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Aside from its various biomedical effects, γ‐oryzanol is a biomarker for heat stress in rice (Oryza sativaL.). Recently, we found that a japonicacultivar Momiroman had the highest γ‐oryzanol among 92 cultivars. In the present study, we aimed to detect the quantitative trait locus (QTLs) for the concentrations of cycloartenyl ferulate (1), 24‐methylenecycloartanyl ferulate (2), and total γ‐oryzanol in the populations from a cross between Momiroman and an indicacultivar Takanari. We detected the QTLs for the concentrations of 1, 2, and/or total γ‐oryzanol on chromosomes 4, 5, 9, and 11. The Momiroman alleles at the QTLs on chromosomes 4, 5, and 11 increased the concentrations of 1, 2, and/or total γ‐oryzanol, whereas the Takanari alleles at the QTLs on chromosome 9 increased. Considering the literature data, the japonicaalleles at the QTLs on chromosomes 4, 5, and 11 may increase the concentrations of 1, 2, and/or total γ‐oryzanol in the indicagenetic background. This study revealed the QTLs for the concentrations of 1, 2, and/or total γ‐oryzanol on chromosomes 4, 5, 9, and 11, providing essential information for the development of cultivars with specific biomedical properties or heat stress resistance.

ABSTRACTDegradation of the appearance of rice grains due to high temperatures during ripening is a serious problem being worsened by global warming. QTLs for tolerance to high temperature during ripening must be identified in order to develop new cultivars efficiently. Here, we developed recombinant inbred lines (RILs) from crosses between the high-temperature-tolerant ‘Genkitsukushi’ and the high-temperature-sensitive ‘Tsukushiroman’. The occurrence of white-back grains is significantly lower in ‘Genkitsukushi’ than in ‘Tsukushiroman’. We evaluated their high-temperature tolerance and performed quantitative trait locus (QTL) analysis. A QTL for white-back grains on chromosome (chr.) 6 and a large-effect QTL on chr. 8 were identified in the two-year experiments. The QTL on chr. 8 was close to one we previously detected using RILs derived from ‘Chikushi 52’ × ‘Tsukushiroman’. The results indicate that the QTL on chr. 6 is unique to ‘Genkitsukushi’ × ‘Tsukushiroman’, and that on chr. 8 could be identical between ‘Genkitsukushi’ and ‘Chikushi 52’. Backcrossed lines which harbor the QTL region on chr. 8 of ‘Genkitsukushi’ in the background of ‘Tsukushiroman’ verified its effects. We also analyzed the marker genotypes near the QTLs on chr. 6 and 8 in relatives of ‘Genkitsukushi’. We discuss the interaction between these QTLs and their relationship with high-temperature tolerance.

ABSTRACTUnderstanding the yield potential and yield-determining factors of recent high-yielding cultivars is essential for further increasing rice yield. In this study, a cross-locational field experiment was conducted across 3 years using ‘Hokuriku 193ʹ (H193), a high-yielding cultivar, at four sites including one in Nagano Prefecture, which is the highest-yielding region in Japan. The highest mean yields of 3 years, 1214 g m−2for brown rice grains and 1586 g m−2for rough grains, were recorded at the Nagano site. The yields from the 17 environments were strongly correlated with spikelet number per square meter while percentage of filled grain was relatively stable, suggesting that sink capacity is the primary determining factor for grain yield of H193. The climatic factors for high spikelet number at the Nagano site can be explained by the high cumulative radiation before heading associated with longer duration until heading by low night temperature. In addition, a large increase in shoot dry weight during grain filling (ΔW) and high radiation use efficiency (ΔW/rad) at the Nagano site could satisfy large source demand by the large sink size. The high ΔW/rad at the Nagano site associated with low night temperature. This study demonstrated high yield potential of H193 and revealed an environment that achieves extra-high yields in H193, which provided insight to attain further increase in rice yield.

