MAPPING QUANTITATIVE TRAIT LOCI FOR SEEDLING VIGOR IN RICE USING RFLPS

Improving seedling vigor is an important objective of modern rice (Ory za sativa L.) breeding programs. The purpose of this study was to iden tify and map quantitative trait loci (QTL) underlying seedling vigor-r elated traits using restriction fragment length polymorphisms (RFLPs). An F-2 population of 204 plants was developed from a cross between a low-vigor japonica cultivar 'Labelle' (LBL) and a high-vigor indica cu ltivar 'Black Gora' (BG). A linkage map was constructed of 117 markers spanning 1496 Haldane cM and encompassing the 12 rice chromosomes wit h an average marker spacing of 14 cM. The length of the shoots, roots, coleoptile and mesocotyl were measured on F-3 families in slantboard tests conducted at two temperatures (18 degrees and 25 degrees C). By means of interval analysis, 13 QTLs, each accounting for 7% to 38% of the phenotypic variance, were identified and mapped in the two tempera ture regimes at a log-likelihood (LOD) threshold of 2.5. Four QTLs con trolled shoot length, 2 each controlled root and coleoptile lengths an d 5 influenced mesocotyl length. Single-point analysis confirmed the p resence of these QTLs and detected additional loci for shoot, root and coleoptile lengths, these latter usually accounting for less than 5% of the phenotypic variation. Only 3 QTLs detected both by interval and single-point analyses were expressed under both temperature regimes. Additive, dominant and overdominant modes of gene action were observed . Contrary to what was predicted from parental phenotype, the low-vigo r LBL contributed 46% of the positive alleles fdr shoot, root and cole optile lengths. Positive alleles from the high-vigor parent BG were id entified for increased root, coleoptile and mesocotyl lengths. However , BG contributed alleles with only minor effects for shoot length, the most important determinant of seedling vigor in water-seeded rice, su ggesting that it would not be an ideal donor parent for introducing fa ster shoot growth alleles into temperate japonica cultivars.