Relaxation of variable chlorophyll fluorescence after illumination of dark-adapted barley leaves as influenced by the redox states of electron carriers

Kinetics of the dark relaxation of variable chlorophyll fluorescence, Fv, were studied after brief illumination of dark-adapted barley leaves in order to understand the rapid reversibility of pulse-induced fluorescence increases, which is observed even when fast linear electron transport to an external electron acceptor is not possible. Four kinetically distinct components were observed which reveal complexity in the oxidation of the reduced primary quinone acceptor of Photosystem II, Q_A ⁻: the slowest component accounted for 4–5% of maximal Fv and had a life-time of several seconds. It is suggested to represent a minor population of inactive Photosystem II centers. The other three components displayed first-order kinetics with half-time of 6–8 ms (`fast' component), 60–80 ms (`middle' component) and 650–680 ms (`slow' component). The fast component dominated Fv when methyl viologen or far-red light accelerated oxidation of plastohydroquinone. It shows rapid oxidation of Q_A ⁻ during electron flow to plastoquinone commensurate with maximum linear electron flow through the electron transport chain. The other two components were observed under conditions of restricted electron flow and excessive reduction of electron carriers. Unexpectedly, the slow component, which is interpreted to reflect the recombination between Q_A ⁻ and an intermediate on the oxidizing side of Photosystem II, saturated already at low irradiances of actinic light when plastoquinone was not yet strongly reduced suggesting that dark-adaptation of leaves results not only in the loss of activity of light-regulated enzymes of the carbon cycle but affects also electron flow from Q_A ⁻ to plastoquinone. KCN poisoning or high temperature treatment of leaves produced a nonexponential pattern of slow Fv relaxation. This effect was largely (heat treatment) or even completely (KCN) abolished by far-red light. This revised version was published online in August 2006 with corrections to the Cover Date.     

Short-term responses of Photosystem I to heat stress

When 23°C-grown potato leaves (Solanum tuberosum L.) were exposed for 15 min to elevated temperatures in weak light, a dramatic and preferential inactivation of Photosystem (PS) II was observed at temperatures higher than about 38°C. In vivo photoacoustic measurements indicated that, concomitantly with the loss of PS II activity, heat stress induced a marked gas-uptake activity both in far-red light (>715 nm) exciting only PS I and in broadband light (350–600 nm) exciting PS I and PS II. In view of its suppression by nitrogen gas and oxygen and its stimulation by high carbon-dioxide concentrations, the bulk of the photoacoustically measured gas uptake by heat-stressed leaves was ascribed to rapid carbon-dioxide solubilization in response to light-modulated stroma alkalization coupled to PS I-driven electron transport. Heat-induced gas uptake was observed to be insensitive to the PS II inhibitor diuron, sensitive to the plastocyanin inhibitor HgCl₂ and saturated at a rather high photon flux density of around 1200 E m^–2 s^–1. Upon transition from far-red light to darkness, the oxidized reaction center P700⁺ of PS I was re-reduced very slowly in control leaves (with a half time t_1/2 higher than 500 ms), as measured by leaf absorbance changes at around 820 nm. Heat stress caused a spectacular acceleration of the postillumination P700⁺ reduction, with t_1/2 falling to a value lower than 50 ms (after leaf exposure to 48°C). The decreased t_1/2 was sensitive to HgCl₂ and insensitive to diuron, methyl viologen (an electron acceptor of PS I competing with the endogenous acceptor ferredoxin) and anaerobiosis. This acceleration of the P700⁺ reduction was very rapidly induced by heat treatment (within less than 5 min) and persisted even after prolonged irradiation of the leaves with far-red light. After heat stress, the plastoquinone pool exhibited reduction in darkness as indicated by the increase in the apparent Fo level of chlorophyll fluorescence which could be quenched by far-red light. Application (for 1 min) of far-red light to heat-pretreated leaves also induced a reversible quenching of the maximal fluorescence level Fm, suggesting formation of a pH gradient in far-red light. Taken together, the presented data indicate that PS I responded to the heat-induced loss of PS II photochemical activity by catalyzing an electron flow from stromal reductants. Heat-stress-induced PS I electron transport independent of PS II seems to constitute a protective mechanism since block of this electron pathway in anaerobiosis was observed to result in a dramatic photoinactivation of PS I.Abbreviations PFD photon flux density - PS Photosystem - Apt and Aox amplitude of the photothermal and photobaric components of the photoacoustic signal, respectively - P700 reaction center pigment of PS I - Fo and Fm initial and maximal levels of chlorophyll fluorescence, respectively - Fv=Fm Fo-variable chlorophyll fluorescence - Q_A primary (stable) electron acceptor of PS II - DCMU (diuron) 3-(3,4-dichlorophenyl)-1,1-dimethylurea - Cyt cytochrome     

