Different characteristics of high yield formation between inbred japonica super rice and inter-sub-specific hybrid super rice
Introduction
Rice is the most important food crop in China, feeding about 65% of the Chinese population (Zhang et al., 2005). Therefore, the steady increase in rice production and yield potential in China will play a key role towards food security and livelihood improvement (Peng et al., 2009). Rice yield increase depends on either the expansion of the rice cultivated area or the production per unit of area. Nonetheless, the increase production due to increased growth area is impinged by the limited availability of arable land and water resources (Cheng et al., 2012). Hence, rice production increase appears possible only through improvements in the yield per unit of area. The use of semi-dwarf rice varieties during the first Green Revolution in the 1960s (Athwal , 1971, Khush, 1999) and the use of heterosis by producing hybrid rice in the late 1970s (Yuan, 2014) allowed for two leaps in rice production in China. Nevertheless, during the last two decades, rice productivity remains stagnant and the rates of annual yield increases have declined since the mid-1980s (Horie et al., 2005).
In order to achieve breakthrough of rice yield increases, a super rice program based on the comprehensive utilization of an ideotype and inter-sub-specific (indica/japonica) heterosis was launched by China’s Ministry of Agriculture in 1996 (Cheng et al., 2007b, Peng et al., 2008). Indica and japonica are the two subspecies of Asian cultivated rice (Oryza sativa) showing significant differences in yield, quality and stress resistance. The use of heterosis between the two subspecies of rice has long been considered a promising approach to further enhance rice yields (Cai et al., 2001, Zhang et al., 1992, Zhao et al., 1999a). Although estimates indicated that the heterosis of inter-sub-specific hybrid between indica and japonica varieties could result in yield potentials of about 30% higher than those of indica or japonica cultivars (Lu et al., 2007), the use of heterosis has encountered some obstacles. The primary hurdle was the partial sterility of spikelets, which caused low seed setting rate in indica/japonica F1 hybrid (Cheng et al., 2007a, Kato et al., 1928). Spikelet sterility in indica/japonica hybrid has been attributed to an allelic interaction at locus S5 on chromosome 6 (Ji et al., 2012, Yang et al., 2012). An allele, S5-n, was detected in some varieties, which was referred to as wide compatibility varieties (WCV) (Qiu et al., 2005) that can produce normal fertile hybrids when crossing with either indica or japonica varieties (Ikehashi and Araki, 1984, Ikehashi and Araki, 1986). By making use of wide compatibility varieties, several inter-sub-specific hybrid varieties with stronger heterosis and normal seed setting rate have been successfully developed, such as Yongyou 6 (Xu et al., 2010), Yongyou 12 (Wang et al., 2014) and Yongyou 15 (Jiang, 2013), which displayed super high yields in the middle and lower reaches of the Yangtze River in China and also have been confirmed as super rice by China’s Ministry of Agriculture.
However, when compared to other types of super rice, the number of inter-sub-specific hybrid super rice between indica and japonica varieties was lower. According to the official report, the number of super rice cultivars confirmed by China’s Ministry of Agriculture from 2005 to 2015 was 118. Among them, there were 28 inbred japonica rice, 1 intra-sub-specific japonica hybrid rice, 8 inbred indica rice, 75 intra-sub-specific indica hybrid rice. Only 6 cultivars were inter-sub-specific hybrid rice between indica and japonica, which accounted for about 5% of the total (Xu and Chen, 2016). Previous work revealed that big panicle with more grains, excellent structure of canopy and strong root system were the important foundation for the high yields shown by inter-sub-specific hybrid rice. However, information on the differences between inter-sub-specific hybrid super rice and other types of super rice, in particular, the single cropping inbred japonica super rice, which was popular in the middle and lower reaches of the Yangtze River in China for its high yield and good quality is very limited.
Here, we used two different types of super rice (with Nanjing 44 and Ningjing 3 as inbred japonica rice cultivars, and Yongyou 12 and Yongyou 15 as inter-sub-specific hybrid rice cultivars) to identify the different characteristics of high yield formation between inter-sub-specific hybrid super rice and inbred japonica super rice cultivars.
Section snippets
Plant material and growth conditions
Two different types of super rice cultivars were used, Nanjing 44 and Ningjing 3 are inbred japonica rice cultivars while Yongyou 12 and Yongyou 15 are inter-sub-specific hybrid rice cultivars. The experiments were conducted on a research farm of Yangzhou University in the Jiangsu province, China (32∘30′N, 119∘25′E, 21 m altitude) during the rice growing season (May–November) of 2012, and repeated in 2013. The soil of the field was sandy loam with 0.14% total N, 87.75 mg kg−1 alkali hydrolysable
Days and climate data of different growth stages
Compared with inbred japonica super rice cultivars (IJSRC), the whole growth period of inter-sub-specific hybrid super rice cultivars (IHSRC) were 25–29 days longer in 2012, and 24–30 days longer in 2013. Among them, stages from sowing to elongating of IHSRC were 3 days shorter in 2012, and 2–3 days shorter in 2013 (Table 1). No differences in active accumulated temperature, effective accumulated temperature, sunshine duration and photosynthetically active radiation were found between IJSRC and
Discussion
Compared with non-super rice cultivars, super rice cultivars have advantages of yield formation (Huang et al., 2011), plant type characters (Zhang et al., 2009b), and dry matter production(Ao et al., 2008), etc. Sufficient total spikelet number was the basis of stable high yield in super rice (Sheehy et al., 2001, Wu et al., 2007). For super rice of Ningjing 1 and Ningjing 3, to achieve more than 11.0 t hm−2 yield of rice, the total spikelet number per square meter should be above 42,000 and to
Conclusions
Yields of IHSRC were higher than those of IJSRC. More total spikelet number due to efficient use of temperature and light during the stage from elongating to heading, stronger ability to tiller, larger leaf area index at each growth stage, higher leaf area duration and above-ground biomass accumulation during the periods from elongating to heading and from heading to maturity, greater photosynthetic rate, chlorophyll content of flag leaf and the root oxidation activity at 30 days after heading,
Acknowledgments
We are grateful for grants from the National Key Research Program of china (2016YFD0300503), the Special Fund for Agro-scientific Research in the Public Interest (201303102), the Key Research Program of Jiangsu Province (BE2016344), the Major Independent Innovation Project in Jiangsu Province (CX(15)1002) and a Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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