ATP-induced photochemical quenching of variable chlorophyll fluorescence

Ternary Interactions and Energy Transfer between Fluorescein Isothiocyanate, Adenosine Triphosphate, and Graphene Oxide Nanocarriers

The interactions of fluorescent probes and biomolecules with nanocarriers are of key importance to the emerging targeted drug delivery systems. Graphene oxide nanosheets (GONs) as the nanocarriers offer biocompatibility and robust drug binding capacity. The interactions of GONs with fluorophores lead to strong fluorescence quenching, which may interfere with fluorescence bioimaging and biodetection. Herein, we report on the interactions and energy transfers in a model ternary system: GONs-FITC-ATP, where FITC is a model fluorophore (fluorescein isothiocyanate) and ATP is a common biomolecule (adenosine-5'-triphosphate). We have found that FITC fluorescence is considerably quenched by ATP (the quenching constant K_SV = 113 ± 22 M^-1). The temperature coefficient of K_SV is positive (α_T = 4.15 M^-1deg^-1). The detailed analysis of a model for internal self-quenching of FITC indicates that the temperature dependence of the net quenching efficiency η for the FITC-ATP pair is dominated by FITC internal self-quenching modes with their contribution estimated at 79%. The quenching of FITC by GONs is much stronger (K_SV = 598 ± 29 M^-1) than that of FITC-ATP and is associated with the formation of supramolecular assemblies bound with hydrogen bonding and π-π stacking interactions. For the analysis of the complex behavior of the ternary system GONs-FITC-ATP, a model of chemisorption of ATP on GONs, with partial blocking of FITC quenching, has been developed. Our results indicate that ATP acts as a moderator for FITC quenching by GONs. The interactions between ATP, FITC, and GONs have been corroborated using molecular dynamics and quantum mechanical calculations.

TBT toxicity on a natural planktonic assemblage exposed to enhanced ultraviolet-B radiation

A microcosm approach was designed to study the combined effects of tributyltin (TBT) from antifouling paints and ultraviolet-B radiation (UVBR: 280-320 nm), on a natural planktonic assemblage (<150 microm) isolated from the St. Lawrence Estuary at the end of the springtime. Microcosms (9l, cylindrical Teflon bags, 75 cm heightx25 cm width) were immersed in the water column of mesocosms (1800 l, polyethylene bags, 2.3 m depth) and exposed to two different UVBR regimes: natural ambient UVBR (NUVBR), and enhanced level of UVBR (HUVBR). During consecutive 5 days, effects of TBT (120 ng l -1) and enhanced UVBR (giving a biologically weighted UVBR 2.15-fold higher than natural light condition) were monitored in the samples coming from following treatments: (i) NUVBR light condition without TBT (NUVBR), (ii) NUVBR light condition with TBT-added (NUVBR+TBT), (iii) HUVBR light condition without TBT (HUVBR) and (iv) HUVBR light condition with TBT-added (HUVBR+TBT). Each treatment was conducted in triplicate microcosms. Different parameters were then measured during 5 days, including TBT analysis, bacterial abundance and productivity, phytoplankton abundance, cellular characteristics and growth rates, as well as in vivo chlorophyll a (Chl a) fluorescence. Following TBT addition (NUVBR+TBT treatment), Chl a concentrations never exceeded 1 microg l-1 whereas final values as high as 54 microg l-1 were observed in TBT-free treatments (NUVBR and HUVBR). TBT addition resulted also in the lost of fluorescence signal of the maximum efficiency of the photosystem II in phytoplankton assemblage. TBT toxicity caused on phytoplankton <20 microm an increase of mean cell size and changes in shape reflected a drastic disturbance of the cell cycle leading to an inhibition of the apparent growth rate. These negative effects of TBT resulted in a final abundance of phytoplankton <20 microm of 591+/-35 cells ml-1 in NUVBR+TBT relative to NUVBR treatment (i.e., 31,846+/-312 cells ml-1). Moreover, when cells were submitted to TBT under enhanced UVBR (HUVBR+TBT treatment), final abundance of phytoplankton <20 microm was only 182+/-90 cells ml-1, with a significant interaction between TBT and UVBR during the last 2 days of the experiment. The same type of interaction was also observed for bacterial abundance in NUVBR+TBT and HUVBR+TBT with stimulation of 226 and of 403%, respectively due to TBT addition relative to NUVBR treatment. When considering bacterial productivity, TBT addition resulted in an inhibition of 32%, and this inhibition was significantly more pronounced under dual stresses (i.e., 77% in HUVBR+TBT). These results clearly demonstrate that the combination of TBT and UVBR stresses have synergistic effects affecting the first trophic levels of the marine food web.

