Activation of non-photochemical quenching in thylakoids and leaves

The mechanism of rapidly-relaxing non-photochemical quenching in two plant species,Chenopodium album L. andDigitalis purpurea L., that differ considerably in their capacity for such quenching has been investigated (Johnson G.N. et al. 1993, Plant Cell Environ.16, 673–679). Illumination of leaves of both species in the presence of 2% O₂ balance N₂ led to the formation of zeaxanthin. When thylakoids were isolated from leaves of each species that had been so treated it was found that inD. purpurea non-photochemical quenching was “activated” relative to the control; a higher level of quenching was found for a given trans-thylakoid pH gradient. No such activation of non-photochemical quenching was observed inC. album. Similar conclusions were drawn when comparing quenching in intact leaves. It is concluded that light activation of quenching is a process that cannot readily be induced inC. album. Measurement of the sensitivity of non-photochemical quenching in leaves ofC. album andD. purpurea to dithiothreitol (DTT; a reagent that inhibits formation of zeaxanthin) showed differences between the two species. In both cases, feeding leaves with DTT inhibited the light-induced formation of zeaxanthin. InC. album this was accompanied by complete inhibition of reversible non-photochemical quenching, whereas inD. purpurea this inhibition was only partial. Data are discussed in relation to studies on the mechanism of quenching and the role of zeaxanthin in this process.     

Influence of ascorbate and the Mehler peroxidase reaction on non-photochemical quenching of chlorophyll fluorescence in maize mesophyll chloroplasts

 Non-photochemical quenching of chlorophyll fluorescence (NPQ) and quantum yield of photosystem II (PSII) were studied with intact mesophyll chloroplasts of maize (Zea mays L.) during the initial minutes of illumination using the pulse-modulated chlorophyll fluorescence technique. Non-photochemical quenching was rapidly reversible in the dark at any point during illumination, which is indicative of energy-dependent dissipation of energy (mediated via thylakoid ΔpH changes and ascorbate-dependent synthesis of zeaxanthin). In chloroplasts suspensions including 15 mM ascorbate in the medium, with addition of oxaloacetate and pyruvate, the PSII yield, rate of reduction of oxaloacetate and phosphorylation of pyruvate reached a maximum after approximately 2 min of illumination. Under these conditions, which promote phosphorylation and a decreased ΔpH across the thylakoid membrane, NPQ rose to a maximum after 2–3 min of illumination, dropped to a minimum after about 6 min, and then increased to a steady-state level. A rather similar pattern was observed when leaves were illuminated following a 30-min dark period. Providing chloroplasts with higher levels of ascorbate (60 mM), prevented the transient drop in NPQ. Anaerobic conditions or addition of potassium cyanide caused a decrease in PSII yield, providing evidence for operation of the ascorbate-dependent Mehler-peroxidase reaction. These conditions also strongly suppressed the transient drop in NPQ. Dithiothreitol, an inhibitor of violaxanthin de-epoxidase, caused a large drop in NPQ even in the presence of high levels of ascorbate. The results suggest that the decline of NPQ occurs in response to an increase in lumen pH after initiation of phosphorylation, that this decline can be suppressed by conditions where ascorbate is not limiting for violaxanthin de-epoxidase, and that the increase of NPQ after such a decline is the result of development of energy dissipation in PSII reaction centers. Received: 13 August 1999 / Accepted: 17 September 1999     

pH sensitivity of chlorophyll fluorescence quenching is determined by the detergent/protein ratio and the state of LHCII aggregation

Here we show how the protein environment in terms of detergent concentration/protein aggregation state, affects the sensitivity to pH of isolated, native LHCII, in terms of chlorophyll fluorescence quenching. Three detergent concentrations (200, 20 and 6 μM n-dodecyl β-d-maltoside) have been tested. It was found that at the detergent concentration of 6 μM, low pH quenching of LHCII is close to the physiological response to lumen acidification possessing pK of 5.5. The analysis has been conducted both using arbitrary PAM fluorimetry measurements and chlorophyll fluorescence lifetime component analysis. The second led to the conclusion that the 3.5 ns component lifetime corresponds to an unnatural state of LHCII, induced by the detergent used for solubilising the protein, whilst the 2 ns component is rather the most representative lifetime component of the conformational state of LHCII in the natural thylakoid membrane environment when the non-photochemical quenching (NPQ) was absent. The 2 ns component is related to a pre-aggregated LHCII that makes it more sensitive to pH than the trimeric LHCII with the dominating 3.5 ns lifetime component. The pre-aggregated LHCII displayed both a faster response to protons and a shift in the pK for quenching to higher values, from 4.2 to 4.9. We concluded that environmental factors like lipids, zeaxanthin and PsbS protein that modulate NPQ in vivo could control the state of LHCII aggregation in the dark that makes it more or less sensitive to the lumen acidification. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: Keys to Produce Clean Energy.     