Grain weight decreases by heat stress during ripening in rice. Phenylpropanoid glycosides and γ‐oryzanol are biomarkers for heat stress. The relationships between grain weight and the concentrations of these compounds should be understood in order to use biomarkers to improve the heat tolerance of rice. In the present study, we aimed to determine the relationships between grain weight and the concentrations of these compounds in the japonicacultivar Koshihikari and the indicacultivar Takanari. In Koshihikari, the concentrations of 3',6‐di‐O‐sinapoylsucrose, 24‐methylenecycloartanyl ferulate, and total γ‐oryzanol had strong negative correlations with grain weight. Conversely, in Takanari, the concentrations of 3',6‐di‐O‐sinapoylsucrose and 3'‐O‐sinapoyl‐6‐O‐feruloylsucrose had negative correlations with grain weight, whereas those of 3',6‐di‐O‐feruloylsucrose and cycloartenyl ferulate had positive correlations with grain weight. Interestingly, the relationships between grain weight and the concentrations of 3',6‐di‐O‐sinapoylsucrose, 24‐methylenecycloartanyl ferulate, and total γ‐oryzanol appeared to be stronger in Koshihikari than in Takanari. The metabolisms of 3',6‐di‐O‐sinapoylsucrose, 24‐methylenecycloartanyl ferulate, and total γ‐oryzanol may be strongly correlated with grain weight reduction caused by heat stress in Koshihikari. The concentrations of 3',6‐di‐O‐sinapoylsucrose, 24‐methylenecycloartanyl ferulate, and total γ‐oryzanol may be powerful tools for improving the heat tolerance of japonicacultivars.

ABSTRACTEnhancing crop traits that increase grain yield under elevated CO2concentrations is an important option for increasing the future productivity of rice. Here, we compared the growth and yield of five varieties with different genetic background under Free-Air CO2Enrichment (FACE) conditions to identify traits responsible for varietal differences in yield increase under elevated CO2. Three high-yielding and two standard rice varieties grown under FACE conditions commonly had (1) shorter growth periods, (2) higher dry matter production, (3) higher numbers of spikelets (sink capacity) and panicles; and (4) higher yield than those grown under ambient CO2. Yield enhancement by elevated CO2(FACE/Ambient), however, differed significantly among varieties, ranging from 1.10 to 1.25. The greater response of the sink capacity, defined as the product of spikelet number and single grain mass, was the main factor involved in yield increase. Three high-yielding varieties (Momiroman, Takanari, and Hokuriku 193) had greater sink capacity than two standard varieties and the sink capacity of these varieties significantly increased under FACE condition. However, yield enhancement in elevated CO2was lower in Hokuriku 193 than in Momiroman and Takanari. In Hokuriku 193, sink production was relatively low while dry matter production was similar to the others. Therefore, larger increase in sink production efficiency per unit of dry matter production under FACE was found to be a particularly important varietal trait, suggesting that efforts to develop varieties suited to the predicted elevated CO2condition should focus on this and the related traits.

ABSTRACTTo enhance the yield potential of rice by breeding, it is important to reveal the genetic factors affecting yield components in high-yielding cultivars. Quantitative trait loci (QTL) analysis for panicle structure and spikelet weight as an index of grain filling was conducted using recombinant inbred lines (RILs) derived from a cross between an indica-dominant high-yielding cultivar, Takanari, and a japonica-dominant high-yielding cultivar, Momiroman in 2012 and 2013 in eastern Japan. The grain-filling ability of Takanari is reported to be better than that of Momiroman. Since grain filling is generally better near the tip of the panicle and decreases as the number of branches from the rachis increases, we classified whole panicles into upper and lower side panicles and spikelets into primary, secondary, and tertiary spikelets according to the number of branches from the rachis. On chromosomes 1, 4, and 6, QTLs regulating the number of spikelets per panicle and panicle structure were detected and were most likely identical to GN1a, SPIKE, and APO1, respectively, which has been previously reported as QTLs regulating the number of spikelets per panicle. Takanari produced much heavier secondary and tertiary spikelets than Momiroman on the lower side panicle. On chromosome 5, novel QTLs regulating spikelet weight were detected. The Takanari allele enhanced secondary and tertiary spikelet weight on the lower side panicle. These results indicate that it may be possible to enhance sink capacity and translocation of source with a combination of novel QTLs detected on chromosome 5 and GN1, APO1, and SPIKE.

