CJ-00214; No of Pages The Crop Journal xxx (2017) xxx–xxx Contents lists available at ScienceDirect The Crop Journal Progress in super-hybrid rice breeding Longping Yuan China National Hybrid Rice Research and Development Center, Changsha 410125, China a r t i c l e i n f o Article history: Received 25 July 2016 Received in revised form August 2016 Accepted November 2016 Available online xxxx Keywords: inter-subspecific heterosis indica japonica morphological characteristics yield increase © 2017 Crop Science Society of China and Institute of Crop Science, CAAS Production and hosting by Elsevier B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Contents Introduction Technical approaches 2.1 Morphological improvement 2.2 Raising the level of heterosis Conclusions References Introduction To meet the food demand of the Chinese people in the 21st century, a super-rice breeding program aimed at increasing rice yield was initiated by the Ministry of Agriculture of China in 1996 It is divided into four phases, with the following yield targets: 10.5 t ha−1 (phase I, 1996– 2000), 12 t ha−1 (phase II, 2001–2005), 13.5 t ha−1 (phase III, 2006– 2015), and 15 t ha−1 (phase IV, 2016–2020) [1] The average yield of super-rice should be verified in two locations of 6.7 each in two consecutive years Through morphological improvement and the use of inter-subspecific (indica/japonica) heterosis, much progress in developing super hybrid rice varieties has been achieved By 2000, several pioneer super hybrids that met the phase I yield target had been developed, and they were released for commercial production in 2001 In recent years E-mail address: lpyuan@hhrrc.ac.cn 0 0 0 the planting area of these hybrids has been approximately million hectares and their average yield has been 8.3 t ha−1 The phase II breeding objective of super-hybrid rice was achieved in 2004 The planting area of phase II hybrids was close to million hectares in 2014 and their average yield was t ha−1 A yield breakthrough in super-rice varieties has been rapidly realized with the great efforts of Chinese rice breeders since 2011 The average yield of the super-hybrid rice Y-U-2 reached 13.9 t ha−1 in a 7.2-ha demonstration trial in 2011 Another new super-hybrid, Y-U-900, yielded 14.8 and 15.4 t ha−1, respectively, in 6.8-ha demonstration trials evaluated in Longhui county, Hunan province in 2013 and in Xupu county, Hunan in 2014 These experimental results mean that the phase III and phase IV breeding objectives of the super-rice breeding program have been achieved Accordingly, the phase V breeding program for super-hybrid rice has been proposed in 2015, with a yield target of 16 t ha− The current landmark variety of super-hybrid rice, Super-1000, was developed, with yield reaching 16.0 t ha−1 in a 6.8-ha demonstration trial in Gejiu county, Yunnan province in 2015 http://dx.doi.org/10.1016/j.cj.2017.02.001 2214-5141/© 2017 Crop Science Society of China and Institute of Crop Science, CAAS Production and hosting by Elsevier B.V This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) Please cite this article as: L Yuan, Progress in super-hybrid rice breeding, The Crop Journal (2017), http://dx.doi.org/10.1016/j.cj.2017.02.001 L YuanThe Crop Journal xxx (2017) xxx–xxx Tall (new type) Super tall 2.0 Plant height (m) Tall (traditional) Semi-tall 1.5 Semi-dwarf 1.0 Dwarf 0.5 (actual) (actual) (actual) (actual) (estimated) (estimated) HI ≈0.3 >0.5 >0.5 >0.5 >0.5 >0.5 Yield level (t ha–1) 3–5 6–8 8–12 12–18 18–20 >20 Fig Trend in plant height for development of super-high-yielding hybrid rice [3] Technical approaches To date, morphological improvement and heterosis use are the only two effective approaches to increasing yield potential in rice breeding, as verified by long-term crop improvement practice [2] Increases in yield potential are very limited without these two approaches Any other breeding approaches and methods, including modern breeding technologies such as genetic engineering, must be combined with favorable morphological characters and strong heterosis; otherwise, there will be no actual contributions to yield increase [3] 2.1 Morphological improvement A plant ideotype is the foundation for super-high yield in rice breeding [4] For example, the high-yielding combination P64S/E32, with striking characteristics, achieved a yield record of 17.1 t ha−1 Based on our studies, a super-high-yielding rice variety displays the following morphological features [5]: (1) Tall erect-leaf canopy [6,7]: The upper three leaf blades should be long, erect, narrow, V-shaped, and thick Long and erect leaves usually present a larger leaf area They can receive light on both sides and will not shade one other from sunlight Thus, light is used more efficiently, and air ventilation is also better within such a canopy Narrow leaves occupy a relatively small space and thus allow a higher effective leaf area index V-shaped leaves make the leaf blade stiffer so that the leaf is not prone to droop Thick leaves have higher photosynthetic function and not readily senesce These morphological features afford a large source of assimilates that are essential to super-high yield (2) Lower panicle position [8–10]: The tip of the panicle should be only 70 cm above the ground during the ripening stage With this architecture, the center of gravity of a plant is low, making Table Comparison of agronomic traits between Super-1000 and Y-U-900 (Changsha, Oct., 2014) Source Yield (t ha−1) HI Plant height (cm) Biomass per culm Stem weight (g 100 cm−1) Super-1000 Y-U-900 (CK) Difference (%) 