Relationship between photosynthetic nitrogen use efficiency and nitrogen allocation in photosynthetic apparatus of winter oilseed rape under different nitrogen levels

【Objectives】Nitrogen is one of the most important factors limiting crop photosynthesis. Besides nitrogen(N)concentration, nitrogen allocation in the photosynthetic apparatus is also an important factor influencing crop photosynthesis. The aim of this study was to explore effects of nitrogen allocation on photosynthetic apparatus at the seedling stage of winter oilseed rape under different N levels, to analyze relationship with photosynthetic nitrogen use efficiency, and to reveal mechanism of photosynthetic nitrogen use efficiency affected by nitrogen. 【Methods】An on-farm experiment was carried out to investigate the effects of different N levels(0, 180 and 360 kg/hm2; marked as N0, N180 and N360, respectively). Some physiological and photosynthetic parameters(e.g., net photosynthetic rate(Pn), nitrogen content, photosynthetic nitrogen use efficiency(PNUE), the maximum rate of carboxylation(Vc max)and the maximum rate of electron transport(Jmax))of the first fully expanded leaf were measured to calculate nitrogen allocation in the photosynthetic apparatus(carboxylation, bioenergetics and light-harvesting components). 【Results】The results show that the N fertilization could significantly enhance crop growth. The leaf number, leaf area and dry matter of N treatment are significantly higher than those of the N0 treatment. Under different N levels, the leaf nitrogen contents and net photosynthetic rates of the N180 and N360 treatments are increased by 155.0%, 157.3% and 57.6%, 56.1%, respectively, compared to those of the N0 treatment. However, photosynthetic nitrogen use efficiencies are significantly decreased, which are 35.6% and 39.6% lower than that of the N0 treatment. The photosynthetic capacity accompanied by the N-fertilizer application is increased, and the maximum net photosynthetic rates(Pn max), carboxylation efficiencies(CE), maximum rates of carboxylation(Vc max)and the maximum electron transport rates(Jmax)of the N180 and N360 treatments are significantly higher than those of the N0 treatment. The application of N fertilizer also has the effects on nitrogen allocation in the photosynthetic apparatus. The fractions of photosynthetic apparatus in the N180 and N360 treatments are decreased by 29.3% and 34.5%, respectively, and the fractions of the carboxylation(PC), bioenergetics(PB)and light harvesting system(PL)are reduced by 24.1%, 23.3%, 34.6% and 31%, 26.7%, 38.5%, respectively. The correlation analysis indicates that the PNUE has very significant positive correlation with partitioning coefficient for the leaf nitrogen in carboxylation and bioenergetics, photosynthetic apparatus, however, it is negatively correlated with the non-photosynthetic components. 【Conclusions】With the increase of N application rate, the photosynthetic nitrogen use efficiency at the seedling stage of winter oilseed rape is decreased which is affected by the nitrogen allocation to photosynthetic apparatus(e.g., carboxylation, bioenergetics and light-harvesting components). Under the appropriate N application rate at the seedling stage, synchronizing the nitrogen allocation of photosynthetic apparatus will be helpful to improve the crop photosynthetic nitrogen use efficiency.