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 02 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 v mable Chl a fluorescence by DCBQ than by DMQ and a decrease in the/7o level with increasing [DCBQ] but not [DMQ] in the 5-150 /aM range. These differential effects confirm the results of Lavergne and Leci, and thus sustain the conclusion that the effects of DMQ and E,CBQ 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 Stern-Volmer behavior. Furthermore, our data confirm that the rate of 02 evolution is higher in the DCBQ-supported E ill reaction than in the DMQsupported Hill reaction. These results can be explained by a more efficient ability of DCBQ to oxidize the pkastoquinol pool or a more efficient ability of DCBQ to displace QB (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. 1~ eywords: Chlorophyll a fluorescence; Active/inactive photosystem ll; Dichlorobenzoquinone; Dimethylbenzoquinone