The relationship between non-photochemical quenching of chlorophyll fluorescence and the rate of photosystem 2 photochemistry in leaves

It has been suggested previously that non-photochemical quenching of chlorophyll fluorescence is associated with a decrease in the rate of photosystem 2 (PS 2) photochemistry. In this study analyses of fluorescence yield changes, induced by flashes in leaves exhibiting different amounts of non-photochemical quenching of fluorescence, are made to determine the effect of non-photochemical excitation energy quenching processes on the rate of PS 2 photochemistry. It is demonstrated that both the high-energy state and the more slowly relaxing components of non-photochemical quenching reduce the rate of PS 2 photochemistry. Flash dosage response curves for fluorescence yield show that non-photochemical quenching processes effectively decrease the relative effective absorption cross-section for PS 2 photochemistry. It is suggested that non-photochemical quenching processes exert an effect on the rate of PS 2 photochemistry by increasing the dissipation of excitation energy by non-radiative processes in the pigment matrices of PS 2, which consequently results in a decrease in the efficiency of delivery of excitation energy for PS 2 photochemistry.     

Rapid light-response curves of chlorophyll fluorescence in microalgae: relationship to steady-state light curves and non-photochemical quenching in benthic diatom-dominated assemblages

Rapid light-response curves (RLC) of variable chlorophyll fluorescence were measured on estuarine benthic microalgae with the purpose of characterising its response to changes in ambient light, and of investigating the relationship to steady-state light-response curves (LC). The response of RLCs to changes in ambient light (E, defined as the irradiance level to which a sample is acclimated to prior to the start of the RLC) was characterised by constructing light-response curves for the RLC parameters α _RLC, the initial slope, ETR_m,RLC, the maximum relative electron transport rate, and E _k,RLC, the light-saturation parameter. Measurements were carried out on diatom-dominated suspensions of benthic microalgae and RLC and LC parameters were compared for a wide range of ambient light conditions, time of day, season and sample taxonomic composition. The photoresponse of RLC parameters was typically bi-phasic, consisting of an initial increase of all parameters under low ambient light (E < 21–181 μmol m⁻² s⁻¹), and of a phase during which α _RLC decreased significantly with E, and the increase of ETR_m,RLC and E _k,RLC was attenuated. The relationship between RLC and LC parameters was dependent on ambient irradiance, with significant correlations being found between α _RLC and α, and between ETR_m,RLC and ETR_m, for samples acclimated to low and to high ambient irradiances, respectively. The decline of α _RLC under high light (Δα _RLC) was strongly correlated (P < 0.001 in all cases) with the level of non-photochemical quenching (NPQ) measured before each RLC. These results indicate the possibility of using RLCs to characterise the steady-state photoacclimation status of a sample, by estimating the LC parameter E _k, and to trace short-term changes in NPQ levels without dark incubation.     

Determination of the quantum efficiency of photosystem II and of non-photochemical quenching of chlorophyll fluorescence in the field

A newly developed portable chlorophyll fluorometer in combination with a special leaf clip holder was used for assessing photosynthetic activity of attached sun leaves of Fagus sylvatica and Cucurbita pepo under field conditions. During diurnal time courses, fluorescence yield, photosynthetic photon flux density (PPFD) incident on the leaf plane, and leaf temperature were measured and quantum efficiency of photosystem II (PS II), apparent relative electron transport rates, and non-photochemical fluorescence quenching (NPQ) calculated. In both species, quantum efficiency followed closely the incident PPFD and no hysteresis could be observed during the day. Apparent electron transport rate showed light saturation above a PPFD of 700 mol m^–2 s^–1 in F. sylvatica, while in C. pepo no saturation was visible up to 1400 mol m^–2 s^–1. NPQ was closely correlated to excessive PPFD calculated from the PS II quantum yield. Maximal NPQ observed was 3.3 Although the beech leaf was exposed for a considerable time to PPFD values of 1400–1500 mol m^–2 s^–1 and leaf temperatures between 30 and 35°C, no obvious signs for sustained photodamage could be observed. The data demonstrate the potential of chlorophyll fluorescence measurements to analyse photosynthetic performance under field conditions with minimal disturbance of the plant. Potential error sources due to the geometry of the leaf clip holder used are discussed.Dedicated to Prof. Dr. F.-C. Czygan on the occasion of his 60th birthday     

