| Rice is the most important and the greatest water-consuming food crop. Extensive researches for drought resistance are extremely necessary in rice. In this study, the recombinant inbred line (RIL) population from a cross between Zhenshan97/Minghui63 was employed in 2004 and 2005, and the mini-core germplasm collection of rice was employed in 2006. A similar water stress for the population was imposed by adjusting the level of water in polyvinyl chloride (PVC) pipes at reproductive stage. Once the leaf rolling index reached grade four, the population were rewatered, then the root-related traits were investigated after two days. The performances of root-related traits under the two water-supply conditions were compared, and the mean membership index value (F0) was used to evaluate total root dry weight (RDWt). Considering the drought resistant mean membership index value of root-related traits (F), relative total biomass (RTB), and relative weight of panicle (RWP) as the index for the drought resistance, respectively, the relationships of root-related traits and drought resistance were further investigated by the method of correlation and path analyses. On the basis of the relationships, the traits closely related to drought resistance were defined. According to F, RTB and RWP values, the RIL population and mini-core germplasm collection were classified, respectively. QTLs for the studied traits were located in the RI population under different cases. The main results are as follows:1. In the RIL population, the closest relationships were observed between RDWt and shallow root dryweight (RDWs) under the three cases (well-watered in 2004 and 2005, water-stressed in 2005). The correlation coefficients are 0.987, 0.985, 0.991, respectively. The broad-sense heritability of RDWs are 0.646, 0.573, 0.584 under three cases, respectively. The volume of shallow root (RVs) was found to had the highest broad-sense heritability (h~2=0.882) in 2004, total root dryweight per tiller number (RDWtTN) (h~2=0.874 and 0.685) under well-watered and water-stressed conditions in 2005, respectively. In the three cases, heritability of ratio of shoot to root (RS) was showed to be the lowest among the studied traits, respectively.The classification for RIL population on the basis of F0 under well-watered condition in 2004 and two water-supply conditions (well-watered and water-stressed conditions) in 2005 demonstrated that R42 and R83 were simultaneously ranked into grade one under the three cases, owning extensive root system lines; R23 and R128 were simultaneously ranked into grade one under the two water-supply conditions in 2005.The effects of root-related traits on drought resistant ability were estimated by correlation and path analyses on the basis of F and RTB as drought resistant index, respectively. The results showed that the RDWs and RDWd had the greatest effect on drought resistance, respectively. The classification for drought resistance of RIL population on the basis of F and RTB, respectively, which revealed that R157 and R204 were simultaneously ranked into grade one under the two cases, owning drought resistant lines.2. A total of 35, 33, 33 QTLs were identified for the studied 14 root-related traits under well-watered in 2004 and two water-supply conditions (well-watered and water-stressed conditions) in 2005, respectively. There is no common QTL for the studied traits under the three cases. When combined data together for mapping QTL, the number of 58 QTLs was detected for 14 root-related traits when considering the different years as environmental factor, and 51 QTLs were detected considering the water-supply conditions as environmental factor. Among the detected QTLs, a total of 19 QTLs were identified simultaneously under the two environmental factors.3. In the mini-core germplasm collection, the closest relationship was observed between RDWt and RDWs under the two cases (well-watered and water-stressed in 2006) with the same correlation coefficients r=0.997. The shallow root dryweight per tiller number (RDWsTN) was found to have the highest broad-sense heritability (h~2=0.956) under well-watered condition, and RDWtTN (h~2=0.939) under water-stressed condition in 2006. The RDWtTN was proved to have the highest broad-sense heritability (h~2=0.908) when water-supply patterns were considered as the environmental factor.Chengnongshuijing, Dongtingwanxian, JWR221, Yangdao2, Zhenxian232, Aimi, Benbanggu, Sanbqsl, Zhonghua8 Qitoubaigu were simultaneously ranked into grade one on the basis of F0 under two water conditions in 2006, owning the extensive root system varieties; Haobayong, Haomake(K), for Hengxianliangchun, Jiangnongzaol only under well-watered condition, and for Yangdao2, Maguzi only under water-stressed condition in 2006. The effects of root-related traits on drought resistant ability were estimated by correlation and path analyses on the basis of F, RWP and RTB as drought resistant index, respectively. The study showed that the RDWs, RDWs, RVd had the greatest effect on drought resistance, respectively. The classification for drought resistance of the mini-core germplasm collection on the basis of F, RWP and RTB, respectively, which indicated that Maguzi was simultaneously ranked into grade one under the three cases, owning drought resistant variety.
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