Japonica Hybrid Research Articles

Grain-Filling Characteristics in Extra-Large Panicle Type of Early-Maturing japonica/indica Hybrids

Early-maturing japonica/indica hybrids (EJIH) have recently been released, performing a yield potential of 13.5 t ha−1 and greater yield increase over conventional japonica rice (CJ) and hybrid indica rice (HI) in production. More spikelets per panicle and improved grain-filling efficiency underlined the basis for the superior yield performance of EJIH. However, few studies are available on the panicle traits and grain-filling characteristics of EJIH, as well as their differences to CJ and HI. In our study, two EJIH, two CJ, and two HI cultivars with similar growth patterns were grown in the same fields. EJIH had a 12.2–18.8% increased (p < 0.05) grain yield relative to CJ and HI, mainly attributed to their higher daily grain yield. Although it had a lower panicle per m2, EJIH exhibited 28.0–38.3% more (p < 0.05) spikelets per m2 from an increase of 58.0–87.8% (p < 0.05) in spikelets per panicle than CJ and HI. Compared with CJ and HI, EJIH had a higher single panicle weight and more grains in the six parts of the panicle, especially in the upper secondary branches (US) and middle secondary branches (MS). EJIH exhibited a higher leaf area index (LAI), leaf area duration (LAD), leaf photosynthetic rate, and SPAD values after heading, which helped increase shoot biomass weight at heading and maturity and post-heading biomass accumulation. For CJ and HI, the grain-filling dynamics of grains in the six parts were all well simulated by the Richards equation. For EJIH, the grain-filling dynamics of grains in the lower secondary branches (LS) were well fitted by the logistics equation, with the Richards equation simulating grain positioning on the other five parts. EJIH had a lower mean grain-filling rate (GRmean) and longer days and grain filling amounts (GFA) during early, middle, and late stages than CJ and HI. Our results suggest EJIH gave a yield advantage over CJ and HI through a higher daily grain yield. The panicle traits and grain-filling characteristics differed greatly among the three cultivar types. Compared with CJ and HI, EJIH had lower GRmean and higher days and more grains in the panicle during early, middle, and late stages, which contributed to an increased GFA after heading, improved filled-grain efficiency, and higher grain yield.

The effects of Rf5 and Rf6 on fertility restoration in Honglian-type cytoplasmic male sterile (CMS) lines of japonica rice (Oryza sativa L. ssp. japonica).

The online version contains supplementary material available at 10.1007/s11032-021-01256-7.

Detection and Dynamic Variation Characteristics of Rice Nitrogen Status after Anthesis Based on the RGB Color Index

We aimed to elucidate the color changes of rice leaves after anthesis and create an algorithm for monitoring the nitrogen contents of rice leaves and of the whole plant. Hence, we aimed to provide a theoretical basis for the precise management of rice nitrogen fertilizer and the research and development of digital image nutrition monitoring equipment and reference. We selected the leaf colors of the main stems of four major rice varieties promoted in production, including Huaidao 5 (late-maturing medium japonica rice), Yangjing 4227 (early maturing late japonica rice), Changyou 5 (late japonica hybrid rice), and Yongyou 8 (late japonica hybrid rice). Under different nitrogen levels, the leaf R, G, and B values of the four rice varieties at different stages after anthesis, the dynamic changes in RGB normalized values, the correlations between RGB normalized values and leaf SPAD values, the leaf nitrogen content and whole plant nitrogen content, and the nitrogen prediction model were studied. The research results demonstrate the following: (1) regardless of nitrogen levels, the leaf of R, G, B, NRI, NGI and NBI of different rice varieties after anthesis followed the order, G > R > B. R, G, NRI, NGI, and days after heading could be fitted according to a logarithmic equation, y = aebx (0.726 ≤ R2 ≤ 0.992); B, NBI, and days after heading could be fitted using a linear equation, y = a + bx (0.863 ≤ R2 ≤ 0.992). Both fitting effects were significant (except NGI). (2) A quadratic function (Y = −1296.192x2 + 539.419x − 10.914; Y = −1173.104x2 + 527.073x − 12.993) was adopted to construct a monitoring model for the NBI and SPAD values of japonica rice and hybrid japonica rice leaves after anthesis and the R2 values were 0.902 and 0.838, respectively. Exponential functions (Y = 5.698e7.261x; Y = 3.371e9.326x) were employed to construct monitoring models of leaf nitrogen content, and the R2 values were 0.833 and 0.706, respectively. Exponential functions (Y = 5.145e4.9143x; Y = 3.966e5.364x) were also used to construct a monitoring model for the nitrogen content of the whole plant, and the R2 values were 0.737 and 0.511, respectively. The results obtained from prediction tests by using Determination Coefficient (R2), Relative Percent Deviation (RPD), and Root Mean Square Error (RMSE) showed that it was feasible, accurate, and efficient to use a scanner for measuring the nitrogen content of rice.

