The effect of thylakoid membrane reorganisation on chlorophyll fluorescence lifetime components: A comparison between state transitions, protein phosphorylation and the absence of Mg²⁺

Room temperature chlorophyll fluorescence lifetime measurements using single photon counting and low-intensity laser excitation have been carried out on photosynthetic systems which have undergone protein reorganisation by an in vivo state 1-state 2 transition, protein phosphorylation and the absence of Mg²⁺. Analysis of the global changes in average lifetime and total fluorescence yield suggest that each treatment brings about a decrease in Photosystem (PS) II absorption cross-section but that this mechanism of energy redistribution accounts for different proportions of the total fluorescence quenching in the various cases. Further analysis of the overall fluorescence decay into individual kinetic components was carried out using a four-exponential model. The state transition did not alter the lifetimes of the four components but decreased the fluorescence yield of the long-lived decay, at both F₀ and F_M, by 24% and increased the yield of the rapid components. Such changes infer that there is a decrease in PS II absorption cross-section and an increase in PS I excitation on going from state 1 to state 2. Furthermore, these alterations show that the 500 ps component (at F₀) gives rise to the 2 ns decay (at F_M). After in vitro protein phosphorylation at 5 mM Mg²⁺, the changes are very similar to those brought abought by a state transition, except that both long-lived kinetic components exhibit a decrease in yield. When protein phosphorylation was carried out at 2 mM Mg²⁺ a slight decrease in the lifetimes of the two slow components was observed, with a further decrease in the yield of the 2.3 ns decay and a larger increase in the yields of the two rapid decays. Although the fluorescence quenching brought about by the absence of Mg²⁺ (57%) was the largest of all the treatments, only a small part could be explained by a decrease in PS II absorption cross-section (17%). The absence of Mg²⁺ led to a decrease in the lifetimes and yields of the two long-lived decays. A careful comparison of the characteristics of the slowest component in the presence and absence of 5 mM Mg²⁺ on closing the PS II traps suggest that this decay has different origins in the two cases.