Devices and methods for room-temperature fluorescence analysis

An overview of the various types of photochemical and non-photochemical fluorescence quenching in vivo is given. Devices and methods are outlined that allow specific information to be obtained from complex fluorescence responses. The importance of correlated measurements of other, independent photosynthesis signals is emphasized. It is shown that a recently introduced pulse-amplitude modulation fluorometer (PAM fluorometer) can also be used with modified emitter-detector units to measure absorbance changes. Examples are given for absorbance changes of the Hill reagent methyl purple, induced by single turnover flashes, and for P 700 absorbance changes measured simultaneously with fluorescence. Correlated P 700 and fluorescence measurements give deeper insights into the control of electron transfer from PQH 2 to cytochrome (cyt) b / f and into the intersystem acceptor-pool size of sun and shade leaves. Possible explanations for differences in pool sizes determined by P 700 and fluorescence measurements are discussed. By using P 700 reduction as an indicator, it is shown that in saturating light the plastoquinone (PQ) pool is already reduced within 50 ms, whereas the last phase of the fluorescence rise (I 2 -P) takes about 300 ms and is paralleled by the re-reduction of P 700 . It is concluded that I 2 -P reflects removal of photochemical quenching at PSI and that 50 ms saturation pulses are appropriate to eliminate the relevant photochemical PSII quenching.

The use of photothermal radiometry in assessing leaf photosynthesis: II. Correlation of energy storage to Photosystem II fluorescence parameters

Following the first part of this work (Malkin et al. (1991) Photosynth Res 29: 87-96), where modulated photothermal radiometry (PTR) was used to measure energy storage (ES) in intact leaves as a function of P700 redox state, we report here on simultaneous ES and fluorescence measurements, which characterize the state of PS II. PTR monitors the conversion of modulated light into heat by measuring the modulated infra-red radiation emitted from the sample. The ratio [PTR+-PTR-]/PTR+, where PTR indicates the PTR signal and the subscripts +,- indicate the presence or absence of saturating background light, is used to quantitate ES. We searched carefully for the right conditions where the background light does not introduce a significant rise in the leaf temperature, which influences the PTR signal as such, otherwise the above ratio deviates from the true ES. Under such conditions, ES and the fluorescence parameters, F (momentary fluorescence level) Fm' (fluorescence of fully reduced PS II reaction centers) were measured during the induction phase of photosynthesis and in the steady state. ES and the parameter γ=(Fm'-F)/Fm', considered by Genty et al. ((1989) Biochim Biophys Acta 990: 87-92) to reflect the yield of PS II, had similar kinetics during the induction phase. Both reached a final maximum plateau after about 4-5 min. of illumination. In different experiments, where the measuring light intensities varied, γ was approximately linearly related to ES. This linear relationship was found in the same way also in steady-state measurements, where these parameters varied by using different background light intensities. Extrapolation to an ES value of zero indicates a finite non-zero value of γ. A possible explanation for this may be found in the existence an electron transport cycle around PS II which does not store energy in the range corresponding to the modulation frequency used (ca. 3.6 Hz).