Transcriptome analysis of flowering regulation by sowing date in Japonica Rice (Oryza sativa L.)

Hybrid japonica cultivars, such as the Yongyou series, have shown high yield potential in the field in both the early and late growing seasons. Moreover, understanding the responses of rice flowering dates to temperature and light is critical for improving yield performance. However, few studies have analyzed flowering genes in high-yielding japonica cultivars. Based on the five sowing date experiments from 2019 to 2020, select the sensitive cultivar Yongyou 538 and the insensitive cultivar Ninggeng 4 and take their flag leaves and panicles for transcriptome analysis. The results showed that compared with sowing date 1 (6/16), after the sowing date was postponed (sowing date 5, 7/9), 4480 and 890 differentially expressed genes (DEGs) were detected in the leaves and panicles in Ninggeng 4, 9275 and 2475 DEGs were detected in the leaves and panicles in Yongyou 538, respectively. KEGG pathway analysis showed that both Ninggeng 4 and Yongyou 538 regulated rice flowering through the plant circadian rhythm and plant hormone signal transduction pathways. Gene expression analysis showed that Os01g0566050 (OsELF3-2), Os01g0182600 (OsGI), Os11g0547000 (OsFKF1), Os06g0275000 (Hd1), and Os09g0513500 (FT-1) were expressed higher and Os02g0771100 (COP1-1) was expressed lower in Yongyou 538 compared with Ninggeng 4 as the climate conditions changed, which may be the key genes that regulate the flowering process with the change of temperature and light resources in sensitive cultivar Yongyou 538 in the late season.

Comparison of grain yield and quality of different types of japonica rice cultivars in the northern Jiangsu plain, China

In recent years, an increasing number of different types of japonica rice cultivars have been released in the southern rice region of China. The grain yield and quality of these new cultivars showed significant differences in large scale planting. However, the causes of the differences remain little known. Therefore, three typical types of japonica rice cultivars were used in this study to investigate their grain yield and quality. A scanning calorimeter (DSC), X-ray powder diffractometer (XRD), rapid viscosity analyzer (RVA) and taste analyzer were used to evaluate the cooking and eating properties. The results showed that the yield of non-soft hybrid japonica rice cultivars was significantly higher than that of non-soft inbred japonica rice cultivars and soft inbred japonica rice cultivars. Soft inbred japonica rice cultivars had a low amylose content and moderate protein content, which are the main reasons for the superior cooking and eating quality. In addition, the relative crystallinity of soft inbred japonica rice cultivars was significantly higher than that of non-soft inbred and non-soft hybrid japonica rice cultivars, which is considered the major factor resulting in higher transition temperature and gelatinization enthalpy (ΔHgel). Non-soft hybrid japonica rice cultivars had a higher number of large starch granules than soft inbred and non-soft inbred japonica rice cultivars. The setback value (SB) and breakdown value (BD), indirectly reflecting the cooking and eating quality of the three types of japonica rice cultivars, also confirmed that soft inbred japonica rice cultivars with a low SB value and a high BD value had better palatability than the other two types. This study provides guidance for future plantation of different types of japonica rice cultivars in large rice-producing areas.

