Kinetic analysis of the light-induced fluorescence quenching in light-harvesting chlorophyll a/b pigment-protein complex of photosystem II

We carried out a kinetic analysis of the light-induced fluorescence quenchi ng (Delta F) of the light-harvesting chlorophyll a/b pigment-protein comple x of photosystem II (LHCII) that was first observed by Jennings et al, (Pho tosynth. Res. 27, 57-64, 1991), We show that during a 2 min light, 2 min da rk cycle, both the light and dark phases exhibit biexponential kinetics; th is is tentatively explained by the presence of two types of light-induced q uenchers in different domains of aggregated LHCII, Quantitative analysis co uld be carried out on the faster kinetic component; the slower component th at was not completed during the measurement was not amenable for quantitati ve analysis. Our analysis revealed that the rate of the light-induced decre ase of the fluorescence yield depended linearly on the light intensity, whi ch shows that the generation of the quencher originates from a reaction tha t is first order with respect to the concentration of the excited domains. As shown by the estimated rate constant, photogeneration of the quencher is a fast reaction that can compete with other excitation-relaxation pathways . Both the decay and the recovery time constants of Delta F depended strong ly on the temperature. Thermodynamic analysis showed that the fast light-in duced decline in the fluorescence was determined by a low fraction of the e xcited states. Recovery was associated with large decrease in the entropy o f activation that indicated the involvement of large structural rearrangeme nts. Macroaggregated LHCII exhibited larger Delta F than small aggregates, which is consistent with the proposed role of aggregated LHCII in thylakoid membranes in nonphotochemical quenching.