Sequential Events in the Photoinhibition of Synechocystis under Sodium Stress.

Synechocystis (PCC 6714) is photoinhibited at low photon flux density when illuminated in medium devoid of Na(+). Photosynthetic activity and variable fluorescence yield were measured in intact cells during the course of inhibition and during recovery from inhibition in order to characterize sequential inhibitory events. Photosystem II antenna chlorophyll becomes functionally disconnected from the reaction center in the first stage of inhibition, although energy is still transferred from phycobilin pigments to the reaction center. This occurs simultaneously with, or subsequent to, the previously observed (J Zhao, JJ Brand [1988] Arch Biochem Biophys 264: 657-664) inactivation of O(2)-evolution units in Na(+)-stressed cells. Addition of Na(+) to the culture medium rapidly restores the O(2)-evolving system, but reassociation of photosystem II antenna chlorophyll is much slower and requires new protein synthesis. A site nearer to the reaction center of photosystem II becomes inactivated in a secondary inhibitory event. Recovery from secondary inhibition occurs slowly and requires protein synthesis. Cells recovering from secondary inhibition produce active photosystem II units with a fully functional complement of antenna chlorophyll. Synechocystis is photoinhibited in complete (Na(+)-containing) medium when illuminated at high photon flux density. These photoinhibited cells exhibit the same recovery behavior as cells at the second stage of inhibition in Na(+)-deficient medium.