Effect of non-lethal low phytic acid mutations on grain yield and seed viability in rice

Low phytic acid (LPA) crops have recently been considered as a potential way to combat nutritional and environmental issues related to seed phytic acid phosphorus (PA-P). Although, a number of LPA mutant lines have been developed in various crops, they are often featured with lower grain yield and seed viability compared with wild type (WT) parents. We recently developed several LPA mutant lines in rice with PA-P reductions varying from 33.8% to 63.6%. In this study, the performance of grain yield and seed viability of these mutants were investigated. Four of the five mutant lines had 12.5–25.6% reductions in grain yield compared to the corresponding WT parental lines. The reduction in grain weight, varying from 5.4% to 10.7%, was found to be the main causative factor of yield reduction. Similarly, LPA mutants had inferior seed viability to their corresponding WT parent varieties; all mutant lines had a significantly lower simplified vigour index (seed germination rate × seedling dry weight) than their parents, with reductions of 7.8–26.3%, although some mutant lines had similar germination rates as their WT parents. The two mutant lines, which had similar germination rates as their WT parent, however, had significantly lower field emergence rates. More pronounced differences of simplified vigour index were observed after artificial aging treatments between four LPA lines from their WT parents, implying that LPA rice seeds were more susceptible to storage than WT. The yield and yield-related traits of F₂ plants and F₃ lines from three crosses were evaluated; the results showed that while LPA was associated with significantly lower grain yield and grain weight than WT sibs, there were also LPA plants and lines that out-performed WT controls. These results implied that the negative effect of LPA mutations on grain yield might be reduced or minimized through cross and selection breeding. The implications of these findings were discussed with regard to LPA rice breeding and potential commercial production.     

Positional variations in phytic acid and protein content within a panicle of japonica rice

Six japonica rice cultivars differing in phytic acid and protein content were used to investigate the effect of grain position in the panicle on grain weight, phytic acid and protein content. There were significant differences in grain weight, albumin, globulin, prolamin, glutelin, and total protein content among the grains within a panicle for all six cultivars, but no differences were found for phytic acid content. An effect of grain position between primary rachises on grain weight and protein content was clearly detected, while that among top, middle and bottom rachis showed a greater change, varying with the cultivar. Albumin and globulin contents tended to be lower in the grains on the primary rachis than in those on the secondary rachis, and glutelin and total protein contents showed the opposite trend in the three cultivars with lower protein content. There was no significant correlation between phytic acid and protein content in the different grain positions within a panicle. The accumulation of phytic acid and protein during grain development appears to be independent.     

Breeding efforts to mitigate damage by heat stress to spikelet sterility and grain quality

Global warming is predicted to aggravate the risk of unstable crop production. It is of great concern that damage to rice spikelet sterility and grain quality will increase, resulting in yield and economic losses. To secure the global food supply and farmers’ income, the development of rice cultivars with heat resilience is a pressing concern. Regarding spikelet sterility, rice cultivars with heat tolerance at different growth stages have been identified in recent years. The early-morning flowering (EMF) trait is effective in heat escape because it shifts the time of day of flowering to earlier in the morning when it is cooler. Although varietal differences are very small, there are some genetic resources for EMF in wild rice accessions. Regarding heat-induced grain chalkiness, heat-tolerant japonica cultivars for mitigating white-back type of chalky grains (WBCG) were found. Quantitative trait loci for heat tolerance at flowering, EMF, and for WBCG in grain quality have been mapped on the rice chromosomes. Further genetic efforts have been successfully connected to the development of near-isogenic lines for each trait with tagged molecular markers. These breeding materials are quite unique and useful in facilitating marker-assisted breeding toward the development of heat-resilient rice in terms of spikelet sterility and grain quality.     

Improvement of bacterial blight resistance of hybrid rice in China using the Xa23 gene derived from wild rice (Oryza rufipogon)

