitory effect on callus and embryo formation. Embryos germinated and formed rooted plantlets on MS solid medium containing 5.0 μM kinetin. The number of plantlets regenerated per root tip explant depended on the concentration of 2, 4-D in the callus initiation medium. The plantlets were established in the soil after acclimatization in a growth cabinet. Somatic embryos were morphologically characterized by scanning electron microscopy(SEM).

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Direct somatic embryogenesis in rice scutella was studied by scanning electron microscopy paying attention to the morphological change of surface structure. Somatic embryos were formed when the scutella of immature embryos of cv. Nipponbare were cultured on MS media containing 2 mg L^-1 2, 4-D. Embryos were then transferred to the media containing 1 mg L^-1 2, 4-D and germinated on hormone-free media. The developmental stages of typical monocotyledonous somatic embryo were classified into proembryo, globular, scutellar and coleoptilar stages by referring to the terminology used for zygotic embryos. Each stage had a characteristic surface structure. The surface of proembryo was smooth. Fibrillar material was observed during the transition from proembryo to globular stage. Fibrillar material was then gradually replaced by a mesh-lide structure at the globular stage. With subsequent development of globular embryo, the apical scutellum region became flattened and the ridge of mesh-like structure became lower. Development of scutellum followed by the emergence of coleoptile from the scutellar notch was observed at the scutellar stage. The coleoptile surface was covered with elongating cells. The coleoptilar stage was characterized by the growth of coleoptile and the development of root. However, aberrant somatic embryos which had leafy scutellum and multiple shoot meristem, were also observed.

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