The effects of low temperature acclimation and photoinhibitory treatments on Photosystem 2 studied by thermoluminescence and fluorescence decay kinetics

The effects of low temperature acclimation and photoinhibitory treatment on Photosystem 2 (PS 2) have been studied by thermoluminescence and chlorophyll fluorescence decay kinetics after a single turnover saturating flash. A comparison of unhardened and hardened leaves showed that, in the hardened case, a decrease in overall and B-band thermoluminescence emissions occurred, indicating the presence of fewer active PS 2 reaction centers. A modification in the form of the B-band emission was also observed and is attributed to a decrease in the apparent activation energy of recombination in the hardened leaves. The acclimated leaves also produced slower Q_A ^– reoxidation kinetics as judged from the chlorophyll fluorescence decay kinetics. This change was mainly seen in an increased lifetime of the slow reoxidation component with only a small increase in its amplitude. Similar changes in both thermoluminescence and fluorescence decay kinetics were observed when unhardened leaves were given a high light photoinhibitory treatment at 4°C, whereas the hardened leaves were affected to a much lesser extent by a similar treatment. These results suggest that the acclimated plants undergo photoinhibition at 4°C even at low light intensities and that a subsequent high light treatment produces only a small additive photoinhibitory effect. Furthermore, it can be seen that photoinhibition eventually gives rise to PS 2 reaction centers which are no longer functional and which do not produce thermoluminescence or variable chlorophyll fluorescence.Abbreviations D1 The 32 kDa protein of Photosystem 2 reaction center - F_m maximum chlorophyll fluorescence yield - F₀ minimal chlorophyll fluorescence yield obtained when all PS 2 centers are open - F_i intermediate fluorescence level corresponding to PS 2 centers which are loosely or not connected to plastoquinone (non-B centers) - F_v maximum variable chlorophyll fluorescence yield (F_v=F_m–F₀) - PS 2 Photosystem 2 - Q_A and Q_B respectively, primary and secondary quinonic acceptors of PS 2 - S1, S2 and S3 respectively, the one, two and three positively charged states of the oxygen evolving system - Z secondary donor of PS 2     

Comparative EPR and thermoluminescence study of anoxic photoinhibition in Photosystem II particles

Photosystem II particles were exposed to 800 W m^–2 white light at 20 °C under anoxic conditions. The F_o level of fluorescence was considerably enhanced indicating formation of stable-reduced forms of the primary quinone electron acceptor, Q_A. The F_m level of fluorescence declined only a little. The g=1.9 and g=1.82 EPR forms characteristic of the bicarbonate-bound and bicarbonate-depleted semiquinone-iron complex, Q_A ^–Fe²⁺, respectively, exhibited differential sensitivity against photoinhibition. The large g=1.9 signal was rapidly diminished but the small g=1.82 signal decreased more slowly. The S₂-state multiline signal, the oxygen evolution and photooxidation of the high potential form of cytochrome b-559 were inhibited approximately with the same kinetics as the g=1.9 signal. The low potential form of oxidized cytochrome b-559 and Signal II_slow arising from Tyr_D ⁺ decreased considerably slower than the g=1.9 semiquinone-iron signal. The high potential form of oxidized cytochrome b-559 was diminished faster than the low potential form. Photoinhibition of the g=1.9 and g=1.82 forms of Q_A was accompanied with the appearance and gradual saturation of the spin-polarized triplet signal of P 680. The amplitude of the radical signal from photoreducible pheophytin remained constant during the 3 hour illumination period. In the thermoluminescence glow curves of particles the Q band (S₂Q_A ^– charge recombination) was almost completely abolished. To the contrary, the C band (Tyr_D ⁺Q_A ^– charge recombination) increased a little upon illumination. The EPR and thermoluminescence observations suggest that the Photosystem II reaction centers can be classified into two groups with different susceptibility against photoinhibition.Abbreviations C band thermoluminescence band associated with Tyr-D⁺Q _a ^– charge recombination - Chl chlorophyll - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - EPR electron paramagnetic resonance - F_o initial fluorescence - F_m maximum fluorescence - Q band thermoluminescence band originating from S₂Q _a -charge recombination - Q _a the primary quinone electron acceptor of PS II - P 680 the primary electron donor chlorophyll of PS II - S₂ oxidation state of the water-splitting system - Phe pheophytin - TL thermoluminescence - Tyr _d redox active tyrosine-160 of the D₂ protein     

