Nitrogen-driven changes in leaf anatomy affect the coupling between leaf mass per area and photosynthesis

Leaf mass per area (LMA) and photosynthetic rate (A) explain fast-slow growth strategies in the worldwide leaf economic spectrum. Nitrogen (N) promotes A and rapid growth, while LMA responds to N supply in a genotype specific manner. Structural traits affect the relationship between LMA and A, and we hypothesized that N supply would affect structural traits and thus the coupling between LMA and A. We tested this hypothesis by measuring A, LMA, anatomical traits, and N allocation to various leaf components in nine Brassica napus cultivars with two N supply levels. Mesophyll cell density (ρcell) and palisade tissue thickness (Tp) predominantly influence the variability in LMA. Enhanced Tp increased LMA, chloroplast surface area exposed to intercellular airspace, and N allocation into Rubisco (Nrub), thereby positively affecting mesophyll conductance (gm) and A. Conversely, ρcell promoted LMA but negatively affected Nrub under N deficiency. Enhanced LMA promoted N allocation to cell wall (Ncw), causing decreased Nrub fraction and consequently A. This negative effect was relieved by the positive effect of Tp on A, which coupled the variation of LMA and A. The plasticity of Tp and ρcell regulating gm and the trade-off between Nrub and Ncw provides insights for simultaneous increases in LMA and A to promote rapid growth and resistance.