On the relationship between chlorophyll fluorescence quenching and the quantum yield of electron transport in isolated thylakoids

The relationship between the empirical fluorescence index ΔF/Fm' and the quantum yield of linear electron flow, Φ(s), was investigated in isolated spinach thylakoids. Conditions were optimised for reliable determination of ΔF/Fm' and Φ(s) with methyl viologen or ferricyanide as electron acceptors under coupled and uncoupled conditions. Ascorbate in combination with methyl viologen was found to stimulate light-induced O2-uptake which is not reflected in ΔF/Fm' and interpreted to reflect superoxide reduction by ascorbate. In the absence of ascorbate, the plot of ΔF/Fm' vs. Φ(s) was mostly linear, except for the range of high quantum yields, i.e. at rather low photon flux densities. With ferricyanide as acceptor, use of relatively low concentrations (0.1-0.3 mM) was essential for correct Fm'-determinations, particularly under uncoupled conditions. Under coupled and uncoupled conditions the same basic relationship between ΔF/Fm' and Φ(s) was observed, irrespective of Φ(s) being decreased by increasing light intensity or by DCMU-addition. The plots obtained with methyl viologen and ferricyanide as acceptors were almost identical and similar to corresponding plots reported previously by other researchers for intact leaves. It is concluded that the index ΔF/Fm' can be used with isolated chloroplasts for characterisation of such types of electron flow which are difficult to assess otherwise, as e.g. O2 dependent flux. The origin of the 'non-linear' part of the relationship is discussed. An involvement of 'inactive' PS II centers with separate units and inefficient QA-QB electron transfer is considered likely.

The relationship between Photosystem II intrinsic quantum yield and millisecond luminescence in thylakoids

The relationship between charge recombination at Photosystem II (PS II), as indicated by millisecond luminescence, and PS II quantum yield was studied in spinach thylakoids during electron flow to methylviologen. Under the low magnesium conditions used, a decrease in quantum yield was observed in the absence of non-photochemical excitation quenching, and therefore cannot be due to a restriction in excitation delivery to the reaction centre. It was found that the decrease of the parameter Φp, which is a measure of the intrinsic quantum yield of 'open' PS II centers, correlates with an increase in luminescence per 'open' center. The relationship between these two parameters was the same whether Φp was manipulated by dissipation of the transthylakoid pH gradient or of the electrical potential. This indicates that the mechanism by which Φp decreases depends in the same way on the two components of the protonmotive force as does the charge recombination at PS II. Calculation of the yield of luminescence with respect to the back reaction will be necessary to determine whether the charge recombination occurs at a sufficiently high rate to be directly responsible for the Φp decrease.

Simultaneous photoreduction and photooxidation of cytochrome b-559 in Photosystem II treated with carbonylcyanide-m-chlorophenylhydrazone

The possibility of a Photosystem II (PS II) cyclic electron flow via Cyt b-559 catalyzed by carbonylcyanide m-chlorophenylhydrazone (CCCP) was further examined by studying the effects of the PS II electron acceptor 2,6-dichloro-p-benzoquinone (DCBQ) on the light-induced changes of the redox states of Cyt b-559. Addition to barley thylakoids of micromolar concentrations of DCBQ completely inhibited the changes of the absorbance difference corresponding to the photoreduction of Cyt b-559 observed either in the presence of 10 μM ferricyanide or after Cyt b-559 photooxidation in the presence of 2 μM CCCP. In CCCP-treated thylakoids, the concentration of photooxidized Cyt b-559 decreased as the irradiance of actinic light increased from 2 to 80 W m(-2) but remained close to the maximal concentration (0.53 photooxidized Cyt b-559 per photoactive Photosystem II) in the presence of 50 μM DCBQ. The stimulation of Cyt b-559 photooxidation in parallel with the inhibition of its photoreduction caused by DCBQ demonstrate that the extent of the light-induced changes of the redox state of Cyt b-559 in the presence of CCCP is determined by the difference between the rates of photooxidation and photoreduction of Cyt b-559 occuring simultaneously in a cyclic electron flow around PS II.We also observed that the Photosystem I electron acceptor methyl viologen (MV) at a concentration of 1 mM barely affected the rate and extent of the light-induced redox changes of Cyt b-559 in the presence of either FeCN or CCCP. Under similar experimental conditions, MV strongly quenched Chl-a fluorescence, suggesting that Cyt b-559 is reduced directly on the reducing side of Photosystem II.

