Junko Yamagishi

QTLs for branching, floret formation, and pre-flowering floret abortion of rice panicle in a temperate japonica × tropical japonica cross

A large panicle with numerous florets is essential for improving rice (Oryza sativa L.) yield. Rice panicle size is determined by such underlying morphogenetic processes as: (1) primary branch formation on the panicle axis; (2) floret formation on the primary branches (mainly determined by the secondary branch formation); and (3) pre-flowering abortion of florets in the panicle. We examined QTLs for these processes to understand how they are integrated into panicle size. We developed 106 backcross-inbred lines (BC1F4) from a cross between ‘Akihikari’ (a temperate japonica) and ‘IRAT109’ (a tropical japonica) and constructed a genetic map. One QTL detected on chromosome 2, with a large effect (R=0.30) on the number of florets per panicle, affected both primary branch formation on the panicle axis and floret formation on the primary branches. In addition, three QTLs that affect only one of these two processes were identified on chromosomes 4, 9, and 11, each having a subsidiary effect on the number of florets per panicle (R2=0.04–0.07). QTLs for pre-flowering floret abortion were detected at three different regions of the genome (chromosomes 1, 10, and 11). This is the first report on QTLs for pre-flowering floret abortion in grasses. The absence of a co-location between QTLs suggests that floret formation and abortion are not directly linked causally. These results demonstrate that studying the partitioning of panicle size into these underlying morphogenetic components would be helpful in understanding the complicated genetic control of panicle size.

Evaluating the resistance of six rice cultivars to drought: restriction of deep rooting and the use of raised beds

Soil water deficits reduce rice (Oryza sativa L.) productivity under upland field conditions. In this study, we constructed screening facilities to evaluate the performance of rice cultivars under drought conditions and to assess the roles of deep roots. Two experiments were conducted with six rice cultivars, including drought-tolerant and drought-susceptible cultivars, grown in two root environments: a root-restricted treatment that restricted rooting depth with water-permeable sheets, and a raised bed that reduced water availability in the surface soil by inserting a gravel layer between the topsoil and subsoil layers to interrupt capillary transport of water. In the root-restricted treatment, in which root growth was restricted to the surface 25-cm layer, leaf water potential decreased faster in cultivars with a large canopy during drought stress, and there was little difference in panicle weight among cultivars. With a normal (unrestricted) root environment, the deepest-rooting cultivar (‘IRAT109’) maintained higher leaf water potential during drought, although panicle weight under drought stress was affected by yield potential as well as by deep rooting. Under the intermittent drought stress in the raised bed, deep-rooting cultivars accumulated more nitrogen and produced more biomass, and the difference in panicle weight between deep-rooting drought-tolerant and shallow-rooting drought-susceptible cultivars was magnified by the raised bed compared with the yield differences under drought in a normal root environment. These results demonstrate that the drought screening facilities we developed can help to identify superior cultivars under upland field conditions without time-consuming measurement of deep root systems.

Growth of Three Rice Cultivars (Oryza sativa L.) under Upland Conditions with Different Levels of Water Supply

Abstract Upland rice production has great potential as a water-saving form of agriculture if yield can be increased and stabilized across a range of environments with different levels of water supply. The objective of this study was to clarify the effects of water supply and plant characteristics on grain yield of rice (Oryza sativa L.) grown under upland conditions. We compared grain yield (ranging from 346-685 g m-2) and yield components of three rice cultivars (‘Yumeno-hatamochi’, YHM; ‘Lemont’, LMT; ‘Nipponbare’, NPB) grown under upland conditions with three water regimes (rain-fed, RU; irrigated, IU; and water deficit during the panicle-formation stage, WD) with those of rice grown under flooded lowland (FL) conditions (ranging from 394-649 g m-2) from 2001 to 2003 at Nishitokyo, Japan. Grain yield and each yield component of NPB in RU were comparable to those in FL when there was ample rain during the 40 days before heading in 2003. However, grain yield of NPB decreased with decreasing water supply during the period of 20-40 days before heading under upland conditions (r = 0.93) as a result of reduced number of spikelets per unit area and reduced harvest index. Water productivity (grain yield per unit water supply) in rice in RU and IU ranged from 0.43 to 1.05 kg m-3 in the three cultivars across the 3 years, and was more than twice the corresponding value in FL. We found a cultivar – water regime interaction for grain yield within each year and a cultivar × environment interaction across all the 5 upland conditions in 2002 and 2003. In FL, NPB and LMT had higher yields than YHM, while LMT had the highest yield under all upland conditions and NPB grain yield under the suboptimal upland environments (i.e. RU and IU in 2002) decreased to the largest extent compared with that under optimal upland environment, i.e. IU in 2003 among the three cultivars. The reasons for the highest grain yield of LMT across upland conditions were maintenance of large panicle and high harvest index. Maximum yield was lowest in YHM. In WD, yield potential and growth recovery, rather than crop growth during water stress, affected the cultivar ranking in terms of grain yield. We conclude that water supply during panicle development is important for maintenance of high yield and that a high potential yield and harvest index, as well as yield stability under different water regimes, are important putative plant characters for developing new elite varieties for water-saving upland rice production.