Analysis of variation patterns in rice milled quality in the offspring of indica–japonica hybridization

Asian cultivated rice (Oryza sativa L.) is divided into two subspecies, indica (O. sativa L. subsp. indica) and japonica (O. sativa L. subsp. japonica). Variation patterns in rice quality become more complex in the offspring produced from indica–japonica hybridization owing to interactions between inter-subspecies components. No QTLs underlying rice milled quality have been successfully cloned; hence, improving rice milled quality is a key dilemma, especially in the offspring of inter-subspecies hybridization. In this study, we analyzed variation patterns in rice milled quality caused by components introduced from indica, including both regulatory genes controlling influence factors (milled grain width and chalkiness) and the frequency of random introductions from the indica genetic backgrounds. Two populations (RILs and BILs) derived from indica–japonica hybridization were used. Both the frequency distribution value of indica components introduced (Di value) and grain-width regulatory genes (GS5 and GW5) had a significant impacted on variation in milled rice width (MRW), and their interaction accounted for the major MRW variance in RILs and BILs. The Di value had a significant impact on variation in chalkiness degree (C) in RILs and BILs; the regulatory gene Chalk5 also participated in the regulation of C in BILs. The Di value further influenced head rice rate (HR), with the average HR gradually decreasing as the Di value increased. Taken together, our results suggest that we should utilize backcrossing or multiple crosses to reduce the Di value in offspring produced by indica–japonica hybridization and promote the use of favorable genes during individual selections.

Assisted selection of eating quality progeny of indica (O. sativa L. ssp. indica) and japonica (O. sativa L. ssp. japonica) hybrids using rice starch properties

Eating quality (EQ) of rice is mainly determined by sensory evaluation using tasting, which is affected by the taster’s geographic residential area, gender, and age. The sensory evaluation protocol also requires rice samples (usually > 500 g) larger than the amount harvested from the early generations of breeding in many cases, thereby hindering the sensory tasting practices. The characteristics of rice starch have a significant impact on EQ; however, information on starch assisted-selection of EQ is insufficient. In this study, the recombinant inbred lines and backcross recombinant inbred lines constructed by the hybridization of japonica (O. sativa L. ssp. japonica) rice (cv. Toyonishiki) and indica (O. sativa L. ssp. indica) rice (cv. Qishanzhan) were used as test materials to analyze and explore the relationships between the amylose content (AC), rapid visco analyzer (RVA) profile characteristics, and the EQ score obtained from sensory tasting assessment. We provided a feasible solution to use rice starch properties to assist in selection of EQ. The results indicated that AC and RVA profile characteristics including trough viscosity (TV), final viscosity (FV), breakdown (BD), and setback (SB) were closely related to the EQ values. Furthermore, an integrated indicator—i.e., starch properties quality (SPQ)—that represented a more comprehensive characterization of rice starch, was derived based on the EQ evaluation hierarchy and weighted AC, TV, FV, BD, and SB values. The correlation between SPQ and EQ was significant (p < 0.001), and was significantly higher compared with using individual indicators. Moreover, the SPQ method requires less rice samples (i.e., 3 g of milled rice flour) and can be applied to assisted-selection for EQ in early generations of breeding. This study provid esessential technical support for the rapid and accurate identification of the EQ of progeny of indica–japonica hybrids.

Compact plant type rice has higher lodging and N resistance under machine transplanting

To identify the major factors that contribute to the difference in lodging among different rice varieties under machine transplanting and their responses to nitrogen (N), field experiments were conducted at Danyang County (a representative eco-site of the Lower Yangtze River) in Jiangsu Province, China in 2017 and 2018, 22 hybrid indica varieties (HIs), 22 inbred japonica varieties (IJs) and two indica japonica hybrid varieties (IJHs) were transplanted by machine with three N rates (N0, N150 and N300, 0, 150 and 300 kg ha–1, respectively). Lodging-related physical parameters, morphological characteristics and apparent transport rates of dry matter were examined. Significant difference in yield was observed among different types of rice, and followed by IJs<HIs<IJHs. The average lodging index (LI) of hybrid varieties (HIs and IJHs) was higher than that of the inbred varieties (IJs) with higher plant height; moreover, lower apparent export rate of dry matter resulted lower LI in IJHs than in HIs. The HIs had a large difference in the LI, which came from the difference in bending stress (BS) induced by the difference in the apparent export rate of dry matter, varieties with lower leaf angle of upper three leaves possess strong lodging resistance capacity; however, the gap among the IJs was due to the difference in the cross section modulus (Z). The LI in the IJs or IJHs increased slightly with the increased N application, and there was no lodging incidence under the high N level, which was due to the low leaf angle and barely changed under high N; there was a significant interaction between varieties (HIs) and N rates in lodging rate and LI, varieties with lower leaf angle of upper three leaves were resistant to high N. These results suggest that compact plant type rice has higher lodging and N resistance at machine-transplanting method.