γ‐Oryzanol and its major components, 24‐methylenecycloartanyl ferulate and cycloartenyl ferulate, which are distributed in the rice bran layer, exert biomedical effects. The objective of the present study was to detect QTLs regulating the concentrations of 24‐methylenecycloartanyl ferulate and cycloartenyl ferulate by using the reciprocal CSSLs derived from a cross between an indica‐type cultivar Takanari and a japonica‐type cultivar Koshihikari, having low and high concentrations, respectively. We detected QTLs for the concentrations of γ‐oryzanol and 24‐methylenecycloartanyl ferulate (μg/grain and/or mg/kg) on chromosome 5 and the concentrations of cycloartenyl ferulate (μg/grain and/or mg/kg) on chromosomes 4 and 11 in reciprocal genetic backgrounds. The concentrations were increased by the Koshihikari allele but decreased by the Takanari allele. In addition, we found a QTL for cycloartenyl ferulate concentration (mg/kg) on chromosome 2 in reciprocal genetic backgrounds. Interestingly, unlike the QTLs on chromosomes 4, 5, and 11, the concentrations were decreased by the Koshihikari allele but increased by the Takanari allele. The favorable alleles from japonica‐type cultivars can be used to improve the lower concentrations of γ‐oryzanol, 24‐methylenecycloartanyl ferulate, and cycloartenyl ferulate in indica‐type cultivars. In addition, the favorable alleles from indica‐type cultivars can be used to further increase the higher concentrations of cycloartenyl ferulate in japonica‐type cultivars. The QTLs detected in this study should contribute to future‐breeding programs can enhance the health benefits conferred by rice.

ABSTRACTTemperatures and solar radiation during ripening critically affect grain appearance in rice (Oryza sativaL.). Climatic factors to induce chalky grains were analyzed under the experimental conditions of high-temperature and shading treatment and also under the ambient condition in a high-temperature-prone region of Japan. The frequency of white-back (WB) and basal-white (BW) grains correlated with temperature and solar radiation, whereas that of milky-white (MW) grains was not correlated, suggesting that complicated climatic factors are involved in the formation of MW grains. Further investigation was carried out to identify the parameters that distinguish perfect and MW grains grown in high-temperature versus those grown in low-solar-radiation conditions. As reported previously, the chalk phenotypes in the transverse section of the MW grains were quite different between environments: oval-shaped chalk for MW grains grown in low-solar-radiation condition and center chalk for MW grains grown in high-temperature condition. Grain hardness and amylopectin chain-length distribution did not explain the difference in MW grains between environments. MW grains subjected to high temperatures had a lower protein content without a consistent reduction in the single-grain weight, whereas those from the low-solar-radiation condition had a lower amylose content with a consistent reduction in the single-grain weight, when compared with perfect grains that developed in either environmental condition. Overall, our results suggest that MW grains are formed through different physiological mechanisms with altered starch and protein synthesis under high-temperature and low-solar-radiation conditions.Abbreviations: BW: basal-white; DAH: days after heading; DP: degree of polymerization; MW; milky-white WB: white-back

AbstractRecent progress in rice genomics has promoted the identification of quantitative trait loci (QTLs) associated with yield and its related traits. SPIKE, a QTL controlling spikelet number per panicle, and GPS, a QTL controlling leaf photosynthesis rate, were identical to NAL1. To assess the effect of SPIKE/GPSon yield potential, we compared DNA sequences of these alleles and conducted yield experiments in the field of Japan using the near-isogenic lines NIL-SPIKE(allele from Daringan in IR 64 genetic background), NIL-GPS(allele from Koshihikari in Takanari genetic background), and a chromosome segment substitution line, SL2115 (allele from Koshihikari in IR 64 genetic background). Despite the two SNPs in the promoter regions between Koshihikari and Daringan, both alleles were effective to increase the number of spikelets per panicle both in Takanari and IR 64 backgrounds. However, the extent of the increase was smaller and unstable in Takanari background than that in IR 64 background. In addition, SPIKE/GPSimproved percentage of filled spikelets only in the IR 64 background. These results suggest that the effects of SPIKE/GPSalleles are similar but are affected by the difference of the genetic backgrounds. Because the increasing effect of spikelets number per panicle was canceled by the decrease of the number of panicles, which seems to be affected by environmental factors, none of NIL-SPIKE, SL2115, or NIL-GPSsignificantly out-yielded their parental cultivars. These results indicate the importance to consider genetic backgrounds and QTL-environment interaction toward the future use of SPIKE/GPS.