Effect of Planting Density on Grain Yield and Water Productivity of Rice (Oryza sativa L.) Grown in Flooded and Non-flooded Fields in Japan

Abstract The effect of planting density on grain yield and water productivity was evaluated in rice (Oryza sativa L.) grown in non-flooded lowland fields in Japan in comparison with flooded fields. One rice cultivar, IR24 was grown both in flooded and non-flooded lowland fields in 2001 and 2002, and only in flooded field in 2003, with different planting densities ranging from 5.6 to 44 hills m-2. Another rice cultivar, Dontokoi was also grown in 2001. Straw mulching treatment was added in non-flooded field in 2002. In non-flooded fields, standing water disappeared from 36 and 8 days after transplanting until maturity in 2001 and 2002, respectively, the mean water content of surface soil during non-flooded period was 72 % g g-1 on a dry basis and 63 % v v-1. Grain yield in flooded fields (637 g m-2; average of 2001 and 2002) was higher than that in non-flooded fields (467 g m-2; average of 2001 and 2002), due to larger spikelet number per panicle in both years, larger 1000 grain weight in 2001, and higher percentage of ripened grains in 2002. Straw mulching tended to increase sink size but reduced percentage of ripened grains, resulting in no yield advantage in 2002. Water productivity in non-flooded fields (0.34 kg m-3; average of 2001 and 2002) was significantly higher than that in flooded fields (0.14 kg m-3). Grain yield increased with higher planting density in flooded fields in 2001 and 2003. In non-flooded fields, however, the effects of planting density on grain yield were little or marginal in both cultivars, due to the trade-off relationship between panicle number and spikelet number per panicle. This study showed that higher planting density would result in higher grain yield in favourable flooded fields, but is not advantageous for higher grain yield under non-flooded lowland fields in Japan in improved cultivars with relatively high tillering and yielding abilities.

Size and activity of shoot apical meristems as determinants of floret number in rice panicles

Abstract The sink capacity (floret number per unit land area) is currently a serious constraint to grain yield production in japonica rice. The size and activity of the early reproductive shoot apex (incipient panicle) are potential determinants of the number of florets generated on the panicle. This hypothesis was tested using eight field-grown japonica rice cultivars (IR65598-112-2, IR65564-44-51, Nipponbare, Akenohoshi, Dobashi 1, Koshihikari, Kochihibiki and Nakateshinsenbon). Using morphometric microscopy, we found that the initial size of the reproductive shoot apex was highly correlated with the number of primary branches, but not with the number of florets per primary branch. The cell division activity of the early reproductive apex examined by in situ hybridization analyses using the histone H4 gene probe as a marker for DNA-replicating cells varied with the cultivar. Akenohoshi had twice as many DNA-replicating cells as Nipponbare and the cell division activity was highly correlated with the number of florets per primary branch, but not with the primary branch number. We concluded that the primary branch number was determined by the initial size of the reproductive apex, and that the floret number per primary branch was determined by the cell division activity in the following apex growth. This result provides the first evidence of a relationship between cell division activity and floret formation in the rice panicle.

Diversity of the rachis-branching system in a panicle in Japonica rice

Abstract Recently, rice varieties having large panicles with many spikelets are expected to produce high yield. However, the ripening, growth pattern, priority to photoassimilate partitioning and final grain weight in each spikelet vary with the position of the spikelet in a panicle. Therefore, not only the panicle size but also rachis-branching system in a panicle is an important factor determining the yield. In this report, we performed principal component analysis to characterize the rachis-branching system in 65 japónica varieties. In the principal component analysis, the proportions of variability explained were 49.6 and 22.2% for the first two attribute components. The first principal component was assumed as the factor of size in number, and the second principal component was assumed as the factor of shape of panicle. The distribution of the scores for each variety in the scatter diagram showed a large diversity in panicle characteristics in japónica varieties. A small number of varieties had scores distributed in the first quadrant of the scatter diagram. These varieties would be high-yielding because the number of spikelets is high in the panicle having a relatively large number of primary rachis-branches compared with the secondary rachis-branches. Some clusters in the scatter diagram were related with their origins.