Simple replacement of violaxanthin by zeaxanthin in LHC-II does not cause chlorophyll fluorescence quenching

Recently, a mechanism for the energy-dependent component (qE) of non-photochemical quenching (NPQ), the fundamental photo-protection mechanism in green plants, has been suggested. Replacement of violaxanthin by zeaxanthin in the binding pocket of the major light harvesting complex LHC-II may be sufficient to invoke efficient chlorophyll fluorescence quenching. Our quantum chemical calculations, however, show that the excited state energies of violaxanthin and zeaxanthin are practically identical when their geometry is constrained to the naturally observed structure of violaxanthin in LHC-II. Therefore, since violaxanthin does not quench LHC-II, zeaxanthin should not either. This theoretical finding is nicely in agreement with experimental results obtained by femtosecond spectroscopy on LHC-II complexes containing violaxanthin or zeaxanthin.     

Use of simultaneous analysis of gas-exchange and chlorophyll fluorescence quenching for analysing the effects of water stress on photosynthesis in apple leaves

Summary A convenient system for the rapid simultaneous measurement of both chlorophyll fluorescence quenching using a modulated light system, and of CO₂, and water vapour exchange by leaves is described. The system was used in a study of the effects of water deficits on the photosynthesis by apple leaves (Malus x domestica Borkh.). Apple leaves were found to have low values of steady-state variable fluorescence, and the existence of significant fluorescence with open traps (F_o) quenching necessitated the measurement and use of a corrected F_o in the calculation of quenching components. Long-term water stress had a marked effect on both gas-exchange and chlorophyll fluorescence quenching. Non-photochemical quenching (qn) in particular was increased in water-stressed leaves, and it was particularly sensitive to incident radiation in such leaves. In contrast, rapid dehydration only affected gas exchange. Relaxation of qn quenching in the dark was slow, taking approximately 10 min for a 50% recovery, in well-watered and in draughted plants, and whether or not the plants had been exposed to high light.     

Analysis of non-photochemical energy dissipating processes in wild type <Emphasis Type="Italic">Dunaliella salina</Emphasis> (green algae) and in <Emphasis Type="Italic">zea1</Emphasis>, a mutant constitutively accumulating zeaxanthin

Generally there is a correlation between the amount of zeaxanthin accumulated within the chloroplast of oxygenic photosynthetic organisms and the degree of non-photochemical quenching (NPQ). Although constitutive accumulation of zeaxanthin can help protect plants from photo-oxidative stress, organisms with such a phenotype have been reported to have altered rates of NPQ induction. In this study, basic fluorescence principles and the routinely used NPQ analysis technique were employed to investigate excitation energy quenching in the unicellular green alga Dunaliella salina, in both wild type (WT) and a mutant, zea1, constitutively accumulating zeaxanthin under all growth conditions. The results showed that, in D. salina, NPQ is a multi-component process consisting of energy- or ΔpH-dependent quenching (qE), state-transition quenching (qT), and photoinhibition quenching (qI). Despite the vast difference in the amount of zeaxanthin in WT and the zea1 mutant grown under low light, the overall kinetics of NPQ induction were almost the same. Only a slight difference in the relative contribution of each quenching component could be detected. Of all the NPQ subcomponents, qE seemed to be the primary NPQ operating in this alga in response to short-term exposure to excessive irradiance. Whenever qE could not operate, i.e., in the presence of nigericin, or under conditions where the level of photon flux is beyond its quenching power, qT and/or qI could adequately compensate its photoprotective function.     

Relationships among violaxanthin deepoxidation,thylakoid membrane conformation,and nonphotochemical chlorophyll fluorescence quenching in leaves of cotton (Gossypium hirsutum L.)