Leaf nitrate accumulation influences the photorespiration of rice (Oryza sativa L.) seedlings

The form of nitrogen (N) supply influences photorespiration in C3 plants, but whether nitrate (NO3−) regulates photorespiration and, if so, the underlying mechanisms for such regulation are still unclear. Three hydroponic experiments were conducted in a greenhouse to investigate the relationships between leaf NO3− concentrations and photorespiration rates in rice (Oryza sativa L.) genotypes cv. ‘Shanyou 63’ hybrid indica and ‘Zhendao 11’ hybrid japonica or using mutants that overexpress NRT2.1 (in cv. ‘Nipponbare’ inbred japonica). We estimated photorespiratory rate from the CO2 compensation point in the absence of daytime respiration (Γ*) using the biochemical model of photosynthesis. Higher Γ* values under high N level or NO3− were significantly and positively correlated with leaf NO3− concentrations. Further elevating leaf NO3− concentrations by either resuming NO3− nutrition supply after N depletion (in cv. ‘Shanyou 63’ hybrid indica and ‘Zhendao 11’ hybrid japonica) or using mutants that overexpress NRT2.1 (in cv. ‘Nipponbare’ inbred japonica) increased Γ* values. Additionally, the activities of leaf nitrate reductase (Nr) and concentrations of organic acids involving in the tricarboxylic acid (TCA) cycle synchronously changed as environmental conditions were varied. Photorespiration rate is related to the leaf NO3− concentration, and the correlation may links to the photorespiration-TCA derived reductants required for NO3− assimilation.

Solar radiation-use characteristics of indica/japonica hybrid rice (Oryza sativa L.) in the late season in southeast China

New indica and japonica hybrid rice cultivars, such as the Yongyou series, provide farmers with very high yield potential. However, information on their canopy light capture and solar radiation use efficiency in the late season is limited. Field experiments were performed to compare the radiation-use parameters of four rice types: indica rice (IR), inbred japonica rice (IJR), hybrid japonica rice (HJR), and hybrid indica/japonica rice (HIJR), from 2016 to 2018 during the late season in Hangzhou, China. The grain yield, aboveground biomass, intercepted solar radiation (SI), and radiation-use efficiency (RUE) of the HIJR were on average respectively 13.4%–53.4%, 14.3%–30.6%, 7.6%–21.4%, and 8.2%–14.9% higher than those of the HJR, IJR, and IR. The leaf area index (LAI) of the HIJR was 18.2%–57.0% greater than that of the IJR and HJR at four growth stages, resulting in respectively 17.8%–38.5% and 10.7%–42.8% greater canopy light interception rates (LIR) and amount of intercepted solar radiation during the vegetative stage. The prolonged grain-filling stage also led to respectively 33.9%–52.6% and 30.5%–51.4% increases in amounts of incident and intercepted radiation for the HIJR relative to the IR during grain filling. These results indicate that the SI superiority of the HIJR was caused by canopy closure as rapid as that of the IR during the vegetative stage (greater LAI and canopy LIR during the growing season) and a grain-filling stage as long as that of the HJR. For grain-filling stage, differences in leaf Pn between HIJR, IR, and IJR were not significant, suggesting that the greater RUE of the HIJR (12.7%–52.8% higher) than that of the other rice types resulted from improved canopy architecture after flowering (FL). Principal components analysis (PCA) revealed that the superiority of the HIJR in terms of solar radiation use resulted from the greater canopy light capture capability of IR and the prolonged growth period (especially during grain filling) of japonica rice in the late growing season.

Agronomic and physiological performance of an indica–japonica rice variety with a high yield and high nitrogen use efficiency

AbstractHybrid rice (Oryza sativa L.) varieties usually need more N application to produce a higher grain yield and exhibit lower N use efficiency (NUE) than inbreds. However, little information is available about the mechanism underlying an indica–japonica rice variety with high yield and high NUE under low N rates. This study investigated this issue. The indica–japonica hybrid rice variety Yongyou 2640 (Y‐2640) and the japonica inbred rice variety Lianjing 7 (L‐7) were grown in a paddy field at N rates of 0 and 200 kg N ha−1. The results showed that the grain yield and internal NUE (grain yield/total N uptake of plants) of Y‐2640 were 16–23% and 9–10%, respectively, higher than those of L‐7. A higher grain yield for Y‐2640 was mainly attributed to the greater spikelet number per unit area that resulted from greater root oxidation activity at panicle initiation and its higher crop growth rate and net assimilation rate before heading. A higher leaf area index and greater root length, root, and shoot biomass were the important agronomic traits of Y‐2640. Higher cytokinin content and the expression of genes involved in the amino acid transporters OsAAP1 and OsLHT1 in the roots and leaves, a higher photosynthetic NUE, and a greater N harvest index were the main physiological traits of Y‐2640. The results suggest that improved agronomic and physiological performance accounts for the high yield and high NUE for the indica–japonica hybrid rice variety.