Effects of dark- and light-induced proton gradients in thylakoids on the Q and B thermoluminescence bands

Thermoluminescence experiments have been carried out to study the effect of a transmembrane proton gradient on the recombination properties of the S₂ and S₃ states of the oxygen evolving complex with Q_A ^- and Q_B ^-, the reduced electron acceptors of Photosystem II. We first determined the properties of the S₂Q_A ^- (Q band), S₂Q_B ^- and S₃Q_B ^- (B bands) recombinations in the pH range 5.5 to 9.0, using uncoupled thylakoids. The, a proton gradient was created in the dark, using the ATP-hydrolase function of ATPases, in coupled unfrozen thylakoids. A shift towards low temperature of both Q and B bands was observed to increase with the magnitude of the proton gradient measured by the fluorescence quenching of 9-aminoacridine. This downshift was larger for S₃Q_B ^- than for S₂Q_B ^- and it was suppressed by nigericin, but not by valinomycin. Similar results were obtained when a proton gradient was formed by photosystem I photochemistry. When Photosystem II electron transfer was induced by a flash sequence, the reduction of the plastoquinone pool also contributed to the downshift in the absence of an electron acceptor. In leaves submitted to a flash sequence above 0°C, a downshift was also observed, which was supressed by nigericin infiltration. Thus, thermoluminescence provides direct evidence on the enhancing effect of lumen acidification on the S₃S₂ and S₂S₁ reverse-transitions. Both reduction of the plastoquinone pool and lumen acidification induce a shift of the Q and B bands to lower temperature, with a predominance of lumen acidification in non-freezing, moderate light conditions.Abbreviations 9-AA 9-aminoacridine - E_A activation energy - F₀ constant fluorescence level - F_M maximum fluorescence, when all PS-II centers are closed - F_V variable fluorescence (F_M–F₀) - PS I, PS II Photosystem I, photosystem II - PQ plastoquinone - TL thermoluminescence     

Thermoluminescence study of the in vivo effects of bicarbonate depletion and acetate/formate presence in the two algae Chlamydobotrys stellata and Chlamydomonas reinhardtii

We investigated the influence of CO₂/HCO₃ ^–-depletion and of the presence of acetate and formate on the in vivo photosynthetic electron transport in the two green algae Chlamydobotrys stellata and Chlamydomonas reinhardtii by means of thermoluminescence technique and mathematical glow curve analysis. The main effects of the removal of CO₂ from the algal cultures was: (1) A shift of the glow curve peak position to lower temperatures resulting from a decrease of the B band and an increase of the Q band. (2) Treatment of CO₂-deficient Chl. stellata with DCMU yielded two thermoluminescence bands in the Q band region peaking at around +12°C and +5°C; in case of Chl. reinhardtii DCMU treatment induced only one band with an emission maximum at +5°C. The presence of acetate or formate in CO₂-depleted algal cultures lowered the intensities of all of the individual TL bands but that of a HT band (TL+37). The effects of CO₂-depletion and of the presence of anions were fully reversible.Abbreviations DCMU 3-(3,4)-dichlorophenyl-1,1-dimethylurea - HT band high temperature TL band - P₆₈₀ reaction center chlorophyll of PS II - Q_A and Q_B primary and secondary quinone acceptors of PS II, respectively - PS II Photosystem II - S_2/3 redox states of the oxygen evolving complex of PS II - TL thermoluminescence     

