Plants primarily incorporate nitrate (NO₃−) and ammonium (NH₄+) as the primary source of inorganic nitrogen (N); the physiological mechanisms of photosynthesis (A) dropdown under NH₄+ nutrition has been investigated in many studies. Leaf anatomy is a major determinant to mesophyll conductance (g_m) and photosynthesis; however, it remains unclear whether the photosynthesis variations of plants exposed to different N forms is related to leaf anatomical variation. In this work, a common shrub, Lonicera japonica was hydroponically grown under NH₄+, NO₃− and 50% NH₄+/NO₃−. We found that leaf N significantly accumulated under NH₄+, whereas the photosynthesis was significantly decreased, which was mainly caused by a reduced g_m. The reduced g_m under NH₄+ was related to the decreased intercellular air space, the reduced chloroplast number and especially the thicker cell walls. Among the cell wall components, lignin and hemicellulose contents under NH₄+ nutrition were significantly higher than those in the other two N forms and were scaled negatively correlated with g_m; while pectin content was independent from N forms. Pathway analysis further revealed that the cell wall components might indirectly regulate g_m by influencing the thickness of the cell wall. These results highlight the importance of leaf anatomical variation characterized by modifications of chloroplasts number and cell wall thickness and compositions, in the regulation of photosynthesis in response to varied N sources.

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