The impact of ozone fumigation and fertilization on chlorophyll fluorescence of birch leaves (Betula pendula)

 The impact of ozone fumigation on chlorophyll a fluorescence parameters and chlorophyll content of birch trees grown at high and low fertilization were studied for 6-, 8-, and 12-week old leaves. Fluorescence parameters were measured with a portable fluorometer with its fibre optics tightly inserted in a gas exchange cuvette at light intensities from 0 to 220 μmol photons m⁻² s⁻¹. Ozone caused significant changes of primary photosynthetic reactions: a decrease of the quantum yield of photosystem II and an increase of non-photochemical quenching. In all leaves a biphasic light response of non-photochemical quenching was observed. Ozone fumigation shifted the onset of the second phase from a PFD of about 60 μmol m⁻² s⁻¹ to about 30 μmol m⁻² s⁻¹. While the fertilizer concentration had no influence on this character, high fertilization supply of plants partially reduced O₃-induced damage. The light responses of Ft, Fm′ and NPQ observed in birch leaves grown in O₃-free air indicate the existence of at least two different processes governing energy conversion of the photosynthetic apparatus at PS II in the range of PFD 0–200 μmol photons m⁻² s⁻¹. The first phase was attributed to a rather slowly relaxing type of non-photochemical quenching, which, at least at low PFD, is thought to be related to a state 1–2 transition. The further changes of the fluorescence parameters studied at higher PFD might be explained by an increase of energy-dependent quenching, connected with the energization of the thylakoid membrane and zeaxanthin synthesis. A major effect of ozone treatment was a lowering of PS II quantum yield. This reflects a reduction of PS II electron transport and corresponds to the reduction of CO₂-fixation observed in ozonated leaves. Received: 24 September 1996 / Accepted: 27 January 1999     

Inhibition of photosynthetic oxygen evolution and electron transfer from the quinone acceptor Q<Subscript>A</Subscript><Superscript>−</Superscript> to Q<Subscript>B</Subscript> by iron deficiency

The effect of iron deficiency on photosynthetic electron transport in Photosystem II (PS II) was studied in leaves and thylakoid membranes of lettuce (Lactuca sativa, Romaine variety) plants. PS II electron transport was characterized by oxygen evolution and chlorophyll fluorescence parameters. Iron deficiency in the culture medium was shown to affect water oxidation and the advancement of the S-states. A decrease of maximal quantum yield of PS II and an increase of fluorescence intensity at step J and I of OJIP kinetics were also observed. Thermoluminescence measurements revealed that charge recombination between the quinone acceptor of PS II, Q_B, and the S₂ state of the Mn-cluster was strongly perturbed. Also the dark decay of Chl fluorescence after a single turnover white flash was greatly retarded indicating a slower rate of Q_A ⁻ reoxidation.     

