Electron Transport-Dependent Chlorophyll-a Fluorescence Quenching by O(2) in Various Algae and Higher Plants

A comparison of chlorophyll-a fluorescence in brown algae (Macrocystis integrifolia, Fucus vesiculosis), green algae (Scenedesmus obliquus, Ulva sp.) and higher plants (bean, corn) show differences in the relative fluorescence intensities and induction time courses which characterize each type of plant. These differences are not reflected in either the maximum fluorescence emission in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (F_max) or the nonvariable fluorescence (F_o). Constancy of F_o and F_max suggests functional similarities of photosystem II and associated antennae pigments in the various classes of plants. The time course differences are observed only in the absence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea and appear, therefore, to be electron transport dependent. During induction, the peak in fluorescence (F_p) is much lower in all of the algae studied than in the higher plants. Exogenous O₂ strongly quenches F_p in all plants studied and our data indicate that the low F_p in the algae can be partially accounted for by endogenous O₂ quenching.     

Phosphorylation of chloroplast membrane proteins partially protects against photoinhibition

Thylakoids isolated from peas (Pisum sativum cv. Kelvedon Wonder) and phosphorylated by incubation with ATP have been compared with non-phosphorylated thylakoids in their sensitivity to photoinhibition by exposure to illumination in vitro. Assays of the kinetics of fluorescence induction at 20° C and the fluorescence emission spectra at-196° C indicate a proportionally larger decrease in fluorescence as a result of photoinhibitory treatment of non-phosphorylated compared with phosphorylated thylakoids. It is concluded that protein phosphorylation can afford partial protection to thylakoids exposed to photoinhibitory conditions.Abbreviations and symbols DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - F ₀ Level of chlorophyll fluorescence when photosystem 2 traps are open - F _m Level of chlorphyll fluorescence when photosystem 2 traps are closed - P Maximum level of fluorescence reached in the absence of DCMU - PSI (II) photosystem I(II)     

Clara cell damage and inhibition of pulmonary mixed-function oxidase activity by naphthalene

The photosystem II electron acceptor 3,6-dichloro-2,5-dimethoxy-p-benzoquinone [DCDMQ] is suggested to replace the second quinone-type two electron acceptor B (or R); the DCDMQ Hill reaction is sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethylurea, but is insensitive to dry heptane extraction of thylakoids and other photosystem II inhibitors. Addition of HCO₃^? to CO₂-depleted thylakoids in silicomolybdate, DCDMQ, diaminodurene and ferricyanide Hill reactions brought about 1,3,10 and 10 fold increase in the electron transport rates; these data confirm that HCO₃^? affects both Q^? to B and B^2? to PQ reactions.     

Age-related differences in frost sensitivity of the photosynthetic apparatus of two Plagiomnium species

Shoots of two species of moss, Plagiomnium undulatum (Hedw.) Kop. and Plagiomnium affine (Funck) Kop., were subjected to freezing at various temperatures. After thawing, the activities of different photosynthetic reactions were determined in relation to the ages of the leaves. Analysis of the fast kinetics of chlorophyll-a fluorescence of individual leaves showed that young and old tissues were considerably less frost tolerant than mature ones. In principle, the pattern of freeze inactivation of photosynthetic reactions resembles that observed in higher plants. The decreases in the amplitude of F_v (variable fluorescence) and the ratio of F_v to F_m (maximum fluorescence) with increasing freezing stress reflect a progressive inactivation of photosystem II (PSII)-mediated electron transport, i.e. inhibition of photoreaction to photochemistry and-or electron donation to the photochemical reaction, and thus a decline in the potential photochemical efficiency of PSII. The insignificant change in the F₀ (constant fluorescence) level during progressive decline of F_v indicates that the excitation-energy transfer between antenna pigments and from those to reaction centres of PSII was little impaired by lethal freezing stress. Sugar analyses of various stem sections showed that ontogenetic variation in the frost tolerance of leaves cannot be attributed to differences in the cellular levels of sucrose, glucose and fructose.Abbreviations and Symbols DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - F_m maximum fluorescence - F₀ constant (initial) fluorescence - F_v variable fluorescence     

The kinetics of photoinhibition of the photosynthetic apparatus in pea chloroplasts

