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

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1 Rice Science, 2012, 19(3): Copyright 2012, China National Rice Research Institute Published by Elsevier BV. All rights reserved Research Advances in High-Yielding Cultivation and Physiology of Super Rice FU Jing, YANG Jian-chang (Key Laboratory of Crop Genetics and Physiology of Jiangsu Province, Yangzhou University, Yangzhou , China) Abstract: 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 t/hm 2 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. Key words: super rice; grain yield; grain filling; crop physiology Rice is one of the most important food crops worldwide, providing 35% 60% of the dietary calories consumed by more than three billion people, and shouldering the responsibility for ensuring world food security (Xie et al, 2006). China is the largest producer and consumer of rice in the world. The rice planting area occupies 30% of the total area of grain crops, and the total rice yield accounts for more than 40% of the total cereal yield. More than half of the population in China lives on rice (Xie, 2007). The breeding and use of semi-dwarf rice from the 1960s to the 1970s and the application of three-line indica hybrid rice in the 1970s resulted in two leaps in rice yield in China. The rice yield in China reached 6.0 t/hm 2 and became the highest yield of any country worldwide in the 1990s (Cheng et al, 1998; Xu, 2006). However, rice production in China has declined since 1997, when total grain yield had reached 200 million tons. Although the rice yield in China has recovered since 2004, there have been no breakthroughs in terms of yield. It is predicted that China s population will reach 1.6 billion by 2030, and crop production must increase by 55% to meet the food demand, even if there is no reduction in the planting area of food crops. Rice accounts for more than 40% of the increase in crop production (Su and Li, 2007). How to achieve the next breakthrough in rice yield is one of the major issues for rice scientists. Received: 21 September 2011; Accepted: 23 December 2011 Corresponding author: YANG Jian-chang (jcyang@yzu.edu.cn) Grain yield of rice can be defined as the product of yield sink capacity and filling efficiency. To further increase yield and improve yield capacity, breeding experts have expanded yield sink capacity. That is, the size of sink organs to be harvested has been maximized, mainly by increasing the number of spikelets per panicle, such as in the new plant type rice of the International Rice Research Institute (IRRI). These hybrid rice varieties, known as super rice or super hybrid rice in China, have large panicles. However, they frequently fail to achieve their high yield potential during production because of poor grain filling. This is mainly attributed to the slow grain filling rate and unfilled grains of the late-flowering inferior spikelets (Peng et al, 1999, 2008; Cheng et al, 2007; Kato et al, 2007). To achieve the third leap forward for rice yield in China, realizing the yield potential of super rice is an important approach to ease the pressure of population growth on the environment and natural resources, and to ensure food security. In this paper, we review the recent advances in research on crop physiology and management of super rice, and suggest further studies on crop physiology and management of super rice. Such research would provide the theoretical basis for maximizing super rice production. Concept of super rice and high yielding potential In 1981, the Japanese government first initiated research on super high-yielding rice breeding and

2 178 Rice Science, Vol. 19, No. 3, 2012 implemented the Reverse 753 Program (Tadaaki, 1988). IRRI launched a new plant type rice breeding program targeting super high-yielding varieties in 1989 (IRRI, 1989). Encouraged by the super rice breeding program of IRRI, the Chinese Ministry of Agriculture began to organize and implement a plan to breed and cultivate Chinese super rice varieties. The plan proposed to develop super rice varieties with the highest yield goal in small area from mid-season or single-season rice crops. As well as yield targets, quality indexes of rice were also proposed; that is, the quality of the north japonica and south indica rice varieties should be the first- and second-grade according to the Ministerial standard, respectively. The plan also proposed that super rice varieties should be resistant to one or two of the local main diseases and insects (Cheng et al, 1998; Xu, 2006). To realize the mega plan, China established the Expert Committee of Super Rice Breeding (Super Rice Breeding Project Evaluation Group) in The super rice breeding project was undertaken by 11 institutions including China National Rice Research Institute, Shenyang Agricultural University, and Guangdong Academy of Agricultural Sciences. The project was granted by the Foundation of the Prime Minister and the National Key Technology Support Program of China in 1998 and 1999, which fully initiated China s super rice research (Pan and Liu, 2007). According to the statistics from the Ministry of Agriculture, up to 80 super rice varieties had been identified by April, 2010, and the planting area was 6.6 million hectares, nearly one-quarter of the rice planting area in