Influence of protein phosphorylation on the electron-transport properties of Photosystem II

Many of the core proteins in Photosystem II (PS II) undergo reversible phosphorylation. It is known that protein phosphorylation controls the repair cycle of Photosystem II. However, it is not known how protein phosphorylation affects the partial electron transport reactions in PS II. Here we have applied variable fluorescence measurements and EPR spectroscopy to probe the status of the quinone acceptors, the Mn cluster and other electron transfer components in PS II with controlled levels of protein phosphorylation. Protein phosphorylation was induced in vivo by varying illumination regimes. The phosphorylation level of the D1 protein varied from 10 to 58% in PS II membranes isolated from pre-illuminated spinach leaves. The oxygen evolution and Q_A ⁻ to Q_B(Q_B ⁻) electron transfer measured by flash-induced fluorescence decay remained similar in all samples studied. Similar measurements in the presence of DCMU, which reports on the status of the donor side in PS II, also indicated that the integrity of the oxygen-evolving complex was preserved in PS II with different levels of D1 protein phosphorylation. With EPR spectroscopy we examined individual redox cofactors in PS II. Both the maximal amplitude of the charge separation reaction (measured as photo-accumulated pheophytin⁻) and the EPR signal from the Q_A ⁻ Fe²⁺ complex were unaffected by the phosphorylation of the D1 protein, indicating that the acceptor side of PS II was not modified. Also the shape of the S₂ state multiline signal was similar, suggesting that the structure of the Mn-cluster in Photosystem II did not change. However, the amplitude of the S₂ multiline signal was reduced by 35% in PS II, where 58% of the D1 protein was phosphorylated, as compared to the S₂ multiline in PS II, where only 10% of the D1 protein was phosphorylated. In addition, the fraction of low potential Cyt b ₅₅₉ was twice as high in phosphorylated PS II. Implications from these findings, were precise quantification of D1 protein phosphorylation is, for the first time, combined with high-resolution biophysical measurements, are discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

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

The dark-relaxation kinetics of variable fluorescence, F_v, in intact green leaves of Pisum stativum L. and Dolichos lablab L. were analyzed using modulated fluorometers. Fast (t_1/2 = 1 s) and slow (t_1/2 = 7–8 s) phases in f_v 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 F_v 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 F_v dark decay indicates that the slow phase develops when a fraction of Q_A ^– (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. F_v rise appears which is prevented by far-red (PSI-absorbed) light. This transient f_v rise reflects the accumulation of Q_A ^– in the dark. The observation of this transient F_v 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 F_v dark-decay kinetics represents the reoxidation of Q_A ^– by the electron-transport chain to PSI, whereas the slow phase is likely to be related to the interaction of Q_A ^– with the donor side of PSII.Abbreviations CCCP carbonylcyanide m-chlorophenylhydrazone - F_O initial fluorescence level - F_v variable fluorescence - P700 primary electron donor of PSI - PSI, II photosystem I, II - Q_A (Q_A ^–) Q_B (Q_B ^–) primary and secondary stable electron acceptor of PSII in oxidized (reduced) state Supported by grant B6.1/88 DST, Govt. of India.     

Electron transport to oxygen mitigates against the photoinactivation of Photosystem II in vivo

The role of electron transport to O₂ in mitigating against photoinactivation of Photosystem (PS) II was investigated in leaves of pea (Pisum sativum L.) grown in moderate light (250 mol m^–2 s^–1). During short-term illumination, the electron flux at PS II and non-radiative dissipation of absorbed quanta, calculated from chlorophyll fluorescence quenching, increased with increasing O₂ concentration at each light regime tested. The photoinactivation of PS II in pea leaves was monitored by the oxygen yield per repetitive flash as a function of photon exposure (mol photons m^–2). The number of functional PS II complexes decreased nonlinearly with increasing photon exposure, with greater photoinactivation of PS II at a lower O₂ concentration. The results suggest that electron transport to O₂, via the twin processes of oxygenase photorespiration and the Mehler reaction, mitigates against the photoinactivation of PS II in vivo, through both utilization of photons in electron transport and increased nonradiative dissipation of excitation. Photoprotection via electron transport to O₂ in vivo is a useful addition to the large extent of photoprotection mediated by carbon-assimilatory electron transport in 1.1% CO₂ alone.Abbreviations Fm, Fo, Fv- maximal, initial (corresponding to open PS II traps) and variable chlorophyll fluorescence yield, respectively - NPQ- non-photochemical quenching - PS- photosystem - Q_A- primary quinone acceptor - qP- photochemical quenching coefficient     

