The ecophysiological linkage of leaf phosphorus (P) to photosynthetic capacity (A max) and to the A max-nitrogen relation remains poorly understood. To address this issue we compiled published and unpublished field data for mass-based A max, nitrogen (N) and P (n = 517 observations) from 314 species at 42 sites in 14 countries. Data were from four biomes: arctic, cold temperate, subtropical (including Mediterranean), and tropical. We asked whether plants with low P levels have low A max, a shallower slope of the A max-N relationship, and whether these patterns have a geographic signature. On average, leaf P was substantially lower in the two warmer than in the two colder biomes, with the reverse true for N:P ratios. The evidence indicates that the response of A max to leaf N is constrained by low leaf P. Using a full factorial model for all data, A max was related to leaf N, but not to leaf P on its own, with a significant leaf N × leaf P interaction indicating that the response of A max to N increased with increasing leaf P. This was also found in analyses using one value per species per site, or by comparing only angiosperms or only woody plants. Additionally, the slope of the A max-N relationship was higher in the colder arctic and temperate than warmer tropical and subtropical biomes. Sorting data into low, medium, and high leaf P groupings also showed that the A max-N slope increases with leaf P. These analyses support claims that in P-limited ecosystems the A max-N relationship may be constrained by low P, and are consistent with laboratory studies that show P-deficient plants have limited ribulose-1,5-bisphosphate regeneration, a likely mechanism for the P influence upon the A max-N relation.