AbstractThe contribution of cell wall components and nonstructural carbohydrate (NSC) to grain filling in rice (Oryza sativaL.) was clarified by investigating the differences in the dynamics of hemicellulose, sugar composition of hemicellulose,β-(1→3),(1→4)-glucan, and NSC among cultivars with different grain filling capacities. This investigation was performed using the stems of standard, high yield and low harvest index (HI) cultivars. Hemicellulose concentration in stems tended to decrease slightly during the grain filling stage. This decrease was attributed to a decrease inβ-(1→3),(1→4)-glucan concentration, which was detected as a decrease in glucose composition of hemicellulose in the stems during the grain filling stage. The rate of decrease and decrease in the amount ofβ-(1→3),(1→4)-glucan in the stems differed among the cultivars. These were higher in high yield and high HI cultivars than in relatively low yield and low HI cultivars. Moreover, a positive correlation was observed between the rate of decrease inβ-(1→3),(1→4)-glucan and NSC, indicating similarities in the dynamics ofβ-(1→3),(1→4)-glucan and NSC among the cultivars. When the top half of panicle was removed,β-(1→3),(1→4)-glucan and NSC concentrations in the culm and leaf sheath did not decrease during the grain filling stage. Therefore, theβ-(1→3),(1→4)-glucan in stems might be one of the sources that supply substrate to panicle as well as NSC.

Rice (Oryza sativaL.) yield is determined by the relationship between sink, source, and plant architecture. ‘Takanari’, a high‐yielding rice variety descended from ‘IR8’ and ‘Tongil’, shows a semidwarf plant stature and has a large sink size and high photosynthesis rate. Previous studies have revealed that alleles of semi‐dwarf1(sd1) and GRAIN NUMBER1A(GN1A), which confer a semidwarf plant type and large panicles, respectively, have been selected repeatedly in breeding programs during the process of breeding for high yield. Until now, however, no genetic factors related to photosynthesis have been identified as having been selected in breeding programs. In this study, we conducted comparative mapping of quantitative trait loci (QTLs) for photosynthesis‐related traits using three F2populations: ‘Koshihikari’ × Takanari, Koshihikari × Tongil, and Koshihikari × IR8. Comparison of QTL locations in the three populations revealed a QTL in the same location on the long arm of chromosome 4 in each one, with the allele for higher photosynthesis‐related traits coming from the higher‐yielding parent of each population. Further genotypic analysis of the Takanari pedigree revealed the same genotype on the long arm of chromosome 4 in varieties related to IR8 and Takanari. These results suggest that breeders have selected the favorable allele of the QTL for photosynthesis on chromosome 4 in several breeding programs for high yield.

AbstractWe previously demonstrated that heat-induced spikelet sterility at anthesis could be mitigated by using an early-morning flowering (EMF) line of Oryza sativaL. cv. Koshihikari crossed with wild rice, O. officinalis.Another microclimate factor, high wind velocity, is known to increase the sterility induced by heat. In this study, we evaluated whether EMF rice could mitigate sterility under the combined stresses of heat and wind. Rice plants were exposed to three levels of wind velocity (1.1, 2.2, 3.4 m s−1) from early-morning until 1500 in a glasshouse, where air temperature reached 30°C at 0800, 34°C at 1000 and 38°C around noon. Under these conditions, sterility steadily increased in Koshihikari, ranging from 28.4 to 86.9% as wind velocity increased. However, in the EMF line, low levels of sterility were observed since most spikelets flowered before 1000 when air temperature reached 35°C, the critical value for causing sterility. These results indicated that the increase in heat-induced spikelet sterility by wind can be potentially mitigated by using EMF rice.

