INSIGHT INTO THE RELATIONSHIP OF CHLOROPHYLL-A FLUORESCENCE YIELD TO THE CONCENTRATION OF ITS NATURAL QUENCHERS IN OXYGENIC PHOTOSYNTHESIS

Fluorescence of chlorophyll a (Chla) is a noninvasive and very sensiti ve intrinsic probe of photosynthesis. It monitors the composition and organization of the photosystems, the exciton energy transfer, the pho tochemistry, and the effects of various types of stress on plants. It is the most used as well as the most abused tool in photosynthesis. Th us, an understanding of its relationship to photosynthesis has been of paramount importance. Both the oxidized primary plastoquinone, Q(A), and the oxidized primary reaction-center Chla, P680+ (for short, P+), are known to be quenchers of Chla fluorescence yield (phi(f)) of photo system II. Flash-number dependence of Chla fluorescence yield shows ei ther a period 4, due to the four-step charge-accumulation process of w ater oxidation (donor side), or period 2 behavior, due to the two-step reduction of the plastoquinone Q(B) (acceptor side) of photosystem II reaction centers. We provide here a further insight into the relation ship of variable Chla fluorescence yield (psi(f)) to the concentration of the two quenchers. The observed time dependence of the ratio of ps i(f) after flash 3 to that after flash 1 (or flash 5) in spinach thyla koids at pH 6 can be explained if we suggest that 1/phi(f) congruent-t o a[PQ(A)] + b[P+] + c, where a, b, and c are constants. From this it follows that the quenching of Chla fluorescence by P680+ after a flash is dependent on Q(A): for low [Q(A)] (when most reaction centers are ClOsed, [PQ(A)] is low) the quenching of Chla fluorescence by P680+ pr edominates, while for high [Q(A)] (when most reaction centers are open ), the quenching of Chla fluorescence is due predominantly to the incr eased concentration of the reduced form of P680 ([P+] is low).