Synergistic or additive interactions among the partners of the legume tripartite symbiotic association (Rhizobium–Arbuscular mycorrhizal fungi–legume) have been shown in most instances to increase legume productivity. Arbuscular mycorrhizal fungi (AMF) promote increased legume biomass production and photosynthetic rates by increasing the ratio of P to N accumulation. An increase in the P content in legume tissue due to the AMF symbiotic association has been consistently associated with an increase in N accumulation and N productivity in legumes with or without a Rhizobium association. Photosynthetic N use efficiency, irrespective of the inorganic source of N is usually enhanced by increased P supply because of the AMF association. Both light-saturated photosynthetic rates and quantum yields increase in legumes in response to increasing N supply due to the Rhizobium symbiotic association. However, the maximum levels achieved for both light-saturated photosynthesis and quantum yield as a function of N supply concentration depend on both P and CO2 supply rates. The N:P supply ratio controls the legume’s growth and photosynthetic response to elevated atmospheric CO2 concentrations. These findings indicate that the N:P:C supply ratio as influenced by the tripartite symbiotic associations plays a fundamental role in controlling the legume’s photosynthetic rate and biomass productivity.