Physiological changes underlying increased photosynthetic-nitrogen use efficiency in response to low-nitrogen conditions in Brassica napus L

Abstract

The photosynthetic-nitrogen use efficiency (PNUE) of Brassica napus 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 Brassica napus 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 CO2 transport and significantly increased stomatal conductance (gs), intercellular carbon dioxide concentration (Ci), mesophyll conductance (gm), and CO2 concentration in chloroplasts (Cc). 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 B. napus under low-nitrogen condition.