PHOTOSYSTEM-2 REACTION CENTERS INACTIVE IN PLASTOQUINONE POOL REDUCTION - A BRIEF OVERVIEW

Photosystem 2 (PS 2) reaction centers inactive in plastoquinone pool r eduction are present in isolated thylakoid membranes, intact chloropla sts, and leaves of dark-adapted plants. Here we describe our recent wo rk investigating the physical and physiological properties that distin guish inactive from active centers. Inactive PS 2 centers are defined by their slow rate of Q(A)- oxidation. They have a competent water oxi dation system and constitute about one-third of the total PS 2 present in dark-adapted leaves and thylakoid membranes. Their effective absor ption cross section for radiant energy utilization is half that of act ive PS 2 centers. Irradiation modifies inactive PS 2 centers in leaves and thylakoid membranes. The modification is manifested by a 50 % dec line of their variable fluorescence and of their contribution to the e lectrochromic shift. In leaves the light-induced modification is rever sible in the dark, whereas it is irreversible in thylakoid membranes.