We evaluated genetic variations in the non-structural carbohydrate (NSC) and the cell-wall components of stem in rice, sorghum, and sugar cane to assess the potential suitability of these gramineous crops for bioethanol production. For NSC, the maximum soluble sugar concentration was highest in sugar cane, followed by sorghum with sucrose. The major NSC in rice was starch, but there were wide variations in the starch to soluble sugar ratios among the cultivars. The total concentration of cell-wall components was negatively correlated with the NSC concentration, indicating competition for carbon sources. Among the cell-wall components, lignin was relatively stable within each group. The major sugar species composing hemicellulose was xylose in all crop groups, but there were differences in composition, with a higher fraction of arabinose and glucose in rice as compared to the other crops. In rice, there was less lignin than in sorghum or sugar cane; this might be advantageous for the efficient saccharification of cellulose.

AbstractThe whole-leaf photosynthetic rate in rice plants is controlled by various physiological processes. In a high-yieldingindicarice variety, Habataki, the leaf photosynthetic rate (LPR) of the uppermost fully expanded leaves was approximately 130 to 140% of that in ajaponicavariety, Sasanishiki, from booting to the early ripening stage. We characterized the difference in the LPR between Habataki and Sasanishiki. Leaves of Habataki contained higher levels of nitrogen and, as a consequence, of Rubisco, and had higher stomatal conductance that was associated with higher hydraulic conductance from roots to leaves than those of Sasanishiki. These features were responsible for the higher LPR of Habataki. An analysis of chromosome segment substitution lines (CSSLs) in which chromosome segments from Habataki were substituted into the genetic background of Sasanishiki showed that three genetic regions on chromosomes 4, 5 and 11 were responsible for the increase in the LPR. Each of these regions was estimated to increase the LPR by 15 to 30%, and we showed that they were associated with higher activity of mesophyll photosynthesis due to higher leaf nitrogen content and greater stomatal conductance. Leaf nitrogen content and stomatal conductance may be useful parameters for further quantitative trait locus analysis of efficient photosynthesis in leaves.

Leaf photosynthesis, an important determinant of yield potential in rice, can be estimated from measurements of chlorophyll content. We searched for quantitative trait loci (QTLs) for Soil and Plant Analyzer Development (SPAD) value, an index of leaf chlorophyll content, and assessed their association with leaf photosynthesis. QTL analysis derived from a cross between japonicacultivar Sasanishiki and high-yielding indicacultivar Habataki detected a QTL for SPAD value on chromosome 4. This QTL explained 31% of the total phenotypic variance, and the Habataki allele increased the SPAD value. Chromosomal segment substitution line (CSSL) with the corresponding segment from Habataki had a higher leaf photosynthetic rate and SPAD value than Sasanishiki, suggesting an association between SPAD value and leaf photosynthesis. The CSSL also had a lower specific leaf area (SLA) than Sasanishiki, reflecting its thicker leaves. Substitution mapping under Sasanishiki genetic background demonstrated that QTLs for SPAD value and SLA were co-localized in the 1,798-kb interval. The results suggest that the phenotypes for SPAD value and SLA are controlled by a single locus or two tightly linked loci, and may play an important role in increasing leaf photosynthesis by increasing chlorophyll content or leaf thickness, or both.

Rice plants are known to accumulate starch in leaf sheaths and culms, and in some cultivars significant amounts of starch are present at the mature stage. This can be considered as potential feedstock for the recovery of fermentable sugars. We isolated starches from the culms of cultivars Yumeaoba, Koshihikari, and Leafstar to investigate their structural and physical features. Yumeaoba culm starch contained 20.2% amylose, whereas Koshihikari and Leafstar contained 25.8% and 25.2%. Yumeaoba culm starch was found by chain-length distribution analysis to contain higher amounts of short chains, resulting in lower gelatinization temperature by 7 °C, as compared to Koshihikari and Leafstar. Consequently, the rate of enzymatic hydrolysis of Yumeaoba culm starches reached maximum at a lower temperature than Leafstar. Rice culm starch, with a lower gelatinization temperature, can provide an advantageous material for feedstock for bioethanol production in terms of energy conservation.

