A thermoluminescence study of Photosystem II back electron transfer reactions in rice leaves – effects of salt stress

The recombination reactions of Photosystem II have been investigated in vivo in rice leaves by using the thermoluminescence (TL) emission technique. Excitation of dark-adapted leaf segments at 0 °C with different number of single turn-over flashes induced the appearance of complex TL glow curves. The mathematical analysis of these curves showed the existence of four TL components: B1-band (temperature maximum, t_max, at 24 °C, originating from S₃Q_B⁻ recombination), B2-band (t_max at 35 °C, from S₂Q_B⁻), AG-band (t_max at 46 °C) and C-band (t_max at 55 °C, from Tyr_D⁺Q_A⁻). Their contributions to the total TL signal were different depending on the number of flashes given. AG-band seems to reflect a special electron transfer from some unknown stroma donor to PS II. Q-band (t_max at 19 °C), originating from S₂Q_A⁻ recombination, was recorded after flashing samples incubated in the presence of DCMU. The recombination halftimes (t_1/2) at 20 °C of S₂Q_A⁻, S₃Q_B⁻, S₂Q_B⁻ and Tyr_D⁺Q_A⁻ were, respectively, 0.8 s, 48 s, 74 s and about 1 h. A sharp AG-band (t_max at 50 °C and t_1/2 of 210 s) could be also observed after illumination of leaves with far-red light and after a dark incubation period of whole plants. Incubation of leaf segments with 0.5 M NaCl abolished the inductions of AG-band by darkness and far-red illumination, significantly decreased Q-band intensity, whereas induced a strong increase in C-band intensity. The possible inhibition of S₂/S₃ formation and quinone oxidation by saline stress are discussed.     

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     

Effects of NaCl Stress on the Structure,Pigment Complex Composition,and Photosynthetic Activity of Mangrove Bruguiera parviflora Chloroplasts

Exposure of two-month-old seedlings of Bruguiera parviflora to NaCl stress (0 to 400 mM) for 45 d under hydroponic culture caused notable disorganisation of the thylakoid structure of chloroplasts in NaCl-treated leaves as revealed from transmission electron microscopy. The absorption spectra of treated and control thylakoid samples were similar having a red peak at 680 nm and Soret peaks at 439 and 471 nm in the blue region of the spectrum. The spectra of treated samples differed from control samples by gradual decrease in absorbance of 100, 200, and 400 mM NaCl treated samples at 471 and 439 nm, which could be due to scattering of radiation in these samples. Thus, absorption characteristics of thylakoid membranes indicated no major alterations in the structural integrity of the photosynthetic membranes during salt stress in B. parviflora. Analysis of pigment protein complexes of thylakoids on non-denaturing gel showed that CP1 complex consisting of photosystem (PS) 1 reaction centre decreased marginally by 19% and the CP47 constituting the core antenna of PS2 declined significantly by 30% in 400 mM NaCl treated samples in respect to control. This decrease in structural core antenna might cause inefficient photon harvesting capacity. However, CP43 content did not alter. An increase in CP2/CP1 ratio from 3.2 in control to 4.0 in 400 mM NaCl treated samples indicated significant structural changes in the thylakoids of salt treated plants. Haem staining of thylakoids revealed significant losses in cytochrome (Cyt)f and Cyt b ₆ contents by NaCl stress. However, Cyt b ₅₅₉ content remained nearly constant in both control and NaCl treated samples. SDS-PAGE of thylakoid proteins showed that the intensity of many of Coomassie stained polypeptide bands ranging from 15–22 and 28–66 kDa regions decreased significantly in NaCl treated samples as compared to control. Electron transport activity of thylakoids, measured in terms of DCPIP photoreduction, was 22% lower in 400 mM NaCl treated plants than in the control ones. Hence, NaCl induces oxidative stress in chloroplasts causing structural alterations in thylakoids. These structural alterations might be responsible for declined efficiency of photosystems and reduced electron transport activity. This revised version was published online in August 2006 with corrections to the Cover Date.     

