The photosynthetic response of leaves to photosynthetic photon flux density (PPFD) may be described by parameters for the rate of dark respiration (R), the initial slope (Ф), the PPFD-saturated rate of net photosynthesis (AmQ) and the apparent convexity (θa). We tested the hypothesis that non-uniformity in stomatal aperture across a leaf results in a clearly lower θa because PPFD saturation will occur at different irradiances in different regions of the leaf. A computer model was constructed to simulate the effects of bell-shaped and other distributions of stomatal conductance for CO2 (gs) across a model leaf. In the model, Ф and AmQ decreased by up to 50% whereas θa decreased by at most 8%, essentially negating the hypothesis. The relationship between photosynthetic rate and gs determined the size of the responses of θa and Ф to non-uniform gs. In support of the model, experiments on sunflower leaves showed that Ф and AmQ decreased by 32 and 52%, respectively, while no measurable change in θa occurred, when abscisic acid was used to induce patchiness in gs. Although not all possible patterns of non-uniform stomatal conductance have been tested, it appears that if large variations in θa occur in nature they do not result from non-uniform gs.