Rice Yield and Physiological Characteristics Under Different Panicle Nitrogen Application Timings in Northeast China

Optional nitrogen (N) application management plays an important role in high yield of rice. The results of this study indicated that the grain yield and related physiological indexes were significantly affected by panicle N application timings for conventional japonica rice and hybrid japonica rice. Increasing the number of panicles is an effective method for improving rice yield in Northeast, China. In addition, delaying the timing of spikelet-protecting fertilizer application appropriately might be of great benefit for improving the yield potential of rice.

Artificial Selection in Domestication and Breeding Prevents Speciation in Rice

Speciation has long been regarded as an irreversible process once the reproductive barriers had been established. However, unlike in natural populations, artificial selection might either accelerate or prevent speciation processes in domesticated species. Asian cultivated rice is a target crop for both domestication and artificial breeding; it contains two subspecies of indica and japonica, which usually produce sterile inter-subspecific hybrids due to reproductive barriers. In this study, we constructed the evolutionary trajectory of a reproductive isolation system S5, which regulates fertility in indica–japonica hybrids via three adjacent genes, based on the data of 606 accessions including two cultivated and 11 wild rice species. Although hybrid sterility haplotypes at S5 lead to establishment of a killer-protector reproductive barrier, origin of wide-compatibility haplotypes by complex hybridization and recombination provides an opposing force to reproductive isolation and thus prevents speciation during domestication. Analysis in a diallel set of 209 crosses involving 21 parents showed that the wide-compatibility genotypes largely rescued fertility of indica–japonica hybrids, indicating that the wide-compatibility gene would enable gene flow to maintain species coherence. This counteracting system indicates that combined effects of natural evolution and artificial selection may result in reversible processes of speciation in rice, which may also have implications for genetic improvement of rice.

Breeding for three-line japonica hybrid rice combinations with high resistant starch content using molecular marker-assisted selection.

Resistant starch (RS) is beneficial for human health, and especially for diabetics. Considering the high cost and low productivity of the Jiangtangdao 1 rice variety with high RS content, breeding high RS rice varieties exhibiting high productivity is essential. A molecular marker-assisted selection strategy was applied to increase RS content in a three-line hybrid rice variety. The functional rice variety Jiangtangdao 1, which contains sbe3-rs (on chr2) that controls the RS content, was used as the high RS content donor parent. Subsequently, male sterile maintainer and restorer lines containing homozygous sbe3-rs were bred using molecular marker-assisted selection combined with traditional breeding methods. The male sterile line was crossed with the restorer lines to identify the optimal hybrid combination with a high RS content. We obtained four combinations for which the yields were >50% higher than those of the control Jiangtangdao 1. In addition, there was no significant difference in the RS content between the combinations and Jiangtangdao 1. The hybrid rice plants with high RS content exhibited favorable agronomic traits and therefore have broad prospects for commercial application.

Effects of Postponing Topdressing-N on the Yield of Different Types of japonica Rice and Its Relationship with Soil Fertility

N fertilizer is usually supplied via multiple applications in rice production in China. Due to the high N-recovery efficiency (NRE) of panicle fertilizer, applying large amounts of fertilizer at the booting stage is considered to be an effective measure of increasing yields, although it has adverse effects on eating quality. In this study, using six inbred and four hybrid japonica varieties, we postponed topdressing-N to increase the ratio of panicle N from 20% to 40% by correspondingly reducing the N amount applied only at the tillering stage. We also analyzed the effects of postponing the topdressing-N on grain yield and dry matter accumulation in both high- and low-fertility blue clayey paddy fields in 2016 and 2017. The effects of postponing topdressing-N applications on japonica rice were related to variety, meteorological conditions, and soil fertility. With respect to the inbred varieties, regardless of whether panicle N was applied as a single or split application, increasing the ratio of panicle N had no effect on the yield components or dry matter accumulation of plants grown in either high- and low-fertility soils. Regarding the hybrid varieties grown in the high-fertility soil, although postponing the topdressing-N application had no effect on yield under good weather conditions (no low-temperature stress during grain-filling), a single application of 40% of the total N at the panicle initiation stage significantly decreased both the dry matter accumulation after heading and the seed-setting rate of varieties that presented long growth periods under low-temperature conditions. With respect to hybrid varieties grown in low-fertility soil, postponing the application of topdressing-N had an adverse effect on the number of effective panicles. Our results suggested that the proportion of panicle N applied to japonica rice should not exceed 30% in clayey paddy fields and that fertilizer management with respect to rice production should be adjusted according to soil type, soil fertility, and variety.