Thermoluminescence properties of the isolated photosystem two reaction centre

Formation of thermoluminescence signals is characteristics of energy- and charge storage in Photosystem II. In isolated D1/D2/cytochrome b-559 Photosystem II reaction centre preparation four thermoluminescence components were found. These appear at -180 (Z band), between -80 and -50 (Z_v band), at -30 and at +35°C. The Z band arises from pigment molecules but not correlated with photosynthetic activity. The Z_v and -30°C bands arise from the recombination of charge pairs stabilized in the Photosystem II reaction centre complex. The +35°C band probably corresponds to the artefact glow peak resulting from a pigment-protein-detergent interaction in subchloroplast preparations (Rózsa Zs, Droppa M and Horváth G (1989) Biochim Biophys Acta 973, 350–353).Abbreviations Chl chlorophyll - Cyt cytochrome - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - D1 psbA gene product - D2 psbD gene product - P680 primary electron donor of PS II - Pheo pheophytin - PS II Photosystem II - Q_A primary quinone acceptor of PS II - Q_B secondary quinone acceptor of PS II - RC reaction centre of PS II - TL thermoluminescence     

A simple model relating photoinhibitory fluorescence quenching in chloroplasts to a population of altered Photosystem II reaction centers

A model is presented describing the relationship between chlorophyll fluorescence quenching and photoinhibition of Photosystem (PS) II-dependent electron transport in chloroplasts. The model is based on the hypothesis that excess light creates a population of inhibited PS II units in the thylakoids. Those units are supposed to posses photochemically inactive reaction centers which convert excitation energy to heat and thereby quench variable fluorescence. If predominant photoinhibition of PS II and cooperativity in energy transfer between inhibited and active units are presumed, a quasi-linear correlation between PS II activity and the ratio of variable to maximum fluorescence, F_VF_M, is obtained. However, the simulation does not result in an inherent linearity of the relationship between quantum yield of PS II and F_VF_M ratio. The model is used to fit experimental data on photoinhibited isolated chloroplasts. Results are discussed in view of current hypotheses of photoinhibition.Abbreviations F_M maximum total fluorescence - F₀ initial fluorescence - F_V maximum variable fluorescence - PS Photosystem - Q_A, Q_B primary and secondary electron acceptors of Photosystem II     

The non-photochemical reduction of plastoquinone in leaves

Although it is generally assumed that the plastoquinone pool of thylakoid membranes in leaves of higher plants is rapidly oxidized upon darkening, this is often not the case. A multiflash kinetic fluorimeter was used to monitor the redox state of the plastoquinone pool in leaves. It was found that in many species of plants, particularly those using the NAD-malic enzyme C₄ system of photosynthesis, the pool actually became more reduced following a light to dark transition. In some Amaranthus species, plastoquinone remained reduced in the dark for several hours. Far red light, which preferentially drives Photosystem I turnover, could effectively oxidize the plastoquinone pool. Plastoquinone was re-reduced in the dark within a few seconds when far red illumination was removed. The underlying mechanism of the dark reduction of the plastoquinone pool is still uncertain but may involve chlororespiratory activity.Abbreviations apparent F_o observed fluorescence yield after dark adaptation - F_m maximum fluorescence when all Q_A is fully reduced - F_o minimum fluorescence yield when Q_A is fully oxidized and non-photochemical quenching is fully relaxed - F_s steady state fluorescence yield - PPFD photosynthetic photon flux density - PQ plastoquinone - Q_A primary quinone acceptor of the Photosystem II reaction center - Q_B secondary quinone acceptor to the Photosystem II reaction center - F F_m minus F_s     

Evidence for the contribution of the S-state transitions of oxygen evolution to the initial phase of fluorescence induction

Fluorescence induction of isolated spinach chloroplasts was measured by using weak continuous light. It is found that the kinetics of the initial phase of fluorescence induction as well as the initial fluorescence level F_j are influenced by the number of preilluminating flashes, and shows damped period 4 oscillation. Evidence is given to show that it is correlated with the S-state transitions of oxygen evolution. Based on the previous observations that the S states can modulate the fluorescence yield of Photosystem II, a simulating calculation suggests that, in addition to the Photosystem II centers inactive in the plastoquinone reduction, the S-state transitions can also make a contribution to the intial phase of fluorescence induction.Abbreviations DCMU 3-(3,4-dichlorophenyl)-1,1-dimethyl urea - F₀ non-variable fluorescence level emitted when all PS II centers are open - F_i initial fluorescence level immediately after shutter open - F_pt intermediate plateau fluorescence level - F_m maximum fluorescence level emitted when all PS II centers are closed - PS II Photosystem II - Q_A primary quinone acceptor of PS II - Q_B secondary quinone acceptor of PS II     