Abstract The kinetics of a range of chlorophyll fluorescence parameters, non-cyclic electron transport and the capacity of the thylakoids to bind Atrazine were examined during photoinhibition treatment of intact pea chloroplasts. Parameters of fluorescence induction of chloroplasts in the presence and absence of 3-(3,4-dichlorophenyl)-1,1-dimethyl urea at 20 °C and at 77 K were determined. The contributions of photochemical and non-photochemical quenching processes to the loss of fluorescence during photoinhibitory treatment were assessed. Two distinct phases of photoinhibitory damage were observed. During the initial 5 min period of exposure to light the minimal fluorescence level (F_o) increased, whilst the maximal fluorescence level (F_P) decreased, both coupled and uncoupled non-cyclic electron transport to methyl viologen decreased and the ability to bind Atrazine to the thylakoids decreased. Fluorescence analyses demonstrated that during this period thylakoids were becoming increasingly less efficient at generating and maintaining a transmembrane proton electrochemical gradient. Photoinhibitory damage that occurred at later times between 5 and 20 min was of a very different nature. Both F_o and F_P declined, a loss of coupled and uncoupled non-cyclic electron transport was observed together with a loss of the capacity to photo-oxidize water. However, no further loss of Atrazine-binding was associated with such changes. A consistent decrease in the quantum yield of non-cyclic electron transport was also observed throughout photoinhibition treatment. The possibility of two distinct mechanisms of photoinhibitory damage to the photosynthetic apparatus is discussed.     

Characterization of a Purified Photosystem II-Phycobilisome Particle Preparation from Porphyridium cruentum

Detergent preparations isolated from thylakoids of the red alga Porphyridium cruentum, in a sucrose, phosphate, citrate, magnesium chloride medium consist of phycobilisomes and possess high rates of photosystem II activity. Characterization of these particles shows that the O₂-evolving activity is stable for several hours and the pH optimum is about 6.5 to 7.2. Response of the system to light, electron donors and acceptors, and inhibitors verify that the observed activity, measured both as O₂ evolution and 2,6-dichlorophenol-indophenol reduction, is due to photosystem II. Furthermore, photosystem II is functionally coupled to the phycobilisome in this preparation since green light, absorbed by phycobilisomes of P. cruentum, is effective in promoting both O₂ evolution and 2,6-dichlorophenol-indophenol reduction. Photosystem II activity declines when light with wavelengths shorter than 665 nm is removed. Both 3-(3,4-dichlorophenyl)-1,1-dimethylurea and atrazine inhibit photosystem II activity in this preparation, indicating that the herbicide binding site is a component of the photosystem II-phycobilisome particle.     

Photosystem II function and herbicide binding sites during photoinhibition of spinach chloroplasts in-vivo and in-vitro

The time courses of some Photosystem II (PS II) parameters have been monitored during in-vivo and in-vitro photoinhibition of spinach chloroplasts, at room temperature and at 10 °C or 0 °C. Exposing leaf discs of low-light grown spinach at 25 °C to high light led to photoinhibition of chloroplasts in-vivo as manifested by a parallel decrease in the number of functional PS II centres, the variable chlorophyll fluorescence at 77K (F _v /F _m ), and the number of atrazine-binding sites. When the photoinhibitory treatment was given at 10 °C, the former two parameters declined in parallel but the loss of atrazine-binding sites occurred more slowly and to a lesser extent. During in-vitro photoinhibition of chloroplast thylakoids at 25 °C, the loss of functional PS II centres proceeded slightly more rapidly than the loss of atrazine-binding sites, and this difference in rate was further increased when the thylakoids were photoinhibited at 0 °C. During the recovery phase of leaf discs (up to 9 h) the increases in F _v /F _m preceded that of the number of functional PS II centres, while only a further decline in the number of atrazine-binding sites was observed. The recovery of variable chlorophyll fluorescence and the concentration of functional PS II centres occurred more rapidly at 25 °C than at 10 °C. These results suggest that the photoinhibition of PS II function is a relatively temperature-independent early photochemical event, whereas the changes in the concentration of herbicide-binding sites appear to be a more complex biochemical process which can occur with a delayed time course.Abbreviations BSA bovine serum albumin - Chl chlorophyll - D1 32kDa herbicide-binding polypeptide in photosystem II and product of the psbA gene - D2 34kDa polypeptide in photosystem II which is the product of the psbD gene - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DCPIP 2,6-dichlorophenolin-dophenol - F ₀, F _v , F _m chlorophyll fluorescence with reaction centres open, variable and maximum fluorescence, respectively - LDS lithium dodecyl sulfate - MES 2-(N-morpholino) ethanesulfonic acid - PSII photosystem II - Q_A, Q_B first and second quinone-type PS II acceptor, respectively     

