Differential effects of dimethylbenzoquinone and dichlorobenzoquinone on chlorophyll fluorescence transient in spinach thylakoids

In isolated thylakoids, the differential effects of 2,5-dimethyl-p-benzoquinone (DMQ) and 2,6-dichloro-p-benzoquinone (DCBQ) on chlorophyll a (Chl a) fluorescence transients and O-2 evolution data have been used to differentiate between active and inactive photosystem II (PSII) centers. This conclusion was challenged by Lavergne and Leci (Photosynth. Res., 35 (1993) 323-343). Thus we have systematically re-investigated this phenomenon using different concentrations of DMQ and DCBQ in thylakoids exposed to various light intensities. We show that the differential effects of DMQ and DCBQ on Chl a fluorescence transients in spinach thylakoids include a larger decrease in the variable Chl a fluorescence by DCBQ than by DMQ and a decrease in the F-0 level with increasing [DCBQ] but not [DMQ] in the 5-150 mu M range. These differential effects confirm the results of Lavergne and Leci, and thus sustain the conclusion that the effects of DMQ and DCBQ on Chl a fluorescence cannot be used to distinguish between active and inactive PSII centers. The data show that the decrease in Chl a fluorescence follows Stem-Volmer behavior. Furthermore, our data confirm that the rate of O-2 evolution is higher in the DCBQ-supported Hill reaction than in the DMQ-supported Hill reaction. These results can be explained by a more efficient ability of DCBQ to oxidize the plastoquinol pool or a more efficient ability of DCBQ to displace Q(B) (secondary bound plastoquinone, a two-electron acceptor of PSII), rather than activating the inactive PSII centers. However, our results do not challenge the existence of inactive PSII.