Length of grain affects the appearance, quality, and yield of rice.  A rice long-grain chromosome segment substitution line Z744, with Nipponbare as the recipient parent and Xihui 18 as the donor parent, was identified.  Z744 contains a total of six substitution segments distributed on chromosomes (Chrs.) 1, 2, 6, 7, and 12, with an average substitution length of 2.72 Mb.  The grain length, ratio of length to width, and 1 000-grain weight of Z744 were significantly higher than those in Nipponbare.  The plant height, panicle number, and seed-set ratio in Z744 were significantly lower than those in Nipponbare, but they were still 78.7 cm, 13.5 per plant, and 86.49%, respectively.  Furthermore, eight QTLs of different traits were identified in the secondary F₂ population, constructed by Nipponbare and Z744 hybridization.  The grain weight of Z744 was controlled by two synergistic QTLs (qGWT1 and qGWT7) and two subtractive QTLs (qGWT2 and qGWT6), respectively.  The increase in the grain weight of Z744 was caused mainly by the increase in grain length.  Two QTLs were detected, qGL1 and qGL7-3, which accounted for 25.54 and 15.58% of phenotypic variation, respectively.  A Chi-square test showed that the long-grain number and the short-grain number were in accordance with the 3:1 separation ratio, which indicates that the long grain is dominant over the short-grain and Z744 was controlled mainly by the principal effect qGL1.  These results offered a good basis for further fine mapping of qGL1 and further dissection of other QTLs into single-segment substitution lines.

An elite backcrossed inbred line Z550 with increased grains per panicle was identified from advanced backcrosses between Nipponbare and Xihui 18 by simple sequence repeat (SSR) marker-assisted selection (MAS).  Z550 carries 13 substitution segments distributed on chromosomes 1, 6, 7, 8, 9, 10, and 12, with an average substitution length of 1.68 Mb.  Compared with the Nipponbare parental line, plant height, panicle length, spikelets per panicle, grains per panicle, and grain weight for Z550 were significantly increased.  While the grain width of Z550 was significantly narrower, and the seed setting ratio (81.43%) was significantly lower than that of Nipponbare, it is still sufficient for breeding purposes.  Quantitative trait loci (QTLs) mapping for important agronomic traits was conducted with the F₂ population derived from Nipponbare crossed with Z550 using the restricted maximum likelihood (REML) method.  A total of 16, including 12 previously unreported QTLs were detected, with contribution rates ranging from 1.46 to 10.49%.  Grains per panicle was controlled by 8 QTLs, 5 of which increased number of grains whereas 3 decreased it.  qGPP-1, with the largest contribution (10.49%), was estimated to increase grains per panicle by 30.67, while qGPP-9, with the minimum contribution rate (2.47%), had an effect of increasing grains per panicle by 15.79.  These results will be useful for further development of single segment substitution lines with major QTLs, and for research of their molecular functions via QTL cloning.