Tohru Kobata

The relationship between dry matter increase of seed and shoot during the seed-filling period in three kinds of soybeans with different growth habits subjected to shading and thinning

Abstract The ratio of dry-matter increase in seed (S) to that in shoot (W), referred to as Rs/W, during the seed-filling period may reflect the balance between the assimilate supply and the sink capacity of harvest organs. In the determinate soybean, cv. Tamahomare, Rs/W during the seed-filling period was nearly the same under various growing conditions including shading and thinning of plants. Therefore, the S in the determinate soybean seems to correlate with the W under various conditions. However, the correlation of the S with the W in indeterminate soybeans in which shoot growth continues during the seed-filling period is unknown. In this study, three soybean cultivars [Glycine max (L.) Merr.], Tamahomare (determinate), Tozan69 (indeterminate) and Peking (semi-indeterminate), were grown under shading and thinning conditions during the seed-filling period. The Rs/w approximated a positive linear regression in the three cultivars. Rs/w was approximately 0.5 in each cultivar. Positive correlations were observed between W and S, pod number or total node number in all cultivars. However, the seed number per pod and individual seed weight were not significantly influenced either by shading or thinning in any cultivar. Hence the rate of partition of assimilate to seeds seems to be determined by pod number. Harvest index was stable except when W was extremely low as when plants were heavily shaded. Our results suggested that the Rs/w, harvest index and the yield-determining processes of the determinate and the indeterminate soybeans are fundamentally the same.

Convenient estimation of unfertilized grains in rice.

Abstract Failure of fertilization in rice is a critical yield-determining factor in plants subjected to temperature or water stress at the early-reproductive stage and in high-yield cultivars bearing heavy spikelets. Although it is important to identify quickly the unfertilized spikelets for research and selection of stress-resistant or high-ripening cultivars from bulksamples, the identification takes time because unfertilized spikelets are usually determined by visual and manual procedures. Our objective was to develop a convenient method to identify unfertilized spikelets in rice. Takanari spikelets at maturity grown in the paddy field were separated into floating and sinking spikelets by different specific gravity solutions of ethanol/water mixture. The unfertilized spikelets were identified by checking the grains inside the spikelets by light penetration and examining the spikelets manually. The percentage of floating spikelets decreased with the increase in ethanol concentration, and that of floating spikelets approximately coincided with the percentage of unfertilized spikelets when the specific gravity was below 0.90×10>3 kg m-3, corresponding to over 70% -ethanol. In a practical range of temperature the specific gravity scarcely changed. In an 80%-ethanol solution, the percentages of floating spikelets in Takanari grown under different nitrogen applications and in rice cultivars having different spikelet size approximately coincided with percentages of unfertilized spikelets, though the percentages of floating spikelets was 5 to 7% higher than the unfertilized spikelets. The use of 70%-ethanol solution increased the difference in some rice cultivars. We concluded that the gravitation method would be convenient for identification of unfertilized spikelets in bulk samples of rice.

The Effect of Different Nitrogen Sources from Urea and Ammonium Sulfate on the Spikelet Number in Egyptian Spring Wheat Cultivars on Well Watered Pot Soils

Abstract The spikelet number (SPN) is an important wheat yield component decided before anthesis. Our objective was to determinethe effect of nitrogen (N) from urea and ammonium sulfate split-applied at different rates before anthesis on the SPN in the recent Egyptian cultivars Sakha93 and Sakha94 and two commonly used cultivars in pots. The response of SPN to applied Nfrom ammonium sulfate was higher than that from urea in all four cultivars used. However, there was no difference in the responses of SPN to absorbed N from the two fertilizers in any of the cultivars. The SPN per applied and absorbed N was thehighest in Sakha94 among the four cultivars. These cultivar differences in the SPN were due to the difference in the spikenumber. The results suggested that the N from ammonium sulfate has a greater effect on SPN than that from urea in recent Egyptian cultivars.

Effect of soil compaction on dry matter production and water use of rice (Oryza sativa L.) under water deficit stress during the reproductive stage.