Period four oscillations in chlorophyll a fluorescence

The discovery of period four oscillations of the fluorescence yield under flashing light demonstrated that not only the redox state of the Photosystem II (PS II) electron acceptor Q_A, but also the oxygen evolving cycle (described by the S states) modulates the fluorescence yield of chlorophyll (Chl). The positive charges accumulated on the donor side of PS II act on the fluorescence yield (measured in the Q_A ⁻ state during a strong flash) through the concentration of the quencher P₆₈₀ ⁺, the oxidized form of PS II reaction center Chl a. However, the period four oscillations of the fluorescence yield detected 1 s after a strong flash (in the P₆₈₀Q_A state) have not yet been fully explained. This revised version was published online in June 2006 with corrections to the Cover Date.     

A model of photosystem II for the analysis of fast fluorescence rise in plant leaves

The polyphasic patterns of fluorescence induction rise in pea leaves in vivo and after the treatment with ionophores have been studied using a Plant Efficiency Analyzer. To analyze in detail photosystem II (PS II) electron transfer processes, an extended PS II model was applied, which included the sums of exponential functions to specify explicitly the light-driven formation of the transmembrane electric potential (ΔΨ(t)) as well as pH in the lumen (pH_L(t)) and stroma (pH_S(t)). PS II model parameters and numerical coefficients in ΔΨ(t), pH_L(t), and pH_S(t) were evaluated to fit fluorescence induction data for different experimental conditions: leaf in vivo or after ionophore treatment at low or high light intensity. The model imitated changes in the pattern of fluorescence induction rise due to the elimination of transmembrane potential in the presence of ionophores, when ΔΨ = 0 and pH_L(t), pH_S(t) changed to small extent relative to control values in vivo, with maximum ΔΨ(t) ∼ 90 mV and ΔΨ(t) ∼ 40 mV for the stationary state at ΔpH ≅ 1.8. As the light intensity was increased from 300 to 1200 μmol m⁻² s⁻¹, the heat dissipation rate constants increased threefold for nonradiative recombination of P680⁺Phe⁻ and by ∼30% for P680⁺Q_A⁻. The parameters ΔΨ, pH_S and pH_L were analyzed as factors of PS II redox state populations and fluorescence yield. The kinetic mechanism of fluorescence quenching is discussed, which is related with light-induced lumen acidification, when ⁺Q_A⁻ and P680⁺ recombination probability increases to regulate the Q_A reduction.     

The effect of hydration on protein flexibility in photosystem II of green plants studied by quasielastic neutron scattering

The effect of hydration on protein dynamics in photosystem II (PS II) membrane fragments from spinach has been investigated by using the method of quasielastic neutron scattering (QENS) at room temperature. The QENS data obtained indicate that the protein dynamics is strongly dependent on the extent of hydration. In particular, the hydration-induced activation of localized diffusive protein motions and Q_A⁻ reoxidation by Q_B in PS II appear to be correlated in their onset at a hydration value of about 45% relative humidity (r.h.). These findings underline the crucial functional relevance of localized diffusive protein motions on the picosecond-timescale for the reactions of light-induced photosynthetic water splitting under formation of plastoquinol and molecular oxygen in PS II of green plants. Advanced neutron scattering and complementary techniques to study biological systems. Contributions from the meetings, “Neutrons in Biology”, STFC Rutherford Appleton Laboratory, Didcot, UK, 11–13 July and “Proteins At Work 2007”, Perugia, Italy, 28–30 May 2007.     

