SLOW EXCITON TRAPPING IN PHOTOSYSTEM-II - A POSSIBLE PHYSIOLOGICAL-ROLE

Photosystem II, which has a primary photochemical charge separation ti me of about 300 ps, is the slowest trapping of all photosystems. On th e basis of an analysis of data from the literature this is shown to be due to a number of partly independent factors: a shallow energy funne l in the antenna, an energetically shallow trap, exciton dynamics whic h are partly 'trap limited' and a large antenna. It is argued that the first three of these properties of Photosystem II can be understood i n terms of protective mechanisms against photoinhibition. These protec tive mechanisms, based on the generation of non photochemical quenchin g states mostly in the peripheral antenna, are able to decrease pheoph ytin reduction under conditions in which the primary quinone, Q(A), is already reduced, due to the slow trapping properties. The shallow ant enna funnel is important in allowing quenching state-protective mechan isms in the peripheral antenna.