Analysis of Light-Induced Depressions of Photosynthesis in Leaves of a Wheat Crop during the Winter

The photosynthetic performances of individual leaves of a wheat (Triticum aestivum cv Bezostaya) crop were assessed daily and throughout individual days during the winter when temperature and light levels were fluctuating. Measurements of chlorophyll fluorescence induction and the maximum quantum yield of O₂ evolution were made on individual leaves. Depressions in the ratio of variable to maximal fluorescence (F_v/F_m) were correlated with low temperatures and high light levels throughout the winter and during the course of individual days. Depressions in F_v/F_m observed in the field during the day were not accompanied by any significant change in the ability of photosystem II complexes to bind 3-(3,4-dichlorophenyl)-1-dimethyl urea, indicating that the depressions in F_v/F_m were not attributable to photodamage to the D1 protein of the photosystem II reaction center. Decreases in F_v/F_m were associated with increases in the rate of dissipation of excitation energy by radiationless decay processes and decreases in the quantum efficiency of CO₂ assimilation, indicative of a rapidly reversible light-induced “downregulation” of photosynthesis. No major changes were observed in the maximum quantum efficiency of O₂ evolution of leaves throughout periods of fluctuating temperature and light, because light-induced depressions in photosynthetic efficiency recovered within the time required to make these measurements.     

Characterization of the photosystem II centers inactive in plastoquinone reduction by fluorescence induction

In order to characterize the photosystem II (PS II) centers which are inactive in plastoquinone reduction, the initial variable fluorescence rise from the non-variable fluorescence level F_o to an intermediate plateau level F_i has been studied. We find that the initial fluorescence rise is a monophasic exponential function of time. Its rate constant is similar to the initial rate of the fastest phase (-phase) of the fluorescence induction curve from DCMU-poisoned chloroplasts. In addition, the initial fluorescence rise and the -phase have the following common properties: their rate constants vary linearly with excitation light intensity and their fluorescence yields are lowered by removal of Mg⁺⁺ from the suspension medium. We suggest that the inactive PS II centers, which give rise to the fluorescence rise from F_o to F_i, belong to the -type PS II centers. However, since these inactive centers do not display sigmoidicity in fluorescence, they thus do not allow energy transfer between PS II units like PS II.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl urea - DMQ 2,5-dimethyl-p-benzoquinone - F_o initial non-variable fluorescence yield - F_m maximum fluorescence yield - F_i intermediate fluorescence yield - PS II photosystem II - Q_A primary quinone acceptor of PS II - Q_B secondary quinone acceptor of PS II     

The effect of high-energy-state excitation quenching on maximum and dark level chlorophyll fluorescence yield

The quenching of variable fluorescence yield (q_N) and the quenching of dark level fluorescence yield (q₀) directly atributable to high-energy-state fluorescence quenching (q_E) was studied to distinguish between energy dissipation in the antenna and light harvesting complexes (antenna quenching) and energy dissipation at the reaction centres (reaction centre quenching). A consistent relationship was obtained between q_N and q₀ in barley leaves, the green alga Dunaliella C9AA and in pea thylakoids with 2,3,5,6-tetramethyl-p-phenylene diamine (DAD) as mediator of cyclic electron flow around PS 1. This correlated well with the relationship obtained using m-dinitrobenzene (DNB), a chemical model for antenna quenching, to quench fluorescence in Dunaliella C9AA or pea thylakoids. The results also correlated reasonably well with theoretical predictions by the Butler model for antenna quenching, but did not correlate with the predictions for reaction centre quenching. It is postulated that q_E quenching therefore occures in the antenna and light harvesting complexes, and that the small deviation from the Butler prediction is due to PS 2 heterogeneity.Abbreviations 9-aa 9-aminoacridine - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - EDTA Ethylenediaminetetra-acetic acid - Hepes 4-(2-hydroxyethyl)-1-piperazineethanesulphonic acid - Mes 2-(N-morpholino) prophanesulfonate - PS 1 photosystem 1 - PS 2 photosystem 2 - Q_A and Q_B primary and secondary stable electron acceptors of photosystem 2 - q_N non-photochemical fluorescence quenching coefficient - q_E high-energy-state fluorescence quenching coefficient - q₀ quenching coefficient for F₀ - F₀ dark level fluorescence yield - F_m maximum fluorescence yield - F_v variable fluorescence yield - F_v/F_m ratio of variable to total fluorescence yield - DAD 2,3,5,6-tetramethyl-p-phenylene diamine - DNB m-dinitrobenzene     

