Time- and reduction-dependent rise of photosystem II fluorescence during microseconds-long inductions in leaves

Lettuce (Lactuca sativa) and benth (Nicotiana benthamiana) leaves were illuminated with 720 nm background light to mix S-states and oxidize electron carriers. Green-filtered xenon flashes of different photon dose were applied and O₂ evolution induced by a flash was measured. After light intensity gradient across the leaf was mathematically considered, the flash-induced PSII electron transport (= 4·O₂ evolution) exponentially increased with the flash photon dose in any differential layer of the leaf optical density. This proved the absence of excitonic connectivity between PSII units. Time courses of flash light intensity and 680 nm chlorophyll fluorescence emission were recorded. While with connected PSII the sigmoidal fluorescence rise has been explained by quenching of excitation in closed PSII by its open neighbors, in the absence of connectivity the sigmoidicity indicates gradual rise of the fluorescence yield of an individual closed PSII during the induction. Two phases were discerned: the specific fluorescence yield immediately increased from F_o to 1.8F_o in a PSII, whose reaction center became closed; fluorescence yield of the closed PSII was keeping time-dependent rise from 1.8F_o to about 3F_o, approaching the flash fluorescence yield F_f = 0.6F_m during 40 μs. The time-dependent fluorescence rise was resolved from the quenching by ³Car triplets and related to protein conformational change. We suggest that Q_A reduction induces a conformational change, which by energetic or structural means closes the gate for excitation entrance into the central radical pair trap-efficiently when Q_B cannot accept the electron, but less efficiently when it can.