Yield potential and nitrogen use efficiency of China's super rice

In 1996, a mega project that aimed to develop rice varieties with super-high yield potential （super rice） was launched by the Ministry of Agriculture （MOA） in China using a combination of the ideotype approach and intersubspecific heterosis. Significant progress has been made in the last two decades, with a large number of super rice varieties being approved by the MOA and the national average grain yield being increased from 6.21 t ha^-1 in 1996 to 6.89 t ha^-1 in 2015. The increase in yield potential of super rice was mainly due to the larger sink size which resulted from larger panicles. Moreover, higher photosynthetic capacity and improved root physiological traits before heading contributed to the increase in sink size. However, the poor grain filling of the later-flowering inferior spikelets and the quickly decreased root activity of super rice during grain filling period restrict the achievement of high yield potential of super rice. Furthermore, it is widely accepted that the high yield potential of super rice requires a large amount of N fertilizer input, which has resulted in an increase in N consumption and a decrease in nitrogen use efficiency （NUE）, although it remains unclear whether super rice per se is responsible for the latter. In the present paper, we review the history and success of China＇s Super Rice Breeding Pro- gram, summarize the advances in agronomic and physiological mechanisms underlying the high yield potential of super rice, and examine NUE differences between super rice and ordinary rice varieties. We also provide a brief introduction to the Green Super Rice Project, which aims to diversify breeding targets beyond yield improvement alone to address global concerns around resource use and environmental change. It is hoped that this review will facilitate further improvement of rice production into the future.

20 years'development of super rice in China--The 20th anniversary of the super rice in China

Rice （Oryza sativa L.） is originated from Asia and more than 90% of rice is produced inAsia. As the most important cereal crop in the world, rice is the staple food for over 1/3 global population, while this proportion is over 60% in Asia （Wan 2010）. In order to maintain the most important role that rice has played in Asian agriculture and ensure food security in the world, especially in Asia,

Effects of Nitrogen Application Rate and Ratio on Lodging Resistance of Super Rice with Different Genotypes

The objective of this study was to determine the morphology mechanism of nitrogen （N） fertilizer rates and ratio on lodging resistance through analying its effects among lodging index （LI）, lodging-related morphological traits and physical strength in basal intemodes by comparing japonica and indica super rice cultivars. Field experiments, with three nitrogen levels （0, 150 and 300 kg ha-L） and two ratios of basal to topdressing （8：2 and 5：5） with two super rice cultivars （Yliangyou 2 and Wuyunjing 23）, were conducted in the Baolin Farm, Danyang Country, Jiangsu Province, China, in 2011 and 2012. Effects of N fertilizer rates and ratios on morphology of whole plant, morphology traits in basal intemodes and culm＇s physical strength parameters were investigated at 20 d after full heading stage. LI of Yliangyou 2 was significant greater than that of Wuyunjing 23 due to larger bending moment by whole plant （WP） with higher plant height and gravity center height. With higher volume of N fertilizer, LI of two super rice cultivars was increased conspicuously. However, no significant effect was detected with increase of panicle fertilizer ratio. The size of breaking strength （M） in basal intemodes was the key factor determining LI among N fertilizer treatments. Correlation analysis revealed that M value was positively related bending stress （BS） of Wuyunjing 23 and section modulus （Z） of Yliangyou 2, respectively. The higher N fertilizer levels induced reduction ofBS of Wuyunjing 23 due to weak culm and leaf sheath plumpness status and reduced Z of Yliangyou 2 owning to small diameter and culm wall thickness, consequently, influencing their M indirectly. These results suggested that breaking strength was the key factor influencing LI with increase of N fertilizer levels. However, the lodging-related morphology mechanism was different with genotypes. Culm wall thickness and diameter in basal internodes of indica super rice and culm and leaf sheath plumpness status of japonica super rice influenced breaking strength, as well as lodging index, respectively.

Effects of nitrogen level on yield and quality of japonica soft super rice

Although studies on the balance between yield and quality of japonica soft super rice are limited, they are crucial for super rice cultivation. In order to investigate the effects of nitrogen application rate on grain yield and rice quality, two japonica soft super rice varieties, Nanjing 9108 （NJ 9108） and Nanjing 5055 （NJ 5055）, were used under seven N levels with the application rates of 0, 150, 187.5,225, 262.5, 300, and 337.5 kg ha^-1. With the increasing nitrogen application level, grain yield of both varieties first increased and then decreased. The highest yield was obtained at 300 kg ha^-1. The milling quality and protein content increased, while the appearance quality, amylose content, gel consistency, cooking/eating quality, and rice flour viscosity decreased. Milling was significantly negatively related with the eating/cooking quality whereas the appearance was significantly positively related with cooking/eating quality. These results suggest that nitrogen level significantly affects the yield and rice quality of japonica soft super rice. We conclude that the suitable nitrogen application rate for japonica soft super rice, NJ 9108 and NJ 5055, is 270 kg ha^-1, under which they obtain high yield as well as superior eating/cooking quality.

