BackgroundA number of field studies have demonstrated that the yield potential of hybrid rice cultivars is higher than that of inbred cultivars, although the magnitude of difference between hybrid and inbred cultivars at different yield levels has not been described. The objective of this study is to compare the yield increase potential at different yield levels between hybrid and conventional rice. Ten field experiments were conducted at five locations in southern China in 2012 and 2013. At each location, two hybrid and two inbred cultivars were grown at three N levels: high (225 kg/hm2), moderate (161–191 kg/hm2) and the control, zero N (0 kg/hm2).ResultsHybrid rice yielded approximately 8 % more grain than did inbred cultivars in Huaiji, Binyang and Haikou; approximately 7 % more in Changsha; and approximately 19 % more in Xingyi. The high grain yields observed for hybrid rice cultivars were attributed to high grain weight and biomass accumulation at maturity. On average, rice yields were approximately 6.0–7.5 t ha−1 (medium yield) in Huaiji, Binyang and Haikou; approximately 9.0 t ha−1 in Changsha (high yield); and approximately 12.0 t ha−1 (super high yield) in Xingyi. The yield gaps among Huaiji, Binyang and Haikou and Changsha were attributed to the differences in spikelets m−2 and biomass production, whereas the yield gap between Changsha and Xingyi was caused by the differences in grain-filling percentage, grain weight and harvest index. The differences in biomass production among sites were primarily due to variation in crop growth rate induced by varied temperatures and accumulative solar radiation.ConclusionsThe yield superiority of hybrid rice was relatively small in comparison with that of inbred cultivars at medium and high yield levels, but the difference was large at super high yield levels. Improving rice yields from medium to high should focus on spikelets m−2 and biomass, whereas further improvement to super high level should emphasize on grain-filling percentage, grain weight and harvest index. Favorable environmental conditions are essential for high yields in hybrid rice.

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Key laboratory of crop cultivation and Farming System, Guangxi university, nanning, P.r. china ABSTRACT Early vigor is important for crop establishment in rice. This study was conducted to determine the seed and seedling traits in relation to early vigor in rice. Laboratory tests and pot experiments were carried out in 2013 and 2014. Morphological and physiological traits of seeds and seedlings were compared between two contrasting rice cultivars, Yuxiangyouzhan with superior early vigor and Huanghuazhan with general early vigor. For seed traits, Yuxiangyouzhan had lower seed hull weight but higher seed amylose content, seed amylase activity, and plumule-radicle ratio than Huanghuazhan, and consequently, Yuxiangyouzhan had about 10% higher germination percentage and velocity and 37% longer plumule than Huanghuazhan. For seedling traits, leaf area, specific leaf weight, leaf chlorophyll and soluble protein contents, leaf net photosynthetic rate, and shootroot ratio were higher in Yuxiangyouzhan than in Huanghuazhan, and as a result, seedling shoot biomass was 23-32% higher in Yuxiangyouzhan than in Huanghuazhan. These results indicate that Yuxiangyouzhan has both superior seed and seedling vigor. The former is attributed to the low mechanical strength of seed hull, high conversion efficiency of seed reserve, and high mobilization of seed reserve to plumule, while the latter is due to large leaf area, high leaf photosynthetic capacity, and high partitioning of dry matter to shoot. This study enriches the physiological understanding of superior early vigor in rice.