A large majority of angiosperms form mutualistic associations with arbuscular mycorrhizal (AM) fungi, which provide plants with nutrient uptake services in return for sugars from photosynthesis. However, evolutionary theory suggests that there should be strong selection for each partner to exploit the other. One way that a mutualism could be reinforced is if enhanced nutrient uptake stimulates plant photosynthesis in excess of the carbon demands of the fungus. Alternatively, plants with high photosynthetic capacity may be more likely to derive benefits from the interaction through luxury carbon donation to the fungus. I will use published information and phylogenetic comparative methods to test the hypothesis that the evolution of a stronger mutualism between plants and fungi is associated with the magnitude of photosynthetic stimulation and/or increased photosynthetic capacity.