Investigation of OCP-triggered dissipation of excitation energy in PSI/PSII-less Synechocystis sp. PCC 6803 mutant using non-linear laser fluorimetry.

In order to prevent photodestruction by high light, photosynthetic organisms have evolved a number of mechanisms, known as non-photochemical quenching (NPQ), that deactivate the excited states of light harvesting pigments. Here we investigate the NPQ mechanism in the cyanobacterium Synechocystis sp. PCC 6803 mutant deficient in both photosystems. Using non-linear laser fluorimetry, we have determined molecular photophysical characteristics of phycocyanin and spectrally distinct forms of allophycocyanin for the cells in non-quenched and quenched states. Our analysis of non-linear fluorescence characteristics revealed that NPQ activation leads to an ~2-fold decrease in the relaxation times of both allophycocyanin fluorescence components, F660 and F680, and a 5-fold decrease in the effective excitation cross-section of F680, suggesting an emergence of a pathway of energy dissipation for both types of allophycocyanin. In contrast, NPQ does not affect the rates of singlet-singlet exciton annihilation. This indicates that, upon NPQ activation, the excess excitation energy is transferred from allophycocyanins to quencher molecules (presumably 3'hydroxyechinenone in the orange carotenoid protein), rather than being dissipated due to conformational changes of chromophores within the phycobilisome core. Kinetic measurements of fluorescence quenching in the Synechocystis mutant revealed the presence of several stages in NPQ development, as previously observed in the wild type. However, the lack of photosystems in the mutant enhanced the magnitude of NPQ as compared to the wild type, and allowed us to better characterize this process. Our results suggest a more complex kinetics of the NPQ process, thus clarifying a multistep model for the formation of the quenching center.

[A new type of adaptation of the cyanobacterium Spirulina platensis to illumination conditions]. / Novyi tip adaptatsii tsianobacterii Spirulina plantensis k usloviiam osveshcheniia.

Incubation of cells of the cyanobacterium Spirulina platensis under conditions of exposure to low-intensity (2-3 microE m-2 s-1) red light, which was predominantly absorbed by photosystem I (PS I), caused atypical adaptation changes. Invariable pigment composition and stoichiometry of photosystems was observed in the cells incubated under these conditions against the background of a decrease in the rate of photosynthetic fixation of CO2 (by one-half) and a 1.5-fold increase in the rate of dark respiration relative to cells incubated under conditions of exposure to green light. Comparison of these data with a high rate of dark relaxation of P700+ in the presence of diuron suggests that deficiency of reduced equivalents at the donor side of PS I in the Spirulina cells exposed to red light is compensated by electron supply from the respiratory chain NAD(P)H dehydrogenase complex.

Interaction of phycobilisomes with photosystem II dimers and photosystem I monomers and trimers in the cyanobacterium Spirulina platensis.

Distribution of phycobilisomes between photosystem I (PSI) and photosystem II (PSII) complexes in the cyanobacterium Spirulina platensis has been studied by analysis of the action spectra of H2 and O2 photoevolution and by analysis of the 77 K fluorescence excitation and emission spectra of the photosystems. PSI monomers and trimers were spectrally discriminated in the cell by the unique 760 nm low-temperature fluorescence, emitted by the trimers under reductive conditions. The phycobilisome-specific 625 nm peak was observed in the action spectra of both PSI and PSII, as well as in the 77 K fluorescence excitation spectra for chlorophyll emission at 695 nm (PSII), 730 nm (PSI monomers), and 760 nm (PSI trimers). The contributions of phycobilisomes to the absorption, action, and excitation spectra were derived from the in vivo absorption coefficients of phycobiliproteins and of chlorophyll. Analyzing the sum of PSI and PSII action spectra against the absorption spectrum and estimating the P700:P680 reaction center ratio of 5.7 in Spirulina, we calculated that PSII contained only 5% of the total chlorophyll, while PSI carried the greatest part, about 95%. Quantitative analysis of the obtained data showed that about 20% of phycobilisomes in Spirulina cells are bound to PSII, while 60% of phycobilisomes transfer the energy to PSI trimers, and the remaining 20% are associated with PSI monomers. A relevant model of organization of phycobilisomes and chlorophyll pigment-protein complexes in Spirulina is proposed. It is suggested that phycobilisomes are connected with PSII dimers, PSI trimers, and coupled PSI monomers.

Analysis of dark-relaxation kinetics of variable fluorescence in intact leaves.