A novel bacterial blight (BB) resistance gene, Xa23, identified from Oryza rufipogon was introgressed into three popular restorer lines (Minghui63, YR293 and Y1671) for wild abortive cytoplasmic male sterility by marker-assisted backcross breeding approach in combination with artificial inoculation and stringent phenotypic selections. The three derived BB resistant restorer lines (Minghui63-Xa23, YR293-Xa23 and Y1671-Xa23) and their hybrid combinations with Zhenshan97A (Shanyou63-Xa23), NongfengA (Fengyou293-Xa23) and Zhong9A (Zhongyou1671-Xa23) demonstrated similar BB resistance spectrum as the donor parent, CBB23 (B). The newly developed BB resistant restorers and their derived hybrids were identical to their respective original versions for agronomic traits especially under disease free condition. However, under severe disease condition, the three BB resistant restorer lines exhibited significantly higher grain weight and spikelet fertility as compared to the respective original restorer lines thus further resulting in BB resistant hybrids with significantly higher grain yields than their respective popular original hybrids. The results indicated that the Xa23 gene could completely express its dominant broad spectrum resistance in different backgrounds of both restorer and male sterile lines across different growth stages, suggesting its immense breeding value in BB resistance improvement for hybrid rice. Moreover, a reasonable utilization and deployment of Xa23 gene for efficient control of BB disease in hybrid rice production was recommended.     

Distribution of phytic acid and mineral elements in three indica rice (Oryza sativa L.) cultivars

Three indica rice cultivars with different grain shapes were selected to determine the milling characteristics and distribution of phytic acid (PA) and six mineral elements including magnesium (Mg), calcium (Ca), manganese (Mn), iron (Fe), zinc (Zn), and selenium (Se). Milling characteristics were quite different among the rice cultivars with different physical dimensions. Similar milling times did not necessarily result in the same degree of milling (DOM) for different rice cultivars. The concentrations of phytic acid and minerals decreased with an increased DOM. These results also showed that phytic acid and minerals except for Zn and Se were not evenly distributed and highly concentrated in the outer layer (0% < DOM < 15%) of the rice kernel. In contrast, Zn and Se seem to be relatively evenly distributed in the grain. Optimum DOM of about 14% was detected for Zhenong 7 (long and slender grain); 10% was found for Zhenong 60 (medium-grain) and 9% for Zhenong 34 (short and round grain). The information generated in this experiment could be useful to optimize milling procedures for maximum removal of phytic acid, minimum mineral losses and weight loss in indica rice cultivars with different grain shapes.     

水稻灌浆期高温对天冬氨酸代谢酶活性及其家族氨基酸含量的影响

 选用粳稻越光和籼稻IR72为材料,研究了灌浆期高温对水稻籽粒中天冬氨酸转氨酶（AAT）、天冬氨酸激酶（AK）活性以及稻米蛋白质组分与天冬氨酸族氨基酸组分含量的影响。结果表明,水稻灌浆期高温下籽粒中AAT和AK活性、天冬氨酸、赖氨酸、苏氨酸、蛋氨酸和异亮氨酸相对含量均显著提高,籽粒中AAT和AK活性与天冬氨酸家族总氨基酸含量分别呈显著和极显著正相关,说明水稻籽粒中AAT和AK对于高温下稻米天冬氨酸家族氨基酸含量的提高具有重要作用。高温有利于稻米中蛋白质组分与天冬氨酸族氨基酸组分相对含量的提高,有利于单粒稻米中谷蛋白与天冬氨酸家族氨基酸组分的积累以及粗蛋白中部分氨基酸比例的改善,但高温降低了每穗稻米中蛋白质组分与部分氨基酸组分的含量。     

外源磷处理对水稻籽粒植酸含量及相关代谢基因表达的影响

利用水稻穗离体培养方法,对不同外源磷浓度下水稻籽粒植酸等磷化物含量的差异及其植酸代谢相关功能基因在灌浆过程中的表达特征进行了探讨。结果表明,水稻籽粒中的植酸、无机磷和总磷含量随外源磷处理浓度上升呈增加趋势,但磷处理对千粒重和单位籽粒中植酸积累量的影响因磷浓度水平而异,高磷处理会导致水稻千粒重和籽粒植酸积累绝对量的显著降低;外源磷处理浓度的上升,不仅会引起水稻籽粒中锌和铁元素含量的显著降低,而且会导致锌、铁营养的生物有效性下降;外源磷处理对水稻籽粒植酸含量的影响与不同磷处理下RINO1基因的相对表达量之间存在较密切关系,中磷浓度(3P)处理会诱导RINO1基因的表达,但高磷浓度(12P)会抑制RINO1基因的表达,RINO1是外源磷浓度处理对水稻籽粒植酸合成代谢过程产生调控作用的一个重要功能基因位点,而IPK2基因与水稻在不同磷浓度处理下的植酸含量变化无直接联系。     

Quality characteristics and field performance of selectable marker-free transgenic rice with antisense Wx gene and improved quality derived from the elite parents of hybrid indica rice

