Assessing the cell wall nitrogen use efficiency - Can the differences between cell wall architectures contribute to...

C4 plants exhibit greater nitrogen use efficiency (NUE) than C3 plants, primarily due to lower ribulose-1,5-bisphosphate carboxylase/oxygenase requirements. However, plant NUE also varies among C4 species, suggesting that other factors, such as cell wall composition, contribute to nitrogen economy. To investigate the contribution of cell wall architecture to nitrogen economy, we compared plant species with distinct cell wall types: eudicots with type I cell walls (T1CW) and grasses with type II cell walls (T2CW), under four nitrogen regimes: deficit, low, medium, and normal. Species with different photosynthetic metabolisms were compared to confirm known differences in photosynthetic nitrogen use efficiency (pNUE), while species with similar metabolism but distinct cell wall types were compared assessing the influence of cell wall on nitrogen economy. The pNUE of C4 grasses was higher than that of C4 eudicots, increasing from +54 % in normal nitrogen to +81 % in nitrogen deficit. C4 grasses presented lower structural nitrogen (-26 %) in normal nitrogen supply, which decreased to -58 % in nitrogen deficit, in comparison to C4 eudicots. An exploratory parameter cwpNUE (photosynthetic rate/structural nitrogen) resulted in a much higher value in C4 grasses (∼1.1 μmol CO₂ s^-1 mmol^-1 cell wall nitrogen) than in the other groups (∼0.5 μmol CO₂ s^-1 mmol^-1 cell wall nitrogen). In turn, the ester-linked ferulic acid increased from +177 % in C4 grasses in normal nitrogen to +362 % in nitrogen deficit, when compared to C4 eudicots. Data supports the hypothesis that in hot, humid tropical environments, nitrogen became a major limiting nutrient for C4 plant growth and development. The overlap between extensins and FA-GAX in crosslinking CW polymers observed in T2CW suggest that nitrogen scarcity may have exerted a selection pressure for adaptations in C4 grasses to contribute to NUE.