The photosynthetic-nitrogen use efficiency (PNUE) of L. is reported to increase under low nitrogen (N) condition. However, the underlying physiological mechanisms are unclear. In this study, the physiological mechanisms underlying increase in the photosynthetic-nitrogen use efficiency of L. under low-nitrogen condition were investigated by assessing the changes in plant architecture, light reception, nitrogen allocation, and leaf tissue structure. The plants exhibited dwarf, upright, and compact phenotype under low-nitrogen condition. Although the total photons received by plants decreased, the average photosynthetic photon flux density remained unchanged. The nitrogen photon reception efficiency (NPRE, calculated as total photons/N accumulation in leaves) was significantly increased by 76.61%–100.63%. The proportion of nitrogen allocated for photosynthesis was increased by 22.06%–38.86%. Moreover, although the leaf thickness remained unchanged, the epidermal thickness increased, and the spongy tissues became thinner. The density of mesophyll cells and chloroplasts significantly increased. Low-nitrogen condition significantly decreased the resistance to CO transport and significantly increased stomatal conductance (g), intercellular carbon dioxide concentration (C), mesophyll conductance (g), and CO concentration in chloroplasts (C). Correlation analysis revealed that light reception, nitrogen allocation in the leaves, and leaf tissue structure were significantly correlated with PNUE. Random forest analysis revealed that nitrogen photon reception efficiency and storage nitrogen were the primary factors positively and negatively impacting photosynthetic-nitrogen use efficiency, respectively. This study enhanced the understanding of the physiological mechanism of increased PNUE of under low-nitrogen condition.

中文翻译：

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甘蓝型油菜响应低氮条件导致光合氮利用效率增加的生理变化

据报道，在低氮 (N) 条件下，L. 的光合氮利用效率 (PNUE) 会增加。然而，潜在的生理机制尚不清楚。本研究通过评估植物结构、光接收、氮分配和叶片组织结构的变化，研究了低氮条件下L. 光合氮利用效率提高的生理机制。低氮条件下植株表现出矮化、直立、紧凑的表型。尽管植物接收的光子总量减少，但平均光合光子通量密度保持不变。氮光子接收效率（NPRE，以总光子数/叶片中氮积累量计算）显着提高了76.61%–100.63%。分配给光合作用的氮比例增加了22.06%~38.86%。而且，虽然叶片厚度没有变化，但表皮厚度增加，海绵组织变薄。叶肉细胞和叶绿体的密度显着增加。低氮条件显着降低了CO运输阻力，显着增加了气孔导度（g）、细胞间二氧化碳浓度（C）、叶肉导度（g）和叶绿体中CO浓度（C）。相关分析表明，光接收、叶片氮分配、叶片组织结构与PNUE显着相关。随机森林分析表明，氮光子接收效率和储存氮分别是对光合氮利用效率产生积极和消极影响的主要因素。本研究加深了对低氮条件下PNUE增加的生理机制的理解。