Spatial Variability Patterns of Wheat Growth and Soil Properties in a Small Field as Affected by Tillage Intensity

Abstract The objective of this study was to obtain information about spatial heterogeneity as a basis for site-specific soil and crop management in small fields. The spatial variability patterns of wheat growth parameters and soil properties, e.g. nitrate, available phosphate, pH, and soil surface hardness, were investigated on a 0.25 ha field of Humic Andosol in 1998 - 1999. Two plots in the field had been tilled with different intensities since autumn 1997. A plot was minimum-tilled with a rotary tiller (MT) and the other plot was conventionally tilled with a moldboard plow and disc-harrowed (CT). Raw data, collected from 100 locations with 2.5 m×5 m spacing in each plot, were divided into trends (large-scale deterministic structure) and residuals (small-scale stochastic structure) by median polishing. Trends were more dominant in CT than in MT. The trend of wheat above-ground biomass at harvest was similar to those of soil nitrate and available phosphate. These trends were partly due to high variability across rows, possibly caused by the same traffic direction of farming operations for several years. The residual data, on the other hand, showed no correlation among the parameters. The spatial dependency of the residual data for soil properties, except for pH, was not apparent for distances larger than the minimum sampling distance (2.5 m). It is concluded that modification of fertilizer application based on the trend data may improve the efficiency of fertilizer use while small-scale site-specific management based on the residual data may be practically difficult. Key words :

Grain Yield and Leaf Area Growth of Direct-Seeded Rice on Flooded and Aerobic Soils in Japan

Abstract Direct-seeding has been proposed as a water- and labor-saving method to grow irrigated rice. Our objective was to compare the effects of flooded and aerobic conditions on the yield stability of direct-seeded rice. We set up four trials in the field: aerobic, near-saturated and flooded soils with direct seeding, and flooded soil with transplanting. Grain yield of direct-seeded rice was comparable to that of transplanted under flooded conditions. However, the yield of direct-seeded rice under aerobic conditions was up to 21% lower than that under flooded conditions. This poor performance was associated with reduced leaf growth during the vegetative stage. Our results indicate that the yield stability of direct-seeded rice could be lowered by the water-saving irrigation, compared with the conventional flooded culture. In order to save irrigation water, physiological research on direct-seeded rice should target the vulnerability of rice to aerobic soils or to soil moisture fluctuations.

Effects of Gibberellic Acid Application on Panicle Characteristics and Size of Shoot Apex in the First Bract Differentiation Stage in Rice

The number of spikelets in a panicle is an important factor in determining the yield of rice. Spikelet differentiation in a panicle is stimulated by gibberellic acid applied at the spikelet differentiation stage (Shimizu, 1965; Shimizu et al., 1966; Kawai et al., 1989; Matsuba, 1991; Yamagishi et al., 1994). Matsuba (1991) mentioned that the observation of fully branched panicles treated with gibberellin would be necessary to understand the potential of panicle growth. The effect of gibberellic acid may be attributed to the increase in cell division in the shoot apical meristem (SAM) at the spikelet differentiation stage. Therefore, we examined the effect of applied gibberellic acid (GA3 ) on SAM at the first bract differentiation stage microscopically and its relation to the panicle characteristics.

Extent and Implications of Weed Spatial Variability in Arable Crop Fields

Summary The integration of technologies and principles to assess, manage, and evaluate spatial and temporal variability has enabled the site-specific application of variable rates of agrochemicals. It is referred to as precision management. This article concerns with the potential of precision management for weed control. Weeds are heter-ogeneously distributed and show different levels of spatial aggregation. A better understanding of weed patchiness is necessary for precision weed management. Studies reviewed in this article showed that significant reduction in the amount of herbicide can be achieved when the spatial variability of weeds is taken into account. It is often difficult, however, to obtain information on the spatial distribution of weeds with accuracy because it depends on monitoring techniques, scales of observation, and statistical methods applied. We conclude that the precision application of herbicides is one approach to managing weeds and it will be possible to achieve a drastic reduction in herbicide applications by using a combination of several different types of weed control strategies. Not only farmers with large fields but also those with small fields should have better access to information on spatial variability to make decisions for weed control.