Evidence from thermoluminescence for bicarbonate action on the recombination reactions involving the secondary quinone electron acceptor of Photosystem II

In this paper, we present the first measurements on thermoluminescence from isolated thylakoids to probe the recombination reactions of S₂ (or possibly S₃) with Q^?_B or Q^?_A, after bicarbonate depletion and its readdition. The effects of bicarbonate depletion on the S₂Q^?_B (or S₃O^?_B) thermoluminescence band was (1) a 6–10°C shift to a higher temperature; (2) a reduction in its intensity upon prolonged depletion; and (3) elimination after the first few flashes of the characteristic period four oscillations in its intensity as a function of the flash number. On the other hand, addition of diuron (3-(3′,4′-dichlorophenyl)-1,1-dimethylurea), which blocks electron flow from Q^?_A to Q_B, produced the same thermoluminescence band, at about + 20°C, assigned to S₂Q^?_A recombination, in both depleted and reconstituted samples. These results suggest (1) the initial effect of bicarbonate depletion is to increase the activation energy for S₂(S₃)Q^?_B recombination; (2) with further depletion, the incidence of this recombination decreases and the cycling of the S₂Q^?_B and S₃Q^?_B recombination is inhibited through effects at the Q_B apoprotein; and (3) the depletion effects are fully reversible. It is suggested that a conformational change of the PS II complex in the region of the Q_B apoprotein is responsible for these effects.     

Total recovery of O2 evolution and nanosecond reduction kinetics of chlorophyll-a II + (P-680+) after inhibition of water cleavage with acetate

Oxygen evolution and reduction kinetics of the photooxidized Chl-a_II ⁺ have been measured in oxygen-evolving complexes from the thermophilic cyanobacterium Synechococcus sp.

1. Incubation of PS II particles with acetate resulted in an inhibition of oxygen evolution and a retardation of the Chl-a_II ⁺=reduction kinetics from the nanosecond range to the microsecond range, indicating a modification of the donor side of photosystem II (PS II).
2. After the first two flashes given to a dark-adapted, acetate treated sample, Chl-a_II ⁺ was re-reduced with a half-life time of 160 μs by a component of the donor side of PS II. Under repetitive excitation Chl-a_II ⁺ was re-reduced in 500 μs by electron back reaction from the primary acceptor Q_A ^- (X-320^-). Obviously, in the presence of acetate only two electrons are available from the donor side.
3. Both oxygen evolution and nanosecond reduction kinetics of Chl-a_II ⁺ were restored to the control level when acetate was removed.
4. The results indicate a tight coupling between O₂ evolution and nanosecond reduction kinetics of Chl-a_II ⁺.
5. The reversible inhibition is probably due to a replacement of Cl^- by acetate within the water splitting enzyme.
6. Due to its strongly retarded kinetics, the reversibly modified system may facilitate investigations of the mechanism of the donor side.

     

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

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

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

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

Uptake of guanine by synchronizedChlorella fusca

1. The transport of guanine in autospores of light-dark synchronizedChlorella fusca was studied using radioactive guanine in the concentration range of 4 nM to 50 μM.
2. The transport system was constitutive, it had high specificity for the permeant, and theQ ₁₀ value was in the range of 1.5 to 2.2. At concentrations lower than 0.2 μM the half saturating constant, S_0.5 was 1 μM both for cells kept in dark and cells kept in light. At higher concentrations the S_0.5 of darkened cells was about 0.23 μM, while that of illuminated cells was unchanged. Only above 0.2 μM guanine did illumination of the cells or addition of glucose increase the transport rate.
3. Guanine which had accumulated did not leak out at temperatures below 45°C or by treatment with 10 μM dinitrophenol, which completely inhibited transport. Furthermore, the accumulated guanine did not exchange with exogenous guanine.
4. The guanine accumulated, more than 10⁵-fold over the external concentration, showing that the transport, was active.
5. The initial transport rate per cell revealed annual fluctuations.