China. These varieties showed high yields, good quality, and disease resistance. Some achieved grain yields of t/hm 2 under special climate conditions in field experiments (Cheng, 2005; Cheng et al, 2005; Xie et al, 2006), which represented great yield potential. Compared with the local conventional high-yielding varieties, super rice varieties showed 8% 20% increases in grain yield (Wu, 2009; Zhang et al, 2009b). In practical terms, this grain yield equated to kg/hm 2 and an average income increase of CNY per hectare, which greatly improved rice production. Morphological and physiological characteristics of super rice Super rice varieties are the combinations of ideal plant types and heterosis, and show high grain yield, good quality, and strong resistance (Min et al, 2002; Cheng, 2005; Cheng et al, 2005; Xie et al, 2006). The main high-yielding characteristics of super rice, as compared with those of conventional varieties, can be summarized as follows: more spikelets per panicle and larger sink size (greater number of spikelets per square meter) (Zhu et al, 2002b; Katsura et al, 2007; Zhang et al, 2009b); larger leaf area index, longer duration of green leaf, greater photosynthetic rate and higher lodging resistance (Wang et al, 2002); greater dry matter accumulation before the heading stage (Zhai et al, 2002; Katsura et al, 2007), greater remobilization of pre-stored carbohydrates from stems and leaves to grains during the grain-filling period (Katsura et al, 2007); and stronger root system and root activity (Zhu et al, 2000, 2001, 2002a; Xu et al, 2010). Yield components Sheehy (2001) reported that the high yield of super rice was attributed to the increased number of spikelets per panicle. Wu et al (2007) observed that the higher total spikelet number resulted in increased grain yield in super rice. They argued that the main strategy to increase the sink capacity was to increase the number of spikelets per panicle. Increases in the number of primary branches and spikelets on the primary and secondary branches contributed to the large panicles. To maximize the yield of super rice, the following attributes can be manipulated by breeding and/or crop management: appropriate panicle number, larger panicles, greater total spikelet number, greater sink capacity, stable grain filling efficiency, and super high-yielding potential. Therefore, to improve the grain yield of super rice, the varieties must have sufficient panicle number, more spikelets per panicle, greater grain filling, and heavier grain weight than conventional varieties (Chen, 2008). Plant type characteristics The technical route for super rice breeding in China is to combine ideal plant types with heterosis. A good plant type ensures that plants receive more light for photosynthesis. Orientation of the flag leaf of rice can significantly affect photosynthetic function of leaves (Chen et al, 2002b). Rice leaves are source organs for producing and exporting assimilates and leaf area is the main index for the source (Yang and Tu, 2003). However, a larger leaf area index benefits the plant in more ways than just increasing photosynthesis. Previous studies showed that the plant type of super rice is better than normal hybrid rice (Xiang and Li, 2003) because of its larger leaf area and smaller leaf angles for the upper three leaves (Wang et al, 2002; Zheng and Huang, 2003; Zou et al, 2003; Cheng et al, 2005; Deng et al, 2005). Moreover, super rice has a moderate plant height and strong lodging resistance

3 FU Jing, et al. Research Advances in High-Yielding Cultivation and Physiology of Super Rice 179 (Zheng and Huang, 2003; Zou et al, 2003). Photosynthetic production and assimilate remobilization The great yield potential of super rice is mainly due to the large sink size; that is, the large panicles. These require adequate sources and a constant input of assimilates, resulting in high grain yield (Zou et al, 2003). Zhai et al (2002) observed that the photosynthetic rate of the flag leaf in super hybrid rice was sufficient to meet the requirements of grain filling. When compared with conventional rice, super hybrid rice had higher chlorophyll content and photosystem II activity, greater photosynthetic rate under various light intensities, and showed stronger resistance to photoinhibition of photosynthesis and to photo-oxidation at noon on a sunny day under strong light conditions (Chen et al, 2002a; Li and Jiao, 2002; Wang et al, 2004b). Further studies showed that super rice had higher superoxide dismutase (SOD) activity and very low malondialdehyde content, and showed a slow decrease in chlorophyll and soluble protein content, SOD activity, and photosynthetic rate in leaves during the grain filling period, which ensured a longer duration of photosynthesis (Xiang and Li, 2003; Deng et al, 2006). The photosynthetic capacity of leaves in super rice varies depending on panicle types. The Activity of ribulose-1,5-bisphosphate carboxylase and chlorophyll content in the flag leaves and the transport and transformation ratio of matter in stems (culms and sheaths) to panicles during grain filling were significantly higher for the varieties with heavy-panicle type than those with mid-panicle and small-panicle types (Ma et al, 2003). When compared with normal hybrid rice, super hybrid rice had a stronger photosynthetic