| The mapping of major and epistatic QTLs associated with rice nitrogen grain production efficiency (NGPE) when cultured in nutrient solution and/or planted in the fields was performed, and their dynamics was analyzed in this research by using a genetically stable population of F9 recombinant inbred lines (RILs), generated by the single seed descent method from the cross between a nitrogen-efficient indica variety, Duochandao, and a nitrogen-inefficient japonica variety, TR22183. In addition, associations between traits related to nitrogen use efficiency (NUE) at different developmental stages were evaluated. The main results were as follows:1. No significant correlations exist between NUE traits at seedling and the highest tillering stages and grain yield (GY), NGPE, nitrogen dry matter production efficiency (NDMPE) at mature stage. While significant or very significant positive correlations with NDMPE were found at the highest tillering stage.2. The correlations between NUE traits at different stages of grain filling and GY, NGPE, and NDMPE were identified as follows: the GY at mature stage showed significantly positive correlation with that at 15 days after heading (DAH), though no significant correlations were found with other traits, and it showed very significantly negative correlations with nitrogen content in both straw and grain, and significantly or very significantly positive correlations with other traits 25 DAH. The NGPE present very significantly negative correlations with grain nitrogen content 15, 25 DAH, or with rice straw nitrogen content 25 DAH, and it did not show obvious correlation with rice straw nitrogen content 15 DAH though very significantly positive correlations with other traits were observed. As for the NDMPE, it showed very significantly positive correlations with GY and NDMPE, while it showed very significantly negative correlations with rice straw or grain nitrogen content 15, 25 DAH. Morever, it presented very significantly positive correlation with NGPE only after 25 days.3. The analysis on the relations between traits associated with NUE at mature stage indicated that rice GY showed very significantly positive correlations with harvest index (HI), nitrogen harvest index (NHI), NGPE, and nitrogen accumulation capacity;while it showed very significantly negative correlations with rice straw or grain nitrogen content. It was showed that the nitrogen content in rice straw had very significantly negative correlation with HI, NHI, NDMPE, and NGPE, and very significantly positive correlation with grain nitrogen content, though no significant correlation with nitrogen accumulation was found. It was also showed that the nitrogen content in rice grain had very significantly negative correlation with NHI, NDMPE, and NGPE, while had no significant correlation with HI. The results indicated that the nitrogen accumulation had very significantly positive correlation with HI, and NHI, and very significantly negative correlation with NDMPE, though it had no significant correlation with NGPE. In addition, there existed very significantly positive correlations between NGPE and HI, NHI, and NDMPE, while very significantly positive correlations were also present between NDMPE and HI, and NHI.4. The correlation analysis for each trait related to NUE at different stages of grain filling showed that very significant correlations were observed between 15 days and 25 DAH, between 15 DAH and mature stage, and between 25 DAH and mature stage, with respect to GY, rice straw nitrogen content, grain nitrogen content, NGPE, NHI, and HI.5. The major QTLs and epistatic QTL pairs for total nitrogen accumulation and/or for NDMPE, and the the QTL involving in epistasis interaction at seedling stage, the highest tillering stage, 15 DAH, 25 DAH, and mature stage were identified, and their chromosomal locations and contributions to phenotypic variation were determined. Meanwhile, the major QTLs and epistatic QTL pairs for GY, rice straw nitrogen content, grain nitrogen content, NGPE, NHI, and HI at 15 DAH, 25 DAH, and mature stage were also identified, and their corresponding chromosomal locations and contributions to phenotypic variation were determined.6. It was found that QTLs controlling different or related traits located in the same or adjacent regions on some chromosomes. Three QTLs, qGW-3, qNCOG-3 and qNHI-3, clustered at 46.8 ~ 49.9 loci on chromosome 1; Four QTLs, qGW-6, qNGPE-6, qNHI-6 and qHI-6 closely located at 7.57 ~ 9.73 loci on chromosome 3, more particularly, all these QTLs were detected 15 DAH, and agronomic traits such as GY, NGPE, NHI and HI, were all controlled by this region. While on chromosome 4, QTLs qGW-7, qNCOS-7, qNCOG-7, qNGPE-7, and qNDMPE-7, closely related to GY, rice straw nitrogen content, grain nitrogen content, NGPE, and NDMPE, distributed at 94.7 ~ 97.2 loci. QTLs controlling GY, NGPE, NHI, and HI, located at 24.07 ~ 31.92 on chromosome 5, were detected at the same developmental stage (15 DAH). Morever, at mature stage, QTLs for GY, rice straw nitrogen content, grain nitrogen content, NGPE were located at 22.4 ~ 23.6 on chromosome 8; while major QTLs for GY, grain nitrogen content, NGPE, were detected 15 DAH at 83.89 ~ 92.49 on chromosome 12.7. The expression patterns of major QTLs and epistatic interaction QTLs for nitrogen use at different developmental stages indicated that the number of major QTLs detected at different stages was much higher than that at a single stage, some QTLs identified at a single stage were also found at several stages, and most QTLs showed their effects only at few stages, and the effect values for each QTL varied at different stages. It was this time-dependent expression of QTLs that make rice nitrogen-use traits varied in a regular way. Furthermore, only one major QTL (qNCOS-4) was identified to control straw nitrogen content and show its effect at 15, 25 DAH, and at mature stage. Meanwhile, dynamic expression analysis on the epistatic interactions for nitrogen-use traits revealed that one QTL for both GY and NHI showed epistatic interaction at 15, 25 DAH, and at mature stage, while two QTLs for HI occurred epistatic interaction and other QTLs could be identified only at specific stages. Further analysis indicated that the number of QTLs for the same trait and their interaction effects varied at different stages.
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