The kinetics and temperature dependencies of development and relaxation of light-induced absorbance changes caused by deepoxidation of violaxanthin to antheraxanthin and zeaxanthin (Z; peak at 506 nm) and by light scattering (S; peak around 540 nm) as well as of nonphotochemical quenching of chlorophyll fluorescence (NPQ) were followed in cotton leaves. Measurements were made in the absence and the presence of dithiothreitol (DTT), an inhibitor of violaxanthin deepoxidase. The amount of NPQ was calculated from the Stern-Volmer equation. A procedure was developed to correct gross AS (S_g) for absorbance changes around 540 nm that are due to a spectral overlap with Z. This protocol isolated the component which is caused by light-scattering changes alone (S_n). In control leaves, the kinetics and temperature dependence of the initial rate of rise in S_n that takes place upon illumination, closely matched that of Z. Application of DTT to leaves, containing little zeaxanthin or antheraxanthin, strongly inhibited both S_n and NPQ, but DTT had no inhibitory effect in leaves in which these xanthophylls had already been preformed, showing that the effect of DTT on A_n and NPQ results solely from the inhibition of violaxanthin deepoxidation. The rates and maximum extents of S_n and NPQ therefore depend on the amount of zeaxanthin (and/or antheraxanthin) present in the leaf. In contrast to the situation during induction, relaxation of Z upon darkening was much slower than the relaxation of S_n and NPQ. The relaxation of S_n and NPQ showed quantitatively similar kinetics and temperature dependencies (Q₁₀=2.4). These results are consistent with the following hypotheses: The increase in lumen-proton concentration resulting from thylakoid membrane energization causes deepoxidation of violaxanthin to antheraxanthin and zeaxanthin. The presence of these xanthophylls is not sufficient to cause S_n or NPQ but, together with an increased lumen-proton concentration, these xanthophylls cause a conformational change, reflected by S_n. The conformational change facilititates nonradiative energy dissipation, thereby causing NPQ. Membrane energization is prerequisite to conformational changes in the thylakoid membrane and resultant nonradiative energy dissipation but the capacity for such changes in intact leaves is quite limited unless zeaxanthin (and/or antheraxanthin) is present in the membrane. The sustained S_n and NPQ levels that remain after darkening may be attributable to a sustained high lumen-proton concentration.Abbreviations A antheraxanthin - DTT dithiothreitol - F, F_m chlorophyll fluorescence yield at actual, full closure of the PSII centers - NPQ nonphotochemical chlorophyll fluorescence quenching - PFD photon flux density - PSII photosystem II - V violaxanthin - Z zeaxanthin - S_n, Z spectral absorbance change caused by light-scattering, violaxanthin deepoxidation We thank Connie Shih for skillful assistance in growing the plants, and for conducting HPLC analyses. A Carnegie Institution Fellowship and a Feodor-Lynen-Fellowship by the Alexander von Humboldt-Foundation to W. B. is gratefully acknowledged. This work was supported in part by Grant No. 89-37-280-4902 of the Competitive Grants Program of the U.S. Department of Agriculture to O.B. This is C. I. W. — D. P. B. Publication No. 1094.     

Continuous recording of photochemical and non-photochemical chlorophyll fluorescence quenching with a new type of modulation fluorometer

A newly developed fluorescence measuring system is employed for the recording of chlorophyll fluorescence induction kinetics (Kautsky-effect) and for the continuous determination of the photochemical and non-photochemical components of fluorescence quenching. The measuring system, which is based on a pulse modulation principle, selectively monitors the fluorescence yield of a weak measuring beam and is not affected even by extremely high intensities of actinic light. By repetitive application of short light pulses of saturating intensity, the fluorescence yield at complete suppression of photochemical quenching is repetitively recorded, allowing the determination of continuous plots of photochemical quenching and non-photochemical quenching. Such plots are compared with the time courses of variable fluorescence at different intensities of actinic illumination. The differences between the observed kinetics are discussed. It is shown that the modulation fluorometer, in combination with the application of saturating light pulses, provides essential information beyond that obtained with conventional chlorophyll fluorometers.     