Abstract The effect of a long term of soil compaction on dry matter production (DMP) and water use in rice cultivated under limited water supply during the reproductive stage is unknown. Our objectives were to determine which of the transpiration (Tr) or water use efficiency (WUE) is dominant in determining DMP under compacted and desiccated soil conditions. When irrigation in the period around the reproductive stage was terminated in artificially compacted and non-compacted fields, the rate of suppression of DMP by soil compaction was similar in the three rice cultivars, but DMP was higher in drought resistant cultivars having deep root density at the heading stage. Six cultivars were grown in pots of 1.0 m in depth containing the soils of three levels of soil bulk density (SBD). Water supply was restricted by keeping the water table in the pot deep without irrigation during the reproductive stage. DMP and Tr in all cultivars decreased with increasing SBD, and a close relationship was seen between DMP and Tr. WUE was thus a fairly stable factor for all cultivars examined. Tr was positively correlated with root length density and was relatively maintained at a high SBD in drought-resistant cultivars having a higher root length density. We concluded that water shortage under compacted soil conditions during reproductive stage suppressed the DMP, and DMP suppression accompanied a reduction of Tr due to poor root development rather than the reduction of WUE. In the drought-resistant cultivars reduction of DMP was relatively small due to their highly developed root systems that allowed high water absorption from the deep layers in the compacted soil.

Molecular physiological aspects of chalking mechanism in rice grains under high-temperature stress

Abstract High-temperature stress during grain filling hastens the growth rate of endosperm and causes grain chalkiness. Scanning microscopy of chalky areas reveals loosely packed, rounded starch granules with occasional small pits. Intensive investigation of the transcriptome, proteome, and metabolome in developing caryopses under high-temperature stress revealed the downregulation of starch synthesis enzymes and the upregulation of α-amylases. High-temperature ripening may unbalance the synthesis and degradation of starch in the developing endosperm cells. In addition to starches, storage proteins are synthesized, assembled, and stored in developing seeds. Several lines of evidence suggest that redox regulation affects seed maturation, including the accumulation of storage starches and proteins, and thus grain quality. A heat-tolerant cultivar of rice shows a characteristic high expression of superoxide dismutase (SOD). H2O2 produced by SOD under high-temperature stress possibly acts as a signal that rapidly can promote the expression of stress-response proteins. Herein, we will discuss the possible molecular physiology of grain chalking under high-temperature stress.

Stay-Green in Rice (Oryza sativa L.) of Drought-Prone Areas in Desiccated Soils

Abstract Stay-green in the post-anthesis period is thought to be an efficient drought-tolerance trait in crops, but its effectiveness in rice (Oryza sativa L.) is unknown. Our objectives were to determine whether the stay-green trait exists in rice cultivars in drought-prone areas. Twenty-four cultivars from Japan and Vietnam were grown in pots of 0.08 m in diameter and 1.00 m deep. At heading, irrigation was terminated in half of the pots and continued in the remaining pots. Every four days during the grain-filling period, we measured the leaf green color with a chlorophyll meter (SPAD), the green leaf area (GLA) and the fraction of transpirable soil water (FTSW). The capacity for maintenance of SPAD-value and GLA in desiccated soils was evaluated by determining the ratio of integrated SPAD-value and GLA in desiccated (D) plants to those in irrigated (I) plants (SPADd/I or GLAd/I, respectively). The SPADd/I and GLAd/I in 24 cultivars showed diverse frequency distributions. Cultivars belonging to higher ranges of the distribution in SPADd/I and GLAd/I tended to show higher ratios of plant dry weight at harvest in D to in I plants. SPADd/I and GLAd/I in the grain-filling period were poorly correlated with those in the seedling period in desiccated soils, and hence the capacity for maintenance of green leaves in the grain-filling period would differ from that in the seedling period. These results suggest that the stay-green trait exists as the capacity for maintaining green leaves and benefits dry matter production in desiccated soils in rice cultivars in drought-prone areas.

Does wetting and redrying the seed before sowing improve rice germination and emergence under low soil moisture conditions

Seedling establishment following germination and emergence is one of the growth processes most susceptible to environmental stresses, including soil desiccation (Steponkus et al., 1980) and low temperature (Nakamura, 1985). For direct sowing of rice on well-drained paddy and upland fields, seedling establishment is sometimes low under fluctuating soil moisture conditions and therefore a technical operation which increases and stabilizes seedling establishment is required (De Datta et al., 1979 ; Shiratsuchi et al., 1997). Seeds which were subjected to wetting and redrying before sowing (seedhardening treatment) were able to emerge better than non-treated seeds in wheat (Henckel, 1964) and carrot (Austin et al., 1969; Currah et al., 1973), but not in barley (Husain et al., 1968). The seed-hardening treatment, therefore, is expected to be a relatively low-cost management practice for improving seedling establishment for some species. It is, however, unknown whether seed-hardening in rice enhances germination, emergence or establishment under drained low soil moisture conditions. Our objective was to investigate the possibility of using a seed-hardening treatment in rice to increase germination and seed emergence in low-moisture soil.

