Anatomically induced changes in rice leaf mesophyll conductance explain the variation in photosynthetic nitrogen use efficiency under contrasting nitrogen supply.

Background The ratio of CO2 mesophyll conductance (gm) to Ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) content has been suggested to positively affect photosynthetic nitrogen use efficiency (PNUE). The anatomical basis of gm has been quantified, but information on the relationship between cell-level anatomies and PNUE is less advanced. Here, hydroponic experiments were conducted in rice plants supplied with ammonium (NH4+) and nitrate (NO3-) under three N levels (low, 0.71 mM; intermediate, 2.86 mM; high, 7.14 mM) to investigate the gas exchange parameters, leaf anatomical structure and PNUE. Results The results showed a lower PNUE in plants supplied with high nitrogen and NH4+, which was positively correlated with the gm/Rubisco ratio. A one-dimensional within-leaf model revealed that the resistance to CO2 diffusion in the liquid phase (rliq) dominated the overall mesophyll resistance (rm), in which CO2 transfer resistance in the cell wall, cytoplasm and stroma were significantly affected by nitrogen supply. The chloroplast surface area exposed to intercellular space (Sc) per Rubisco rather than the gm/Sc ratio was positively correlated with PNUE and was thus considered a key component influencing PNUE. Conclusion In conclusion, our study emphasized that Sc was the most important anatomical trait in coordinating gm and PNUE with contrasting N supply.