The dark-relaxation kinetics of variable fluorescence, Fv, in intact green leaves of Pisum stativum L. and Dolichos lablab L. were analyzed using modulated fluorometers. Fast (t1/2 = 1 s) and slow (t1/2 = 7-8 s) phases in fv dark-decay kinetics were observed; the rate and the relative contribution of each phase in total relaxation depended upon the fluence rate of the actinic light and the point in the induction curve at which the actinic light was switched off. The rate of the slow phase was accelerated markedly by illumination with far-red light; the slow phase was abolished by methyl viologen. The halftime of the fast phase of Fv dark decay decreased from 250 ms in dark-adapted leaves to 12-15 ms upon adaptation to red light which is absorbed by PSII. The analysis of the effect of far-red light, which is absorbed mainly by PSI, on Fv dark decay indicates that the slow phase develops when a fraction of QA (-) (the primary stable electron acceptor of PSII) cannot transfer electrons to PSI because of limitation on the availability of P700(+) (the primary electron donor of PSI). After prolonged illumination of dark-adapted leaves in red (PSII-absorbed) light, a transient. Fv rise appears which is prevented by far-red (PSI-absorbed) light. This transient fv rise reflects the accumulation of QA (-) in the dark. The observation of this transient Fv rise even in the presence of the uncoupler carbonylcyanide m-chlorophenyl hydrazone (CCCP) indicates that a mechanism other than ATP-driven back-transfer of electrons to QA may be responsible for the phenomenon. It is suggested that the fast phase in Fv dark-decay kinetics represents the reoxidation of QA (-) by the electron-transport chain to PSI, whereas the slow phase is likely to be related to the interaction of QA (-) with the donor side of PSII.

[Effects of pyridazinones and cerulenin on the biosynthesis and functional state of photosystem 2 in barley leaves]. / Vliianie piridazinonov i tserulenina na biosintez i funktsional'noe sostoianie fotosistemy 2 v list'iakh iachmenia.

The effects of pyridazinones and cerulenin on the formation of photosystem 2 in etiolated barley leaves and its functional state in green leaves were studied. It was shown that the reaction centers of photosystem 2 in greening leaves are formed after 2.5--3.0 hrs of illumination independently of herbicide treatment. In the leaves greening in the presence of pyridazinones and cerulenin and in green leaves treated by these substances a change in the chlorophyll state takes place, which is detected by a shortwave shift of the low temperature fluorescence maximum at 740 nm. Long-term treatment of greening and green leaves by pyridazinones increases variable fluorescence at 20 degrees. The inhibition of carotene synthesis by pyridazinones SAN 6706 and SAN 9789 during the greening is accompanied by a decrease of variable fluorescence at - 196 degrees and of its reversible part, as well as by an appearance of a light-induced dip of the fluorescence yield at 20 degrees slowly reversible in the dark. It is suggested that pyridazinones produce a 3-fold effect on the photosynthetic apparatus: 1) they block electron transport in the acceptor part of photosystem 2 and affect directly the reaction centers of this photosystem; 20 SAN 6706 and SAN 9789 inhibit carotene biosynthesis during greening of leaves, resulting in a formation of a photounstable pigment apparatus with a low amount of the reaction centers of photosystem 2; 3) pyridazinones which damage the membrane structure and probably the lipid composition of chloroplasts cause significant changes of the chlorophyll state; this effect is similar to that exerted by cerulenin.

[Effect of pyridazinone herbicides on the photosynthetic electron transport chain of chloroplasts and Chlorella]. / Deistvie piridazinovykh gerbitsidov na fotosinteticheskuiu tsep' perenosa élektrona khloroplastov i khlorelly.

In order to establish the site of pyridazinone herbicides action on the photosynthetic electron transport chain, their effect on the photochemical activity of chloroplasts and Chlorella was studied. It was shown that these compounds similar to diuron inhibit the delta F of chloroplasts but enhance the delta F and cause the disappearance of slow transient processes in Chlorella and change the light-off time course of delta F both in Chlorella and in the chloroplasts. The inhibiting effect is observed at herbicide concentration of 5 x 10(-6) M and is maximal at 10(-4) M. However, in contrast to diuron the herbicides enhance the msec afterglow in Chlorella cells; besides, even at concentration as high as 10(-4) M they only partly block photosynthetic oxygen evolution and the light-induced change of pH. Pyridazinone herbicides retard the delay of light-off delta F at-196 degrees C more efficiently than diuron. It is suggested that the herbicides under study inhibit the photosynthetic electron transport chain, however less efficiently than diuron; the inhibiting effect is decreased in the following order: SAN 9785, SAN 6706, SAN 9789. The herbicides affect mainly the acceptor part of the photosystem 2, retarding the electron transport from the intermediary acceptor to plastoquinone. In addition these herbicides may also have other sites of action in the region of photosystem 2.

[Nature of changes in the fluorescence process of P700-enriched chloroplast fragments]. / Pripoda izmenenii bykhoda fluorestsentsii fragmentov khloroplastov, obogashchennykh P700.

Spectral and photochemical properties of P700-enriched chloroplast fragments, obtained by ether treatment of liophylized digitonin fragments, were studied. It was shown that time course of fluorescence changes of isolated fragments (in contrast to digitonin fragments) at 20 degrees does not correspond to time course of absorption changes at 700 nm. Differences in low temperature fluorescence spectra of fragments, initially distinguished by redox states of photosystem 1 reaction centers were found. However, the fragments under study were incapable of light-induced changes of fluorescence yield at--196 degrees, independently of spectral region of measured fluorescence (lambda greater than 660 nm or lambda greater than 710 nm), though these fragments reveal phototransformation of P700. Thus changes in the low temperature fluorescence spectra cannot be accounted for by redox changes of P700. The fragments, isolated by ether treatment at 20 degrees as well at--196 degrees do not reveal light-induced fluorescence changes caused by redox changes of P700. The fluorescence changes observed may be due to accessory photoprocesses of chlorophyllprotein complex.