Light acclimation maintains the redox state of the PS II electron acceptor Q2 within a narrow range over a broad range of light intensities

Chrysanthemum inducum-hybrid `Coral Charm', Hibiscus rosa-sinensis L. `Cairo Red' and Spathiphyllum wallisii Regel `Petit' were grown in natural light in a greenhouse at three levels of irradiance using permanent shade screens. Light acclimation of photosynthesis was characterized using modulated chlorophyll a fluorescence of intact leaves. A close correlation was found between the degree of reduction of the primary electron acceptor Q_A of Photosystem II (PS II) approximated as the fluorescence parameter 1−q_P, and light acclimation. The action range of 1−q_P was 0–0.4 from darkness to full irradiance around noon, within the respective light treatments in the greenhouse, indicating that most PS II reaction centres were kept open. In general, the index for electron transport (ETR) measured by chlorophyll fluorescence was higher for high-light (HL) than intermediate-(IL) and low-light (LL) grown plants. However, HL Chrysanthemum showed 40% higher ETR than HL Hibiscus at light saturation, despite identical redox states of Q_A. The light acclimation of the non-radiative dissipation of excess energy in the antenna, NPQ, varied considerably between the species. However, when normalized against q_P, a strong negative correlation was found between thermal dissipation and ETR measured by chlorophyll fluorescence. To be able to accommodate a high flux of electrons through PS II, the plants with the highest light-saturated ETR had the lowest NPQ/q_P. The possibility of using chlorophyll fluorescence for quantification of the energy balance between energy input and utilization in PS II in intact leaves is discussed. This revised version was published online in June 2006 with corrections to the Cover Date.     

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     

Effect of monochromatic UV-B radiation on electron transfer reactions of Photosystem II

The adverse effect of low intensity, small band UV-B irradiation (λ = 305 ± 5 nm, I = 300 mW m⁻²) on PS II has been studied by comparative measurements of laser flash-induced changes of the absorption at 325 nm, ΔA₃₂₅(t), as an indicator of redox changes in Q_A, and of the relative fluorescence quantum yield, F(t)/F_o, in PS II membrane fragments. The properties of untreated control were compared with those of samples where the oxygen evolution rate under illumination with continuous saturating light was inhibited by up to 95%. The following results were obtained: a) the detectable initial amplitude (at a time resolution of 30 μs) of the 325 nm absorption changes, ΔA₃₂₅, remained virtually invariant whereas the relaxation kinetics exhibit significant changes, b) the 300 μs kinetics of ΔA₃₂₅ dominating the relaxation in UV-B treated samples was largely replaced by a 1.3 ms kinetics after addition of MnCl₂, c) the extent of the flash induced rise of the relative fluorescence quantum yield was severely diminished in UV-B treated PS II membrane fragments but the relaxation kinetics remain virtually unaffected. Based on these results the water oxidizing complex (WOC) is inferred to be the primary target of UV-B impairment of PS II while the formation of the ‘stable’ radical pair P680^+·Q_A ^−● is almost invariant to this UV-B treatment. This revised version was published online in June 2006 with corrections to the Cover Date.     

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     

Coupled cyclic electron transport in intact chloroplasts and leaves of C3 plants: Does it exist? if so,what is its function?

Transthylakoid proton transport based on Photosystem I-dependent cyclic electron transport has been demonstrated in isolated intact spinach chloroplasts already at very low photon flux densities when the acceptor side of Photosystem I (PS I) was largely closed. It was under strict redox control. In spinach leaves, high intensity flashes given every 50 s on top of far-red, but not on top of red background light decreased the activity of Photosystem II (PS II) in the absence of appreciable linear electron transport even when excitation of PS II by the background light was extremely weak. Downregulation of PS II was a consequence of cyclic electron transport as shown by differences in the redox state of P700 in the absence and the presence of CO₂ which drained electrons from the cyclic pathway eliminating control of PS II. In the presence of CO₂, cyclic electron transport comes into play only at higher photon flux densities. At H⁺/e=3 in linear electron transport, it does not appear to contribute much ATP for carbon reduction in C3 plants. Rather, its function is to control the activity of PS II. Control is necessary to prevent excessive reduction of the electron transport chain. This helps to protect the photosynthetic apparatus of leaves against photoinactivation under light stress.     