AbstractThe objective of this study was to clarify the effects of soil temperature in the stage from late tillering to panicle initiation (SI) and during the grain-filling stage (SII) on grain setting, dry matter production, photosynthesis, non-structural carbohydrate (NSC), xylem exudation and abscisic acid (ABA) levels in rice (Oryza sativaL. cv. Koshihikari). Rice plants were exposed to four different soil temperatures during SI or SII: 17.5, 25, 31.5 and 36.5°C (ST18, ST25, ST32 and ST37, respectively). The yield, yield components, grain filling and quality in SI were negatively influenced by high soil temperature of 37°C. On the other hand, there was no significant difference in those characters among temperature treatments in SII. The root/shoot ratio was smallest in the ST37 plants in both SI and SII, mainly due to their lighter root weight. At 7 days after initiation of treatment (DAT) in both SI and SII, the photosynthetic and xylem exudation rate tended to increase slightly as soil temperature increased up to 32°C. At 21 DAT, however, the photosynthetic rate was lowest in ST37, with concurrent decrease of diffusion conductance and SPAD value. In addition, decrease of NSC concentration in stem and xylem exudation rate, and increase of ABA level in leaves and xylem exudate were observed in ST37 plants at 21 DAT. These results suggested that high soil temperature before heading especially influenced yield, grain quality and plant growth. Possible mechanisms of the effect of soil temperature are discussed.

Soft carbohydrates, defined as readily-recoverable carbohydrates viamere extraction from the biomass or brief enzymatic saccharification, were found in significant amounts in rice straw as forms of free glucose, free fructose, sucrose, starch, and β-1,3-1,4-glucan. In this study, we investigated their amounts in rice straw (defined as culm and leaf sheath), and developed an easy method for glucose and fructose recovery from them with heat-pretreatment and subsequent 4-h enzymatic saccharification with an enzyme cocktail of cellulase and amyloglucosidase. The recovery of glucose and fructose exhibited good correlation with the amounts of soft carbohydrates. The maximum yields of glucose and fructose in the rice straw per dry weight at the heading stage and the mature stage were 43.5% in cv. Habataki and 34.1% in cv. Leafstar. Thus, rice straw with soft carbohydrates can be regarded as a novel feedstock for economically feasible production of readily-fermentable glucose and fructose for bioethanol.

AbstractThe objective of the present study was to identify the limiting factors in biomass productivity of new tropical rice lines (panicle weight type) by analyzing the relationship between root and shoot activity. Five field experiments using three new lines, IR65598-112-2, IR65600-42-5-2, and IR68544-29-2-1-3-1-2, and one of the highest-yielding indica rice varieties (panicle number type), IR72, were conducted at the International Rice Research Institute from 1997 to 2002. Specific absorption rate of N (SARN , mg N g-1root dry weight d-1), specific stem sap exudation rate (sap exudation rate, g exudate g-1root dry weight 12 h-1) and specific root respiratory rate (root respiratory rate, μmol O2g-1root dry weight h-1) were determined as indices of root activity. Relative growth rate (RGR, mg g-1d-1) was used as an index of shoot activity. Compared with IR72, the new lines showed lower RGR and SARNvalues during the early growth stages. In contrast, during the late growth stages, these activities were higher in the new lines than in IR72. The SARBand sap exudation rate showed a linear correlation with RGR at successive growth stages, regardless of the genotypes and growth conditions. These findings indicated that active root-shoot interaction was maintained throughout the growth period in high-yielding tropical rice, including the new lines and IR72. Therefore, it was considered that SARNand the sap exudation rate are useful indices of root activity regulating RGR in the new lines. However, the root respiratory rate could not be used for selection, because the relationship between RGR and the root respiratory rate changed with the growth conditions. Our findings support the hypothesis that root and shoot activities during maturation are important factors affecting the productivity of the new lines, which have not yet been able to attain the targeted yield.