advantage. Using a 14 C radioisotope tracer technique, Yan et al (2001) observed greater photosynthetic capacity and more photosynthetic products in the heavy-panicle type varieties Liangyoupeijiu, Yayou 162, and Teyou 124 than in the normal hybrid rice variety, Shanyou 63. Super rice showed a longer photosynthetic function, although super rice varieties exhibited a lower remobilization of assimilates from stems to panicles than Shanyou 63. The amount and the rate of assimilates remobilized from stems to panicles during grain filling are significantly and positively correlated with grain weight, and sucrose phosphate synthase (SPS) plays an important role in the remobilization of assimilates. Therefore, the enhancement of SPS activity by physiological regulation would increase remobilization of assimilates from vegetative tissues to grains, consequently increasing the yield potential of super rice varieties (Mark et al, 1987). The rapid expansion of leaf area and great leaf area index resulted in a large vegetative biomass, which may contribute to the increase in grain yield in super rice (Zou, 2007). An increase in biomass is the basis for achieving high yields (Chen et al, 1995). The proportion of dry matter production in super rice was appropriate at each growth stage; that is, 20% 25% at the jointing stage, 50% at the booting stage, 60% 70% at the full heading stage, and 30% 40% from heading to maturity, if the proportion was taken as 100% at maturity (Ling et al, 1993; Zou et al, 2001; Wang et al, 2006). Although the remobilization of assimilates from stems to grains was not very high, there was high dry matter production during the mid- and late-growth periods in super rice, which could meet the requirement for assimilates for grain filling and also contribute to a strong resistance to lodging and delayed senescence (Zou et al, 1997; Sun et al, 2010). Root traits Roots are extremely important plant organs. They anchor the plant, uptake nutrients and water, and synthesize plant hormones, organic acids and amino acids. Root morphology and physiology are balanced with those of the shoot (Inukai et al, 2004). Stronger root activity and greater ability to absorb nutrients can benefit shoot growth and development. Therefore, strong root activity of rice is the basis for vigorous growth during the early growth stage and guarantees grain filling during the late-growth stage. Zou et al (2003) reported that super hybrid rice had a stronger root system, greater emergence vigor of new roots, greater root number, higher root volume, and better root quality than conventional rice varieties. As well, super rice had more roots at the booting stage than at the tillering or maturity stage. Zhu et al (2002b) investigated the root traits of super hybrid rice and reported that Xieyou 9308 had larger root biomass and a larger ratio of deep roots to total roots compared with Shanyou 63. They observed that lateral distribution of roots increased with increasing soil depth, and that the root mass tended to be away from the center of plants. Root weight density was the highest at the soil surface, and gradually decreased with increasing soil depth. The greatest root biomass was at the heading stage and it decreased after heading. The ratio of deep roots to total roots increased throughout the growing period. The root biomass showed the fastest increase from the tillering stage to the panicle initiation stage, and roots grew downwards

4 180 Rice Science, Vol. 19, No. 3, 2012 during this period. It is proposed that deep roots are closely correlated with grain yield. Cutting off roots at the panicle initiation stage decreased the spikelet number per panicle, and cutting of roots at the heading stage decreased the seed-setting rate. There are reports that abscisic acid (ABA) concentration in root sap and in leaves was significantly lower in super rice than in check varieties, however, the concentration of cytokinins [zeatin (Z) + zeatin riboside (ZR)] was significantly higher in super rice than in check varieties (Zhu et al, 2000, 2001, 2002a). This was consistent with the higher leaf photosynthetic rate for super rice varieties (Cao et al, 2004). Zhang et al (2009b) observed that shoot dry weight, root dry weight, and root length density were significantly greater in super rice varieties of Liangyoupeijiu and Huaidao 9 than in the check varieties Yangdao 6 and Yangfujing 8 throughout the growth season. The differences in root/shoot ratio between super rice varieties and check varieties were not significant. Root oxidation activity per plant and (Z + ZR) content in the root were significantly greater in super rice varieties than in check varieties before the heading stage. However, during the mid- and lategrain filling periods, root oxidation activity and (Z + ZR) content in the root were significantly lower in super rice varieties than in check varieties, which may be due to the growth characteristics of more tillers and larger panicles in super rice. This could lead to competition for carbohydrates between panicles and roots, and results in fewer assimilates transported to roots during the late grain filling period. The lower supply of assimilates to roots would lead to earlier root senescence, and consequently, earlier shoot senescence. The earlier root senescence in super rice varieties would