     

Reverse electron flow in chloroplasts

Energy dependent reverse electron flow reactions in isolated thylakoids provide a unique tool to study, in the dark, the coupling between the ATP synthase, proton transport and the electron transfer system. Appropriate experimental conditions have been established to follow experimentally the following reactions:

1. ATP driven proton uptake into the inner-thylakoid space, which requires preactivation of the ATP synthase.
2. ATP driven reverse electron transport, which involves proton transport as an intermediate, and results in the reduction of Q_A by an externally added electron donor.
3. ATP driven luminescence, which requires the presence of an oxidized partner on the water side of photosystem II, and involves electron transport from Q_B to Q_A.
4. ΔpH driven reverse electron flow, which does not require the participation of the ATP synthase, and uses reduced intermediates between the two photosystems as electron donors for the reduction of Q_A.
5. ΔpH driven luminescence which again uses reduced intermdiates between the two photosystems as electron donors for Q_A reduction, and requires the presence of an oxidized partner on the water side of photosystem II.

Several of these reactions have been shown to occur in intact chloroplasts and may provide an important regulatory mechanism in vivo.     

Floristic composition and phenology of temperate grasslands of Western Himalaya as affected by scraping,fire and heavy grazing

This paper reports the analysis of floristic composition and phenology of protected, annually scraped and fired, and heavily grazed grasslands community of Western Himalaya.

1. A total of 19, 20 and 23 species were recorded at protected (S₁), annually scraped and fired (S₂) and heavily grazed (S₃) sites respectively.
2. Domination of therophytic flora as indicated by biological spectrum study, was noted in all the three sites.
3. Density of plants reached a maximum in the rainy season and amounted to be 4300.5, 3439.8 and 3298.0 tillers/m² on S₁, S₂ and S₃ respectively.
4. Maximum total basal cover was also recorded in the rainy season and amounted to be 769.9, 3390.6 and 2295.7 cm²/m² on S₁, S₂ and S₃ respectively.
5. Correlation between density and basal cover was positive on all three sites.

     

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     

Habitability of the early earth: Clues from the physiology of nitrogen fixation and photosynthesis

In the absence of direct evidence concerning the nature of the early Earth environments, it is acceptable under the uniformitarian principle to attempt to define primitive habitats from modern procaryotic physiology. Combining the rock and fossil record with present phylogenetic reconstuctions, application of this paleoecological approach to the evolutionary biochemistry and physiology of nitrogen fixation and photosynthesis leads to several inferences about the nature of Archean environments:

1. To stimulate nitrogenase evolution and avoid its repression, the activity of the NH ₄ ⁺ ion was less than 10^?3, and probably lower.
2. To be consistent with a moderately protective ozone screen, while not also repressing nitrogenase activity, incursions of abiotic dissolved oxygen at levels in the range 10^?1.2?10^?3.5 PAL would have been acceptable.
3. To induce the formation and activity of RuBP carboxylase, the pCO₂ was less than 100 PAL.
4. To support Photosystem I activity, sulfide concentrations of at least 10^?4 M were present in the photic zone.
5. To avoid a too-rapid oxidation of sulfide, the pH was probably between 6–7, where H₂S exceeds HS^?.

Evolutionary ‘pressure’ to stimulate the later development of oxygenic photosynthesis (Photosystem II), would require several subsequent habitat modifications:

1. Lowering the sulfide to < 10^?4 M to inhibit Photosystem I.
2. Raising the pH above neutral (HS^? > H₂S), to mediate more rapid oxidation of HS^?.
3. Maintaining either an illumination below 300–400 lux (to avoid photosynthetic O₂ self-repression of nitrogen fixation), or an adequate local source of combined nitrogen (aNH ₄ ⁺ > 10^?4) to repress nitrogen fixation entirely.

     

Effects of hydroxylamine and silicomolybdate on the decay in delayed light emission in the 6–100 μs range after a single 10 ns flash in pea thylakoids

Measurements are reported on μs delayed light emission, following a single 10 ns excitation flash, in Alaska pea thylakoids treated with hydroxylamine (NH₂OH) or with silicomolybdate.