Light-dependent degradation of the Q(B)-protein in isolated pea thylakoids

The 32 000-dalton Q_B-protein of photosystem II (PS II) is rapidly damaged and removed from isolated pea thylakoids during incubation in the light resulting in a loss of photosynthetic electron flow through PS II. This in vitro photoinhibition is similar to that previously reported with intact Chlamydomonas cells. The damage occurs at a faster rate in vitro, however, due to the inability of isolated thylakoids to synthesize replacement Q_B-protein. The removal of the damaged Q_B-protein does not require any soluble components of the chloroplast stroma and is unaffected by the protease inhibitors phenyl-methylsulfonylfluoride or antipain. Unlike the effect of trypsin, no low mol. wt. membrane-bound or soluble fragments of the labelled Q_B-protein could be identified either by autoradiography or immunologically using polyclonal antibodies specific for the Q_B-protein. The lightinduced damage to the Q_B-protein (indicated by a loss of Q_B functional activity), preceded the removal of the protein from the membrane. We conclude that photodamage of the Q_B-protein generates a conformational change which renders the protein susceptible to attack by a highly efficient, intrinsic membrane protease.     

Proline in relation to free radical production in seedlings of Brassica juncea raised under sodium chloride stress

The production of malondialdehyde (MDA) was higher in cotyledons from NaCl-raised Brassica juncea seedlings than in control seedlings. Light accelerated the MDA-producing capacity of thylakoids isolated from both control and treated seedlings. When exposed to strong white light (920 mol photons m^-2 s^-1) the thylakoids from NaCl seedlings produced nearly 5 times more MDA than control thylakoids. In the cotyledons of NaCl seedlings, the proline level was 24-fold higher than in controls. The presence of proline during exposure of thylakoids to white light decreased MDA levels. The reduction in MDA production was higher in the thylakoids of NaCl seedlings than of controls. It is proposed that proline accumulation has an adaptive significance as it lowers the generation of free radicals and thus reduces the lipid peroxidation linked membrane deterioration under stress.     

Afterglow thermoluminescence band as a possible early indicator of changes in the photosynthetic electron transport in leaves

The afterglow (AG) band of thermoluminescence (TL) has been investigated in leaves of Arabidopsis thaliana. Excitation of dark-adapted leaves with two saturating single turn-over flashes induced the appearance of a complex TL glow curve that could be well simulated by three components: the two components, B₁ and B₂, of the usually called B-band, peaking at 18 and 26 °C, respectively, and a band with t_max at 41 °C, which we attributed to an AG emission. Illumination of dark-adapted leaves with 720 nm monochromatic and FR lights generated the emission of a sharp single band peaking also around at 41 °C, that it is usually assigned to an AG emission band. Dark-incubation of whole plants increased the intensity of AG-band in TL curves induced by two flashes and, in parallel, decreased B-bands. Selective illumination of leaves with light mostly absorbed by PS II (650 nm light) completely abolished the AG-band induced by two flashes, B-band being the only TL band observed. The single AG-band induced by 720 nm light was abolished if leaves were also illuminated with 650 nm light. On the other hand, AG-band could be restored if 650 nm illuminated leaves were afterwards illuminated with 720 nm light. The changes in the intensity of B and AG bands induced by selective illuminations seem to be related to alterations in the redox state of Q_B and plastoquinone pool.     

Fractional control of photosynthesis by the QB protein,the cytochrome f/b 6 complex and other components of the photosynthesic apparatus

In order to obtain information on fractional control of photosynthesis by individual catalysts, catalytic activities in photosynthetic electron transport and carbon metabolism were modified by the addition of inhibitors, and the effect on photosynthetic flux was measured using chloroplasts of Spinacia oleracea L. In thylakoids with coupled electron transport, light-limited electron flow to ferricyanide was largely controlled by the Q_B protein of the electron-transport chain. Fractional control by the cytochrome f/b ₆ complex was insignificant under these conditions. Control by the cytochrome f/b ₆ complex dominated at high energy fluence rates where the contribution to control of the Q_B protein was very small. Uncoupling shifted control from the cytochrome f/b ₆ complex to the Q_B protein. Control of electron flow was more complex in assimilating chloroplasts than in thylakoids. The contributions of the cytochrome f/b ₆ complex and of the Q_B protein to control were smaller in intact chloroplasts than in thylakoids. Thus, even though the transit time for an electron through the electron-transport chain may be below 5 ms in leaves, oxidation of plastohydroquinone was only partially responsible for limiting photosynthesis under conditions of light and CO₂ saturation. The energy fluence rate influenced control coefficients. Fractional control of photosynthesis by the ATP synthetase, the cytochrome f/b ₆ complex and by ribulose-1,5-bisphosphate carboxylase increased with increasing fluence rates, whereas the contributions of the Q_B protein and of enzymes sensitive to SH-blocking agents decreased. The results show that the burdens of control are borne by several components of the photosynthetic apparatus, and that burdens are shifted as conditions for photosynthesis change.Abbreviations Chl chlorophyll - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DNP-INT 2,4-dinitro phenylether of 2-iodo-4-nitrothymol - pCMBS p-chloromercuribenzosulfonate     