Control of the light harvesting function of chloroplast membranes: the LHCII-aggregation model for non-photochemical quenching

Dissipation of excess excitation energy within the photosystem II light-harvesting antenna (LHCII) by non-photochemical quenching (NPQ) is an important photoprotective process in plants. An update to a hypothesis for the mechanism of NPQ [FEBS Letters 292, 1991] is presented. The impact of recent advances in understanding the structure, organisation and photophysics of LHCII is assessed. We show possible locations of the predicted regulatory and quenching pigment-binding sites in the structural model of the major LHCII. We suggest that NPQ is a highly regulated concerted response of the organised thylakoid macrostructure, which can include different mechanisms and sites at different times.     

On the relationship between the non-photochemical quenching of the chlorophyll fluorescence and the Photosystem II light harvesting efficiency. A repetitive flash fluorescence induction study

Plants respond to excess light by a photoprotective reduction of the light harvesting efficiency. The notion that the non-photochemical quenching of chlorophyll fluorescence can be reliably used as an indicator of the photoprotection is put to a test here. The technique of the repetitive flash fluorescence induction is employed to measure in parallel the non-photochemical quenching of the maximum fluorescence and the functional cross-section (sigma(PS II)) which is a product of the photosystem II optical cross-section a(PS II) and of its photochemical yield Phi(PS II) (sigma (PS II) = a(PS II) Phi(PS II)). The quenching is measured for both, the maximum fluorescence found in a single-turnover flash (F(M) (ST)) and in a multiple turnover light pulse (F(M) (MT)). The experiment with the diatom Phaeodactylum tricornutum confirmed that, in line with the prevalent model, the PS II functional cross-section sigma (PS II) is reduced in high light and restored in the dark with kinetics and amplitude that are closely matching the changes of the F(M) (ST) and F(M) (MT) quenching. In contrast, a poor correlation between the light-induced changes in the PS II functional cross-section sigma (PS II) and the quenching of the multiple-turnover F(M) (MT) fluorescence was found in the green alga Scenedesmus quadricauda. The non-photochemical quenching in Scenedesmus quadricauda was further investigated using series of single-turnover flashes given with different frequencies. Several mechanisms that modulate the fluorescence emission in parallel to the Q(A) redox state and to the membrane energization were resolved and classified in relation to the light harvesting capacity of Photosystem II.     

Temperature dependence of violaxanthin de-epoxidation and non-photochemical fluorescence quenching in intact leaves ofGossypium hirsutum L. andMalva parviflora L.

The temperature dependence of the rate of de-epoxidation of violaxanthin to zeaxanthin was determined in leaves of chilling-sensitive Gossypium hirsutum L. (cotton) and chilling-resistant Malva parviflora L. by measurements of the increase in absorbance at 505 nm (A ₅₀₅) and in the contents of antheraxanthin and zeaxanthin that occur upon exposure of predarkened leaves to excessive light. A linear relationship between A ₅₀₅ and the decrease in the epoxidation state of the xanthophyll-cycle pigment pool was obtained over the range 10–40° C. The maximal rate of de-epoxidation was strongly temperature dependent; Q₁₀ measured around the temperature at which the leaf had developed was 2.1–2.3 in both species. In field-grown Malva the rate of de-epoxidation at any given measurement temperature was two to three times higher in leaves developed at a relatively low temperature in the early spring than in those developed in summer. Q₁₀ measured around 15° C was in the range 2.2–2.6 in both kinds of Malva leaves, whereas it was as high as 4.6 in cotton leaves developed at a daytime temperature of 30° C. Whereas the maximum (initial) rate of de-epoxidation showed a strong decrease with decreased temperature the degree of de-epoxidation reached in cotton leaves after a 1–2 · h exposure to a constant photon flux density increased with decreased temperature as the rate of photosynthesis decrease. The zeaxanthin content rose from 2 mmol · (mol chlorophyll)^–1 at 30° C to 61 mmol · (mol Chl)^–1 at 10° C, corresponding to a de-epoxidation of 70% of the violaxanthin pool at 10° C. The degree of de-epoxidation at each temperature was clearly related to the amount of excessive light present at that temperature. The relationship between non-photochemical quenching of chlorophyll fluorescence and zeaxanthin formation at different temperatures was determined for both untreated control leaves and for leaves in which zeaxanthin formation was prevented by dithiothreitol treatment. The rate of development of that portion of non-photochemical quenching which was inhibited by dithiothreitol decreased with decreasing temperature and was linearly related to the rate of zeaxanthin formation over a wide temperature range. In contrast, the rate of development of the dithiothreitol-resistant portion of non-photochemical quenching was remarkably little affected by temperature. Evidently, the kinetics of the development of non-photochemical quenching upon exposure of leaves to excessive light is therefore in large part determined by the rate of zeaxanthin formation. For reasons that remain to be determined the relaxation of dithiothreitolsensitive quenching that is normally observed upon darkening of illuminated leaves was strongly inhibited at low temperatures.Abbreviations and Symbols Chl chlorophyll - DTT dithiothreitol - EPS epoxidation state - NPQ non-photochemical chlorophyll fluorescence quenching - PFD photon flux density - PSII photosystem II - F, F_m fluorescence emission at the actual, full closure of the PSII centers C.I.W.-D.P.B. Publication No. 1092We thank Connie Shih for skillful assistance in growing the plants, for conducting the HPLC analyses, and for preparing the figures. A Carnegie Institution Fellowship and a Feodor-Lynen-Fellowship by the Alexander von Humboldt-Foundation to W.B. is gratefully acknowledged. This work was supported by Grant No. 89-37-280-4902 of the Competitive Grants Program of the U.S. Department of Agriculture to O.B.     