1. In thylakoids treated with 2 mM NH₂OH in the light, or in the dark, the quantum yield of delayed light emission is considerably enhanced. A 10 μs lifetime component of delayed light emission is not significantly changed, whereas a 50–70 μs lifetime component is increased. MnCl₂ and diphenylcarbazide are unable to reverse the above effects of NH₂OH treatment. Thus Mn²⁺ and diphenylcarbazide must not donate electrons directly to reaction center II but on the oxygen-evolution side of the NH₂OH block.
2. When the closed form of photosystem II reaction centers (P₆₈₀Q^-), where P₆₈₀ is the reaction center chlorophyll and Q is a ‘stable’ electron acceptor, is generated by preillumination of NH₂OH-treated thylakoids with diuron present, the μs delayed light emission is inhibited, but a low level residual delayed light emission remains. Possible origins of this emission are discussed. It is believed that the best explanation for residual DLE is the existence of another acceptor besides Q that partakes in charge separation and rapid dissipative recombination when the reaction center is in the P₆₈₀Q^- state.
3. The quantum yield of delayed light emission from ‘closed’ reaction centers (P₆₈₀ ⁺Q^-) that have all charge stabilization reactions (i.e., flow of electrons to P₆₈₀ ⁺ and out of Q^-) blocked by NH₂OH treatment and addition of diuron is 1.1×10^-3 for components measured in a range from 6 to 400 μs and extrapolated to zero time.
4. The addition of silicomolybdate, which accepts electron from Q^-, causes delayed light emission in the μs range to be greatly inhibited.

     

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     

Sensitivity changes of photoreceptor cells of Hirudo medicinalis caused by changes in extracellular calcium concentration

Extracellular recordings from the vacoule of photoreceptor cells of Hirudo medicinalis L. were performed using microelectrodes. The cells were adapted by white light flashes given at constant intervals (20 s). Response height versus relative intensity curves obtained from the same cell in physiological saline (PS) and in bathing solutions of either a) lowered calcium contents (2 ΜM/1 or less) or b) raised calcium contents (15 mM/1) were compared. The cells' adaptation state in PS was operationally defined by the ratio Q=h _A /h _S where h _A is the response height evoked by the adapting flashes, and h _S is the corresponding saturation response height. Sensitivity changes were measured by the half saturation intensity shift. Lowering extracellular calcium resulted in:

1. The response height increased and the shape of the response became more rounded and prolonged.
2. The total resistance between the vacuole and outside decreased from 8.2±1.4 MΩ (n=6) in PS to 4.6±0.4 MΩ (n=5). The resistance was independent of the cells' adaptation state.
3. A change of the cells' sensitivity occured either in direction to light adaptation or in direction to dark adaptation. It depended functionally on the ratio Q:

a) if Q was less or equal to about 0.6 the cells' sensitivity increased. b) if Q was greater than 0.6 the cells' sensitivity diminished. Raising extracellular calcium decreased the sensitivity of all cells tested independent of their adaptation states in PS. The results can be interpreted under the assumptions that 1. the sensitivity of leech photoreceptor cells is inversely proportional to the intracellular free calcium concentration and Z. intracellular calcium can interact with extracellular calcium in relatively dark adapted cells whereas in relatively light adapted cells the raise of intracellular free calcium is mainly effected by a release from intracellular stores. It is assumed that a Q value of about 0.6 separates relatively light adapted cells from relatively dark adapted cells.     