also be partially attributed to a larger canopy. The irradiance in the canopy at 20 cm above the ground was 10% 12% less in super rice than in check varieties. Because the main supply of carbohydrates for rice roots is derived from lower leaves in plants, the lower irradiance at the level of the lower leaves in the super rice canopy could decrease photosynthesis in these leaves, thereby reducing root activity and leading to earlier root senescence and lower and unstable seed-setting rate. Problems and research prospects Main problems Poor grain-filling and unstable seed-setting rate of inferior spikelets Super rice varieties have great yield potential. However, they show two main problems during production: first, there is wide variation in grain filling between superior spikelets (usually located on apical primary branches, earlier flowering grains) and inferior spikelets (usually located on proximal secondary branches, later flowering grains) (Mohapatra et al, 1993; Yang et al, 2000; Yang and Zhang, 2010); second, these varieties can show a low and/or unstable seed-setting rate, that is, the seedsetting rate differs in the same super rice variety among different years or regions (Du et al, 2006; Ao et al, 2008; Yang and Zhang, 2010). These problems hinder the realization of high yield potential for super rice varieties and limit their applications in rice production. However, these issues also affect rice quality, especially processing quality and appearance. Because large amounts of water and nutrients are required for differentiation and growth of inferior spikelets, poor grain filling and grain weight of inferior spikelets also seriously affects water and nutrient-use efficiencies. There have been many studies on the causes of poor grain filling and grain weight of inferior spikelets, but the results and conclusions differ markedly because of differences in research materials and methods. The main arguments and conclusions are as follows: 1) Photosynthetic products are preferentially supplied to superior spikelets, and grain-filling of some inferior spikelets is decreased or terminated when there is an insufficient supply of assimilates (Wang et al, 1962; Murty and Murty, 1982). 2) There is not necessarily a link between the substrate concentration in grains and differences in the development of superior and inferior spikelets (Mohapatra et al, 1993; Yang et al, 2006), and the main reason for poor grain filling of inferior spikelets is their low sink strength (low endosperm cell division rate and activities of enzymes involved in sucrose-to-starch conversion) or low biochemical efficiency of converting sucrose into starch during the early grain filling stage (Mohapatra et al, 1993; Liang et al, 2001; Yang et al, 2003). 3) The higher ratio of inhibitory hormones (ABA and ethylene) to growth promoting hormones (auxin, cytokinin and gibberellins) in grains inhibits the filling of inferior spikelets (Waters et al, 1984; Tao et al, 2003). 4) ABA can promote grain filling, and lower ABA content, higher ethylene evolution rate, or a lower ratio of ABA to 1-aminocyclopropane-1-carboxylic acid (ACC, the ethylene precursor) (ABA/ACC) are closely associated with slow grain filling and low grain weight of inferior spikelets (Yang et al, 2006; Zhang et al, 2009a). 5) Low expression of genes encoding enzymes involved in cell wall physiology, including invertase, vacuolar invertase, adenosine diphosphate glucose pyrophosphorylase (AGPase), and starch synthase of inferior spikelets during the early grain filling period account for poor grain filling (Ishimaru et al, 2005),

5 FU Jing, et al. Research Advances in High-Yielding Cultivation and Physiology of Super Rice 181 but different and even opposite results have been reported (Hirose et al, 2002). 6) Polyamines play a role in regulating grain filling of rice. In some super rice varieties, the lower spermidine (Spd) and spermine (Spm) concentrations and lower ratio of Spd to putrescine (Put) (Spd/Put) and Spm/Put in inferior spikelets are an important physiological reason for the low grain filling rate and low grain weight of inferior spikelets (Yang et al, 2008; Tan et al, 2009). It is clear that we have not yet fully understood the mechanism underlying the poor grain-filling in inferior spikelets of super rice. Low use efficiencies of water and nutrients High grain yields of super rice in trials and demonstrations are usually achieved with high inputs of fertilizer and water. The authors investigated the yield and fertilizer and water inputs for nine super rice varieties in five rice-planting regions (Prefecture-level cities) in Jiangsu Province, China. We observed that the average yield of super rice varieties was 9.62 t/hm 2, which was 11% higher than that of six conventional high-yielding varieties. The irrigation water for super rice varieties was 7696 m 3 /hm 2 and nitrogen application rate was 284 kg/hm 2, which were 14% and 12% higher, respectively, than those required for conventional rice varieties. The use efficiency of irrigation water and nitrogen partial productivity (grain yield/amount of nitrogen) in super rice varieties were decreased by 3.1% and 1.5%, respectively, when compared with those of check varieties (data not shown). These results suggest that the grain yield of super rice is high, but the water and