Response curves of the rate of net CO2 uptake and of fluorescence quenching (photochemical, qp, and non-photochemical, qE) to photon irradiance were obtained for leaves of eight species. Their photosynthetic capacity varied greatly due to genetic (sun/shade species) and environmental factors (growth under different light and nutrient regimes). The two fluorescence parameters were studied in response to the proportion of excess light at each irradiance, which is given by 1 -Aa/Ap, where Aa is the actual and Ap the potential rate of photosynthesis (assuming that the high photon yield of CO2 uptake seen at low light occurred at all irradiances). All leaves exhibited a very similar response of either 1-qp or qE to 1 - Aa/Ap indicating that the differences in 1 - qp or qE among leaves at a given irradiance can be explained exclusively by differences in the balance between absorbed light and electron transport. In leaves of all species qE, which is primarily a reflection of radiationless dissipation of excitation energy, responded linearly to 1 -Aa/Ap, except for a component already present under conditions in which light is limiting (i.e. when Aa=Ap). By contrast, the reduction state of the primary electron acceptor of photosystem II, 1 - qp, when expressed as a function of 1 - Aa/Ap, increased in all leaves much less than expected if it were solely a reflection of the balance between light absorption and photochemistry. Radiationless energy dissipation is a process which has the potential to maintain Q in a low reduction state.