Agronomic and physiological performance of indica/japonica hybrid rice cultivar under low nitrogen conditions

It is generally believed that indica/japonica hybrid rice (IJHR) cultivars have a higher yield potential under high nitrogen (N) supply, but it is not established whether IJHR could obtain a relatively high yield under low N conditions. In order to fill this gap of knowledge, six rice cultivars were examined in this study, consisting of three IJHR cultivars and three japonica hybrid rice (JHR) cultivars, all of which are prevalent in the lower Yangtze River plain. These cultivars were compared for grain yield and other closely-related agronomic and physiological traits under two N input treatments: one being a low application of N (LN) and the other a normal application of N (NN), in 2016 and 2017 in Hangzhou, Zhejiang Province, China. Compared to JHR, IJHR showed grain yield increases of 15% under NN and 25% under LN. Relative to JHR, there are four main agronomic and physiological traits which are associated with IJHR’s superior yield performance under LN conditions. Firstly, IJHR has less redundant vegetative growth, evidenced by a higher percentage of productive tillers. Secondly, IJHR has the larger sink size, resulting from higher spikelet numbers per panicle and sufficient panicles per m2. Thirdly, IJHR exhibited increased shoot activity during the grain filling period, indicated by an increased accumulation of non-structural carbohydrate (NSC) in the stems at heading time, a higher percentage of NSC remobilization from stem to grains during the ripening period, higher enzyme activity in the grains’ sucrose-to-starch conversion process, and greater photosynthesis production during the ripening period. Finally, IJHR was seen to have improved root morpho-physiological traits, characterized by greater root biomass, deeper soil distribution at heading time, and higher root oxidation activity (ROA) during the ripening period. Our results suggest that IJHR’s improved agronomic and physiological traits will lead to improved yields under low N input conditions.

Favorable Alleles of GRAIN-FILLING RATE1 Increase the Grain-Filling Rate and Yield of Rice.

Hybrid rice (Oryza sativa) has been cultivated commercially for 42 years in China. However, poor grain filling still limits the development of hybrid japonica rice. We report here the map-based cloning and characterization of the GRAIN-FILLING RATE1 (GFR1) gene present at a major-effect quantitative trait locus. We elucidated and confirmed the function of GFR1 via genetic complementation experiments and clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9) gene editing in combination with genetic and molecular biological analyses. In addition, we conducted haplotype association analysis to mine the elite alleles of GFR1 among 117 rice accessions. We observed that GFR1 was constitutively expressed and encoded a membrane-localized protein. The allele of the rice accession Ludao (GFR1 Ludao) improved the grain-filling rate of rice by increasing Rubisco initial activity in the Calvin cycle. Moreover, the increased expression of the cell wall invertase gene OsCIN1 in the near isogenic line NIL-GFR1 Ludao promoted the unloading of Suc during the rice grain-filling stage. A yeast two-hybrid assay indicated that the Rubisco small subunit interacts with GFR1, possibly in the regulation of the rice grain-filling rate. Evaluation of the grain-filling rate and grain yield of F1 plants harboring GFR1 Ludao and the alleles of 20 hybrids widely cultivated commercially confirmed that favorable alleles of GFR1 can be used to further improve the grain-filling rate of hybrid japonica rice.