Triplet-minus-singlet absorbance difference spectra of reaction centers of Rhodopseudomonas sphaeroides R-26 in the temperature range 24–290 K measured by Magneto-Optical Difference Spectroscopy (MODS)

The recently developed technique of Magneto-Optical Difference Spectroscopy (MODS) [10] has been applied to reaction centers (RC) of the photosynthetic bacterium Rhodopseudomonas sphaeroides R-26. Absorbance changes induced by a magnetic field are measured as a function of wavelength yielding the triplet-minus-singlet (T-S) absorbance difference spectrum. (T-S) spectra thus obtained have been measured from 24–290 K. Going from low to high temperature the (T-S) spectra show the following features:

1. A rapid decrease of positive absorption bands at 809 and 819 nm.
2. A slow appearance of a band shift at 798 nm.
3. A shift of the peak wavelength of the Q_y absorbance band of the primary donor P-860 from 992 to 861 nm, and of its Q_x band from 603 to 600 nm.

The spectra at 24, 66, 116, and 290 K have been analyzed by Gaussian deconvolution. The 800 nm region of the spectrum at 24 K can be decomposed in a combination of two band shifts and an appearing band. The temperature dependence of the spectra in this region is well explained by spectral broadening of the two shifting bands combined with a decrease in intensity of the appearing band when the temperature increases. The two shifting bands in the 800 nm region are identified as the two bands at 803 and 813 nm which together make up the 800 nm band in the absorption spectrum and are assigned to the two accessory RC bacteriochlorophylls (BChls). The band shift of the 813 nm pigment is appreciably larger than that of the 803 nm pigment. The appearing band at 808 nm is attributed to monomeric absorption of ³P-860, the triplet state being localized on one BChl. We find no evidence for admixture of a charge transfer (CT) state of ³P-860 with one of the accessory BChls at higher temperature.     

Thermoluminescence and Delayed Luminescence Investigation of the Green Alga,Chlamydobotrys stellata

Thermoluminescence and delayed luminescence investigations of the autotrophically and photoheterotrophically cultivated green alga, Chlamydobotrys stellata, demonstrated that both the thermoluminescence and delayed luminescence yields are much lower in the photoheterotophic algae than in the autotrophic ones due to an efficient luminescence quenching of unknown mechanism. The relative contributions of the so called Q (S₂Q^?_A charge recombination) and B (S₂Q^?_B and S₃Q^?_B charge recombinations) thermoluminescence bands to the glow curve as well as the Q_A(S₂Q^?_B charge recombination) and Q_B (S₂Q^?_B and S₃Q^?_B charge recombinations) delayed luminescence components to the delayed luminescence decay of autotrophically and photoheterotrophically cultivated Chl. stellata were compared using a computer assisted curve resolution method. It was found that, while in the autotrophic cells the area of the B band was considerably larger than of the Q band, in photoheterotrophic cells the Q band was more effectively charged than the B band. In the delayed luminescence decay curves measured in the seconds to minutes time region the amplitude of the Q_A component relative to that of the Q_B component was larger in the photoheterotrophic cells than in the autotrophic ones. These observations demonstrate that, after light-induced charge separation in the photosystem II reaction centers of autotrophic cells, electrons are “quasipermanently” stored mainly in the secondary quinone acceptor pool, Q_B but in the nonquenched photosystem II reaction centers of photoheterotrophic cells the main reservoir of electrons is the primary quinone acceptor, Q_A. This behaviour indicates an inhibition of electron transport in the photoheterotrophic alga at the level of the secondary quinone acceptor, Q_B.     

Charge accumulation and recombination in Photosystem II studied by thermoluminescence. I. Participation of the primary acceptor Q and secondary acceptor B in the generation of thermoluminescence of chloroplasts