The quantum efficiency of photosystem II and its relation to non-photochemical quenching of chlorophyll fluorescence; the effect of measuring-and growth temperature

The relation between the quantum yield of oxygen evolution of open photosystem II reactions centers (_p), calculated according to Weis and Berry (1987), and non-photochemical quenching of chlorophyll fluorescence of plants grown at 19°C and 7°C was measured at 19°C and 7°C. The relation was linear when measured at 19°C, but when measured at 7°C a deviation from linearity was observed at high values of non-photochemical quenching. In plants grown at 7°C this deviation occurred at higher values of non-photochemical quenching than in plants grown at 19°C. The deviations at high light intensity and low temperature are ascribed to an increase in an inhibition-related, non-photochemical quenching component (q_I).The relation between the quantum yield of excitation capture of open photosystem II reaction centers (_exe), calculated according to Genty et al. (1989), and non-photochemical quenching of chlorophyll fluorescence was found to be non-linear and was neither influenced by growth temperature nor by measuring temperature.At high PFD the efficiency of overall steady state electron transport measured by oxygen-evolution, correlated well with the product of q _N and the efficiency of excitation capture (_exe) but it deviated at low PFD. The deviations at low light intensity are attributed to the different populations of chloroplasts measured by gas exchange and chlorophyll fluorescence and to the light gradient within the leaf.Abbreviations F₀ basic fluorescence - F₀ basic fluorescence, thylakoid in energized state - F_m maximal fluorescence - F_m maximum fluorescence in energized state - F_s steady state fluorescence - F_v maximal variable fluorescence - PFD photon flux density - PS II_rc Photosystem II reaction center - q_F0 quenching of basic fluorescence - q_E energy related quenching - q_N non-photochemical quenching:-q_f-total quenching - q_I inhibition-related quenching - q_p photochemical quenching - q_r quenching due to state transition - R_d dark respiration - _p PS II efficiency of excitation capture of open PS II_rc - _pe extrapolated minimal value of _p - _p0 extrapolated maximal value of _p - _si quantum efficiency of linear electron transport, calculated from gas exchange measurements based on incident light - _sf quantum efficiency of linear electron transport, calculated from fluorescence measurements, based on incident measuring light     

Non-photochemical quenching of chlorophyll fluorescence in the green alga Dunaliella

The relaxation of the non-photochemical quenching of chlorophyll fluorescence has been investigated in cells of the green alga Dunaliella following illumination. The relaxation after the addition of DCMU or darkening was strongly biphasic. The uncoupler NH₄Cl induced rapid relaxation of both phases, which were therefore both energy-dependent quenching, qE. The proportion of the slow phase of qE increased at increasing light intensity. In the presence of the inhibitors rotenone and antimycin the slow phase of qE was stabilised for in excess of 15 min. NaN₃ inhibited the relaxation of almost all the qE. The implications of these results are discussed in terms of the interpretation of the non-photochemical quenching of chlorophyll fluorescence in vivo and the mechanism of qE.Abbreviations PS II Photosystem II - qQ photochemical quenching of chlorophyll fluorescence - qNP non-photochemical quenching of chlorophyll fluorescence - qE energy-dependent quenching of chlorophyll fluorescence - F _m maximum level of chlorophyll fluorescence for dark adapted cells - F _m level of fluorescence at any time when qQ is zero     

