Effects of dynamic changes in leaf functional nitrogen on photosynthetic rate and photosynthetic nitrogen use efficiency in Brassica napus

Rapeseed (<italic>Brassica napus</italic> L.) is China’s leading oilseed crop, and high yield depends heavily on nitrogen (N) fertilizer inputs; however, nitrogen use efficiency remains relatively low. Rapeseed biomass is primarily derived from photosynthesis. Enhancing the net photosynthetic rate (<italic>P</italic>_n) and photosynthetic nitrogen use efficiency (PNUE) may provide a feasible approach to simultaneously improve yield and nitrogen use efficiency, thereby reducing N fertilizer application, increasing rapeseed production, and supporting sustainable development of the rapeseed industry. Here, we conducted a hydroponic experiment with different N levels using two rapeseed germplasms that differed significantly in leaf area and <italic>P</italic>_n. During leaf development, we quantified dynamic changes in leaf N content, photosynthetic capacity, functional N fractions, and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) characteristics to elucidate how N allocation regulates <italic>P</italic>_n and PNUE. Leaf N content declined with growth, but <italic>P</italic>_n did not decrease consistently, and <italic>P</italic>_n increased with higher N supply. Photosynthetic N (<italic>N</italic>_psn) and storage N (<italic>N</italic>_store) were the dominant components of leaf N, accounting for 83.12%-97.61%. During leaf expansion, <italic>N</italic>_psn changed little whereas <italic>N</italic>_store declined markedly, which helped maintain <italic>P_n</italic> at lower total leaf N and improved PNUE. When leaf N content fell below 1.77 g m^-2, both <italic>N</italic>_psn and<italic> N</italic>_store decreased linearly, leading to significant reductions in <italic>P</italic>_nand PNUE. During leaf senescence, the decline in <italic>N</italic>_psn was 6.24%-19.07% greater than that of <italic>N</italic>_store, resulting in pronounced decreases in both <italic>P</italic>_n and PNUE. Rubisco content was significantly and positively correlated with <italic>N</italic>_psn. The germplasm Yunyou 9, which had higher <italic>P</italic>_n, showed higher <italic>N</italic>_psn and Rubisco content than Fuyou 3, which had larger leaf area, with increases of 5.63%-40.37% and 0.38%-38.02%, respectively. Both <italic>N</italic>_psn and <italic>N</italic>_store increased with increasing N supply, and <italic>P</italic>_n increased with <italic>N</italic>_psn. Overall, during leaf development, Rubisco helped maintain <italic>N</italic>_psn while <italic>N</italic>_store declined, supporting higher <italic>P</italic>_n and improved PNUE. Therefore, optimizing within-leaf N allocation by precisely regulating Rubisco content may be an effective strategy to further enhance rapeseed photosynthetic capacity and nitrogen use efficiency.