The crop sciences hold the key to feeding the 10 billion people on earth by the middle of the 21st century and to doing so sustainably and in the face of climatic changes which may adversely affect food production, particularly at low latitudes. Increased food production will have to come mainly from greater yields per crop, especially of the staple cereals, given the continuing loss of prime arable land to urbanization. Advances and innovations in agronomy, and the extension of irrigation, have made major contributions to the yield iracreases since 1960, and must be maintained. Plant breeding has played a key role, partly through improved resistance to pests and diseases and tolerance of various stresses, and partly through raising the yield potential. Among the small grain cereals the rise in yield potential has largely been due to increase in the harvest index associated with dwarf stature, made necessary by cheaper nitrogenous fertilizers and made possible by effective herbicides. In maize, on the other hand, the route to higher yield potential has been quite different. But in both cases synergistic interactions between plant breeding and agronomic advances have been a crucial element. Looking to the future, agronomic intensification and innovation will doubtless continue, and protective plant breeding will become more closely targeted and effective, but for how much longer yield potential can be raised is uncertain, especially for those crops in which the rise in harvest index has been the main component so far.
Lower leaf water potential (Ψ1) was observed in young rice plants grown in wet and highly compacted soil. Our objectives were to establish why the reductions of Ψ1 occurred and to find the effect of plant hydraulic conductance on Ψ1. One seedling of a lowland cultivar Nipponbare and an upland cultivar Senshou were grown in plastic pipes with a diameter of 5.4 cm and a height of 45 cm filled with a soil mixture. Soil bulk density (SBD) was set at four levels and wet soil conditions were maintained. In a growth chamber Ψ1, transpiration rate per unit leaf area (TL), leaf area and root length were measured at 35 days after sowing. In both cultivars, plant hydraulic conductance (CP=-TL/Ψ1) decreased as SBD increased and there was a positive correlation between CP and root length per unit leaf area. The greater decreases in root length than in leaf area, in the soil with a high bulk density, were suggested to reduce CP, thus resulting in lower Ψ1. In both cultivars, root hydraulic conductance per unit root length (CR), estimated using a pressure-flux method, increased with increase in SBD. The increases in CR were accompanied by the increases in TL per unit root length at a high SBD. We suggest that the suppression of root length in rice by highly compacted soil causes lower CP which in turn reduces Ψ1, even if the soil is wet. CR, water absorption rate per unit root length and the diameter of the primary roots, clearly increased under highly compacted soil conditions, but this might not be able to compensate for the greater reduction in root length than in leaf area, and so may not permit CP and Ψ1 to be maintained.
The growth and panicle characters of bread wheat (Triticum aestivum L.), in which only a single primary seminal root was allowed to grow by suppressing the growth of nodal roots, were analyzed in pots. The growth of the aboveground parts was not inferior to that of the control until the panicle formation stage due to vigorous growth of the primary seminal root. At the booting stage, however, the T-R ratio in weight was doubled as compared with the control, although the ratio of leaf area to root length was hardly changed. The grain weight per plant of the treated plants was about 80% of that in the controls, and the grains in the treated plants were more vitreous than the control. These results indicate that the primary seminal root remained functional until the ripening period in the plants without development of any other roots.
The soil sheath (SS) distribution and its change with the passage of time in maize root systems were investigated. Maize seeds were planted in deep pots, and the roots were sampled at 2, 3, 4 and 6 weeks after sowing. The distribution of SS on the axile roots and the 1st order lateral roots was examined quantitatively. On axile roots, as classified by the nodes from which they emerged, younger roots were more thickly sheathed than older ones. A similar tendency was observed in the 1st order lateral roots. Significant correlations were observed between the thickness of SS, and the root length or the diameter in one part of the axile roots. L-type 1st order lateral roots (branched 1st order laterals) were sheathed more than S-type lateral roots (non-branched). Quantitative analysis revealed that the amount of SS was closely related to the root length, thickness, branching capacity, and root age.