Grain yield and water use efficiency of super rice under soil water deficit and alternate wetting and drying irrigation

This study investigated if super rice could better cope with soil water deficit and if it could have better yield performance and water use efficiency （WUE） under alternate wetting and drying （AWD） irrigation than check rice. Two super rice cultivars and two elite check rice cultivars were grown in pots with three soil moisture levels, well watered （WW）, moderate water deficit （MWD） and severe water deficit （SWD）. Two cultivars, each for super rice and check rice, were grown in field with three irrigation regimes, alternate wetting and moderate drying （AWMD）, alternate wetting and severe drying （AWSD） and conventional irrigation （CI）. Compared with that under WW, grain yield was significantly decreased under MWD and SWD treatments, with less reduction for super rice than for check rice. Super rice had higher percentage of productive tillers, deeper root distribution, higher root oxidation activity, and greater aboveground biomass production at mid and late growth stages than check rice, especially under WMD and WSD. Compared with CI,AWMD increased, whereasAWSD decreased grain yield, with more increase or less decrease for super rice than for check rice. Both MWD and SWD treatments and eitherAWMD orAWSD regime significantly increased WUE compared with WW treatment or CI regime, with more increase for super rice than for check rice. The results suggest that super rice has a stronger ability to cope with soil water deficit and holds greater promising to increase both grain yield and WUE by adoption of moderate AWD irrigation.

Advances and prospects of super rice breeding in China

Super rice breeding in China has been very successful over the past 3 decades, and the Chinese government has made great efforts to support breeding and cultivation of both conventional and hybrid super rice. In this review, we focus on the progress in and potential of super rice breeding. After the establishment of the breeding theory and strategy of ＂generating an ideotype with strong heterosis through inter-subspecies hybridization, by using gene pyramiding to combine elite traits through composite-crossing to breed super rice varieties with both ideotype and strong hybrid vigor＂, a series of major breakthroughs have been achieved in both conventional and super hybrid rice breeding. A number of new genetic materials with ideotype have been created successfully, and the Ministry of Agriculture of China has approved 156 novel super rice varieties and combinations for commercialization. During the Developing the Super Rice Varieties Program, great attention has also been paid to the integration and demonstration of the rice production technology. Collaboration between industry and university researchers has led to technological innovations and initiation of a demonstration system for super hybrid rice. With widespread cultivation of super rice with higher quality and yield, as well as resistance or tolerance to abiotic or biotic stresses, the yield of rice production per unit has reached a new level. In addition to increased quality and yield, hybrid rice breeding has also led to improvements in many other agronomic traits, such as resistance to pests and diseases, resistance to lodging, and optimized light distribution in population. Achievements in super rice breeding and innovation in rice production have made major contributions to the progress in rice sciences and worldwide food security.

Changes in Enzyme Activities Involved in Starch Synthesis and Hormone Concentrations in Superior and Inferior Spikelets and Their Association with Grain Filling of Super Rice

The changes in activities of key enzymes involved in sucrose-to-starch conversion and concentrations of hormones in superior and inferior spikelets of super rice were investigated and their association with grain filling was analyzed.Four super rice cultivars,Liangyoupeijiu,IIyou 084,Huaidao 9 and Wujing 15,and two high-yielding and elite check cultivars,Shanyou 63 and Yangfujing 8,were used.The activities of sucrose synthase （SuSase）,adenosine diphosphoglucose pyrophosphorylase （AGPase）,starch synthase （StSase） and starch branching enzyme （SBE）,and the concentrations of zeatin ＋ zeatin riboside （Z ＋ ZR）,indole-3-acetic acid （IAA） and abscisic acid （ABA） in superior and inferior spikelets were determined during the grain filling period and their relationships with grain filling rate were analyzed.Maximum grain filling rate,the time reaching the maximum grain-filling rate,mean grain filling rate and brown rice weight for superior spikelets showed a slight difference between the super and check rice cultivars,but were significantly lower in the super rice than in the check rice for inferior spikelets.Changes of enzyme activities and hormone concentrations in grains exhibited single peak curves during the grain filling period.The peak values and the mean activities of SuSase,AGPase,StSase and SBE were lower in inferior spikelets than in superior ones,as well as the peak values and the mean concentrations of Z ＋ ZR and IAA.However,the peak value and the mean concentration of ABA were significantly higher in inferior spikelets than in superior ones and greater in the super rice than in the check rice.The grain filling rate was positively and significantly correlated with the activities of SuSase,AGPase and StSase and the concentrations of Z ＋ ZR and IAA.The results suggested that the low activities of SuSase,AGPase and StSase and the low concentrations of Z ＋ ZR and IAA might be important physiological reasons for the slow grain filling rate and light grain weight of inferior spikelets in super rice.

