| Seedling vigor is one of the major determinants for stable stand establishment in rice (Oryza sativa L.), especially in direct seeding cropping system. In the current study, mapping of QTL for seedling vigor related traits was conducted to dissect the genetic basis of seedling vigor in rice using recombinant inbred lines derived from a cross between Lemont (japonica) and Teqing (indica).1, The recombinant inbred lines were assessed for four seedling vigor traits by paper-roll tests at three temperatures of 25℃, 20℃ and 15℃, respectively. Using a linkage map with 198 marker loci, the main-effect QTL for the traits were mapped by composite interval mapping. A total of 34 QTL for the four seedling vigor traits were identified. Of these QTL, the majority (82%) was clustered within five genomic regions, designated as QTL qSV-3-1, qSV-3-2, qSV-5 qSV-8-1, and qSV-8-2, respectively. All of these five QTL had small individual effects on the traits, explaining 3.1 to 15.8% of the phenotypic variation with a mean of 7.3%. QTL qSV-3-1, qSV-3-2 and qSV-8-1 showed almost consistent effects on the traits across all the three temperatures while QTL qSV-5 and qSV-8-2 produced effects mainly at the normal temperatures of 20℃ and 25℃. Thus significant genotype x environmental temperature interactions appeared to be QTL-specific.2, The paper plate test was newly develop and was succefully used in evaluating the field seedling establishment characteristics in rice. This newly developed method has proved to be effective in controlling environmental error and useful in testing field seedling establishment characteristics in many plant crops like rice.3, All the RILs and their parents were evaluated for the field seedling establishment characteristics in both upland and flooding field conditions by paper plate tests.The parent Teqing was superior to the parent Lemont in all the traits investigated. Continuous variation and transgressive segregation in both directions were observed in the RIL population. In the upland treatment, 9 main effect QTL for 3 seedling establishment related traits were detected, each explaining 5.7~14.0%of the trait variation and the positive alleles all contributed by the parent Teqing. In the floodingtreatment, 8 main effect QTL for 4 seedling establishment related traits were mapped, also each with small effect on the trait. On the basis of co-location analysis of all the QTL detected, it was found that two QTL, qSE-1-1 and qSE-3-2, showed additive effects on both coleoptile and seedling emergence only under the flooding field condition, while the other two QTL, qSE-5-1 and qSE-JO, produced additive effects on the multiple seedling establishment related traits under both the upland and flooding field conditions.Comparison of QTL for lab germinability and field establishment characteristics revealed that: a) there were four chromosomal regions, two on chromosome 3 and other two on chromosome 5, which showed additive effects on both lab germinability and field establishment characteristics, b) there were two chromosomal regions on chromosome 8 displaying effects only on lab germinability while the other two, each on chromosomes 1 and 10, produced effects only on field establishment characteristics.4^ Cold tolerance at early seedling stage of rice is one of the major determinants for the stable stand establishment in temperate and high-elevation areas. In the current study, the recombinant inbred lines were evaluated for the cold tolerance at early seedling stage by the paper-roll tests in a two-replication trial with 10 d and 13 d treatments at 10°C, respectively. Composite interval mapping was conducted to locate both main-effect and digenic epistatic QTL for the trait. Three main-effect QTL were identified. The comparison of the QTL identified in these two cold treatments resulted in an intriguing finding that seedling cold tolerance in the 10 d cold treatment was regulated by many loci each with minor effect while that in the 13 d cold treatment was controlled by a major QTL as well as minor ones. Of the three main-effect QTL, QTL qSCT-U, closely linked to microsatellite marker RM202 on chromosome 11, was found to increase its additive effect from 4.07% to 10.11% (seedling survival percentage) as the duration of cold stress was prolonged from 10 d to 13 d. In the 13 d cold treatment, QTL qSCT-11 was detected at a very high LOD score of 19, explaining up to 30% of the phenotypic variation. The identification of QTL for the trait is important for the development of rice cultivars with a broader climatic adaptation.
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