Functional effects of chemical modification of unstacked and stacked chloroplasts with glutaraldehyde.

Although glutaraldehyde alkylates protein NH₂ groups to the same extent in unstacked and stacked thylakoids, the photosynthetic electron transport of the stacked membranes is always more inhibited. Inhibition of photosystem II electron transport, measured in the presence of lipophilic Hill oxidants, is 20–30% in unstacked and 60–70% in stacked thylakoids. Photosystem I electron transport is nearly completely inhibited in both preparations, but in the case of stacked thylakoids maximal inhibition occurs at a lower glutaraldehyde level than in unstacked thylakoids. In contrast, the photooxidation of the reaction center chromophore of photosystem I (P700) is unaffected by the glutaraldehyde treatment of either stacked or unstacked chloroplasts. The results are discussed with regard to the accessibility of membrane sites to exogenous electron transport cofactors, in view of the observation that N-methylphenazonium methosulfate, a quencher of electronically excited chlorophyll a, partitions more easily into the pigment domains of the glutaraldehyde-fixed unstacked thylakoids.     

Structural and functional self-organization of Photosystem II in grana thylakoids

The biogenesis of the well-ordered macromolecular protein arrangement of photosystem (PS)II and light harvesting complex (LHC)II in grana thylakoid membranes is poorly understood and elusive. In this study we examine the capability of self organization of this arrangement by comparing the PSII distribution and antenna organization in isolated untreated stacked thylakoids with restacked membranes after unstacking. The PS II distribution was deduced from freeze-fracture electron microscopy. Furthermore, changes in the antenna organization and in the oligomerization state of photosystem II were monitored by chlorophyll a fluorescence parameters and size analysis of exoplasmatic fracture face particles. Low-salt induced unstacking leads to a randomization and intermixing of the protein complexes. In contrast, macromolecular PSII arrangement as well as antenna organization in thylakoids after restacking by restoring the original solvent composition is virtually identical to stacked control membranes. This indicates that the supramolecular protein arrangement in grana thylakoids is a self-organized process.     

Activity of the natural algicide, cyanobacterin, on angiosperms

Cyanobacterin is a secondary metabolite produced by the cyanobacterium (blue-green alga) Scytonema hofmanni. The compound had previously been isolated and chemically characterized. It was shown to inhibit the growth of algae at a concentration of approximately 5 micromolar. Cyanobacterin also inhibited the growth of angiosperms, including the aquatic, Lemna, and terrestrial species such as corn and peas. In isolated pea chloroplasts, cyanobacterin inhibited the Hill reaction when p-benzoquinone, K₃Fe(CN)₆, dichlorophenolindophenol, or silicomolybdate were used as electron acceptors. The concentration needed to inhibit the Hill reaction in photosystem II was generally lower than the concentration of the known photosystem II inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethyl urea. Cyanobacterin had no effect on electron transport in photosystem I. The data indicate that cyanobacterin inhibits O₂ evolving photosynthetic electron transport in all plants and that the most probable site of action is in photosystem II.     

