Light Response of CO₂ Assimilation, Reduction Stage of Q and Radiationless Energy Dissipation in Intact Leaves

Abstract

Response curves of the rate of net CO₂ uptake and of fluorescence quenching (photochemical, q_p, and non-photochemical, q_E) 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 -A_a/A_p, where A_a is the actual and A_p 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-q_p or q_E to 1 - A_a/A_p indicating that the differences in 1 - q_p or q_E 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 q_E, which is primarily a reflection of radiationless dissipation of excitation energy, responded linearly to 1 -A_a/A_p, except for a component already present under conditions in which light is limiting (i.e. when A_a=A_p). By contrast, the reduction state of the primary electron acceptor of photosystem II, 1 - q_p, when expressed as a function of 1 - A_a/A_p, 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.