Breadfruit (Artocarpus altilis (Parkinson) Fosberg) is a prolific tropical tree producing highly nutritious and voluminous carbohydrate-rich fruits. Already recognized as an underutilized crop of high potential, breadfruit could ameliorate food insecurity and protect against climate-related productivity shocks in undernourished equatorial regions. However, a lack of fundamental knowledge impedes widespread agricultural adoption, from modern agroforestry to plantation schemes. Here, we used a multi-environment breadfruit variety trial across the Hawaiian Islands to determine photosynthetic limitations, understand the role of site conditions or varietal features, and define their contributions to agronomic efficiency. Photosynthetic rates were dependent on location and variety, and strongly correlated with fruit yield (r2 = 0.80, P <
0.001). Photochemistry was suitable to full-sunlight conditions, with a saturation point of 1545 photosynthetically active radiation, Vcmax of 151 μmol m-2 s-1 and Jmax of 128 μmol m-2 s-1, which are high-end compared with other tropical and temperate tree crops. However, limitations on CO2 assimilation were imposed by stomatal characteristics, including stomatal density (P <
0.05) and diurnal oscillations of stomatal conductance (>
50% reductions from daily maxima). These constraints on CO2 diffusion are likely to limit maximum productivity more than photochemistry. Our results comprise the first comprehensive analysis of breadfruit photosynthesis, successfully link ecophysiology with fruit yield, and identify vital traits for future research and management optimization.