Liquid Culturing of Detached Panicles of Rice : Cooled Culture Solutions Extend the Period of Growth

Grain dry-matter increase (GDI) in rice depends highly on assimilate supply from plant parts other than grains. Assimilate supplys contribution to GDI through the panicle organ is often ignored because the net photosynthetic rate in the rice panicle is very low (Tsuno et al., 1975; Ishihara et al., 1990). In order to investigate the effect of nutritional and environmental factors on GDI, it is essential to strictly control the supply of nutrients to grains as well as the panicle environment. If grains on detached panicles cultured under the conditions of controlled nutrient supply and environment grow almost equally as well as those on field-grown plants, we can clarify the factors controlling GDI more easily. As a crossbreeding technique, water culture of detached panicles with leaves attached has been used to harvest set grains (Syakudo, 1958), but it is unknown how rice grains on the panicles cultured in the solution with complete nutrients can grow and what conditions are needed for the detached panicles to achieve GDI similar to that under field conditions. Using the method of liquid culture previously used for wheat (Sing and Jenner, 1983) , we tried culturing· of detached panicles of nee.

Assimilate Supply as a Yield Determination Factor in Spring Wheat under High Temperature Conditions in the Mediterranean Zone of South-East Turkey

Abstract Wheat in the Mediterranean zone often encounters high temperatures during the terminal growth stage. This study investigated whether assimilate supply by source-function plays a dominant role in determining grain production in spring wheat exposed to high terminaltemperatures in the Mediterranean zone of south-east Turkey. The spring wheat cultivar Adana99 was grown under irrigated conditions sowing according to the current schedule (CS plants) and late-sowing (LS plants) for two years. Grain yield of CS plants, which were subjected to relatively lower temperatures during the terminal growth stage, was higher than that of LS plants, which were subjected to relatively higher temperatures. A high temperature in the post-anthesis period accelerated leaf senescence and reduced radiation-use efficiency. Halving the plant density at anthesis, which increases assimilate supply to grains, significantly increased the grain dry weight (GDW) and whole plant dry weight (WPDW) in the post-anthesis period. Grain filling percentage (F%, observed/final GDW) in thinned plants, which was regarded as potential F%, showed a single logistic equation based on cumulative temperature after anthesis, regardless of post-anthesis temperatures. In the LS plants, the daily increase in WPDM ( Δwpdw) decreased, but the peak of an increase rate of potential GDW estimated from F% was shifted to earlier-filling stage, resulting in reduced GDW at harvest. The final GDW in both years closely correlated with the estimated Δwpdw in the grain-filling period. These results suggest that source-function for assimilate supply in the post-anthesis period is one of the important yield-determining processes in spring wheat subjected to high temperatures.

Effct of Hardening, Wetting and Redrying before Sowing, on Germination and Seedling Emergence of a Japanese Wheat Variety Norin 61 in Desiccated Soil

Abstract Seed hardening, wetting and redrying of the seed before sowing, promotes germination under low soil-moisture conditions in wheat varieties bred and grown in semi-arid areas. It is unknown, however, whether this is also the case in wheat varieties cultivated in a temperate humid zone. We examined the effect of the seed hardening on germination and seedling emergence under low soil-moisture conditions in a wheat variety, Norin 61, which is cultivated over a wide area of the temperate humid zone in Japan. We first determined the optimum temperature for germination (23° C) and the critical range of soil-moisture (GR) for germination (water-holding capacity of 24%; – 0.30 MPa). Then, we imbibed the seeds of Norin 61 for various periods followed by air-drying, and compared the germination of these seeds and their seedling emergence with those in non-treated seeds at 23°C under GR condition. Imbibition of the seeds for 24 h, which increased the water content to 64%, followed by redrying significantly accelerated the germination and seedling emergence under such conditions. Even in the field, the seed hardening treatment significantly accelerated germination and seedling emergence under both irrigated and non-irrigated conditions. We concluded that seed hardening treatment is available to accelerate the germination of a wheat variety of the temperate humid zone in Japan, Norin 61.