Modification of room-temperature picosecond chlorophyll fluorescence kinetics in green algae by photosystem II trap closure

Room-temperature single-photon timing measurements on green algae at low excitation energies indicate at least three kinetic components of chlorophyll fluorescence when the reaction centres of photosystem (PS) II are open (F_o) with lifetimes of approx. 50 ps (fast), 200 ps (middle) and 450 ps (slow). The absence, in green algae, of a long-lifetime component (1.4 ns) is at variance with previous reports. Closing the reaction centres gave rise to a similar increase in both the middle and slow lifetimes, by a factor of between 4 and 5, to produce lifetimes of approx. 850 ps and 2 ns, respectively, when all traps were closed (F_max). The yield and lifetime of the fast component were found to be independent of PS II centre closure. The yields of the middle and slow components were both modified by reduction of the primary stable electron acceptor of PS II (Q_a) with the slow-component yield increasing by a greater degree than the middle-component yield. It was also observed that as PS II reaction centres were closed to photochemistry the fluorescence yield and average lifetime increased proportionally. This trend and the similar increases in the middle and slow lifetimes are indicative of a well-connected system of PS II units favouring exciton transfer between PS II centres. We suggest that the fast kinetic component arises mainly from PS I while the middle and slow components arise from different types of fluorescing unit located in the granal lamellae and associated with system of well-connected PS II.