Abstract: 【Objective】 This study was carried out to investigate the effects of nitrogen allocation on the photosynthetic apparatus of leaves at different positions in winter oilseed rape under suitable nitrogen level at seedling stage and to analyze the limiting factors that affect the photosynthetic nitrogen use efficiency by nitrogen nutrition, and provide a theoretical basis for the reasonable application of nitrogen fertilizer.【Method】A field experiment was carried out with designed four nitrogen levels (0, 45, 180, 360 kg·hm^-2, marked as N₀, N₄₅, N₁₈₀ and N₃₆₀, respectively). The dry matter at seedling stage and seed yield were determined. Then the N₀ and N₁₈₀ treatments were selected, the plant leaves were equally divided into 3 groups (upper, middle and lower) by leaf number from top to bottom. Some physiology and photosynthetic parameters (e.g., net photosynthetic rate (Pn _max), nitrogen content (N_A), chlorophyll content (Cc) and soluble protein nitrogen content (N_S)) tested on leaves at different positions were measured to calculate nitrogen allocation in the photosynthetic apparatus (carboxylation, bioenergetics and light-harvesting components). 【Result】 Seed yield was increased significantly by nitrogen fertilization, compared with N₀ treatment, N₄₅, N₁₈₀ and N₃₆₀ were increased by 170%, 505.6% and 604.1%, respectively and the dry matter accumulation was consistent with the yield. Compared with N₀ treatment, N_A, Cc and Pn _max were significantly increased in different leaf positions by nitrogen fertilization, but photosynthetic nitrogen use efficiency (PNUE) has significantly declined in the upper and middle leaves. For nitrogen allocation on the photosynthetic apparatus, the proportion of the carboxylation (P_C), bioenergetics (P_B) and light harvesting system (P_L) in the upper and middle leaves of N₁₈₀ treatment were lower than that of N₀ treatment, but the nitrogen contents of those components were higher than that of N₀ treatment and the increasing extent of the components were 20.6%, 11.8% and 28.8%, respectively. The ratio between soluble protein nitrogen content (N_S) and non-soluble protein nitrogen content (N_non-S) was not significantly influenced by whether nitrogen fertilization or not in the same position leaves, but the photosynthetic component partitioning in N_S or N_non-S was significantly influenced, which the ratio of nitrogen content of the carboxylation (N_C) to N_S was an average of 83.4% in all leaves in N₀ treatment, while that of the N₁₈₀ treatment was only 60.3%. Based on boundary line approach to quantify the influence of each component of the photosynthetic apparatus for PNUE, the results indicated that the influence degrees of P_C and P_B were 26.8% and 42.6%, significantly higher than that of P_L. The influence of nitrogen nutrition on PNUE was dominated by P_C and P_B, which the average proportion reached 77.8%. P_C was the main limiting factor of PNUE in upper leaves, which the influence degree reached 83.3%, while dominated by P_B and P_L in the lower leaves.【Conclusion】 Nitrogen fertilizer had a significant effect on increasing yield of winter rape and the optimum nitrogen application rate was 180 kg·hm^-2. More nitrogen was allocated to photosynthetic apparatus by plant, and the photosynthetic nitrogen can be degraded earlier in the lower leaves under nitrogen deficiency. Suitable nitrogen level can maintain the distribution of the photosynthetic proteins within the types of their proteins. The effect of nitrogen nutrition on photosynthetic nitrogen use efficiency is dominated by allocation of nitrogen to the carboxylation and bioenergetics. The dominant effect by the distribution of the carboxylation was transformed to the dominant effect by the distribution of the light harvesting system and bioenergetics along with the decline of leaf position.

Key words: winter oilseed rape, nitrogen, leaf position, nitrogen allocation, photosynthetic nitrogen use efficiency