In the glow curves of chloroplasts excited by a series of flashes at +1°C the intensity of the main thermoluminescence band appearing at +30°C (B band; B, secondary acceptor of Photosystem II) exhibits a period-4 oscillation with maxima on the 2nd and 6th flashes indicating the participation of the S₃ state of the water-splitting system in the radiative charge recombination reaction. After long-term dark adaptation of chloroplasts (6 h), when the major part of the secondary acceptor pool (B pool) is oxidized, a period-2 contribution with maxima occurring at uneven flash numbers appears in the oscillation pattern. The B band can even be excited at ?160°C as well as by a single flash in which case the water-splitting system undergoes only one transition (S₁ → S₂). The experimental observations and computer simulation of the oscillatory patterns suggest that the B band originates from charge recombination of the S₂B^? and S₃B^? redox states. The half-time of charge recombination responsible for the B band is 48 s. When a major part of the plastoquinone pool is reduced due to prolonged excitation of the chloroplasts by continuous light, a second band (Q band; Q, primary acceptor of Photosystem II) appears in the glow curve at +10°C which overlaps with the B band. In chloroplasts excited by flashes prior to DCMU addition only the Q band can be observed showing maxima in the oscillation pattern at flash numbers 2, 6 and 10. The Q band can also be induced by flashes after DCMU addition which allows only one transition of the water-splitting system (S₁ → S₂). In the presence of DCMU, electrons accumulate on the primary acceptor Q, thus the Q band can be ascribed to the charge recombination of either the S₂Q^? or S₃Q^? states depending on whether the water-splitting system is in the S₂ or the S₃ state. The half-time of the back reaction of Q^? with the donor side of PS II (S₂ or S₃ states) is 3 s. It was also observed that in a sequence of flashes the peak positions of the Q and B bands do not depend on the advancement of the water-splitting system from the S₂ state to the S₃ state. This result implies that the midpoint potential of the water-splitting system remains unmodified during the S₂ → S₃ transition.     

Inactivation of photosynthetic oxygen evolution by UV-B irradiation: A thermoluminescence study

The influence of UV-B irradiation on photosynthetic oxygen evolution by isolated spinach thylakoids has been investigated using thermoluminescence measurements. The thermoluminescence bands arising from the S₂Q_B ^- (B band) and S₂Q_A ^– (Q band) charge recombination disappeared with increasing UV-B irradiation time. In contrast, the C band at 50°C, arising from the recombination of Q_A ^- with an accessory donor of Photosystem II, was transiently enhanced by the UV-B irradiation. The efficiency of DCMU to block Q_A to Q_B electron transfer decreased after irradiation as detected by the incomplete suppression of the B band by DCMU. The flash-induced oscillatory pattern of the B band was modified in the UV-B irradiated samples, indicating a decrease in the number of centers with reduced Q_B. Based on the results of this study, UV-B irradiation is suggested to damage both the donor and acceptor sides of Photosystem II. The damage of the water-oxidizing complex does not affect a specific S-state transition. Instead, charge stabilization is enhanced on an accessory donor. The acceptor-side modifications decrease the affinity of DCMU binding. This effect is assumed to reflect a structural change in the Q_B/DCMU binding site. The preferential loss of dark stable Q_B ^- may be related to the same structural change or could be caused by the specific destruction of reduced quinones by the UV-B light.Abbreviations Chl chlorophyll - DCMU 3-(3,4,-dichlorophenyl)-1,1-dimethylurea - PS II Photosystem II - Q_A first quinone electron acceptor of PS II - Q_B second quinone electron acceptor of PS II - Tyr-D accessory electron donor of PS II - S₀-S₄ charge storage states of the water-oxidizing complex     

Polarization sensitivity in crayfish lamina monopolar neurons

1. Polarization sensitivity (PS) was examined in photoreceptors and lamina monopolar cells (LMCs) in two species of crayfish, Procambarus clarkii and Pacifasticus leniusculus. The measurements were made with intracellular recordings and broad field illumination.
2. PS is about 40% greater in Pacifasticus than in Procambarus (Table 1). In both species the LMC stationary PS profiles (estimated with flashes) are similar to those of receptors (Figs. 1 and 2). Both receptor and LMC sensitivity profiles are well described by cos² θ functions (Fig. 3). PS was observed in all receptors and 78% of LMCs.
3. When stimulated with a rotating polarizer, receptors and LMCs exhibit membrane potential modulation with phase predicted by the stationary PS profile (Fig. 5). In photoreceptors, the polarization-elicited percent modulation falls off steeply as intensity increases. The LMC modulation is stronger than that in receptors and relatively insensitive to the mean intensity (Figs. 6 to 8). For low intensities the LMC modulation is 100%. The LMC dynamic behavior is consistent with either an opponency mechanism or strong but polarization-insensitive lateral inhibition.
4. Receptors and LMCs exhibit steady-state differential sensitivity to stationary e-vector orientation (Fig. 9).
5. About 10% of the LMC neurons exhibit PS maxima separated by 90°. These results imply a nonlinear summation of signals from orthogonal receptor channels (Fig. 10).