Combined effect of shading time and nitrogen level on grain filling and grain quality in japonica super rice

There is limited information about the combined effect of shading time and nitrogen （N） on grain filling and quality of rice. Therefore, two japonica super rice cultivars, Nanjing 44 and Ningjing 3, were used to study the effect of shading time and N level on the characteristics of rice panicle and grain filling as well as the corresponding yield and quality. At a low N level （150 kg N ha^-1, 150N）, grain yield decreased （by 21.07-26.07%） under the treatment of 20 days of shading before heading （BH） compared with the no shading （NS） treatment. These decreases occurred because of shortened panicle length, decreased number of primary and secondary branches, as well as the grain number and weight per panicle. At 150N, in the treatment of 20 days of shading after heading （AH）, grain yield also decreased （by 9.46-10.60%） due to the lower grain weight per panicle. The interaction of shading and N level had a significant effect on the number of primary and secondary branches. A high level of N （300 kg N ha^-1, 300N） could offset the negative effect of shading on the number of secondary branches and grain weight per panicle, and consequently increased the grain yield in both shading treatments. In superior grains, compared with 150N NS, the time to reach 99% of the grain weight （T99） was shortened by 1.6 to 1.7 days, and the grain weight was decreased by 4.18-5.91% in 150N BH. In 150N AH, the grain weight was 13.39-13.92% lower than that in 150N NS due to the slow mean and the maximum grain-filling rate （GRmean and GRmax）. In inferior grains, grain weight and GRmean had a tendency of 150N NS〉150N BH〉150N AH. Under shaded conditions, 300N decreased the grain weight due to lower GReen both in superior and inferior grains. Compared with 150N NS, the milling and appearance qualities as well as eating and cooking quality were all decreased in 150N BH and 150N AH. Shading with the high level of 300N improved the milling quality and decreased the number of chalky rice kernels, but the eating and cooking quality was reduced with increased chalky area and overall chalkiness. Therefore, in the case of short term shading, appropriate N fertilizer could be used to improve the yield and milling quality of rice, but limited application of N fertilizer is recommended to achieve good eating and cooking quality of rice.

Research progress on the breeding of japonica super rice varieties in Jiangsu Province,China

In this study we report the results of a decade-long breeding program for japonica super rice made by Nanjing Branch of Chinese National Center for Rice Improvement in Jiangsu Academy of Agricultural Sciences. We concluded that selection of parents with good comprehensive traits and complementary advantages and disadvantages of both parents in the hybrid combination, and early selection of high heritability traits in earlier segregating generations could significantly improve the breeding efficiency. The use of closely-linked functional markers in pyramiding of multiple genes could greatly increase breeding efficiency, avoiding time-consuming and laborious steps that were used in traditional breeding program. It is also important to coordinate the yield components with variety characteristics such as yield stability, wide adaptability, lodging resistance, and an attractive grain appearance during late growth stage of rice.

Super Rice Cropping Will Enhance Rice Yield and Reduce CH_4 Emission:A Case Study in Nanjing,China

A pot experiment was performed to learn the differences in plant productivity and OH4 emission between two rice cultivars, super rice variety Ningjing 1 and traditional variety Zhendao 11, which were currently commercially appUed in Nanjing, China. Similar seasonal changes of CH4 emission fluxes and soil solution CH4 contents were found between the tested cultivars. Although there was no significant difference in plant biomass production between the cultivars, the grain yield of Ningjing 1 was significantly higher by 35.0% （P 〈 0.05） than that of Zhendao 11, whereas the total CH4 emission from Ningjing 1 was 35.2% lower （P 〈 0.05）. The main difference in the amounts of CH4 emission between the cultivars occurred in the period from the tillering stage to the heading stage. The biomass-scaled and yield-scaled CH4 emissions were respectively 3.8 and 5.2 mg/g for Ningjing 1, significantly lower than those for Zhendao 11 （7.4 and 12.8 mg/g, respectively）. According to the relationships between the plant growth characteristics and the CH4 emission, a stronger root system contributed mainly to the lower CH4 emission of Ningjing 1, as compared with Zhendao 11. Our results demonstrated that super rice has advantages not only in grain productivity but also in CH4 emission mitigation. Further expansion of super rice cropping will enhance rice yield and reduce greenhouse gas emission in China.

