1. Net photosynthetic capacity (Amax, defined as light-saturated net photosynthesis under near optimal ambient environmental conditions) of mature leaves often depends on the level of leaf nitrogen (N), but an assortment of relationships between these variables has been observed in studies of diverse plant species. Variation in leaf structure has been identified as an important factor associated with differences between the area- and mass-based expressions of the Amax–N relationship. In this paper we test the hypothesis that leaf structure, quantified using a measure of leaf area displayed per unit dry mass invested [specific leaf area (SLA)], is more than just a conversion factor, but itself can influence Amax–N relationships. We test this using several kinds of comparisons, based on field data for 107 species from sites representing six biomes and on literature data for 162 species from an equally diverse set of biomes.
2. Species and genera with thicker and/or denser leaves (lower SLA) consistently have flatter slopes of the Amax–N (mass-based) relationship than those with higher SLA. These and all other contrasts usually applied as well using area-based expressions, although such relationships were less consistent and weaker overall. A steeper slope indicates greater incremental change in Amax per unit variation in N.
3. Functional groups (e.g. needle-leafed evergreen trees, broad-leafed trees or shrubs, forbs) show the same patterns: groups with lower SLA have lower Amax–N slopes. Functional groups differ in mean leaf traits as well as in Amax–N relationships. Forbs have the highest SLA and mass-based N and Amax, followed by deciduous species (whether needle-leafed or broad-leafed, shrub or tree), with lowest values in evergreen species (again regardless of leaf type or functional group).
4. Interspecific variation in mass-based Amax is highly significantly related to the combination of leaf N and SLA (r2 = 0·86). At any value of leaf N, Amax increases with increasing SLA and at any value of SLA, Amax increases with increasing leaf N. Because this relationship, between Amax and the combination of N and SLA, is similar in two independent data sets, and as well, across broad taxonomic and geographic gradients, we hypothesize that it is universal in nature. Therefore, for broad interspecific contrasts among dicotyledons in any biome, we can reasonably well predict Amax based on the combination of SLA and leaf N. These findings have important implications for convergent evolution of leaf adaptation and great potential utility in models of global vegetation functioning.