Light-induced fluorescence quenching in the light-harvesting chlorophyll a/b protein complex

Summary Irradiation of the principal photosystem II light-harvesting chlorophyll-protein antenna complex, LHC II, with high light intensities brings about a pronounced quenching of the chlorophyll fluorescence. Illumination of isolated thylakoids with high light intensities generates the formation of quenching centres within LHC II in vivo, as demonstrated by fluorescence excitation spectroscopy. In the isolated complex it is demonstrated that the light-induced fluorescence quenching: a) shows a partial, biphasic reversibility in the dark; b) is approximately proportional to the light intensity; c) is almost independent of temperature in the range 0–30°C; d) is substantially insensitive to protein modifying reagents and treatments; e) occurs in the absence of oxygen. A possible physiological importance of the phenomenon is discussed in terms of a mechanism capable of dissipating excess excitation energy within the photosystem II antenna.Abbreviations chla chlorophyll a - chlb chlorophyll b - F₀ fluorescence yield with reaction centers open - F_m fluorescence yield with reaction centres closed - F_i fluorescence at the plateau level of the fast induction phase - LHC II light-harvesting chlorophyll a/b protein complex II - PS II photosystem II - PSI photosystem I - Tricine N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycine     

Characterization of the non-photochemical quenching of chlorophyll fluorescence that occurs during the active accumulation of inorganic carbon in the cyanobacterium Synechococcus PCC 7942

Previous work has shown that the maximum fluorescence yield from PS 2 of Synechococcus PCC 7942 occurs when the cells are at the CO₂ compensation point. The addition of inorganic carbon (C_i), as CO₂ or HCO₃ ^–, causes a lowering of the fluorescence yield due to both photochemical (q_p) and non-photochemical (q_N) quenching. In this paper, we characterize the q_N that is induced by C_i addition to cells grown at high light intensities (500 mol photons m^–2 s^–1). The C_i-induced q_N was considerably greater in these cells than in cells grown at low light intensities (50 mol photons m^–2 s^–1), when assayed at a white light (WL) intensity of 250 mol photons m^–2 s^–1. In high-light grown cells we measured q_N values as high as 70%, while in low-light grown cells the q_N was about 16%. The q_N was relieved when cells regained the CO₂ compensation point, when cells were illuminated by supplemental far-red light (FRL) absorbed mainly by PS 1, or when cells were illuminated with increased WL intensities. These characteristics indicate that the q_N was not a form of energy quenching (q_E). Supplemental FRL illumination caused significant enhancement of photosynthetic O₂ evolution that could be correlated with the changes in q_p and q_N. The increases in q_p induced by C_i addition represent increases in the effective quantum yield of PS 2 due to increased levels of oxidized Q_A. The increase in q_N induced by C_i represents a decrease in PS 2 activity related to decreases in the potential quantum yield. The lack of diagnostic changes in the 77 K fluorescence emission spectrum argue against q_N being related to classical state transitions, in which the decrease in potential quantum yield of PS 2 is due either to a decrease in absorption cross-section or by increased spill-over of excitation energy to PS 1. Both the C_i-induced q_p (t _0.5<0.5 s) and q_N (t _0.51.6 s) were rapidly relieved by the addition of DCMU. The two time constants give further support for two separate quenching mechanisms. We have thus characterized a novel form of q_N in cyanobacteria, not related to state transitions or energy quenching, which is induced by the addition of C_i to cells at the CO₂-compensation point.Abbreviations BTP- 1,3-bis[tris(hydroxymethyl)-methylaminopropane] - Chl- chlorophyll - C_i- inorganic carbon (CO₂+HCO₃ ^–+CO₃ ^2–) - DCMU- 3-(3,4-dichlorophenyl)-, 1-dimethylurea) - F- chlorophyll fluorescence measured at any time in the absence of a saturating flash - F_o- chlorophyll fluorescence with only the weak modulated measuring beam on - F_M'- chlorophyll fluorescence during a saturating flash - F_M- maximum chlorophyll fluorescence, measured in the presence of WL and FRL at the CO₂-compensation point or in the presence of DCMU - F_V- variable fluorescence (= F_M'–F₀) - FRL- supplemental illumination with far red light - MB- modulated measuring beam of the PAM fluorometer - MV- methyl viologen - PAM- pulse amplitude modulation - PFD- incident photon flux density - PS 1, 2- Photosystems 1 and 2 - Q_A- primary electron-accepting plastoquinione of PS 2 - q_N- non-photochemical quenching of chlorophyll fluorescence - q_p- photochemical quenching of chlorophyll fluorescence; rubisco-ribulose bisphosphate carboxylase/oxygenase - SF- saturating flash (600 ms duration) - WL- white light illumination     