fertilizer use efficiencies are not significantly improved, or even decreased, compared with conventional varieties. The relationship between root morphology/physiology and absorption and utilization of water and nutrients in super rice is not fully understood, and few studies have been conducted on water and nutrient absorption in super rice. In recent years, several water-saving irrigation techniques, such as alternate wetting and drying irrigation, moderate wetting irrigation, controlled irrigation, and non-flooded mulching cultivation, have been trialed. These methods significantly increased water use efficiency (Lin et al, 2004; Wang et al, 2004a; Peng et al, 2006b; Wang et al, 2007; Zhang et al, 2009c), but the grain yield varied with soil texture and with the level of soil drainage and precipitation during the rice growth season (Tabbal et al, 2002; Belder et al, 2004; Tuong et al, 2005; Yang et al, 2007). There are several nutrient management techniques, such as controlling total nitrogen rate and regulating application at each growth stage according to crop needs, site- specific nutrient management, precision and quantitative application of fertilizers, and soil testing and fertilizer recommendation (Peng et al, 2006a; Zhang et al, 2006; Ling, 2007). These techniques largely focus on increasing fertilizer use efficiency while retaining current yield levels. The current water and fertilizer management of super rice is still the same as that used for conventional high-yielding rice varieties, which might limit yield potential in super rice (Peng et al, 2009). Research prospects for crop physiology of super rice The lower yield potential from super rice is due to poor grain filling and low grain weight of inferior spikelets. Techniques to address these issues, such as appropriate use of irrigation and fertilizers, have been reported. However, there is still a long way to go to understand the mechanism underlying poor filling of inferior spikelets, and to define the best times to supply water and nutrients to super rice to achieve its full yield potential. Therefore, the following two aspects of super rice are suggested for further research. First, research should focus on the physiological reasons for poor grain filling, the unstable seed-setting rate, and the low grain weight of inferior spikelets in super rice. Such research should include studies on the changes of hormonal concentrations and enzymatic activities for sucrose-to-starch conversion in superior and inferior spikelets during endosperm development, differences in mrna expression of genes related to hormones and enzymes, and the relationships between these biochemical processes and the development and filling of superior and inferior spikelets. This would illustrate the intrinsic factors that determine poor grain filling in inferior spikelets. Studies should be carried out to understand the changes in hormonal concentrations in functional organs (roots, stems and leaves), physiological traits of roots and ultra-structural features in root tip cells, photosynthetic characteristics of leaves, transportation characteristics of matter in culms and sheaths, and ultra-structure features in the phloem and other vascular tissues in the primary and secondary branches and in different parts of rice panicles. Such research would clarify the physiological reasons for poor grain filling and low grain weight in inferior spikelets by combining the methods at the whole plant level, organ level, and cellular level (including physiology, biochemistry, and molecular biology) (Yang, 2010). Second, research should focus on the relationships between morphological and physiological traits of roots and water and nutrient absorption and utilization

6 182 Rice Science, Vol. 19, No. 3, 2012 in super rice. It is necessary to understand root growth and development, morphology, and distribution; the quantities, components, and physiology of root exudates; and the water and nutrient requirements of super rice. This information would enable us to unravel the mechanisms underlying root-shoot interactions and root-soil interactions to achieve high yields, and to establish strategies and techniques to improve yield and water and nutrient use efficiencies in super rice. Ultimately, biological processes and mechanisms involved in high yields and high water- and nutrientuse efficiencies could be illustrated, and new technological approaches could be created to achieve high yields and water- and nutrient-use efficiencies in super rice. ACKNOWLEDGEMENTS This work was supported by grants from the National Natural Science Foundation of China (Grant Nos and ), the National Basic Research Program (Grant Nos. 2009CB and 2012CB114306), the National Key Technology Support Program of China (Grant No. 2011BAD16B14), and the Natural Science Foundation of Jiangsu Province, China (Grant No. BK ). REFERENCES Ao H J, Wang S H, Zou Y B, Peng S B, Tang Q Y, Fang Y X, Xiao A M, Chen Y M, Xiong C M Study on yield stability and dry matter characteristics of super hybrid rice. Sci Agric Sin, 41(7): (in Chinese with English Belder P, Bouman B A M, Cabangon R, Lu G A, Quilang E J P, Li Y H, Spiertz J H J, Tuong T P Effect of water-saving irrigation on rice yield and water use in typical lowland conditions in Asia. 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