A New Mechanism for Adaptation to Changes in Light Intensity and Quality in the Red Alga Porphyra perforata: III. Fluorescence Transients in the Presence of 3-(3,4-Dichlorophenyl)-1,1-dimethylurea

In the red alga Porphyra perforata, the level of chlorophyll fluorescence in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) decreased during illumination of the thallus. The results showed that: (a) this decay was related to the photooxidative activity of photosystem I; (b) Q, the primary electron acceptor of photosystem II, became oxidized during the decay of the fluorescence; (c) reagents which inhibit the back reaction of photosystem II inhibited the decay.

From these results, it is suggested that, when conditions in the chloroplasts of this red alga become too oxidative, excess light energy can be converted to heat as a result of an accelerated back reaction of photosystem II. This may be one of the mechanisms by which this alga can cope with the high salt and high light conditions that can occur in its natural habitat.

     

State 1-State 2 Transitions in a Unicellular Green Algae : Analysis of In Vivo Chlorophyll Fluorescence Induction Curves in the Presence of 3-(3,4-Dichlorophenyl)-1, 1-dimethylurea (DCMU)

A study has been made on the State 1-State 2 transitions exhibited by the unicellular green algae Chlorella pyrenoidosa. Chlorophyll fluorescence induction curves from algae adapted to State 1 or State 2 have been analyzed and a comparison made with similar curves produced by decreasing the intensity of light going to the photosystem II reaction centers. In both cases, quenching of the maximum fluorescence yield (F_m) and the initial fluorescence yield (F_o) were observed so that the F_v/F_m ratio and the area above the induction curve (A_max) remained constant. The State 1-State 2 transition also produced changes in the β_max component indicative of some alteration within photosystem II organization. The implications of these experiments on the in vivo mechanism for energy redistribution between the two photosystems are discussed in terms of changes in absorption cross-section rather than being due to spillover from photosystem II to photosystem I. These changes may reflect the phosphorylation of the light-harvesting chlorophyll a/b protein complex and its subsequent migration away from the photosystem II core leading to its closer association with photosystem I.     

Effects of dehydration on the electron transport of Chlorella. An in vivo fluorescence study

Chlorella was used to study the effects of dehydration on photosynthetic activities. The use of unicellular green algae assured that the extent of dehydration was uniform throughout the whole cell population during the course of desiccation. Changes in the activities of the cells were monitored by measurements of fluorescence induction kinetics. It was found that inhibition of most of the photosynthetic activities started at a similar level of cellular water content. They included CO₂ fixation, photochemical activity of Photosystem II and electron transport through Photosystem I. The blockage of electron flow through Photosystem I was complete and the whole transition occurred within a relative short time of dehydration. On the other hand, the suppression of Photosystem II activity was incomplete and the transition took a longer time of dehydration. Upon rehydration, the inhibition of Photosystem II activity was fully reversible when samples were in the middle of the transition, but was not thereafter. The electron transport through Photosystem I was also reversible during the transition, but was only partially afterward.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl urea - F_m maximum fluorescence yield - F₀ non-variable fluorescence level emitted when all PS II centers are open - F_v variable part of fluorescence - PS photosystem - Q_A primary quinone acceptor of Photosystem II     

Interaction of exogenous quinones with membranes of higher plant chloroplasts: modulation of quinone capacities as photochemical and non-photochemical quenchers of energy in Photosystem II during light-dark transitions

Light modulation of the ability of three artificial quinones, 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB), 2,6-dichloro-p-benzoquinone (DCBQ), and tetramethyl-p-benzoquinone (duroquinone), to quench chlorophyll (Chl) fluorescence photochemically or non-photochemically was studied to simulate the functions of endogenous plastoquinones during the thermal phase of fast Chl fluorescence induction kinetics. DBMIB was found to suppress by severalfold the basal level of Chl fluorescence (F_o) and to markedly retard the light-induced rise of variable fluorescence (F_v). After irradiation with actinic light, Chl fluorescence rapidly dropped down to the level corresponding to F_o level in untreated thylakoids and then slowly declined to the initial level. DBMIB was found to be an efficient photochemical quencher of energy in Photosystem II (PSII) in the dark, but not after prolonged irradiation. Those events were owing to DBMIB reduction under light and its oxidation in the dark. At high concentrations, DCBQ exhibited quenching behaviours similar to those of DBMIB. In contrast, duroquinone demonstrated the ability to quench F_v at low concentration, while F_o was declined only at high concentrations of this artificial quinone. Unlike for DBMIB and DCBQ, quenched F_o level was attained rapidly after actinic light had been turned off in the presence of high duroquinone concentrations. That finding evidenced that the capacity of duroquinone to non-photochemically quench excitation energy in PSII was maintained during irradiation, which is likely owing to the rapid electron transfer from duroquinol to Photosystem I (PSI). It was suggested that DBMIB and DCBQ at high concentration, on the one hand, and duroquinone, on the other hand, mimic the properties of plastoquinones as photochemical and non-photochemical quenchers of energy in PSII under different conditions. The first model corresponds to the conditions under which the plastoquinone pool can be largely reduced (weak electron release from PSII to PSI compared to PSII-driven electron flow from water under strong light and weak PSI photochemical capacity because of inactive electron transport on its reducing side), while the second one mimics the behaviour of the plastoquinone pool when it cannot be filled up with electrons (weak or moderate light and high photochemical competence of PSI).     

