Energy dissipation in photosynthesis: Does the quenching of chlorophyll fluorescence originate from antenna complexes of photosystem II or from the reaction center?

Dissipation of light energy was studied in the moss Rhytidiadelphus squarro sus (Hedw.) Warnst., and in leaves of Spinacia oleracra L, and Arabidopsis thaliana (L.) Heynh., using chlorophyll fluorescence as an indicator reacti on. Maximum chlorophyll fluorescence of 3-(3,4-dichloropheny1)-1,1-dimethyl urea (DCMU)-treated spinach leaves, as produced by saturating light and stu died between +5 and -20 degreesC, revealed an activation energy DeltaE of 0 .11 eV. As this suggested recombination fluorescence produced by charge rec ombination between the oxidized primary donor of photosystem II and reduced pheophytin. a mathematical model explaining fluorescence, and based in par t on known characteristics of primary electron-transport reactions, was dev eloped. The model permitted analysis of different modes of fluorescence que nching, two localized in the reaction center of photosystem II and one in t he light-harvesting system of the antenna complexes. It predicted differenc es in the relationship between quenching of variable fluorescence F, and qu enching of basal, so-called Fo fluorescence depending on whether quenching originated from antenna complexes or from reaction centers. Such difference s were found experimentally, suggesting antenna quenching as the predominan t mechanism of dissipation of light energy in the moss Rhytidiadelphus, whe reas reaction-center quenching appeared to be important in spinach and Arab idopsis. Both reaction-center and antenna quenching required activation by thylakoid protonation but only antenna quenching depended on or was strongl y enhanced by zeaxanthin. De-protonation permitted relaxation of this quenc hing with half-times below 1 min. More slowly reversible quenching, tentati vely identified as so-called q(1) or photoinhibitory quenching, required pr otonation but persisted for prolonged times after de-protonation. It appear ed to originate in reaction centers.