A MAJOR LOCUS FOR SUBMERGENCE TOLERANCE MAPPED ON RICE CHROMOSOME-9

Submergence stress is a widespread problem in rice-growing environment s where drainage is impeded. A few cultivars can tolerate more than 10 days of submergence, but the genes conferring this tolerance have not been identified. We used random-amplified polymorphic DNA (RAPD) and restriction fragment length polymorphism (RFLP) markers to map submerg ence tolerance in 169 F-2 plants and the resulting F-3 families of a c ross between a tolerant indica rice line, IR40931-26: and a susceptibl e japonica line, PI543851. IR40931-26 inherited strong submergence tol erance from the unimproved cultivar FR13A. Eight-day old F-3 seedlings were submerged for 14-16 days in 55-cm deep tanks, and tolerance was scored after 7 days recovery on a scale of 1 (tolerant) to 9 (suscepti ble). The tolerant and susceptible parents scored 1.5 and 8.4, respect ively, and the F-3 means ranged from 1.6 to 8.9. Two bulks were formed with DNA from F-2 plants corresponding to the nine most tolerant and the nine most susceptible F-3 families. Of 624 RAPD primers used to sc reen the bulks, five produced bands associated with either tolerance o r susceptibility. These markers were mapped to a region of chromosome 9 by linkage to RFLP markers. A submergence tolerance quantitative tra it locus (QTL), here designated Sub1, was located ca. 4 cM from the RF LP marker C1232 and accounted for 69% of the phenotypic variance for t he trait.