Quantitative trait loci for sink size and ripening traits in rice (Oryza sativa L.)

Quantitative trait loci (QTLs) that affect sink size and ripening, which are often negatively correlated, were analyzed in two inbred lines of rice (Oryza sativa L.) derived from crosses between the semi-dwarf indica and japonica cultivars. Recombinant inbred lines (RILs) from Milyang23 (indica)/Akihikari (japonica), and back-crossed inbred lines (BCILs) from Sasanishiki (japonica)/Habataki(indica)//Sasanishiki///Sasanishiki were studied over a period of two years. A major QTL that was related to the number of spikelets per panicle was found in the same region of chromosome 1 in both populations. The indica allele increased the number of spikelets and reduced the ripening percentage. The indica allele of the QTL, which was found in both populations but at different locations on chromosome 6, also increased the number of spikelets per panicle, although to a lesser extent than in the case of the QTL on chromosome 1. In this instance, the number of spikelets per panicle did not have a negative effect on the ripening percentage. The increased number of spikelets produced by the QTL on chromosome 6 was due mainly to an increase in the number of primary rachis branches. In contrast, the effect of the QTL on chromosome 1 relied on an increase in the number of secondary rachis branches. In addition, dry matter production tended to increase when the QTL region of chromosome 6 belonged to the indica genotype. Therefore, both panicle structure and source productivity might contribute to the increase of the sink size without reducing the ripening percentage. We also found two loci on chromosomes 11 and 12 of the RILs that were associated with the percentage of incompletely filled spikelets, but which did not affect the sink size. It is considered that these QTLs could contribute significantly to the breeding of cultivars with increased sink size and increased ripening.