Effects of N application strategies on N leaching loss in paddy soil and N use characteristics in different super hybrid rice cultivars

A rational nitrogen (N) application strategy effectively improves grain yield and N use efficiency (NUE) in the production of super rice (Oryza sativa L.). This study explored the effects of different N management strategies on N leaching loss in paddy soil and N use characteristics in different kinds of super hybrid rice cultivars. Two rice cultivars, Zhongzheyou 1 (ZZY1, a super rice of hybrid indica) and Yongyou 12 (YY12, a super rice of hybrid japonica), were chosen as representatives, and four N input levels (CK-0, N135-135, N195-195 and N255-255 kg N ha-1) with three application strategies (P1-60%:30%:10%, P2-50%:30%:20% and P3-40%:30%:30% with split N applied proportionally at the basal, tillering and panicle initiation stages, respectively) were set as treatments. An increase in the total N application rate and split proportion of basal fertilization resulted in higher N loss via leaching during the early growing stage of rice. Improving the split proportion of the second topdressing applied at the panicle initiation stage was beneficial to promoting the total N uptake and the amount of biomass accumulation, which ultimately enhanced the grain yield and NUE. YY12 obtained a 0.4–10.0% higher grain yield and 3.6–18.7% higher N recovery efficiency (REN) than that of ZZY1. Furthermore, YY12 showed greater adaptation at the high N level than did ZZY1, and allocating more N during the mid–late stage had a significant advantage in achieving high yield potential and improving NUE for YY12 compared to ZZY1. Thus, the results suggested that the N255 level with the P3 strategy was the optimal N management strategy that could optimize trade-offs between grain yield and NUE for YY12, while the N195 level with the P2 or P3 strategy was more appropriate for ZZY1.

Exploring the Relationships Between Yield and Yield-Related Traits for Rice Varieties Released in China From 1978 to 2017.

Despite evidence from previous case studies showing that agronomic traits partially determine the resulting yield of different rice (Oryza sativa L.) varieties, it remains unclear whether this is true at the ecotype level. Here, an extensive dataset of the traits of 7686 rice varieties, released in China from 1978 to 2017, was used to study the relationship between yield and other agronomic traits. We assessed the association between yield and other agronomic traits for four different rice ecotypes, i.e., indica inbred, indica hybrid, japonica inbred, and japonica hybrid. We found that associations between agronomic traits and yield were ecotype-dependent. For both the indica inbred and indica hybrid ecotypes, we found that greater values of certain traits, including the filled grain number per panicle, 1000-grain-weight, plant height, panicle length, grains per panicle, seed setting rate, long growth period, low panicle number per unit area, and low seed length/width ratio, have accounted for high grain yield. In the japonica inbred and japonica hybrid ecotypes, we found that only high panicle number per unit area and long growth period led to high grain yield. Indirectly, growth period consistently had a positive effect on yield in all ecotypes, and plant height had a positive effect on yield for the indicas and japonica inbred only. Plant height had a negative effect for the japonica hybrid. Altogether, our findings potentially have valuable implications for improving the breeds of rice ecotypes.

Quantitative Trait Locus Mapping for Yield-Associated Agronomic Traits in a BC2F6 Population of Japonica Hybrid Rice Liaoyou 5218

RAD-seq method is a recently developed, cost-effective, and high-throughput approach for detecting genetic variability based on single-nucleotide polymorphisms (SNPs) and high-density genetic map. This study aimed to construct the quantitative trait locus (QTL) mapping for yield-associated agronomic traits in rice using a BC2F6 population which was derived from japonica hybrid rice Liaoyou 5218. Liaoyou 5218 were firstly crossed to female parent 5216A, and the subsequent self-crossed BC1F6 population was backcrossed to 5216A. The 167 BC2F6 breeding lines showed different agronomic traits from parental Liaoyou 5218 and C418. RAD-seq and bioinformatics methods were used to identify high-quality SNPs in the 167 BC2F6 breeding lines, which generated 40968 SNP markers on 12 chromosomes in rice. Linkage and QTL mapping was constructed, and 14 QTLs related to 6 agronomic traits were identified, including 4, 3, and 4 QTLs on chr03, 09, and 10, respectively. Among the yield-associated QTLs mapping genes, ITPK3 and EGY3 were related to plant height; CYP724B1, GAPC2, TRS120, BADH1, AOX1a, AOX1b, and COLD1 were associated with average panicle length; ACT2 and BAMY1 were associated with 1000-grain weight and tiller number per plant, respectively. We suggested that the 14 QTLs in the BC2F6 breeding lines derived from Liaoyou 5218 might be of important values for the identification and marker-assisted selection of candidate genes in rice breeding.