| Drought stress is a major constraint to rice production. In order to improve rice drought resistance, it is important to study the mechanism of rice drought resistance and to perform molecular marker-assisted breeding.By comparing the results of correlation, path analysis and QTL mapping of traits associated with rice drought resistance, index of drought resistance(IDR), yield per plant(YP), 1000-grain weight(TGW), basal root thickness(BRT) were selected as target traits and their QTLs were used for marker-assisted breeding (MAS). Four segregating populations (YIBC1upland, YIBC1lowland, JIF2upland, JIBC1upland), derived from crosses between a japonica upland rice variety IRAT109 as donor parent with drought resistance and two japonica lowland rice varieties with better grain quality and sensitive to drought, were used for verification of MAS effects of target QTLs. Target QTL near-isogenic lines (QTL-NILs) were also developed through the foreground selection for target QTLs and background selection for genetic background of the recurrent parent. The main results were as follows:Using three segregating populations under upland condition (YIBC1, JIBC1 and JIF2), QTL effects were verified through marker-assisted selection (MAS). The results showed that QTLs conferring thicker BRT, bigger TGW and higher YP were genetically stabilized in different populations, with different genetic backgrounds. The differences between the means of BRT, TGW and YP of individuals carrying and not carrying donor favorable alleles at the target QTLs flanking markers loci were significant at 5% or 1% level. The effects of target QTLs for BRT, TGW and YP were 5.05-8.16%, 13.94-18.15%, 34.89-58.88% respectively.Three populations under upland condition were also used to compare QTL effects based on one or both of the flanking markers of target QTLs. For the QTL of BRT, QTL effects based on the left side marker (RM1136) and right side marker (RM273) were equal to that based on both flanking markers; For target QTLs of YP and TGW, QTL effects based on the right side marker (RM527) was more effective than that based on the left side marker (RM541), nearly equal to that based on both of the flanking markers. Effects of target QTLs were also studied in backcross population YIBCi under lowland condition. The results showed that the differences between the means of BRT, TGW and YP of individuals with and without donor favorable alleles at the target QTLs flanking markers loci were not significant, suggesting that these QTLs could only be expressed under upland condition.MAS effects based on the genotypes of individuals selected from three populations of YIBC1upland, YIBC1lowland, JIF2upland were studied for two generations (BC1F1, BC1F2, F2, F3) in Beijing and Hainan under upland condition. The results showed that MAS effects based on QTL of BRT was 4.56-28.80%; the indirect MAS effects of YP and TGW were -15.75-96.47% and -0.47-36.52% respectively.When MAS for QTLs of YP, TGW, IDR were carried out, MAS effects were 7.05-136.79% for YP, 0.51-23.07% for TGW respectively. Comprehensive evaluations of individuals (BC1F3, F4) selected based on the genotypes in Hainan werealso carried out in Beijing under upland condition. The results indicated that phenotypic values of target traits of the selected individuals were much exceeded the control. For individuals selected with QTL of BRT, the percentages exceeding the control were 7.04-31.88% for BRT, 38.29-113.78% for YP, 21.08-33.30% for TGW and 90.71-270.04% for IDR. Sixteen elite lines were obtained; for individuals selected with QTLs of YP,TGW and IDR, the percentages exceeding the control were 13.36-190.79% for YP, 16.21-29.95% for TGW and 125.64-410.26% for IDR. Twenty elite lines were selected. Target QTL-NILs were obtained through (l)the foreground selection for target QTLs and background selection for the recurrent parent of three generations of BC1F1, BC2F1,. BC3F1 and (2) recovering ratio of genetic background( RRGB)>93.75% and phenotypic values of traits related to drought resistance of rice were h...
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