Characterization of the alterations of the chlorophyll a fluorescence induction curve after addition of Photosystem II inhibiting herbicides

The effects of Photosystem II inhibiting herbicides, 3-(3,4-dichlorophenyl)-1,1-dimethylurea (diuron), atrazine and two novel 2-benzylamino-1,3,5-triazine compounds, on photosynthetic oxygen evolution and chlorophyll a fluorescence induction were measured in thylakoids isolated from Chenopodium album (wild type and atrazine-resistant plants) and cyanobacterial intact cells. The resistant plants have a mutation of serine for glycine at position 264 of the D1 protein. Diuron and two members of a novel class of 2-benzylamino-1,3,5-triazine compounds were almost as active in wild-type as in atrazine-resistant thylakoids, indicating that the benzylamino substitution in the novel triazines may be important for the lack of resistance in these atrazine-resistant plants. The inhibition by the herbicides of oxygen evolution in the cyanobacteria was somewhat lower than in the thylakoids of Chenopodium album wild type, probably caused by a slower uptake in the intact cells. The so-called OJIP fluorescence induction curve was measured during a one second light pulse in the absence and in the presence of high concentrations of the four herbicides. In the presence of a herbicide we observed an increase of the initial fluorescence at the origin (Fo′), a higher J level, and a decreased steady state at its P level (Fp). The increase to Fo′ and the decreased leveling Fp are discussed. After dark adaptation about 25% of the reaction centers are in the S₀ state of the oxygen evolving complex with an electron on the secondary electron accepting quinone, Q_B. The addition of a herbicide causes a transfer of the electron on Q_B to the primary quinone acceptor, Q_A, and displacement of Q_B by the herbicide; the reduced Q_A leads to a higher Fo′. The decrease of Fp in the presence of the herbicides is suggested to be caused by inhibition of the photo-electrochemical stimulation of the fluorescence yield. This revised version was published online in August 2006 with corrections to the Cover Date.     

Usefulness of thermoluminescence in herbicide research

Many herbicides of different chemical structure inhibit photosynthetic electron flow by interrupting the photosyn‐thetic electron flow by interrupting the photosynthetic electron transport chain between the primary acceptor (Q_A) and the secondary acceptor (Q_B) of photosystem 2 (PS2). Thermoluminescence (TL) originates from PS2, and the bands of the glow curve can be related to the charge recombination between positively charged donors and negatively charged acceptors. The glow curve of TL is strongly influenced by addition of PS2 herbicides. The herbicide treatment shifts the peak position and activation energy of the TL band related to Q_A, suggesting that herbicide binding affects the midpoint redox potential not only of Q _B but also that of Q_A. On the basis of the band shift the herbicides of various chemical structures can be classified into different “thermodynamical” groups which relfect the differences in the binding properties of these herbicides. As a new approach TL seems to be a useful technique in studying the mechanism and site of action of herbicides that inhibit electron transport of PS2.     

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     

Evaluation of photosynthetic activities in thylakoid membranes by means of Fourier transform infrared spectroscopy

Light-induced Fourier transformed infrared (FTIR) difference spectroscopy is a powerful method to study the structures and reactions of redox cofactors involved in the photosynthetic electron transport chain. So far, most of the FTIR studies of the reactions of oxygenic photosynthesis have been performed using isolated photosystem I (PSI) and photosystem II (PSII) preparations, which, however, could be modified during isolation procedures. In this study, we developed a methodology to evaluate the photosynthetic activities of thylakoids using FTIR spectroscopy. FTIR difference spectra upon successive flashes using thylakoids from spinach exhibited signals typical of the S-state cycle at the Mn₄CaO₅ cluster and Q_B reactions in PSII with period-four and -two oscillations, respectively. Similar measurement in the presence of an artificial quinone as an exogenous electron acceptor showed features specific to the S-state cycle. Simulations of the oscillation patterns provided the quantum efficiencies of the S-state cycle and electron transfer in PSII. Moreover, FTIR measurement under continuous illumination on thylakoids in the presence of DCMU showed signals due to Q_A reduction and P700 oxidation simultaneously. From the relative amplitudes of marker bands of Q_A^? and P700⁺, the molar ratio of photoactive PSII and PSI centers in thylakoids was estimated. FTIR analyses of the photo-reactions in thylakoids, which are more intact than isolated photosystems, will be useful in investigations of the photosynthetic mechanism especially by genetic modification of photosystem proteins.     

