QTLs for cell-membrane stability mapped in rice (Oryza sativa L.) under drought stress

Cell-membrane stability (CMS) is considered to be one of the major selectio n indices of drought tolerance in cereals. In order to determine which geno mic region is responsible for CMS, 104 rice (Oryza sativa L.) doubled haplo id (DH) lines derived from a cross between CT9993-5-10-1-M and IR62266-42-6 -2 were studied in the greenhouse in a slowly developed drought stress envi ronment. Drought stress was induced on 50-day-old plants by withholding wat er. The intensity of stress was assessed daily by visual scoring of leaf wi lting and by measuring leaf relative water content (RWC). The leaf samples were collected from both control (well-watered) and stressed plants (at 60- 65% of RWC), and the standard test for CMS was carried out in the laborator y. There was no significant difference (P>0.05) in RWC between the two pare ntal lines as well as among the 104 lines, indicating that all the plants w ere sampled at a uniform stress level. However, a significant difference (P <0.05) in CMS was observed between the two parental lines and among the pop ulation. No significant correlation was found between CMS and RWC, indicati ng that the variation in CMS was genotypic in nature. The continuous distri bution of CMS and its broad-sense heritability (34%) indicates that CMS sho uld be polygenic in nature. A linkage map of this population comprising of 145 RFLPs, 153 AFLPs and 17 microsatellite markers was used for QTL analysi s. Composite interval mapping identified nine putative QTLs for CMS located on chromosomes 1, 3, 7, 8, 9, 11 and 12. The amount of phenotypic variatio n that was explained by individual QTLs ranged from 13.4% to 42.1%. Four si gnificant (P<0.05) pairs of digenic interactions between the detected QTLs for CMS were observed. The identification of QTLs for this important trait will be useful in breeding for the improvement of drought tolerance in rice . This is the first report of mapping QTLs associated with CMS under a natu ral water stress condition in any crop plants.