Improving Drought Tolerance Of Japonica Rice By BC Breeding And QTL Pyramiding

Drought stress and water shortage are the most important factors threatening crop production in many areas of the world. In China, agriculture uses～70% of its total fresh water resources and rice production alone takes up～70% of the water resources for agriculture. In North China, rice acreage has nearly doubled recently as the dramatically increased demand for high quality japonica rice in China. This has created a serious problem in competing the limited water resources in the area. Thus, development and adoption of high yielding, drought tolerant (DT) or water-use-efficient (WUE) japonica varieties are the most attractive solution to this problem. Thus, two major challenges facing plant breeders for developing high yielding japonica rice cultivars with desirable levels of DT and WUE are: where is the useful genetic variation for DT/WUE and what breeding strategy should be used.In this study, we used BC breeding to improve japonica hybrid restorer line, C418 by mining the diversity hidden in the indica gene pool. Five BC₂F₂ bulk populations were developed by crossing C418 (the recurrent parent) with 5 indicia donors followed by two rounds backcrossing. The BC₂F₂ populations were subjected to two rounds of selection under severe drought, resulting in the development of 113 DT introgression lines (ILs). Evaluation was made for the lines about introgression effects. Also, we mapped the QTLs for drought tolerance and analyzed the effects of DT QTLs. Furthermore, we conducted 4 pyramiding populations using 8 superior lines improved for DT followed by severe drought selection performed for F₂. QTLs mapping and analyzing of DT genetic network were made for pyramiding DT lines. The main results summarized as follows:1. Evaluation was made for 113 lines about introgression effects. Progeny testing across 2 diverse environments allowed the identification of 46 (40.7%) superior C418-ILs, demonstrating the power of BC breeding in exploiting favorable variation hidden in the indica gene pool for improving DT of japonica rice. Increased-height and accelerated-heading were 2 correlated changes in these ILs, suggesting the possible involvement of GA pathway and drought escaping. Considerable variations in yield components among these ILs indicate the presence of different DT mechanisms. The C418-ILs developed in this study were supposed to provide useful materials for dissection of DT and yield, and for further pyramiding breeding in rice.2. We mapped the QTLs for drought tolerance of 113 lines and analyzed the effects of DT QTLs.We found that the percent of super introgressin loci in small populations were much larger than that in large population. So we should consider the proper size of populations when we select the DT lines. Many QTLs mapped in the 113 lines were concurrence in two populations and these loci were more reliable for breeder. But, the gene functions of these loci are mostly close and we have to avoid selecting these loci to increase the improving effects when we perform pyraming breeding. We also found the DT QTLs with positive additive effect for yield and these QTLs will be used to improve DT further. At the same time, somes QTLs with positive dominant effect for yield were mapped under 4 conditions and these QTLs will be used to study dominant effect of QTLs further which are very useful for Hybrid rice breeding.3. QTLs mapping and analyzing of DT genetic network were made for pyramiding DT lines. DT QTLs of X² test and yield QTLs of One-way ANOVA were found overlapping in every population which was desired by us. The results also verified the conclusion that some DT QTLs were positive additive effect for yield and another DT QTLs were negative additive effect for yield. Genetic networks of pyramiding populations on DT indicated that AL1s were all in the chief positions in 4 populations and AL2, AL3, AL4, M1, M3, M4, M5 and M6 of population I , AL2, M1, M2 and M3 of population II, M1, M2, M3, AL2 and AL3 of population III, M1,M2,M3,M4,M5,M6,M7, M8, M9,M10,M12,M14 and M16 of population IV were in the upstream of the relationship which had the highest regulating ability to the genetic network of DT. The functions of these locus or genes are very essential in the DT genetic network. We should pay attentions to track and select these locus in our breeding practice.