Photochemical reactions of chlorophyll in dehydrated Photosystem II: two chlorophyll forms (680 and 700 nm)

Lichens and phototolerant poikilohydric mosses differ from spinach leaves, fern fronds or photosensitive mosses in that they show strongly decreased F_o chlorophyll fluorescence after drying. This desiccation-induced fluorescence loss is rapidly reversible under rehydration. Fluorescence emission from Photosystem II at 685 nm was decreased more strongly by dehydration than 720 nm emission. Reaction centers of Photosystem II lose activity on dehydration and regain it on hydration. Heating of desiccated lichens increased F_o chlorophyll fluorescence. The activation energy for the reversible part of the temperature-dependent fluorescence increase was 0.045 eV, which corresponds to the energy difference between the 680 and 697 nm absorption bands. In desiccated chlorolichens such as Parmelia sulcata, heating induces the appearance of positive variable fluorescence related to the reversible reduction of Q_A due to overcoming the energy barrier. This is interpreted to provide information on the mechanism of photoprotection: energy is dissipated by changing Chl680 or P680 into a chlorophyll form, which absorbs at 700 nm and emits light at 720 nm (Chl-720 or P680(700)) with a low quantum yield. Dissipation of light energy in this trap is activated by desiccation.     

Effect of complete extraction and re-addition of manganese on thermoluminescence of pea Photosystem II preparations

Thermoluminescence of Photosystem II particles isolated from pea chloroplasts using digitonin and Triton X-100 was measured after 1 min illumination at a certain temperature (T(ex)) followed by illumination during cooling (40 Cdeg/min) to a lower temperature. Glow curves of the particles are characteristic of the photosynthetic oxygen-evolving material studied earlier. Complete (more than 95%) removal of Mn from the Photosystem II particles abolishes thermoluminescence bands around 0° C, related to the oxygen-evolving system, but the thermoluminescence bands peaking around -30°C (TL(-30)), -55°C (TL_ (-55)) and between-68 and -85° C, depending on Tex(TLv), remain unaltered. The bands are characterized by different dependence on T,x. The TL(-30), TL(-55) and TL v bands can also be observed in the glow curve of isolated pea and spinach chloroplasts. Re-addition of MnCI (2) (2 μM, corresponding to nearly 4 Mn atoms per reaction center of Photosystem II) to the Mn-depleted particles does not reactivate the thermoluminescence bands around 0° C. However, it does lead to suppression of TL(-30) accompanied by parallel activation of TL(-55), revealing competition of the TL (-30) and TL(-55) for charges generated by the reaction center. These data, as well as the results on the effect of inhibitors and electron donors to Photosystem II, show that positive charges contributing to the TL(-30), TL (-55) and TL v thermoluminescence bands are located on secondary electron donors of Photosystem II which do not require Mn and are located closer to the reaction center than the Mn-containing, water-oxidizing enzyme.     

Spectroscopy of non-photochemical and photochemical quenching of chlorophyll fluorescence in leaves; evidence for a role of the light harvesting complex of Photosystem II in the regulation of energy dissipation

Dissipation of absorbed excitation energy as heat, measured by its effect on the quenching of chlorophyll fluorescence, is induced under conditions of excess light in order to protect the photosynthetic apparatus of plants from light-dependent damage. The spectral characteristics of this quenching have been compared to that due to photochemistry in the Photosystem II reaction centre using leaves of Guzmania monostachia. This was achieved by making measurements at 77K when fluorescence emission bands from each type of chlorophyll protein complex can be distinguished. It was demonstrated that photochemistry and non-photochemical dissipation preferentially quench different emission bands and therefore occur by dissimilar mechanisms at separate sites. It was found that photochemistry was associated with a preferential quenching of emission at 688 nm whereas the spectrum for rapidly reversible non-photochemical quenching had maxima at 683 nm and 698 nm, suggesting selective quenching of the bands originating from the light harvesting complexes of Photosystem II. Further evidence that this was occurring in the light harvesting system was obtained from the fluorescence excitation spectra recorded in the quenched and relaxed states.Abbreviations pH transthylakoid pH gradient - F_o minimum level of chlorophyll fluorescence when Photosystem II reaction centres are open - F_m maximum level of fluorescence when Photosystem II reaction centres are closed - F_v variable fluorescence F_m minus F_o - F'_o F_o in any quenched state - F_m F_m in any quenched state - LHCII light harvesting complexes of Photosystem II - PSI Photosystem I - PS II Photosystem II - qN non-photochemical quenching of chlorophyll fluorescence - qE non-photochemical quenching of chlorophyll fluorescence that occurs in the presence of a pH     