Protective Effect of Triacontanol Against Acidic Mists in Samanea saman (Jacq.) Merrill Seedlings: Differential Responses in Growth, 14CO2 Fixation,Ribulose-1,5-Bisphosphate Carboxylase,and Electron Transport Activities

Seedlings of tropical leguminous tree Samanea saman (Jacq.) Merrill were exposed for 7 d to acidic mist (AM, induced by H₂SO₄) of pH 5.6, 4.0, and 2.0. AM significantly reduced seedling growth (root and shoot length, leaf density, leaf area, fresh and dry mass accumulation) and photosynthetic activities. In thylakoids isolated from leaves treated at pH 4.0 and 2.0 a decrease in the activities of photosystem (PS) 2 and whole chain electron transport was observed, but PS1 activity did not change. When the seedlings were subsequently sprayed with triacontanol (TRIA), the AM effect was partially or completely reversed indicating that TRIA can protect from AM effects. The artificial electron donors, di-phenylcarbazide (DPC) and hydroxylamine (NH₂OH), markedly restored the loss of PS2 activity in AM (pH 2.0) treated leaves. This is the first report of alleviating the AM by TRIA in tropical tree seedlings.     

Contributions of the free oxidized and QB-bound plastoquinone molecules to the thermal phase of chlorophyll-a fluorescence

Variable chlorophyll a (Chl a) fluorescence is composed of a photochemical and a thermal phases of similar amplitudes. The photochemical phase can be induced by a saturating single turnover flash (STF) and reflects the reduction of the Photosystem II (PS II) Q_A primary electron acceptor. The thermal phase requires multiple turnover flash (MTF) and is somehow related to the reduction of the plastoquinone (PQ) molecules. This article aimed to determine the relative contributions of the Q_B-bound and the free oxidized PQ molecules to the thermal phase of Chl a fluorescence. We thus measured the interactive effects of exogenous PQ (PQex), of an inhibitor (DCMU) acting at the Q_B site of PS II and of an artificial quencher, 2-methyl-1,4-naphtoquinone, on Chl a fluorescence levels induced by STF (F_F) and MTF (F_M) in spinach thylakoids. We observed that: (1) the incorporation of PQex in thylakoids stimulated photosynthetic electron transport but barely affected F_F and F_M in the absence of DCMU; (2) DCMU significantly increased the amplitude of F_F but slightly quenched F_M; (3) 2-methyl-1,4-naphtoquinone quenched F_M to a larger-extent than F_F; (4) DCMU increased the quenching effects of PQex on F_F and F_M and also, of methyl-1,4-naphtoquinone on F_F. These results indicate that: (1) the Q_B-bound and the free PQ molecules contribute to about 56% and 25%, respectively, to the thermal phase Chl a fluorescence in dark-adapted thylakoids; and (2) the thermal phase of Chl a fluorescence is more susceptible than the photochemical phase to the non-photochemical quenching effect of oxidized quinones. This revised version was published online in June 2006 with corrections to the Cover Date.     

Multiple sites of retardation of electron transfer in Photosystem II after hydrolysis of phosphatidylglycerol

Phosphatidylglycerol (PG), containing the unique fatty acid Δ3, trans-16:1-hexadecenoic acid, is a minor but ubiquitous lipid component of thylakoid membranes of chloroplasts and cyanobacteria. We investigated its role in electron transfers and structural organization of Photosystem II (PSII) by treating Arabidopsis thaliana thylakoids with phospholipase A₂ to decrease the PG content. Phospholipase A₂ treatment of thylakoids (a) inhibited electron transfer from the primary quinone acceptor Q_A to the secondary quinone acceptor Q_B, (b) retarded electron transfer from the manganese cluster to the redox-active tyrosine Z, (c) decreased the extent of flash-induced oxidation of tyrosine Z and dark-stable tyrosine D in parallel, and (d) inhibited PSII reaction centres such that electron flow to silicomolybdate in continuous light was inhibited. In addition, phospholipase A₂ treatment of thylakoids caused the partial dissociation of (a) PSII supercomplexes into PSII dimers that do not have the complete light-harvesting complex of PSII (LHCII); (b) PSII dimers into monomers; and (c) trimers of LHCII into monomers. Thus, removal of PG by phospholipase A₂ brings about profound structural changes in PSII, leading to inhibition/retardation of electron transfer on the donor side, in the reaction centre, and on the acceptor side. Our results broaden the simple view of the predominant effect being on the Q_B-binding site.     