We examined the involvement of genetic and environmental factors in the variations and associations of the rate and duration of grain filling and the final grain weight in rice cultivars, instead of the simple phenotypic correlation and variance. The estimations were conducted by analyses of variance and covariance in a two-way classification, using ten rice cultivars cultivated under three environments at different locations and years. Also examined were the differences in these parameters for various grain positions on a panicle, the grains on primary rachis-branches and the grains on secondary rachis-branches. In both grain positions in a panicle, the rate and duration of grain filling differed widely with the environment and cultivar, and showed a very high negative environmental correlation between these two traits. The final grain weight (= filling rate × filling duration) showed a much smaller variation due to environmental fluctuations compared with the rate and duration of filling, which reflects the negatively associated environmental changes in the rate and duration of grain filling. The duration of filling, especially that in the grains on secondary rachis-branches, was more influenced by non-genetic factors than the rate of filling. The genetic correlation between the rate and duration of grain filling was not so high. The final grain weight, on the other hand, was strongly and genetically correlated only with the filling rate, not with the filling duration. This may have resulted from some constraints which do not allow longer filling durations to attain higher final grain weights.
The change in the vertical distribution of leaf N concentration from the vegetative period to maturity were examined to evaluate the effect of the leaf N gradient on dry matter production in rice (Oryza sativa L.). Cultivar 'Nipponbare' was grown in culture solutions at two N levels in 1993 and three N levels in 1994. The samples were taken three times in 1993 and six times in 1994 by a stratified clipping method, and leaf area, leaf mass, leaf N concentration on a dry weight basis (NW) and on a leaf area basis (NA) were determined for each stratum. During the active tillering stage, both NW and NA showed a steep vertical gradient with a high N concentration in the upper strata, and the gradient was steeper in a low N solution. The distributions became fairly uniform during the reproductive phase (from panicle initiation to heading), even though leaf area index (LAN) was as high as 6. After heading, NA showed a steep vertical gradient with a concave shape that was similar to the light distribution, mostly because of the presence of thicker leaves in the upper strata. The gradient was the largest at the later stages of grain filling, in which NA of the uppermost leaves was higher than the average by as much as 40%. Nevertheless, the simulated advantage of biomass production due to the non-uniform NA distribution during the ripening period was less than 8% compared with the uniform N distribution in the canopy. The NW profile did not agree with the light distribution: It was convex and conserved over the sampling periods when expressed in relative values. The rate of NW decrease in all the strata after heading could be expressed as a linear function of the initial level of NW around heading, so that NW in the upper leaves decreased faster.
Differentiation and development of floral organs of soybean (Glycine max (L.) Merr. cv. "Touzan 69") were classified into nine stages; pre-differentiation (I), floral differentiation (II), calyx formation (III), petal formation (IV), stamen formation (V), pistil formation (VI), ovule-anther formation (VII), pollen-embryo sac formation (VIII), and flowering (IX) . The floral organs differentiated from the lower to higher nodes on the main stem. The developmental rate was higher at stages II∼VI than at stages VII∼IX, and at a higher node. At the same node, the floral organs differentiated first in the basal raceme followed by the secondary and tertiary racemes in this order and the developmental rate was the highest in the tertiary raceme. The floral organs differentiated first in the secondary raceme at the 4th node. However most of the floral primordia that had differentiated in the secondary or tertiary racemes did not develop into flowers. The factors determining the floral organs are unknown, but this indicates that the sink-capacity in each node of soybean plant is extremely large.