Mechanisms for controlling balance between light input and utilisation in the salt tolerant alga Dunaliella C9AA

The yield of photosynthetic O2 evolution was measured in cultures of Dunaliella C9AA over a range of light intensities, and a range of low temperatures at constant light intensity. Changes in the rate of charge separation at Photosystem I (PS I) and Photosystem II (PS II) were estimated by the parameters ΦPS I and ΦPS II . ΦPS I is calculated on the basis of the proportion of centres in the correct redox state for charge separation to occur, as measured spectrophotometrically. ΦPS II is calculated using chlorophyll fluorescence to estimate the proportion of centres in the correct redox state, and also to estimate limitations in excitation delivery to reaction centres. With both increasing light intensity and decreasing temperature it was found that O2 evolution decreased more than predicted by either ΦPS I or ΦPS II. The results are interpreted as evidence of non-assimilatory electron flow; either linear whole chain, or cyclic around each photosystem.

O2-dependent electron flow, membrane energization and the mechanism of non-photochemical quenching of chlorophyll fluorescence

Recent progress in chlorophyll fluorescence research is reviewed, with emphasis on separation of photochemical and non-photochemical quenching coefficients (qP and qN) by the 'saturation pulse method'. This is part of an introductory talk at the Wageningen Meeting on 'The use of chlorophyll fluorescence and other non-invasive techniques in plant stress physiology'. The sequence of events is investigated which leads to down-regulation of PS II quantum yield in vivo, expressed in formation of qN. The role of O2-dependent electron flow for ΔpH- and qN-formation is emphasized. Previous conclusions on the rate of 'pseudocyclic' transport are re-evaluated in view of high ascorbate peroxidase activity observed in intact chloroplasts. It is proposed that the combined Mehler-Peroxidase reaction is responsible for most of the qN developed when CO2-assimilation is limited. Dithiothreitol is shown to inhibit part of qN-formation as well as peroxidase-induced electron flow. As to the actual mechanism of non-photochemical quenching, it is demonstrated that quenching is favored by treatments which slow down reactions at the PS II donor side. The same treatments are shown to stimulate charge recombination, as measured via 50 μs luminescence. It is suggested that also in vivo internal thylakoid acidification leads to stimulation of charge recombination, although on a more rapid time scale. A unifying model is proposed, incorporating reaction center and antenna quenching, with primary control of ΔpH at the PS II reaction center, involving radical pair spin transition and charge recombination to the triplet state in a first quenching step. In a second step, triplet excitation is trapped by zeaxanthin (if present) which in its triplet excited state causes additional quenching of singlet excited chlorophyll.

The mechanisms contributing to photosynthetic control of electron transport by carbon assimilation in leaves

'Photosynthetic control' describes the processes that serve to modify chloroplast membrane reactions in order to co-ordinate the synthesis of ATP and NADPH with the rate at which these metabolites can be used in carbon metabolism. At low irradiance, optimisation of the use of excitation energy is required, while at high irradiance photosynthetic control serves to dissipate excess excitation energy when the potential rate of ATP and NADPH synthesis exceed demand. The balance between ΔpH, ATP synthesis and redox state adjusts supply to demand such that the [ATP]/[ADP] and [NADPH]/[NADP(+)] ratios are remarkably constant in steady-state conditions and modulation of electron transport occurs without extreme fluctuations in these pools.

Regulation of electron transport in maize mesophyll chloroplasts: The relationship between chlorophyll a fluorescence quenching and O2 evolution

The relationship between the redox state of the primary electron acceptor of photosystem II (QA) and the rate of O2 evolution in isolated mesophyll chloroplasts from Zea mays L. is examined using pulse-modulated chlorophyll a fluorescence techniques. A linear relationship between photochemical quenching of chlorophyll fluorescence (qQ) and the rate of O2 evolution is evident under most conditions with either glycerate 3-phosphate or oxaloacetate as substrates. There appears to be no effect of the transthylakoid pH gradient on the rate of electron transfer from photosystem II into QA in these chloroplasts. However, the proportion of electron transport occurring through cyclic-pseudocyclic pathways relative to the non-cyclic pathway appears to be regulated by metabolic demand for ATP. The majority of non-photochemical quenching in these chloroplasts at moderate irradiances appeared to be "energy"-dependent quenching.