The optimal nitrogen (N) managements and the selection of genotypes with high N use efficiency (NUE) play a vital role in rice production aiminged at high yield and high NUE. Two rice cultivars, one with high-NUE (Dexiang 4103) and the other with low-NUE (Yixiang 3724) were used in pot and field experiments in 2013 and 2014. The total N fertilizer applied was 180 kg ha−1 of urea and three treatments were included: 1) 50% basal dressing, 30% topdressing at 7 d after transplanting (DAT), and 20% topdressing at 4th leaves emerged from the top (N1), 2) 30% basal dressing, 30% topdressing at 7 DAT, 40% topdressing was split into two equal applications at 4th and 2nd leaves emerged from the top, 3) 20% basal dressing, 20% topdressing at 7 DAT, 60% topdressing was split into two equal applications at 4th and 2nd leaves emerged from the top, respectively. Double isotope tracing technique of 13C and 15N and physiological-biochemical analysis were used to study the accumulation, translocation, distribution of N and photosynthate, and the correlation between morphology and physiological-biochemical characteristics and their relationships with grain yield. There were significant effects of cultivars and N application modes on grain yield, as well as the absorption and translocation of N and photosynthate from full-heading to maturity stage. Compared with N1and N3, N2 treatment with high-NUE was the best model in this paper referred as the variety and N application coupling model, which could improve N accumulation in rice plant after anthesis, increase photosynthetic rate, activities of ribulose 1,5-bisphosphate carboxylase, and glutamine synthetase in flag leaves, promote accumulation and translocation of photosynthate and N, and then improve the yield and NUE. Double isotope labeling results showed that the accumulation amount of photosynthetic products and N in rice plant with high-NUE was 7.78–12.75 mg 13C plant-1 and 15.14–18.78 mg 15N plant-1 higher, the translocation amount of photosynthate and N in leaves with high-NUE was 1.70–2.93 mg 13C plant-1, 2.21–4.55 mg 15N plant-1 higher, the translocation amount of photosynthate and N in leaf sheaths with high-NUE was 1.70–2.93 mg 13C plant-1, 0.05–1.14 mg 15N plant-1 higher than those with low-NUE, respectively. From full-heading to maturity stage, 13C photosynthate in the spike with high-NUE and low-NUE respectively increased by 31.04–44.68 mg 13C plant-1(accounting for 42.04%–46.38% of total amount of 13C) and 24.94–34.26 mg 13C plant-1 (accounting for 36.45%–41.36% of total amount of 13C), while, 15N accumulation in the spike with high-NUE and low-NUE respectively increased by 35.56–46.58 mg 15N plant-1(accounting for 61.82%–82.93% of total amount of 15N) and 27.37–31.57 mg 15N plant-1 (accounting for 58.04%–68.31% of total amount of 15N). Compared with low-NUE, the high NUE rice cultivar is more beneficial to the accumulation and translocation of photosynthate, with higher N absorption capacity, stronger N translocation ability, and stronger C and N metabolism capacity in leaves, satisfying the N demand of plants during their grain forming stage, which is the important reason for high-NUE rice cultivar further to increase yield and NUE. From, the information of C/N ratio in different organs after-anthesis, combined with the results of high yield and high NUE in different varieties under N2 treatment, from full-heading to maturity stage, the C/N increasing range in leaf and panicle is the same as the reducing range in leaf sheaths and roots. Therefore, C/N ratio in different organs from full-heading to maturity stage might be a candidate indicator for high yield and high NUE in rice production.
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