Relation Between the Heat-Induced Increase of F0 Fluorescence and a Shift in the Electronic Equilibrium at the Acceptor Side of Photosystem 2

F₀ fluorescence and thermoluminescence (TL) were recorded simultaneously on various dark-adapted leaf samples. Above 40 °C, a sharp peak of TL coincided with the onset of the heat-induced F₀ rise. It results from a back-transfer of an electron from the secondary Q_B ^-to the primary acceptor Q_A of photosystem 2, followed by a luminescence-emitting recombination with Tyr-D¹. This demonstrates that the critical temperature at which the F₀ starts rising also corresponds to a shift towards the left of the Q_A↔Q_B ^- equilibrium. This revised version was published online in August 2006 with corrections to the Cover Date.     

Light‐independent thermoluminescence from thylakoids of greening barley leaves. Evidence for involvement of oxygen radicals and free chlorophyll

A study was conducted to identify biophysical markers which change in response to changing chlorophyll organization during plant development. When heated to around 70°C in the dark, barley thylakoids emit red thermoluminescence (TL). This is a pure chemiluminescence signal and distinct from the lower-temperature TL bands of thylakoids that are seen only with preillumination. The development of the light-independent, 70°C TL band was investigated following transfer of dark-grown barley leaves to the light. Because of the rapidly increasing chlorophyll content of the plastid membrane, the TL signal was normalized against either chlorophyll or tissue mass of the starting material. In either case, the extent of the TL signal reached a maximum in the early hours of greening and then declined. The drop in signal over 20 h was approximately 50% for TL per unit tissue mass, and well over 90% for TL per unit chlorophyll. Exposure of plastid membrane samples to hydrogen peroxide for several minutes caused a large increase in light-independent TL, while addition of ascorbate caused substantial quenching. The fluorescence profiles of dark-grown barley leaves were recorded following transfer to the light. Basal fluorescence (F₀) reaches a substantial level after just seconds of illumination. Over the next few hours, F₀ increases only slightly and then starts to decline. The decline in F₀ is correlated with an increase in variable fluorescence (F_v) which indicates the appearance of active photosystem II. It is concluded that the early peak in F₀ reflects a state in which the leaves contain a maximum amount of disorganized chlorophyll. Considering the TL and fluorescence data together, we propose the following: When chlorophyll first appears in the system, it is not properly assembled into the complexes that offer photochemical or non-photochemical quenching of the excited state. Thus, fluorescence and parallel chlorophyll triplet formation are prevalent. The triplets cause generation of active oxygen resulting in lipid peroxidation and/or other radical-generating processes. When the membranes are heated, increased interaction of the radicals with chlorophyll generates chemiluminescence. We thus conclude that light-independent thermoluminescence is a marker for actual damage arising from poor chlorophyll organization and propose that this parameter might be usefully applied for assessing stress effects.     

