Mapping and identifying quantitative trait loci associated with drought resistance in rice (Oryza sativa L.) under different water stress conditions

Four BC2F3 populations derived from crosses of two recurrent parents, IR64 and Teqing, with two donor parents, Binam and Type 3 selected for drought resistance from lowland and upland water stress environments in BC2F2 generation and two BC2F2 backcrosses derived from crosses of the same recurrent parents with one donor parent, Binam, selected for grain quality from nonstress irrigated environment in BC2F2 were used to genotype and map the quantitative trait loci associated with drought resistance in rice. Drought resistance was evaluated in the BC2F4 populations grown under the same nonstress and water stress environments given above using five characters, namely grain yield per plant, days to heading, plant height, panicle length, and panicle number as basis. Analyses of variance indicated highly significant genotypic effects and genotype by stress interaction for all traits measured. Populations derived from the recurrent parent Teqing had higher grain yields per plant under both lowland and upland stress environments as compared to the populations derived from the recurrent parent IR64, which indicated that the former had more QTLs associated with drought resistance than the latter. Populations derived from the donor parent Binam and selected from lowland or upland environment generally had higher grain yields per plant, indicating that Binam conferred more favorable alleles associated with drought resistance. Lines selected from upland stress environments, in general, had the highest grain yields per plant under both lowland and upland stress environments probably because the QTLs affecting drought resistance could be easily expressed under upland environment allowing the breeder to easily select individuals with drought resistance. Upland stress environment as environment for selection, therefore, resulted in more effective selection for drought resistance. A total of 233 QTLs affecting the traits grain yield per plant (55 QTLs), days to heading (82), and plant height (96) were identified in the six population under three environments. For 22 of those for grain yield per plant, the introgression alleles come from the donor parents, increasing grain yield per plant from 0.56% to 69.6% for an average of 25.8%; for 62 of the QTLs for days to heading, the introgression alleles decreased days to heading from 0.14% to 6.7% for an average of 2.2%; and for 45 of plant height QTLs, the introgression alleles from the donor parents decreased plant height from 0.31% to 14.5% for an average of 4.3%. These results showed that exploiting positive alleles associated with drought resistance from some varieties in advanced backcross populations would be an effective approach in breeding for drought resistance trait in rice. Some QTLs in the same population were detected in different environments. Sometimes, QTLs mapped on some chromosome regions in the same population in different stress environments varied in their actions and effects. Considerable heterosis has been observed in many cases, which can be exploited for further increasing the level of drought resistance of rice varieties under water stress environments