Actions of tetraphenylboron on the electron flow in Photosystem II of isolated chloroplasts

Abstract

Isolated chloroplasts from pokeweed ( Phytolacca americana) were used in studies of the actions of tetraphenylboron on photosynthetic electron transport. Any effects on sites which couple electron transport to photophosphorylation, could be eliminated by the use of uncoupled chloroplasts. Very low concentrations of tetraphenylboron were found to strongly inhibit the reoxidation in the dark of the reduced primary acceptor Q in chloroplasts poisoned with 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Apparently, the tetraphenylboron anion has a high affinity to the oxygen evolving reaction complex where it is readily photoxidized. Slightly higher concentrations of tetraphenylboron inhibited steady-state rates of oxygen evolution measurably, and induced rapid and irreversible photodestructions of Photosystem II. System I mediated electron transport was not affected by these photoinhibitory processes. In many respects, the effects were similar to those observed in the presence of desaspidin or carbonylcyanide m-chlorophenylhydrazone. However, much lower concentrations of tetraphenylboron were required, and its actions were more restricted to Photosystem II. These properties may make tetraphenylboron a valuable tool in studies on the reaction mechanism of the oxygen-evolving apparatus.