Thermoluminescence studies on the function of Photosystem II in the desiccation tolerant lichen Cladonia convoluta

The effect of desiccation and rehydration on the function of Photosystem II has been studied in the desiccation tolerant lichen Cladonia convoluta by thermoluminescence. We have shown that in functional fully hydrated thalli thermoluminescence signals can be observed from the recombination of the S₂₍₃₎Q_B ^– (B band), S₂Q_A ^– (Q band), Tyr-D⁺Q_A ^– (C band) and Tyr-Z⁺(His⁺)Q_A ^– (A band) charge stabilization states. These thermoluminescence signals are completely absent in desiccated thalli, but rapidly reappear on rehydration. Flash-induced oscillation in the amplitude of the thermoluminescence band from the S₂₍₃₎Q_B ^– recombination shows the usual pattern with maxima after 2 and 6 flashes when rehydration takes place in light. However, after rehydration in complete darkness, there is no thermoluminescence emission after the 1 st flash, and the maxima of the subsequent oscillation are shifted to the 3rd and 7th flashes. It is concluded that desiccation of Cladonia convoluta converts PS II into a nonfunctional state. This state is characterized by the lack of stable charge separation and recombination, as well as by a one-electron reduction of the water-oxidizing complex. Restoration of PS II function during rehydration can proceed both in the light and in darkness. After rehydration in the dark, the first charge separation act is utilized in restoring the usual oxidation state of the water-oxidizing comples.Abbreviations Chl chlorophyll - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DT desiccation tolerant - PS II Photosystem II - TL thermoluminescence - P₆₈₀ reaction center Chl of PS II - Q_A and Q_B puinone electron acceptors of PS II - S₀,...,S₄ the redox states of the water-oxidizing complex - Tyr-Z and Tyr-D redox-active tyrosine electron donors of PS II     

Thermoluminescence and fluorescence study of changes in Photosystem II photochemistry in desiccating barley leaves

An effect of desiccation (a decrease of relative water content from 97% to 10% within 35 h) on Photosystem II was studied in barley leaf segments (Hordeum vulgare L. cv. Akcent) using chlorophyll a fluorescence and thermoluminescence (TL). The O-J-I-P fluorescence induction curve revealed a decrease of F_P and a slight shift of the J step to a shorter time with no change in its height. The analysis of the fluorescence decline after a saturating light flash revealed an increased portion of slow exponential components with increasing desiccation. The TL bands obtained after excitation by continuous light were situated at about –27°C (Z_v band – recombination of P680⁺Q_A ⁻), –14 °C (A band – S₃Q_A ⁻), +12 °C (B band – S_2/3Q_B ⁻) and +45 °C (C band – TyrD⁺Q_A ⁻). The bands related to the S-states of oxygen evolving complex (A and B) were reduced by desiccation and shifted to higher and lower temperatures, respectively. In accordance with this, the band observed at about +27 °C (S₂Q_B ⁻) after excitation by 1 flash fired at –10 °C and band at about +20 °C (S_2/3Q_B ⁻) after 2 flashes decreased with increasing water deficit and shifted to lower temperatures. A new band around 5 °C appeared in both regimes of TL excitation for a relative water content of under 42% and was attributed to the Q band (S₂Q_A ⁻). It is suggested that under desiccation, an inhibition of the formation of S₂- and S₃-states in OEC occurred simultaneously with a lowering of electron transport on the acceptor side of PS II. The temperature down-shift of the TL bands obtained after the flash excitation was induced at the initial phases of water stress, indicating a decrease of the activation energy for the S_2/3Q_B ⁻recombination. This revised version was published online in June 2006 with corrections to the Cover Date.     