In rice grains, high amylose content (AC) is correlated with poor grain quality, particularly in indica hybrid rice. To obtain indica hybrid rice with improved cooking and eating qualities, we introduced the antisense Waxy (Wx) gene into 2 elite parental lines of indica hybrid rice by using co-transformation methods. Subsequently, we selected several elite homozygous transgenic lines that did not contain the selectable marker. The expression of the endogenous Wx gene of the selected transgenic lines was significantly downregulated, resulting in low AC in the mature seeds; moreover, the AC in some lines reduced to the level observed in glutinous rice. With the decrease in AC, the gel consistency of the transgenic rice became softer, and the gelatinization temperature tended to be higher than those of the wild types, especially in the case of the Longtefu-derived transformants. We also analyzed the pasting properties of the selected transgenic low-AC lines, and we noted an improvement in the pasting properties of the transgenic rice lines. The results from a field trial indicated that the grain weights of the transgenic lines with lower AC exhibit remarkable reduction compared with those of the wild types.     

Agronomic performance of rice breeding lines selected based on plant traits or grain yield

Selection for yield per se has greatly contributed to yield improvement in many crops. It is expected that selection based on plant traits is more effective in increasing crop yield potential. This study was conducted to compare the effectiveness of trait-based and yield-based selection in increasing rice yield and to determine whether lines with ideotype traits have the potential to express higher yield under optimal crop management conditions. Lines were selected based on plant traits or on grain yield measured in a breeder's replicated yield trial. The main target traits for selection were plant height, leaf and panicle morphology, grain size, total dry weight, and grain-filling percentage. Yield performance of trait-based selection was compared with that of yield-based selection in an agronomic trial with optimum crop management for three seasons. Trait-based selection increased leaf area index and total dry weight but reduced spikelet number per m² and harvest index compared with yield-based selection. Consequently, selection based on plant traits did not increase grain yield compared with selection based on yield per se. In one of the three seasons, yield of trait-based selection was significantly lower than that of yield-based selection. Among all tested breeding lines, maximum yield was produced by yield-based selection and minimum yield came from trait-based selection. These results suggest that lines with ideotype traits did not express higher grain yield than lines selected based on yield per se under optimal crop management conditions, and yield-based selection was as effective in increasing rice grain yield as trait-based selection in the late generations of the breeding cycle.     

Effect of salinity on yield and 2-acetyl-1-pyrroline content in the grains of three fragrant rice cultivars (Oryza sativa L.) in Camargue (France)

Aromatic quality of rice grains is known to vary greatly with environmental factors and cultivation methods. Among the environmental factors, soil salinity is thought to have a positive impact on the content of 2-acetyl-1-pyrroline (2AP) in grains, the key volatile compound of rice aroma. This study compared 2AP content in grains of three improved fragrant rice (Oryza sativa L.) varieties grown in two fields, differing mainly in their soil salinity level. The impact of salinity on yield and main yield components was also investigated to understand the relationship between aromatic quality and yield build-up. Soil salinity was monitored by measuring the electrical conductivity (EC) of soil solution samples extracted every week. 2AP content in grains was determined by a newly developed stable isotope dilution method involving solid-phase microextraction (SPME) and GC MS/MS analysis. The results showed an increase of 2AP content in grains with salinity for the three varieties. The relationship between 2AP and mean EC of the crop fitted a single model for the three varieties (R² = 0.728). In contrast, the impact of salinity on yield and yield components differed greatly between the three varieties. One variety appeared to be very sensitive to salt stress, with significant yield loss up to 40%, while the two other varieties proved to be resistant to the salinity levels experienced by the plants, with no significant yield loss or even higher yields in saline conditions. Nevertheless, the three varieties presented a significant negative correlation between 1000 grain weight (TGW) and the mean EC of the crop, and between TGW and 2AP content. It was concluded that the increase of 2AP content with salinity resulted partially from a 2AP concentration mechanism in smaller size grains. The direct effect of salinity on 2AP synthesis through stimulation of the proline metabolism is further discussed.     