Characterization of photosystem II heterogeneity in response to high salt stress in wheat leaves (<Emphasis Type="Italic">Triticum aestivum</Emphasis>)

The effect of high salt stress on PS II heterogeneity was investigated in wheat (Triticum aestivum) leaves. On the basis of antenna size, PS II has been classified into three forms, i.e., α, β, and γ centers while on the basis of electron transport properties of the reducing side of the reaction centers, two distinct forms of PS II have been suggested, i.e., Q_B reducing centers and Q_B non-reducing centers. The chlorophyll a (Chl a) fluorescence transients, which can quantify PS II behavior, were recorded using PEA to derive OJIP in vivo with high time resolution and further analyzed according to JIP test. Our results showed that with an increase in the salt concentration during growth, the number of Q_B non-reducing centers increased. In antenna size heterogeneity the number of β and γ centers increased while the number of α centers decreased. A change in the energetic connectivity between the PS II units was also observed. Recovery studies showed that antenna heterogeneity was completely recovered from damage at 0.5 M NaCl concentration and partially recovered at 1 M NaCl concentration while reducing side heterogeneity showed no recovery at all after 0.5 M onwards.     

Pitfalls, artefacts and open questions in chlorophyll thermoluminescence of leaves or algal cells

Thermoluminescence of intact photosynthetic organisms, leaves or algal cells, raises specific problems. The constitutive S_2/3Q _B ^? B bands constitute major probes of the state of photosystem II in vivo. The presence of a dark-stable acidic lumen causes a temperature downshift of B bands, specially the S₃ B band, providing a lumen pH indicator. This is accompanied by a broadening of the S₃ B band that becomes an envelope of elementary B bands. The occasional A_T, Q and C bands are briefly examined in an in vivo context. It is emphasized that freezing below the nucleation temperature is not necessary for physiological studies, but a source of artefacts, hence should be avoided. In intact photosynthetic structures, a dark-electron transfer from stroma reductants to the quinonic acceptors of photosystem II via the cyclic/chlororespiratory pathways, strongly stimulated by moderate warming, gives rise to the afterglow (AG) luminescence emission that reflects chloroplast energy status. The decomposition of complex TL signals into elementary bands is necessary to determine the maximum temperature T _m and the area of each of them. A comparison of TL signals after 1 flash and 2 flashes prevents from confusing the three main bands observed in vivo, i.e. the S₂ and S₃ B bands and the AG band. Finally, the thermoluminescence bands arising sometimes above 50 °C are mentioned. The basic principles of (thermo)luminescence established on isolated thylakoids should not be applied directly without a careful examination of in vivo conditions.     

Studies on the mechanism of photosystem II photoinhibition I. A two-step degradation of D1-protein

The role of D1-protein in photoinhibition was examined. Photoinhibition of spinach thylakoids at 20°C caused considerable degradation of D1-protein and a parallel loss of variable fluorescence, Q_B-independent electron flow and Q_B-dependent electron flow. The breakdown of D1-protein as well as the loss of variable fluorescence and Q_B-independent electron flow were largely prevented when thylakoids were photoinhibited at 0°C. The Q_B-dependent electron flow markedly decreased under the same conditions. This inactivation may represent the primary event in photoinhibition and could be the result of some modification at the Q_B-site of D1-protein. Evidence for this comes from fluorescence relaxation kinetics following photoinhibition at 0°C which indicate a partial inactivation of Q_A ^--reoxidation. These results support the idea of D1-protein breakdown during photoinhibition as a two step process consisting of an initial inactivation at the Q_B-site of the protein followed by its degradation. The latter is accompanied by the loss of PS II-reaction centre function.Abbreviations Asc ascorbate - p-BQ 1, 4-benzoquinone - DAD diaminodurene - DPC diphenylcarbazide - DQH₂ duroquinole - Fecy ferricyanide - MV methylviologen - Q_A primary quinone acceptor of PS II - Q_B secondary quinone acceptor of PS II - SiMo silicomolybdate     

Oxygen-evolving system and secondary quinonic acceptors are highly reduced in dark adapted Euglena cells: A thermoluminescence study