Light-harvesting chlorophyll a-b complex requirement for regulation of Photosystem II photochemistry by non-photochemical quenching

Recently, it has been suggested (Horton et al. 1992) that aggregation of the light-harvesting a-b complex (LHC II) in vitro reflects the processes which occur in vivo during fluorescence induction and related to the major non-photochemical quenching (qE). Therefore the requirement of this chlorophyll a-b containing protein complex to produce qN was investigated by comparison of two barley mutants either lacking (chlorina f2) or depressed (chlorina¹⁰⁴) in LHC II to the wild-type and pea leaves submitted to intermittent light (IL) and during their greening in continuous light. It was observed that qN was photoinduced in the absence of LHC II, i.e. in IL grown pea leaves and the barley mutants. Nevertheless, in these leaves qN had no (IL, peas) or little (barley mutants) inhibitory effect on the photochemical efficiency of Q_A reduction measured by flash dosage response curves of the chlorophyll fluorescence yield increase induced by a single turn-over flash During greening in continuous light of IL pea leaves, an inhibitory effect on Q_A photoreduction associated to qN developed as Photosystem II antenna size increased with LHC II synthesis. Utilizing data from the literature on connectivity between PS II units versus antenna size, the following hypothesis is put forward to explain the results summarized above. qN can occur in the core antenna or Reaction Center of a fraction of PS II units and these units will not exhibit variable fluorescence. Other PS II units are quenched indirectly through PS II-PS II exciton transfer which develops as the proportion of connected PS II units increases through LHC II synthesis.     

The role of carbon dioxide and oxygen in determining chlorophyll fluorescence quenching during leaf development

Chlorophyll fluorescence emission at 680 nm (F680) and the rate of CO₂ fixation were measured simultaneously in sections along the length of wheat and maize leaves. These leaves possess a basal meristem and show a gradation in development towards the leaf tip. The redox state of the primary electron acceptor, Q, of photosystem II was estimated using a non-invasive method. Distal mature leaf sections displayed typical F680 induction curves which were generally anti-parallel with CO₂ fixation and during which Q became gradually oxidised. In leaf-base sections net assimilation of CO₂ was not detectable, F680 quenched slowly and monotonously without displaying any of the oscillations typical of mature tissue and Q remained relatively reduced. Sections cut from mid-regions of the leaf showed intermediate characteristics. There were no major differences between the wheat and maize leaf in the parameters measured. The results support the hypothesis that generation of the transthylakoid proton gradient and associated ATP production is not a major limitation to photosynthesis during leaf development in either C₃ or C₄ plants. Removal of CO₂ from the mature leaf sections caused little change in steady-state F680 and produced about 50% reduction of Q. When O₂ was then removed, F680 rose sharply and Q became almost totally reduced. In immature tissue unable to assimilate CO₂, removal of O₂ alone caused a similar large rise in F680 and reduction of Q whilst removal of CO₂ had negligible effects on F680 and the redox state of Q. It is concluded that in leaf tissue unable to assimilate CO₂, either because CO₂ is absent or the tissue is immature, O₂ acts as an electron acceptor and maintains Q in a partially oxidised state. The important implication that O₂ may have a role in the prevention of photoinhibition of the photochemical apparatus in the developing leaf is discussed.Abbreviations F680 chlorophyll fluorescence emission at 680 nm - PSI photosystem I - PSII photosystem II - Q PSII primary electron acceptor - pH transthylakoid proton gradient