PSII Complexes with Destabilized Primary Quinone Acceptor of Electrons in Dark-Adapted Chlorella

Abstract

Incubation of the alga Chlorella pyrenoidosa Chick in darkness (at 37°C) for 24 h did not change the initial (F 0) and maximum (F m) yield of chlorophyll fluorescence in diuron-treated cells. In dark-incubated alga, the contribution of the slow (rise time 10–15 min) phases to the kinetics of F m rise and, correspondingly, to variable fluorescence F v (where F v = F m – F 0) increased twofold. In addition, F m was attained at higher concentrations of diuron, which inhibits electron transfer between the primary (Q A) and secondary (Q B) quinone acceptors of electron in the PSII. Inhibition of photosynthetic electron transfer with o-phenanthroline, which, at high concentrations, competitively replaces both Q B and Q A, decreased F m yield due to selective suppression of the slow phase of fluorescence rise. It was assumed that the slow phase in the kinetics of F m rise reflects the functioning of PSII complexes with destabilized Q A. Such destabilization can result from the modification of the major PSII proteins (D1 and D2) in dark-adapted Chlorella cells.