Cyclic electron flow around Photosystem II in vivo

The oxygen flash yield (Y_O2) and photochemical yield of PS II (_{PS II}) were simultaneously detected in intact Chlorella cells on a bare platinum oxygen rate electrode. The two yields were measured as a function of background irradiance in the steady-state and following a transition from light to darkness. During steady-state illumination at moderate irradiance levels, Y_O2 and _{PS II} followed each other, suggesting a close coupling between the oxidation of water and Q_A reduction (Falkowski et al. (1988) Biochim. Biophys. Acta 933: 432–443). Following a light-to-dark transition, however, the relationship between Q_A reduction and the fraction of PS II reaction centers capable of evolving O₂ became temporarily uncoupled. _{PS II} recovered to the preillumination levels within 5–10 s, while the Y_O2 required up to 60 s to recover under aerobic conditions. The recovery of Y_O2 was independent of the redox state of Q_A, but was accompanied by a 30% increase in the functional absorption cross-section of PS II (_{PS II}). The hysteresis between Y_O2 and the reduction of Q_A during the light-to-dark transition was dependent upon the reduction level of the plastoquinone pool and does not appear to be due to a direct radiative charge back-reaction, but rather is a consequence of a transient cyclic electron flow around PS II. The cycle is engaged in vivo only when the plastoquinone pool is reduced. Hence, the plastoquinone pool can act as a clutch that disconnects the oxygen evolution from photochemical charge separation in PS II.Abbreviations ADRY acceleration of the deactivation reactions of the water-splitting enzyme (agents) - Chl chlorophyll - cyt cytochrome - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - F_O minimum fluorescence yield in the dark-adapted state - F_I minimum fluorescence yield under ambient irradiance or during transition from the light-adapted state - F_M maximum fluorescence yield in the dark-adapted state - F_M maximum fluorescence yield under ambient irradiance or during transition from light-adapted state - F_V, F_V variable fluorescence (F_V=F_M–F_O ; F_V=F_M–F_I) - FRR fast repetition rate (fluorometer) - _{PS II} quantum yield of Q_A reduction (_{PS II}=(F_M – F_O)/F_M or _{PS II})=(F_M= – F_I=)/F_M=) - LHCII Chl a/b light harvesting complexes of Photosystem II - OEC oxygen evolving complex of PS II - P680 reaction center chlorophyll of PS II - PQ plastoquinone - POH₂ plastoquinol - PS I Photosystem I - PS II Photosystem II - RC II reaction centers of Photosystem II - PS II the effective absorption cross-section of PHotosystem II - TL thermoluminescence - Y_O2 oxygen flash yield The US Government right to retain a non-exclusive, royalty free licence in and to any copyright is acknowledged.     

The effect of UV-B radiation on photosynthesis and respiration of phytoplankton,benthic macroalgae and seagrasses

Several species of marine benthic algae, four species of phytoplankton and two species of seagrass have been subjected to ultraviolet B irradiation for varying lengths of time and the effects on respiration, photosynthesis and fluorescence rise kinetics studied. No effect on respiration was found. Photosynthesis was inhibited to a variable degree in all groups of plants after irradiation over periods of up to 1 h and variable fluorescence was also inhibited in a similar way. The most sensitive plants were phytoplankton and deep-water benthic algae. Intertidal benthic algae were the least sensitive to UV-B irradiation and this may be related to adaptation, through the accumulation of UV-B screening compounds, to high light/high UV-B levels. Inhibition of variable fluorescence (F_v) of the fluorescence rise curve was a fast and sensitive indicator of UV-B damage. Two plants studied, a brown alga and a seagrass, showed very poor recovery of F_v over a period of 32 h.Abbreviations F_m- fluorescence yield with reaction centres closed - F_o- fluorescence yield with reaction centres open - F_v- variable fluorescence - PAR- photosynthetically active radiation - P680- primary donor of Photosystem II - O- primary quencher of Photosystem II - Q_A- primary quinone acceptor of Photosystem II - UV-B- ultraviolet B     

Compared thermoluminescence characteristics of pea thylakoids studied in vitro and in situ (in leaves). The effect of photoinhibitory treatments