Photoinactivation of photosystem II by cumulative exposure to short light pulses during the induction period of photosynthesis

Photoinactivation of Photosystem (PS) II in vivo was investigated by cumulative exposure of pea, rice and spinach leaves to light pulses of variable duration from 2 to 100 s, separated by dark intervals of 30 min. During each light pulse, photosynthetic induction occurred to an extent depending on the time of illumination, but steady-state photosynthesis had not been achieved. During photosynthetic induction, it is clearly demonstrated that reciprocity of irradiance and duration of illumination did not hold: hence the same cumulative photon exposure (mol m^–2) does not necessarily give the same extent of photoinactivation of PS II. This contrasts with the situation of steady-state photosynthesis where the photoinactivation of PS II exhibited reciprocity of irradiance and duration of illumination (Park et al. (1995) Planta 196: 401–411). We suggest that, for reciprocity to hold between irradiance and duration of illumination, there must be a balance between photochemical (qP) and non-photochemical (NPQ) quenching at all irradiances. The index of susceptibility to light stress, which represents an intrinsic ability of PS II to balance photochemical and non-photochemical quenching, is defined by the quotient (1-qP)/NPQ. Although constant in steady-state photosynthesis under a wide range of irradiance (Park et al. (1995). Plant Cell Physiol 36: 1163–1169), this index of susceptibility for spinach leaves declined extremely rapidly during photosynthetic induction at a given irradiance, and, at a given cumulative photon exposure, was dependent on irradiance. During photosynthetic induction, only limited photoprotective strategies are developed: while the transthylakoid pH gradient conferred some degree of photoprotection, neither D1 protein turnover nor the xanthophyll cycle was operative. Thus, PS II is more easily photoinactivated during photosynthetic induction, a phenomenon that may have relevance for understorey leaves experiencing infrequent, short sunflecks.Abbreviations D1 protein psbA gene product - DTT dithiothreitol - F_v, F_m, F_o variable, maximum, and initial (corresponding to open traps) chlorophyll fluorescence yield, respectively - NPQ non-photochemical quenching - PS Photosystem - Q_A primary quinone acceptor of PS II - qP photochemical quenching coefficient     

Enhanced photosystem 2 thermostability during leaf growth of Elm (Ulmus pumila) seedlings

We examined photosynthetic activities and thermostability of photosystem 2 (PS2) in leaves of elm (Ulmus pumila) seedlings from initiation to full expansion. During leaf development, net photosynthetic rate (P _N) increased gradually and reached the maximum when leaves were fully developed. In parallel with the increase of P _N, chlorophyll (Chl) content was significantly elevated. Chl a fluorescence measurements showed that the maximum quantum yield of PS2 (ϕ_PS2), the efficiency a trapped exciton, moved an electron into the electron transport chain further than Q_A ⁻ (Ψ_o), and the quantum yield of electron transport beyond Q_A (ϕ_Eo) increased gradually. These results were independently confirmed by our low irradiance experiments. When subjected to progressive heat stress, the young leaves exhibited considerably lower ϕ_PS2 and higher minimal fluorescence (F₀) than the mature leaves, revealing the highly sensitive nature of PS2 under heat in the newly initiating leaves. Further analysis showed that PS2 structure in the newly initiating leaves was strongly altered under heat, as evidenced by the increased fluorescence signals at the position of the K step. We therefore demonstrated an inhibition in the oxygen-evolving complex (OEC) in the young leaves. This resulted in decrease in amount of the functional PS2 reaction centres and relative increase in the PS2 reaction centres with inhibited electron transport at the acceptor side under heat. We suggest that the enhanced thermostability of PS2 during leaf development is associated with improved OEC stability.     

Photophysical processes of energy conversion in thylakoid membranes of <Emphasis Type="Italic">Chlamydomonas reinhardtii</Emphasis> mutants D1-R323H,D1-R323D,and D1-R323L