High Temperature During Rice Grain Filling Enhances Aspartate Metabolism in Grains and Results in Accumulation of Aspartate-Family Amino Acids and Protein Components

Global warming causes the exacerbation of rice growing environment, which seriously affects rice growth and reproduction, and finally results in the decrease of rice yield and quality. We investigated the activities of aspartate metabolism enzymes in grains, and the contents of Aspartate-family amino acids and protein components to further understand the effects of high temperature (HT) on rice nutritional quality during rice grain filling. Under HT, the average activities of aspartate aminotransferase (AAT) and aspartokinase (AK) in grains significantly increased, the amino acid contents of aspartate (Asp), lysine (Lys), threonine (Thr), methionine (Met) and isoleucine (Ile) and the protein contents of albumin, globulin, prolamin and glutelin also significantly increased. The results indicated that HT enhanced Asp metabolism during rice grain filling and the enhancement of Asp metabolism might play an important role in the increase of Asp-family amino acids and protein components in grains. In case of the partial appraisal of the change of Asp-family amino acids and protein components under HT, we introduced eight indicators (amino acid or protein content, ratio of amino acid or protein, amino acid or protein content per grain and amino acid or protein content per panicle) to estimate the effects of HT. It is suggested that HT during rice grain filling was benefit for the accumulation of Asp-family amino acids and protein components. Combined with the improvement of Asp-family amino acid ratio in grains under HT, it is suggested that HT during grain filling may improve the rice nutritional quality. However, the yields of parts of Asp-family amino acids and protein components were decreased under HT during rice grain filling.     

Influence of environmental temperature during grain filling period on granule size distribution of rice starch and its relation to gelatinization properties

High temperature (HT) is the major environmental factor affecting grain starch properties of cooking rice cultivars. However, little information has been available on the effect of environmental temperature on the starch granule size distribution of rice grains. In this paper, five indica rice genotypes, including the wild type (9311) and its four mutants differing in amylose content (AC), were used to investigate the effect of environmental temperature on the starch granule size distribution, as well as its relation to AC and gelatinization properties of rice starch. Two temperature treatments (HT and NT) at filling stage were imposed to rice plants under the controlled temperature chambers. The result showed that HT increased the average diameter of starch granules and enhanced the proportion of large starch granules (LSG, D > 2.6 μm) by number, volume and surface area, respectively. However, influence of HT on GT and starch granule size distribution was relatively independent of their alteration in AC level for different rice genotypes. Therefore, HT-induced increase in the average diameter of starch granules and LSG percentage was strongly responsible for the higher starch gelatinization temperature and inferior cooked palatability of HT-ripening rice grains, which be not inherently associated with their varying AC level.     

Agronomic Performance of F1 Hybrids of Rice (Oryza sativa L.) in Japonica-Indica Crosses

Summary

Generally F₁ hybrids from crosses between japonica and indica rice show variable degrees of sterility. Previous studies revealed that among the indica rice, the Aus type has sexual affinity with japonica cultivars giving higher fertility in F₁ hybrids. Therefore, we made an effort to obtain higher grain yield by producing F₁ hybrids between japonica and Aus-type indica. A field experiment was conducted to investigate the heterosis in these F₁ hybrids for dry matter accumulation, spikelet fertility, yield and yield attributes, to find cross combinations with higher grain yield, and examined the relationships of grain yield with some agronomic characters. F₁ hybrids flowered earlier than their parent cultivars. Heterosis for dry matter accumulation was positive at the panicle initiation stage, at heading and at maturity. The average heterosis for number of panicles, grain and straw yield per plant, number of spikelets per panicle and 1000-grain weight was positive. A few F₁ hybrids showed positive heterosis for number of filled grains per panicle, spikelet fertility and harvest index; but the average heterosis for these traits was negative. Among the yield components, a higher number of panicles per plant and higher number of spikelet per panicle contributed to higher grain yield in F₁ hybrids. A significant and positive relationship existed between spikelet fertility and grain yield per plant. The higher grain yield was attributed to higher dry matter production rather than higher harvest index in F₁ hybrids, although low spikelet fertility limited yield potential in some F₁ hybrids.     

QTL mapping for crude protein and protein fraction contents in rice (Oryza sativa L.)

Improvement of rice storage proteins is important in rice breeding for high nutritional quality. Seventy-one recombinant inbred lines (RIL) derived from a cross between japonica variety Asominori and indica variety IR24 were used to study the inheritance of crude protein and protein fraction contents in rice. A total of 16 QTL were identified and mapped on eight chromosomes. Several QTL affecting contents of different protein fractions were mapped in the same chromosomal region. In particular, two QTL with a significant contribution were identified to simultaneously affect prolamin and glutelin contents. One QTL denoted as qCP-12 affecting crude protein content (CP) was located in the same region as QTL qGLT-12 affecting glutelin content, in agreement with the positive correlation between glutelin level and protein content. QTL with larger genetic effects were further confirmed using two sets of chromosome segment substitution lines (CSSL), where Asominori and IR24 were used as the recurrent parents. By QTL comparative analysis, two QTL for CP, three for globulin content and one for prolamin content were located in the vicinity of CP QTL previously identified in polished rice. Application of these results in rice breeding is discussed.     

