Abstract— The intensity of chlorophyll a fluorescence during the early part of fluorescence induction at O, initial fluorescence, and P, peak fluorescence, was higher during the day phase of the circadian cycle than during the night phase in continuous light (LL) conditions and was positively correlated with the rate of oxygen evolution. The circadian rhythm in fluorescence in LL persisted in the presence of 10μM 3–(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU), which blocks electron flow from photo‐system (PS) II in photosynthesis. The rhythmic changes in fluorescence intensity are consistent with a lower rate constant for radiationless transitions during the day phase than during the night phase of the circadian rhythmicity. The circadian changes in the intensity of fluorescence were abolished at 77K, which may indicate the importance of structural changes in membranes in circadian oscillations.
Phycobilisomes from the blue-green alga Nostoc sp. are known to contain the phycobiliproteins: c-phycoerythrin (c-PE), c-phycocyanin (c-PC) and four forms of allophycocyanin (APC I. 11, 111, and B). We have made a detailed study of the effects of the intensity of a single 6 ps excitation pulse on the decay kinetics and the yield of fluorescence in the individual isolated phycobiliproteins at pH 7 and 23°C. The risetime of the fluorescence of c-PE, c-PC and APC was < 12 ps. We found that the decay of the fluorescence was exponential at intensities of 5 l O I 4 photons/cm2 in all the phycobiliproteins; the lifetimes being 1552 + 31 ps for c-PE, 21 1 1 f 83 ps for c-PC, 1932 f 165 ps for APC 1. 1870 _+ 90 ps for APC 11,1816 5 88 ps for APC 111, (1869 5 62 ps for the averaged APCs I, 11, and 111), and 2667 5 233 ps for APC B. We also found that the fluorescence decay became non-exponential in c-PE at excitation intensities > l o t 4 photons/cm*, but was exponential for all the other phycobiliproteins even at a pulse intensity of lOI5 photons/cm2. The relaxation times of c-PE and c-PC decreased with excitation intensity above lOI4 photonslcm'. For c-PE and c-PC the relative fluorescence vs excitation intensity was readily described by a relationship derived for a model in which exciton-exciton annihilation occurs. In APC the fluorescence yield and relaxation time were only slightly dependent on the excitation intensity. The results are interpreted to indicate the occurrence of singlet-singlet annihilation intramolecularly among the several phycobilin chromophores within the individual phycobiliprotein molecules in solution. The s toftransfer time is less than 12ps in c-PC.
An attempt to characterize the mechanism of inhibition of photosynthetic electron transport in isolated pea chloroplasts by the herbicide 4,6-dinitro-o-cresol (DNOC) by a comparison with the effects of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) revealed the following:1. The percent inhibition of oxygen evolution by a given herbicide concentration is the same at various light intensities except at very low intensities where the percent inhibition becomes larger. The same results are obtained with the herbicide DCMU.2. The concentration of DCMU causing 50% inhibition of oxygen evolution decreases with de creasing chloroplast (and thus of chlorophyll) concentration. W ith DNOC, the relative decrease is much less than with DCMU. At the inhibited molecule, there appears to be a cooperative binding of DCM U with two binding sites and a noncooperative binding of DNOC with only one binding site.3. The chlorophyll a fluorescence induction is influenced by DNOC in the same characteristic way as it is by D C M U : both herbicides cause a faster rise in fluorescence yield than in control chloroplasts, although a higher concentration of the former is required for the same effect.4. The chlorophyll fluorescence emission spectra at 77 °K show a slight decrease in the bands at 685 and 735 nm, and no or only a very slight decrease at 695 nm upon addition of high con centrations of either D CM U or DNOC before the onset of illumination.5. The degree of polarization of chlorophyll a fluorescence is lower after addition of DCM U or DNOC upon excitation by 460 or 660 nm light.It is concluded that, although the chemical structure of DNOC is completely different from that of DCM U, its site and mechanism of inhibition is similar to that of DCMU. Both herbicides inhibit electron transport between the primary electron acceptor of photosystem II and the plastoquinone pool. This causes a closing of the reaction centers of photosystem II. However, the interaction with the inhibited molecule is different for the two herbicides.The herbicide 4,6-dinitro-o-cresol (DNOC) is a potent inhibitor of photosynthesis. Van Rensen et al. [1] showed that the uncoupled Hill reaction (with ferricyanide as an electron acceptor) is in hibited 50 percent with 1 j u m DNOC. This herbicide has no influence on the silicomolybdate-mediated Hill reaction in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCM U), but inhibits the phenylenediamine-mediated photoreduction of ferri cyanide in the presence of the plastoquinone anta * On leave from the
Abstract. –The absorption and fluorescence characteristics of subchloroplast particles highly enriched in P700 (1 P700 to 10–15 chlorophyll a molecules) and of two artificial systems are presented here. The fluorescence characteristics measured were excitation and emission spectra, and the degree of polarization of fluorescence. The model systems studied were chlorophyll a and pheophytin a in a polystyrene matrix, and a colloidal mixture of these two pigments with bovine serum albumin. Effects of 0.5% sodium dodecyl sulfate on the optical properties of the P700‐enriched particles are also described. The importance of pigment‐pigment and pigment‐protein interactions in determining the fluorescence properties of these particles are discussed. The possible role of pheophytin in these preparations needs further investigation.
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