Breeding Methodology and Practice of Super Rice in China

To meet the growing demand for rice production, the Ministry of Agriculture, P. R. China initiated a nationwide project on the development of super rice in 1996. Through efforts in research, super rice breeding in China has gained significant advances in the aspects of breeding theories, creation of breeding materials and selection and promotion of elite rice varieties. A creative super rice breeding path of the construction of harmonious plant type combined with the utilization of heterosis was proposed. Super rice plays more and more important roles in rice production. Totally, 71 super rice varieties in China have been successfully developed and released to production. These varieties have 12 t/hm2 yield potential in on-farm demonstration fields, 8% to 15% higher than the check varieties with high resistance to pest and medium to high level of grain quality, the total growing area of which has reached 23.7 million hectares, and rice grain production increased by 17.7 million tons from 1996 to 2009. This paper reviewed the development and achievements of super rice breeding in aspects of the breeding methodology and breeding practices, summarized the successful experience of super rice breeding in China and proposed the developmental strategies of super rice for the future.

Erect panicle super rice varieties enhance yield by harvest index advantages in high nitrogen and density conditions

The erect panicle （Ep） type is an important characteristic for japonica super rice in Northeast China and plays a significant role in enhancing yield. The Ep type is considered to be a genetic ideotype resource to the japonica super rice group by virtue of its agronomic advantages such as grain number per panicle and biomass. This study addresses the effects of nitrogen and planting density conditions on yielding performance regarding panicle type （PT） using the recombinant inbred line （RIL） population derived from the cross between an Ep variety Liaogeng 5 and non-Ep variety Wanlun 422. The genetics underlying the Ep type proved to be robust not only for panicle-type optimization but also plant height, panicle length, flag leaf length and seed density. We also found that regardless of nitrogen and density, correlation between harvest index （HI） and plant height was not significant in Ep type whatever the nitrogen and density. The application of Ep type provides a potential strategy for yield improvement by increasing biomass through HI maintainable in rice.

Genetical and Physiological Basis of Plant Type Model of Erect and Large Panicle Japonica Super Rice in Northern China

The historical changes in rice yields across China were explored. The physiological mechanisms and genetic basis of the erect and large panicle super-high-yield plant type model for breeding japonica super rice were analyzed mainly on the panicle type, number of large vascular bundles （LVB） in the panicle neck, and the panicle type index （PTI）. In the production point of view, we suggested that, for the breeding of super-high-yield japonica rice, the erect panicle types with more LVB numbers in the panicle neck and superior upper grains in the secondary branches would be the key factors. The information has potential significance in the rice breeding and productivity not only in China but also throughout the rice production areas of the world.

Research Advances in High-Yielding Cultivation and Physiology of Super Rice

In 1996,China launched a program to breed super rice or super hybrid rice by combining intersubspecific heterosis with ideal plant types.Today,approximately 80 super rice varieties have been released and some of them show high grain yields of 12-21 t/hm2 in field experiments.The main reasons for the high yields of super rice varieties,compared with those of conventional varieties,can be summarized as follows：more spikelets per panicle and larger sink size （number of spikelets per square meter）;larger leaf area index,longer duration of green leaf,greater photosynthetic rate,higher lodging resistance,greater dry matter accumulation before the heading stage,greater remobilization of pre-stored carbohydrates from stems and leaves to grains during the grain-filling period;and larger root system and greater root activity.However,there are two main problems in super rice production：poor grain-filling of the later-flowering inferior spikelets （in contrast to earlier-flowering superior spikelets）,and low and unstable seed-setting rate.Here,we review recent research advances in the crop physiology of super rice,focusing on biological features,formation of yield components,and population quality.Finally,we suggest further research on crop physiology of super rice.

