Mapping QTLs for cold tolerance at germination and the early seedling stage in rice (Oryza sativa L.)

Cold tolerance is an important breeding target in rice production. We studied quantitative trait loci (QTLs) controlling cold tolerance at germination (CTG) and early seedling (CTS) stages, using recombinant inbred lines derived from a japonica x indica cross. CTG was evaluated based on the percentage rate of germination at 15 degrees C for 12 days after pre-incubation of imbibed seeds at 20 degrees C for 2 days. For CTS, seven-day-old seedlings grown at 25 degrees C were subjected to two consecutive periods of three-day cold stress at 4 degrees C with an intervening eight-day recovery at 25 degrees C. CTS evaluation was according to an arbitrary five-point rating system at the fifth day of recovery after each stress period. No correlations were found between CTG and CTS, while a weak correlation was detected between CTS after the first and second stress. By the composite interval mapping, five QTLs for CTG explaining 5.7%-9.3% of the total phenotypic variance (PVE) and nine for CTS with PVE of 5.8%-35.6% were detected. Only one of these QTLs was common, indicating growth-stage specificity of cold tolerance. Four of the five QTLs after the second cold stress were different from the ones after the first cold stress. Analysis of variance test showed significant interactions between alleles at the QTL sites and the two stress conditions with respect to the mean CTS scores. A possible involvement of cold acclimation and usefulness of japonica germplasms in breeding for cold tolerance in indica rice was discussed.