     

Impairment of photosystem 2 activity at the level of secondary quinone electron acceptor in chloroplasts treated with cobalt, nickel and zinc ions

The toxicity of heavy metals on photosystem 2 photochemistry, was investigated by monitoring Hill activity, fluorescence, and thermoluminescence properties of photosystem 2 (PS 2) in pea (Pisum sativum L. cv. Bombay) chloroplasts. In Co²⁺-, Ni²⁺- or Zn²⁺-treated chloroplasts 2,6-dichlorophenolindophenol-Hill activity was markedly inhibited. Addition of hydroxylamine which donates electrons close to PS 2 reaction center did not restore the PS 2 activity. Co²⁺-, Ni²⁺ or Zn²⁺ also inhibited PS 2 activity supported by hydroxylamine in tris (hydroxymethyl)aminomethane (Tris)-inactivated chloroplasts. These observations were confirmed by fluorescence transient measurements. This implies that the metal ions inhibit either the reaction center or the components of PS 2 acceptor side. Flash-induced thermoluminescence studies revealed that the S₂Q^?_A charge recombination was insensitive to metal ion addition. The S₂Q^?_B charge recombination, however, was inhibited with increase in the level of Co²⁺, Ni²⁺ or Zn²⁺. The observed sensitivity of S₂^?_B charge recombination in comparison to the stability of S₂Q^?_A recombination suggests that the metal ions inhibit at the level of secondary quinone electron acceptor. Q_B. We suggest that Co²⁺, Ni²⁺ or Zn²⁺ do not block the electron flow between the primary and secondary quinone electron acceptor, but possibly, directly modify Q_B site, leading to the loss of PS 2 activity.     

Comparative kinetic light-scattering and -absorption photometry

A recent development of kinetic light-scattering and -absorption photometry is described. Essential points are:

1. In the scattering experiment, amplitude resolution of 2 · 10^?5 (single flash) by application of a differential detector, stability of the same magnitude due to optical compensation, high intensity at the detector due to special optics for the scattered light and semiconductor sources.
2. In the absorption measurement, elimination of scattering contributions by the dual wavelength-method and by high aperture optics.
3. Simultaneous measurement of absorption and scattering. The application of the method is described in using signals from isolated bovine rod outer segments. A reliable procedure is described by the use of which the originally measured light-scattering effects can be split up into single signals.

The method allows comparative kinetic analysis of absorption and scattering signals. The possible causal connections between pigment and membrane structure processes can be selected.     

Charakterisierung eines phagenähnlichen Partikels aus Zellen von Nitrobacter

1. Polyhedral particles were isolated from cells of Nitrobacter winogradskyi and of Nitrobacter strains K₁, K₄ and α₁. Their physical and biological properties are characterized.
2. The investigated strains contain polyhedral particles, 1000–1200 Å in size. With increasing age of the culture more particles are found in cells of Nitrobacter. Simultaneously the number of colony producing nitritoxidants decreases.
3. In strain α₁ the loss of the capability to form colonies is connected with partial lysis of the cell and release of particles.
4. A homogeneous fraction of particles was obtained by zone density gradient centrifugation in Tris-Mg-SH-buffer.
5. The polyhedral particles have a sedimentation coefficient of s _w,20 ⁰ =825S and a CsCl-buoyant density of ?²⁵ g/cm³.
6. Based on the determined properties the particles are classified as phage-like Nitrobacter particles Nb₁.