14.19 14.14 0.35 0.58 0.57 1.75 118.00 125.20 –5.75 12.28 12.26 0.16 8.42 7.53 11.82 HI: harvest index the plant highly resistant to lodging Lodging resistance is also one of the essential characters required for breeding superhigh-yielding rice (3) Greater panicle size [8]: Grain weight per panicle should be around g, and the number of panicles about 250 per square meter Theoretically, the yield potential is about 15 t ha−1 in this case 2.2 Raising the level of heterosis Our studies indicate that the heterosis level in rice shows the following general trend [11]: indica/japonica N indica/javanica N japonica/ javanica N indica/indica N japonica/japonica Indica/japonica hybrids possess a very large sink and rich source, of which the yield potential is 30% higher than that of indica/indica hybrids used commercially For this reason, the use of indica/japonica heterosis has become our focus in developing super hybrid rice However, there are many challenges in developing indica/japonica hybrids, a key one being low seed set [12] By use of the wide compatibility (WC) gene (Sn5) and the intermediate-type male parent instead of typical japonica varieties [13], several inter-subspecific hybrid varieties with stronger heterosis and normal seed set have been successfully developed Grain yield is the product of harvest index (HI) and biomass As HI has already reached a very high level (above 0.5), further improvement of the rice yield ceiling should rely on an increase in biomass [2,14] From a morphological viewpoint, increasing plant height is an effective and feasible way to increase biomass Our experience in super-hybrid rice breeding has indicated a general trend: the greater the plant height, the higher are the biomass and grain yield, provided that the HI remains above 0.5 and the plant is resistant to lodging [15] This trend is illustrated in Fig Another effective approach to increasing biomass is increasing the thickness of the stem Comparing a promising new hybrid, Super1000, with a super-hybrid, Y-U-900 (Table 1), we found that the height of Super-1000 was 7.2 cm less than that of Y-U-900, its biomass per culm was almost the same as that of Y-U-900, and Super-1000 had thicker and heavier stems The advantage of this approach is that the developed hybrids are highly resistant to lodging However, it is more difficult to increase stem thickness than plant height Conclusions The development of science and technology is endless Pursuing higher and higher crop yields is an eternal theme Rice still has great Please cite this article as: L Yuan, Progress in super-hybrid rice breeding, The Crop Journal (2017), http://dx.doi.org/10.1016/j.cj.2017.02.001 L YuanThe Crop Journal xxx (2017) xxx–xxx potential for yield increases Our further objective is to achieve a yield of 17 t ha−1 within two or three years Super-hybrid rice has a very bright future and can make a great contribution to world food security and peace References [1] L.P Yuan, Developing super hybrid rice for the food security of China, Hybrid Rice 30 (3) (2015) 1–2 (in Chinese) [2] G.H Ma, L.P Yuan, Hybrid rice achievements, development and prospect in China, J Integr Agric (2015) 197–205 [3] L.P Yuan, Conceiving of breeding further super-high-yield hybrid rice, Hybrid Rice 27 (6) (2012) 1–2 (in Chinese with English abstract) [4] C.M Donald, The breeding of crop ideotypes, Euphytica (1968) 385–403 [5] L.P Yuan, Breeding of super hybrid rice, in: S.B Peng, B Hardy (Eds.), Rice Research for Food Security and Poverty Alleviation, International Rice Research Institute, Los Baños, Philippines 2001, pp 143–149 [6] E.H Murchie, Y Chen, S Hubbart, S.B Peng, P Horton, Interactions between senescence and leaf orientation determine in situ patterns of photosynthesis and photoinhibition in field-grown rice, Plant Physiol 119 (1999) 553–564 [7] P Horton, Prospects for crop improvement through the genetic manipulation of photosynthesis: morphological and biochemical aspects of light capture, J Exp Bot 51 (2000) 475–485 [8] S.B Peng, G.S Khush, P Virk, Q.Y Tang, Y.B Zou, Progress in ideotype breeding to increase rice yield potential, Field Crops Res 108 (2008) 32–38 [9] T.L Setter, E.A Conocono, J.A Egdane, M.J Kropff, Possibility of increasing yield potential of rice by reducing panicle height in the canopy I Effects of panicles on light interception and canopy photosynthesis, Funct Plant Biol 22 (1995) 441–451 [10] T.L Setter, E.A Conocono, J.A Egdane, Possibility of increasing yield potential of rice by reducing panicle height in the canopy II Canopy photosynthesis and yield of isogenic lines, Funct Plant Biol 23 (1996) 161–169 [11] L.P Yuan, Recent Progress in breeding super hybrid rice in China, in: Y.X Lu (Ed.), Science Progress in China, Elsevier Science Ltd., Oxford 2003, pp 231–236 [12] L.Y Chen, Y.H Xiao, W.B Tang, D.Y Lei, Practices and prospects of super hybrid rice breeding, Rice Sci 14 (2007) 71–77 [13] Q Ji, J.F Lu, Q Chao, M.H Gu, M.L Xu, Delimiting a rice wide-compatibility gene Sn5 to a 50 kb region, Theor Appl Genet 111 (2005) 1495–1503 [14] J.F Ying, S.B Peng, Q.R He, H Yang, C.D Yang, R.M Visperas, K.G Cassman, Comparison of high-yield rice in tropical and subtropical environments: I Determinants of grain and dry matter yields, Field Crops Res 57 (1998) 71–84 [15] H.F Deng, Studies on the Objective Traits of Super Hybrid Rice in the Yangtze River BasinPh.D Dissertation Hunan Agricultural University, 2008 Please cite this article as: L Yuan, Progress in super-hybrid rice breeding, The Crop Journal (2017), http://dx.doi.org/10.1016/j.cj.2017.02.001