Characteristics of thermoluminescence glow curves were compared in three types of Euglena cells: (i) strictly autotrophic, Cramer and Myers cells; (ii) photoheterotrophic cells sampled from an exponentially growing culture containing lactate as substrate repressing the photosynthetic activity; (iii) semiautotrophic cells, sampled when the lactate being totally exhausted, the photosynthesis was enhanced.In autotrophic and semiautotrophic cells, composite curves were observed after series of two or more actinic flashes fired at –10°C, which can be deconvoluted into a large band peaking in the range 12–22°C and a smaller one near 40°C, This second band presents the characteristics of a typical B band (due to S_2/3Q_B ^- recombination), whereas the first one resembled the band, shifted by -15–20°C, which is observed in herbicide resistant plants. The amplitude of this major band, which was in all cases very low after one flash, exhibited oscillations of period four but rapidly damping, with maxima after two and six flashes. In contrast, photoheterotrophic Euglena displayed single, non-oscillating curves with maxima in the range 5–10°C.In autotrophic and semiautotrophic cells, oxidizing pretreatments by either a preillumination with one or more (up to twenty-five) flashes, or a far-red preillumination in the presence of methylviologen, followed by a short dark period, induced thermoluminescence bands almost single and shifted by +3–5°C, or +12°C, respectively. In autotrophic cells, far-red light plus methyl viologen treatment induced a band peaking at 31°C, as in isolated thylakoids from Euglena or higher plants, while it had barely any effect in photoheterotrophic cells.Due to metabolic activities in dark-adapted cells, a reduction of redox groups at the donor and acceptor sides of PS II dark-adapted cells is supposed to occur. Two different explanations can be proposed to explain such a shift in the position of the main band in dark-adapted autotrophic control. The first explanation would be that in these reducing conditions a decreasing value of the equilibrium constant for the reaction: S_nQ_A ^-Q_BS_nQ_AQ_B ^-, would determine the shift of the main TL band towards low temperatures, as observed in herbicide resistant material. The second explanation would be that the main band would correspond to peak III already observed in vivo and assigned to S_2/3Q_B ^2- recombinations.Abbreviations CM Cramer and Myers - D₁ a 32 kDa protein component of the PS II reaction center, psbA.gene product - D₂ a 34 kDa protein component of the PS II reaction center, psbD gene product - FR lar-red illumination - Lexpo and Lstat cells from lactate culture samples at exponential and stationary phase of growth - MV methylviologen - pBQ parabenzoquinone - PQ plastoquinone - PS II photosystem II - Q_A primary quinone electron acceptor - Q_B secondary quinone electron acceptor - TL thermoluminescence     

Studies on plant growth-regulating substances

The dwarfing of seedlings of Phaseolus vulgaris cv. Masterpiece by treatment with solutions of 4-chlorobenzyltributylammonium bromide (B₄) as a foliar spray or by application to the roots has been demonstrated. Dwarfing resulted from a reduction of internodal and petiolar lengths but there was no evidence of delayed leaf or flower development. A reduction in transpiration rate was accompanied by some protection against wilting and an increased ability to recover from this condition. No changes in transpiration coefficient were observed. The primary leaves of treated seedlings retained chlorophyll longer than those of control seedlings, but in other leaves, chlorophyll and carotenoid pigment levels tended to fall following treatment with B₄. Evidence was obtained that B₄ treatment delayed root nodule formation, an effect which could be associated with the antibacterial properties of quaternary ammonium salts.     

Photosynthetic electron transport in thylakoids from cotton cultivars (Gossypium sp.) differing in tolerance to prometryn

A method to determine photosynthetic electron transport in thylakoid membranes is described for Gossypium barbadense (cv. Pima S-7) and G. hirsutum (cv. DP 5415). These cultivars differed markedly in tolerance to prometryn, a PS II inhibitor. The rates of photosynthetic electron transport obtained were 245 mole oxygen mg^–1 chl h¹. Plant age and leaf size influenced the activity of the thylakoid preparations. Thylakoids from leaves of plants 24 to 37 d and 50–70 mm in diameter had the highest activities; thylakoids from cotyledons, fully expanded leaves and young leaves had low activity. Thylakoids from both species had similar photosynthetic activities and I₅₀'s for prometryn, atrazine and diuron. Thus, tolerance to prometryn was not due to differential binding at D1 protein.Abbreviations PSII photosystem II - DAP day after planting - DQ duroquinone - DBMIB dibromothymoquinone - DMBQ 2,5-dimethyl-p-benzoquinone - I₅₀ concentration to inhibit reaction by 50% - Q_A quinone A - Q_B quinone B