Chlamydomonas reinhardtii mutants D1-R323H, D1-R323D, and D1-R323L showed elevated chlorophyll fluorescence yields, which increased with decline of oxygen evolving capacity. The extra step K ascribed to the disturbance of electron transport at the donor side of PS II was observed in OJIP kinetics measured in mutants with a PEA fluorometer. Fluorescence decay kinetics were recorded and analyzed in a pseudo-wild type (pWt) and in mutants of C. reinhardtii with a Becker and Hickl single photon counting system in pico- to nanosecond time range. The kinetics curves were fitted by three exponentials. The first one (rapid, with lifetime about 300 ps) reflects energy migration from antenna complex to the reaction center (RC) of photosystem II (PS II); the second component (600–700 ps) has been assigned to an electron transfer from P680 to Q_A, while the third one (slow, 3 ns) assumingly originates from charge recombination in the radical pair [P680^+• Pheo^−•] and/or from antenna complexes energetically disconnected from RC II. Mutants showed reduced contribution of the first component, whereas the yield of the second component increased due to slowing down of the electron transport to Q_A. The mutant D1-R323L with completely inactive oxygen evolving complex did not reveal rapid component at all, while its kinetics was approximated by two slow components with lifetimes of about 2 and 3 ns. These may be due to two reasons: a) disconnection between antennae complexes and RC II, and b) recombination in a radical pair [P680^+• Pheo^−•] under restricted electron transport to Q_A. The data obtained suggest that disturbance of oxygen evolving function in mutants may induce an upshift of the midpoint redox potential of Q_A/Q_A⁻ couple causing limitation of electron transport at the acceptor side of PS II.     

Inhibition of Photosystem II activity by saturating single turnover flashes in calcium-depleted and active Photosystem II

Inhibition of Photosystem II (PS II) activity induced by continuous light or by saturating single turnover flashes was investigated in Ca²⁺-depleted, Mn-depleted and active PS II enriched membrane fragments. While Ca²⁺- and Mn-depleted PS II were more damaged under continuous illumination, active PS II was more susceptible to flash-induced photoinhibition. The extent of photoinactivation as a function of the duration of the dark interval between the saturating single turnover flashes was investigated. The active centres showed the most photodamage when the time interval between the flashes was long enough (32 s) to allow for charge recombination between the S₂ or S₃ and Q_B ⁻ to occur. Illumination with groups of consecutive flashes (spacing between the flashes 0.1 s followed by 32 s dark interval) resulted in a binary oscillation of the loss of PS II-activity in active samples as has been shown previously (Keren N, Gong H, Ohad I (1995), J Biol Chem 270: 806–814). Ca²⁺- and Mn-depleted PS II did not show this effect. The data are explained by assuming that charge recombination in active PS II results in a back reaction that generates P₆₈₀ triplet and thence singlet oxygen, while in Ca²⁺- and Mn-depleted PS II charge recombination occurs through a different pathway, that does not involve triplet generation. This correlates with an up-shift of the midpoint potential of Q_A in samples lacking Ca²⁺ or Mn that, in term, is predicted to result in the triplet generating pathway becoming thermodynamically less favourable (G.N. Johnson, A.W. Rutherford, A. Krieger, 1995, Biochim. Biophys. Acta 1229, 201–207). The diminished susceptibility to flash-induced photoinhibition in Ca²⁺- and Mn-depleted PS II is attributed at least in part to this mechanism. This revised version was published online in June 2006 with corrections to the Cover Date.     

Toxic effects of antimony on photosystem II of <Emphasis Type="Italic">Synechocystis</Emphasis> sp. as probed by in vivo chlorophyll fluorescence

It has been demonstrated that antimony (Sb) at concentrations ranging from 1.0 to 10.0 mg L⁻¹ inhibits O₂ evolution. Deeper insight into the influence of Sb on PSII was obtained with measurements of in vivo chlorophyll fluorescence. The donor and the acceptor sides of PSII were shown to be the target of Sb. Sb treatment induces inhibition of electron transport from Q_A⁻ to Q_B/Q_B⁻ and accumulation of P₆₈₀⁺. S₂(Q_AQ_B)⁻ charge recombination and oxidation by PQ9 molecules became more important in Q_A⁻ reoxidation as the electron transfer in PSII was inhibited. Sb exposure caused a steady increase in the proportion of PSII_X and PSII_β. These changes resulted in increased fluxes of dissipated energy and decreased index of photosynthesis performance, of maximum quantum yield, and of the overall photosynthetic driving force of PSII.