Are seeding densities an opportunity to increase grain yield of winter wheat in a living mulch of white clover?

Optimum plant densities are a key to maximise yields in most crops. However, such information is often lacking for more environmentally sound cropping systems, such as living mulches (LM) for small grains. In 2004 and 2005, three trials were conducted in the Swiss Midlands on fields managed in accordance with the Swiss organic farming guidelines. The objective of the study was to determine whether seeding density of winter wheat (Triticum aestivum L.) is a relevant factor for determining grain yield in a white clover (Trifolium repens L.) living mulch. The winter wheat cv. Titlis was directly sown in wide spaced rows (0.375 m) at densities of 300 (LM300), 450 (LM450) or 600 (LM600) viable grains m⁻² in a white clover living mulch established at a seeding rate of 15 kg ha⁻¹. A bare soil control treatment with a wheat density of 450 viable grains m⁻² (BS450) was also included in the trials. Mean grain yields of LM300, LM450, and LM600 never reached the values observed in BS450. This was mainly due to a lower ear density, which, nevertheless, increased linearly with the seeding density within the living mulch in all trials, but the rate of increase depended on the environment. The decrease of the grain weight brought about by the increasing seeding density had only a marginal impact on the grain yield, which was increased from 1.31, 1.98, and 4.09 Mg ha⁻¹ (LM300) to 1.97, 2.64, and 4.75 Mg ha⁻¹ (LM600) for each of the three trials in the study. Significantly higher protein contents were observed for LM300 compared to the higher densities in the living mulch and to BS450. Our research showed that an increase of the seeding density is an effective mean to increase the grain yield in living mulch systems with white clover. However, it is likely that the control of the living mulch to reduce competition with the main crop is a more relevant factor.     

Waxy endosperm accompanies increased fat and saccharide contents in bread wheat (Triticum aestivum L.) grain

The contents of fat, starch, pentosan, fructan, β-glucan and several mono- and oligosaccharides in grain were evaluated to find out the possible effects of the Wx-D1 gene of bread wheat using two sets of near-isogenic waxy and non-waxy lines and two low-amylose mutant lines with a common genetic background of Kanto 107. These materials have two non-functional Wx-A1b and Wx-B1b alleles in common. Waxy near-isogenic lines with a non-functional Wx-D1d allele showed consistently increased contents of fat, total fructan, β-glucan, glucose, fructose, sucrose, 1-kestose, 6-kestose, neokestose, nystose and bifurcose compared with non-waxy lines with a functional Wx-D1a allele throughout three growing/harvest seasons. Starch and total pentosan contents were inconsistently influenced by the allelic status of the Wx-D1 locus, while water-soluble pentosan and raffinose contents were not affected. The compositional changes of a low-amylose mutant line with an almost non-functional Wx-D1f allele were closely similar to those of waxy near-isogenic lines, while significantly different changes were barely observed in another low-amylose mutant line with a partly functional Wx-D1g allele in two seasons. These results showed that the Wx-D1 gene has pleiotropic effects on the fat and saccharide contents of bread wheat grain.     

Old and modern durum wheat varieties from Italy and Spain differ in main spike components

Four field experiments comparing 24 durum wheat varieties grown at different periods during the 20th century in Italy and Spain were carried out to assess the changes caused by breeding activities on the number of grains per main spike and its determinants: number of spikelets per spike, number of grains per spikelet, fertile flowering and grain setting. Increases of 0.14 grains spike⁻¹ year⁻¹ (0.43% year⁻¹ in relative terms) and 0.08 grains spike⁻¹ year⁻¹ (0.22% year⁻¹) were observed in Italian and Spanish varieties, respectively. The overall change in the number of grains per spike in Italian germplasm (29.5%) was due to increases in both, the number of spikelets per spike (7.5%) and the number of grains per spikelet (20.3%), while in Spanish varieties the increase in the number of grains per spike (19.5%) was only attributed to the improvement of the number of grains per spikelet. The increase in the number of fertile florets per spike (about 12%) was similar in both countries, but while it explained more than 70% of the changes in the number of grains per spike in Spanish varieties, grain setting was responsible for most of the improvement in the number of grains per spike in the Italian germplasm. The percentage of florets setting grains was 68 and 64% in modern Italian and Spanish varieties, respectively. Most of the changes in the number of grains per spikelet were found in the upper part of the spike on Italian varieties, whilst they were more evenly distributed in the Spanish ones. The main achievement derived from the introduction of the Rht-B1 dwarfing gene was an increase in the number of grains per spikelet, but it did not have any effect on the number of spikelets on the main spike. The lack of genetic associations between grain setting and both the number of spikelets per spike and the number of fertile florets per spike suggests that future yield gains may be obtained through increases in the three components independently.     