Characteristics of thermoluminescence (TL) glow curves were studied in thylakoids (isolated from pea leaves) or in intact pea leaves after an exposure to very high light for 2 min in the TL device. The inhibition of photosynthesis was detected as decreases of oxygen evolution rates and/or of variable fluorescence.In thylakoids exposed to high light, then dark adapted for 5 min, a flash regime induced TL glow curves which can be interpreted as corresponding to special B bands since: 1) they can be fitted by a single B band (leaving a residual band at –5°C) with a lower activation energy and a shift of the peak maximum by –5 to –6°C and, 2) the pattern of oscillation of their amplitudes was normal with a period of 4 and maxima on flashes 2 and 6. During a 1 h dark adaptation, no recovery of PS II activity occurred but the shift of the peak maximum was decreased to –1 to –2°C, while the activation energy of B bands increased. It is supposed that centers which remained active after the photoinhibitory treatment were subjected to reversible and probably conformational changes.Conversely, in intact leaves exposed to high light and kept only some minutes in the dark, TL bands induced by a flash regime were composite and could be deconvoluted into a special B band peaking near 30°C and a complex band with maximum at 2–5°C. In the case of charging bands by one flash, this low temperature band was largely decreased in size after a 10 min dark adaptation period; parallely, an increase of the B band type component appeared. Whatever was the flash number, bands at 2–5°C were suppressed by a short far red illumination given during the dark adaptation period and only remained a main band a 20°C; therefore, the origin of the low temperature band was tentatively ascribed to recombinations in centers blocked in state S₂Q_A ^–Q_B ^2–. In vivo, the recovery of a moderately reduced state in the PQ pool, after an illumination, would be slow and under the dependence of a poising mechanism, probably involving an electron transfer between cytosol and chloroplasts or the so-called chlororespiration process.Abbreviations E_a- activation energy - FR- far-red - MV- methylviologen - pBQ- p-benzoquinone - PQ- plastoquinone - PS II- Photosystem II - Q_A- primary quinone electron acceptor of PS II - Q_B- secondary quinone electron acceptor of PS II - TL- thermoluminescence     

The chlorophyll a fluorescence induction pattern in chloroplasts upon repetitive single turnover excitations: Accumulation and function of QB-nonreducing centers

The increase of chlorophyll fluorescence yield in chloroplasts in a 12.5 Hz train of saturating single turnover flashes and the kinetics of fluorescence yield decay after the last flash have been analyzed. The approximate twofold increase in F_m relative to F_o, reached after 30-40 flashes, is associated with a proportional change in the slow (1-20 s) component of the multiphasic decay. This component reflects the accumulation of a sizeable fraction of Q_B-nonreducing centers. It is hypothesized that the generation of these centers occurs in association with proton transport across the thylakoid membrane. The data are quantitatively consistent with a model in which the fluorescence quenching of Q_B-nonreducing centers is reversibly released after second excitation and electron trapping on the acceptor side of Photosystem II.     

Photosystem I-dependent cyclic electron transport is important in controlling Photosystem II activity in leaves under conditions of water stress

Leaves of the C₃ plant Brassica oleracea were illuminated with red and/or far-red light of different photon flux densities, with or without additional short pulses of high intensity red light, in air or in an atmosphere containing reduced levels of CO₂ and/or oxygen. In the absence of CO₂, far-red light increased light scattering, an indicator of the transthylakoid proton gradient, more than red light, although the red and far-red beams were balanced so as to excite Photosystem II to a comparable extent. On red background light, far-red supported a transthylakoid electrical field as indicated by the electrochromic P₅₁₅ signal. Reducing the oxygen content of the gas phase increased far-red induced light scattering and caused a secondary decrease in the small light scattering signal induced by red light. CO₂ inhibited the light-induced scattering responses irrespective of the mode of excitation. Short pulses of high intensity red light given to a background to red and/or far-red light induced appreciable additional light scattering after the flashes only, when CO₂ levels were decreased to or below the CO₂ compensation point, and when far-red background light was present. While pulse-induced light scattering increased, non-photochemical fluorescence quenching increased and F₀ fluorescence decreased indicating increased radiationless dissipation of excitation energy even when the quinone acceptor Q_A in the reaction center of Photosystem II was largely oxidized. The observations indicate that in the presence of proper redox poising of the chloroplast electron transport chain cyclic electron transport supports a transthylakoid proton gradient which is capable of controlling Photosystem II activity. The data are discussed in relation to protection of the photosynthetic apparatus against photoinactivation.Abbreviations F, F_M, F'_M, F"_M, F₀, F'₀ chlorophyll fluorescence levels - _exc quantum efficiency of excitation energy capture by open Photosystem II - _{PS II} quantum efficiency of electron flow through Photosystem II - P₅₁₅ field indicating rapid absorbance change peaking at 522 nm - P₇₀₀ primary donor of Photosystem I - Q_A primary quinone acceptor in Photosystem II - Q_N non-photochemical fluorescence quenching - Q_q photochemical quenching of chlorophyll fluorescence