Low temperature or cold stress is one of the major abiotic stresses limiting rice (Oryza sativa L.) production and productivity in the temperate rice growing regions as well as in tropical high lands worldwide. Low temperature at the reproductive stage causes high sterility and decreases production. In this study, we assessed recombinant inbred lines (RILs) that possessed cold‐tolerance genes and/or quantitative trait loci (QTL) from the donor line IR66160‐121‐4‐4‐2 in the genetic background of a cold‐sensitive japonica cultivar, Geumobyeo. The selected 15 RILs with QTL for cold tolerance were phenotyped for three main agronomic traits—culm length (CL), days to heading (DTH), and spikelet fertility (SF)—which were most affected during cold stress. The RILs with cold‐tolerant and cold‐sensitive parents were evaluated under cold‐water (18–19°C) irrigation in the field and cold‐air temperature (17–18°C) in the temperature‐controlled greenhouse. The RILs showed significant differences in these traits compared to the cold‐sensitive parent. Traits CL and DTH exhibited positive correlation with SF in the selected breeding lines. The SF of the selected breeding lines was higher (51–81%) than that of the cold‐sensitive parent, Geumobyeo (7%). Our results confirmed that cold tolerance was associated with SF but the traits CL and DTH were differently associated with cold tolerance. The cold‐tolerant breeding lines selected in this study had at least one of the three QTL associated with cold tolerance. The breeding lines confirmed to have cold tolerance are useful to breed cold‐tolerant cultivars and increase our understanding of the mechanism of cold tolerance in rice.

Cold tolerance at the seedling stage of rice is an important phenotypic trait that causes normal plant growth and stable rice production in temperate regions as well as tropical high-lands in Asia and Africa. In order to find quantitative trait loci (QTLs)/ genes associated with cold tolerance, we constructed a linkage map using 153 recombinant inbred lines (RILs) derived from a cross between a cold-tolerant temperate japonica cultivar, Geumobyeo, and a coldsensitive tropical japonica breeding line, IR66160-121-4-4-2. The RILs were phenotyped for cold tolerance or sensitivity based on the degrees of cold tolerance as cold tolerance indices at the seedling stage. The seedlings for cold-tolerance/-sensitive traits were scored on the 7th day of the recovery period at 25°C after cold treatment at 10°C. Two QTLs (qCTS4a and qCTS4b) associated with cold tolerance at the seedling stage were identified on the long and short arms of chromosome 4 with an LOD score of 2.89 and 2.75, respectively, using composite interval mapping. The QTLs were flanked by simple sequence repeat (SSR) markers RM3648-RM2799 and RM3375a-RM558 that explained 8.3 and 7.8% of the total phenotypic variation, respectively. Seven of the selected RILs expressed cold tolerance at both the seedling and reproductive stages. The SSR markers associated with the QTLs will be useful for tracking favorable QTLs/genes into cold-sensitive elite cultivars and may have potential for pyramiding different QTLs for the improvement of cold tolerance in rice.