Hormonal changes are related to the poor grain filling in the inferior spikelets of rice cultivated under non-flooded and mulched condition

Non-flooded plastic mulching cultivation (PM) of rice can save much irrigation but usually exhibits a poor grain filling and low grain weight when compared to traditional flooding cultivation (TF). This study measured the variations of plant hormones during grain filling and investigated whether they were related to the grain-filling problem under mulching in a field experiment. Hormonal levels in the grains and the grain development of both superior (early flowered) and inferior spikelets (later flowered) were monitored continuously during the grain filling period. The contents of indole-3-acetic acid (IAA), zeatin riboside (ZR) and abscisic acid (ABA) were higher in superior grains than in inferior ones at early grain-filling stage. For individual spikelets, the peaks of IAA and ZR contents appeared just before the peak grain-filling rate but the peak ABA content matched it. The earlier flowered superior spikelets did not show much difference in grain filling characteristics and hormonal changes under the two cultivation treatments. Distinctively with the inferior spikelets in a panicle, PM led to lower grain weight, higher grain-filling rate at early stage and shorter active grain-filling period than the TF. PM also resulted in less IAA and ZR but more ABA contents in the grains than TF at early and middle grain filling stages in these inferior spikelets. Peaks of these hormone contents in the PM inferior grains usually appeared earlier and also disappeared more quickly than those in TF inferior grains. When exogenous IAA was applied to plants at the initial grain-filling stage, IAA and ZR were increased and ABA was reduced in the inferior grains. The treatment prolonged the active grain-filing period and reduced the peak grain-filling rate of inferior grains, similarly as the case with TF. As a result, IAA spraying enhanced the weight of PM inferior grains but reduced that of TF ones. Exogenous ABA spraying led to the opposite effects to those of IAA. The results suggest that the shortened grain-filling period in the inferior grains under PM is related to the increased ABA and reduced IAA and ZR in the grains. Regulation of the ratio of ABA to IAA in grains could potentially increase the weight of inferior grains under the water-saving cultivation with film mulching.     

Pre-anthesis non-structural carbohydrate reserve in the stem enhances the sink strength of inferior spikelets during grain filling of rice

Sink strength plays an important role in grain filling of cereals but how it is related to the pre-anthesis non-structural carbohydrate (NSC) reserves is not clear. This study investigated if and how an increase in NSC reserves could enhance sink strength, and consequently improve grain filling of later-flowering inferior spikelets (in contrast to the earlier flowering superior spikelets) for rice varieties with large panicles. Two “super” rice varieties (the recently bred high-yielding rice) and two New Plant Type (NPT, named in IRRI for the extra-large panicle) rice lines were compared with two elite inbred varieties under field-grown conditions. Three nitrogen (N) treatments, applied at the stages of panicle initiation, spikelet differentiation or both, were adopted with no N application during the mid-season as control. Both super rice and NPT rice showed a greater yield capacity as a result of a larger panicle than the elite inbred rice. However, a lower percentage of filled grains limited the realization of higher yield potential in super rice and especially in NPT rice, due to their lower grain filling rate and the smaller grain weight of their inferior spikelets. The low grain filling rate and small grain weight of inferior spikelets are mainly attributed to a poor sink strength as a result of small sink size (small number of endosperm cells) and low sink activity, e.g. low activities of sucrose synthase (SuSase) and adenosine diphosphoglucose pyrophosphorylase (AGPase). The amounts of NSC in the stem and NSC per spikelet at the heading time are significantly and positively correlated with sink strength (number of endosperm cells and activities of SuSase and AGPase), grain filling rate, and grain weight of inferior spikelets. Nitrogen application at the spikelet differentiation stage significantly increased, whereas N application at the panicle initiation or at both panicle initiation and spikelet differentiation stages, significantly reduced, NSC per spikelet at the heading time, sink strength, grain filling rate, and grain weight of inferior spikelets in super rice. The results suggest that pre-anthesis NSC reserves in the stem are closely associated with the sink strength during grain filling of rice, and N application at the spikelet differentiation stage would be a good practice to increase pre-anthesis NSC reserves, and consequently to enhance sink strength for rice varieties with large panicles, such as super rice varieties.