Understanding of dynamics of N derived from organic N sources in soil is required for the development of sustainable agricultural systems. The aim of this paper is to compare, using pot experiments, the fate of N from urea (UF) and organic N sources such as rice straw compost (RC) and cattle compost (CC) using 15N labeled materials in paddy soil planted with rice. Two soils with a history of long-term applications of chemical fertilizers (LTCN) and organic N sources, i.e. straw compost +soybean cake, (LTON), were also compared. Nitrous oxide emissions were monitored during the growing period. Yield and N uptake of rice were higher in LTON soil than LTCN soil with no significant interaction with N applications. Chemical fertilizer increased yield and N uptake with a recovery rate by rice of 36 to 45%. Nitrogen recovery from RC and CC by rice was less than 10%. When recovery of N in soil was included with that recovered in the plant, 70% and 61% of applied N in the UF treatment was recovered from the LTCN and LTON soils, respectively. In comparison, more than 95% of applied N was recovered in the plant and soil for the RC and CC treatments. There was a sharp increase in N2O emission during the aerated period in nonplanted pots regardless of whether supplemental N was added, and this was associated with the increase in NO3- in soil solution at 0.5 cm depth. There was a much lower N2O emission in planted pots than nonplanted pots with no significant difference among the LTCN and LTON soils or the N treatments. The results indicated that the application of organic N source provided lower N supply to the plant than urea, but also could reduce N loss because of higher retention in the soil. Long-term continuous application of organic N sources enlarged the labile N pool without increasing N2O emission. Nitrous oxide emission was important during the mid-season aerated period from pot experiments and was partly related to the concentration of NO3- and the rate of nitrification.

AbstractA field experiment was conducted to characterize root growth and water uptake from soil layers with different N levels by upland rice (Oryza sativaL.) and maize (Zea maysL.) under severe water stress (no water supply) and mild water stress (intermittent water supply). Under mild stress conditions, medium N application led to the increase of the total root length but to the decrease of the root length/leaf area ratio compared with the no-N treatment in rice. Water was extracted from soil layers in a pattern approximately proportional to the root length distribution under mild stress. Both in rice and maize, a major portion of the water was extracted from the 0–20 cm depth. Application of N increased the total water uptake, which was attributed to the greater root length and higher specific water extraction rate per unit length of root. Specific water extraction rate can vary to some extent, depending on the transpiration demand of the shoots. In rice, water uptake, dry matter production in shoot, and root length were largely suppressed under severe stress compared with mild stress. In maize, water uptake under severe stress was maintained at a level similar to that under mild stress. This was attributed to the higher extraction from the deep soil below the 40 cm depth, which was supported by the increased root distribution and increased specific water extraction rate per unit root length at the depth under severe stress compared with mild stress. The lower water-capturing capacity of rice than that of maize under severe stress was characterized by a lower response to the enhancement of the morphological growth of the roots in deeper soil layers and the limited ability of water extraction per unit length of root, especially in the deeper layers.

abstractNodal root anatomy was compared among twelve upland and lowland rice (Oryza sativeL.) varieties with tropical origin which were grown in hydroponic culture and under field conditions. The traditional upland japonica varieties showed the largest diameter of root, stele, and xylem vessel followed by modern upland varieties. There was a clear varietal difference in the ratio of stele to root diameter, which was associated with the genetic group rather than with the ecosystems. The japonica varieties had a significantly larger stele diameter relative to the root diameter than indica and aus varieties. The indica and aus varieties displayed more xylem vessels per unit area of stele than the japonica varieties, but the diameter of xylem vessel was smaller. Equivalent xylem vessel diameter (De) was more dependent on the number of xylem in the indica varieties than in the japonica varieties. Distinctly different types of sclerenchyma anatomy were identified among the varieties. The development of sclerenchyma was classified into four different types based on thickening of cell wall in the outer cortical parenchyma and the number of sclerenchymatous cell layers. Like the xylem anatomy, the varietal differences in sclerenchyma development were more associated with genetic group rather than the ecotype. The japonica varieties had higher frequency of the types which have a doubled cell layer in sclerenchyma with thick cell wall than indica and aus. The difference among the genetic groups was nearly consistent across growing conditions, aerobic and submerged soils. These results indicated that sclerenchyma development is controlled by a genetic factor.