Photooxidation reactions of diphenylcarbazide and their DCMU-sensitivity in thylakoids of the blue-green alga Oscillatoria chalybea

Thylakoids of Oscillatoria chalybea are able to split water. The Hill reaction of these thylakoids is sensitive to DCMU. Diphenylcarbazide can substitute for water as the electron donor to photosystem II with these fully functioning thylakoids. However, the diphenylcarbazide photooxidation is completely insensitive to 3-(3,4-dichlorophenyl)-N-N-dimethyl urea (DCMU) at high diphenylcarbazide concentrations. In with Tris-treated Oscillatoria thylakoids the water splitting capacity is lost and diphenylcarbazide restores electron transport through photosystem II as occurs with higher plant chloroplasts. However, also these photoreactions are insensitive to DCMU. If diphenylcarbazide acts in Oscillatoria as an electron donor to photosystem II the result suggests that diphenylcarbazide feeds in its electrons behind the DCMU inhibition site. This in turn indicates that in Oscillatoria the site of inhibition of DCMU is on the donor side of photosystem II.Abbreviations Used DCMU 3-(3,4-dichlorophenyl)-N-N-dimethyl urea - DPC diphenylcarbazide - DCPiP 2,6-dichlorophenol indophenol - TMB tetramethyl benzidine - A-2-sulf anthraquinone-2-sulfonate     

State transitions,photosystem stoichiometry adjustment and non-photochemical quenching in cyanobacterial cells acclimated to light absorbed by photosystem I or photosystem II

Cells of the cyanobacterium Synechococcus 6301 were grown in yellow light absorbed primarily by the phycobilisome (PBS) light-harvesting antenna of photosystem II (PS II), and in red light absorbed primarily by chlorophyll and, therefore, by photosystem I (PS I). Chromatic acclimation of the cells produced a higher phycocyanin/chlorophyll ratio and higher PBS-PS II/PS I ratio in cells grown under PS I-light. State 1-state 2 transitions were demonstrated as changes in the yield of chlorophyll fluorescence in both cell types. The amplitude of state transitions was substantially lower in the PS II-light grown cells, suggesting a specific attenuation of fluorescence yield by a superimposed non-photochemical quenching of excitation. 77 K fluorescence emission spectra of each cell type in state 1 and in state 2 suggested that state transitions regulate excitation energy transfer from the phycobilisome antenna to the reaction centre of PS II and are distinct from photosystem stoichiometry adjustments. The kinetics of photosystem stoichiometry adjustment and the kinetics of the appearance of the non-photochemical quenching process were measured upon switching PS I-light grown cells to PS II-light, and vice versa. Photosystem stoichiometry adjustment was complete within about 48 h, while the non-photochemical quenching occurred within about 25 h. It is proposed that there are at least three distinct phenomena exerting specific effects on the rate of light absorption and light utilization by the two photoreactions: state transitions; photosystem stoichiometry adjustment; and non-photochemical excitation quenching. The relationship between these three distinct processes is discussed.Abbreviations Chl chlorophyll - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - F relative fluorescence intensity at emission wavelength nm - F _o fluorescence intensity when all PS II traps are open - light 1 light absorbed preferentially by PS I - light 2 light absorbed preferentially by PS II - PBS phycobilisome - PS photosystem