Developing green super rice varieties with high nutrient use efficiency by phenotypic selection under varied nutrient conditions

The development of green super rice varieties with improved nutrient use efficiency(NuUE)is a vital target area to increase yield and make it more stable under rainfed conditions.In the present study, we followed an early backcross(BC) breeding approach by using a highyielding and widely adapted Xian variety, Weed Tolerant Rice 1(WTR-1), as a recipient and a Geng variety, Hao-An-Nong(HAN), as a donor.Starting from the BC1F2 generation, the BC population went through one generation of selection under irrigated, low-input, and rainfed conditions, followed by four consecutive generations of screening and selection for high grain yield(GY) under six different nutrient conditions(NPK, 75 N,-N,-P,-NP, and-NPK), leading to the development of 230 BC1F6 introgression lines(ILs).These 230 ILs were evaluated under the same six nutrient conditions for 13 agro-morphological and grain yield component traits in comparison to four checks and parents.Significant trait variations were observed between the treatments and ILs.Positive correlations were identified for GY with biomass, panicle length, flag-leaf area, flag-leaf width, filled grain number per panicle,1000-grain weight, and tiller number under-N,-P,-NP, and-NPK conditions.Out of 230 ILs,12 were identified as promising under two or more nutrient deficiency conditions.The results demonstrated an efficient inter-subspecific BC breeding procedure with a first round of selection under rainfed-drought conditions, followed by four generations of progeny testing for yield performance under six nutrient conditions.The promising ILs can be useful resources for molecular genetic dissection and understanding the physiological mechanisms of NuUE.

CLIMATIC RISK ZONING OF DOUBLE CROPPING SUPER RICE CULTIVATION IN HUNAN PROVINCE

Based on meteorological data including daily sunshine duration, temperature and precipitation from 97 meteorological stations in Hunan province during the period of 1981—2010, in combination with the field experiment in different places at different sowing dates, the precise climatic risk zoning of double cropping super rice cultivation has been studied by using the spatial interpolation method and other Geographical Information System(GIS)technologies. Three key climatic factors were selected including chilling in May, high temperature heat damage during July to early August and low temperature damage in autumn in this study. Furthermore, based on the analysis of climatic conditions suitable for double cropping super rice cultivation and climatic disasters, 8-22 ℃ active accumulated temperature, sunshine duration from late March to October, climatic risk index of the low temperature in autumn, and climatic risk index of chilling in May were selected as key climatic factors to study the precise agro-meteorological regionalization of double cropping super rice in Hunan province. The results showed that: the high-yielding zones of double cropping super rice in Hunan were mainly located in Zhuzhou, Hengyang, Yongzhou and Chenzhou City, the moderate-yielding zones were primarily located in the east and north reaches of Dongting Lake,together with most of Changsha, Zhuzhou and Xiangtan City, and other regions in Hunan were not suitable for double cropping super rice. These findings can provide valuable information for the large-scale cultivation of double cropping super rice in Hunan province.

Super Rice Breeding in China

1. Dernand for super high yield rice in ChinaRice is one of the main staple food in China. Theperformance of rice sector in production and yield had been

Contribution and Prospect of Erect Panicle Type to Japonica Super Rice

Over the past 30 years,super rice played an important role in boosting rice yield.The phenotype of erect panicle(EP)architecture controlled by dense and erect panicle 1(dep1)is the typical characteristic of super rice,and the phenotype has been used in rice breeding for nearly a century.In this review,the molecular genetic basis of EP phenotype,and mechanism of how dep1 affects rice carbon,nitrogen metabolism and grain quality(grain shape and taste quality)were discussed.In addition,we discussed the possible improvement strategies of rice yield and quality.This review provides a quick overview of the whole process for rice quality formation,as well as suggestions and ideas for future research on rice quality improvement.

Studies on the Morphological Characters of South China Double Cropping Super Rice at the Active Tillering Stage

The morphological characters of the newly released super hybrid rice in South China, Yueza 122, Fengyou 428, Peiza 67, and super conventional varieties, Guangchao 3, Shengtai 1 at the active tillering stage in both early and late crops were investigated. Using the analysis of variance and other statistic methods, the ideal morphological characteristics of South China double cropping super-rice at the active tillering stage were determined. They are 55-60 cm in the early crop and 60-76 cm in the late crop for the ideal plant height, about 15 tillers in the early crop and 14-19 tillers in the late crop for the ideal tiller number per plant, 40-44 cm in the early crop and 42-60 cm in the late crop for the ideal length of the first leaf under the top leaf, 1.2-1.4 cm in the early crop and 1.2-1.3cm in the late crop for the ideal width of the first leaf under the top leaf, 39- 44 cm in the early crop and 37-43 cm in the late crop for the ideal length of the second leaf under the top leaf, 1.1-1.4 cm in the early crop and about 1.1 cm in the late crop for the ideal width of the second leaf under the top leaf, 22-58o in the early crop and 4- 12o in the late crop for the ideal top leaf angle at the active tillering stage. Based on these results, a new concept of “dynamic plant type structure” for South China double cropping super rice breeding was suggested.