Faculty of Agriculture, Tokyo University of Agriculture & Technology / Faculty of Agriculture, Tokyo University of Agriculture & Technology / Faculty of Agriculture, Tokyo University of Agriculture & Technology / Faculty of Agriculture, Tokyo University of Agriculture & Technology / Faculty of Agriculture, Tokyo University of Agriculture & Technology
We developed a new direct hill seeding method, paper-mulched direct seeding (PMDS). In this method, recycled-paper sheets with rice (Oryza Sativa L.) seeds affixed are spread over a flooded paddy field just after puddling and leveling. Three or five rice seeds were sandwiched between biodegradable gauze or non-woven mesh, fixed to a hole (25 mm diameter) arranged on the sheet at a distance corresponding to the planting density. The sheet gave a very high percentage (98.3%) of germination in water culture of cultivar, Koshihikari. Koshihikari sown by the PMDS method showed a relatively high percentage of seedling establishment, ranging from 70 to 80%. Moreover, a high stability of seedling establishment was obtained by the PMDS method as compared with lowland hill seeding of non-germinated seeds without use of the oxygen supplying chemical, calcium peroxide. Frequencies of vacant hills in the PMDS plot of 5 cultivars, Akitakomachi, Dontokoi, Hinohikari, Hitomebore and Koshihikari, were very low, ranging from 0.40 to 2.33%. Although no herbicide was applied in the PMDS plot, only a few weeds, i.e., Scripus juncoides Roxb. var. ohwians T. koyama and Eleocharis kuroguwai Ohwi were observed, indicating that herbicides are not required in the PMDS. Little lodging was seen in the PMDS plot. The brown rice yield by the PMDS was 615 g m-2. These results clearly demonstrate that the PMDS is useful for hill seeding in flooded paddy fields.
Natural Farming (NF), characterized by no external input such as chemical fertilizers and biocides, is a farming method aiming to produce safe food by an environmentally sound method. Examination of the long-term change in the productivity under NF will provide us with important information for the low input and sustainable rice production. Yield surveys were conducted in 17 prefectures of three climatically different districts (the Tohoku, Kinki, and Chugoku) in Japan in 1990 and 542 sets of data were collected from NF paddies, differing in years (l-50) afterconvertion to NF (YAC). The average brown rice yields of NF ranged from 352gm-2 in Okayama to 504gm-2 in Akita. The variation in NF yield generally reflected the average regional yield by conventional farming (CF) (p<0.05), but the yield gap between NF and CF was larger, the higher the CF yield. The average yield gap for the overalldata was about -66gm-2, 13% of the regional average in 1990. Although no significant increase in yield with YAC was observed in the data for each district, the yield gap was narrowed particularly after around 15 YAC onward in the Tohoku district. The yield in the Chugoku district did not show any notable increase with the extension of YAC. The yield gain with extension of YAC was modest, but the level of productivity was maintained at about 79-98% of the regional average.
Genotypic and phenotypic variances, and covariances were estimated among five characteristics of 12 early maturing genotypes of grain sorghum in a double cropping of spring and fall season in Japan. The differences among genotypes and genotype×season interaction were significant for almost all characteristics. A large proportion of the phenotypic variance, which was attributable to genotypic variance, was obtained for plant height, 100 grain weight and grain yield. The highest genotype×environment interaction was detected for days to heading. Harvest index had a negative correlation with plant height and positive correlation with grain yield, phenotypically and genotypically. This correlation analysis may provide useful information in the selection of sorghum genotypes with high yield and short plant height for a double cropping system in Japan.
Somaclonal variation in heading date, protein content in brown rice and regeneration ability was examined in regenerants derived from anther culture of rice (Oryza sativa L.). Regenerants were highly homozygous, and the variances in culm length and 1000 grain weight among panicle-row lines of A2 (the second generation after chromosome duplication of the progeny derived from anther culture) were smaller than those in their parents. Some A2 regenerants headed earlier than their F2 population. In some regenerants derived from anther culture of cv. Asahi, protein content in brown rice was lower, and the heading date was earlier than those in cv. Asahi. By successive anther culture (using regenerant successively) of cv. Shirayukihime (developed by anther culture), regenerants with higher regeneration ability than that of Shirayukihime were obtained. Therefore, anther culture was proven to be quite useful as a mutation breeding method.