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

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

Coregulation of electron transport through PS I by Cyt b 6 f,excitation capture by P700 and acceptor side reduction. Time kinetics and electron transport requirement

Regulation of electron transport rate through Photosystem I (PS I) was investigated in intact sunflower leaves. The rate constant of electron donation via the cytochrome b ₆ f complex (k_q, s^–1) was obtained from the postillumination P700⁺ reduction rate, measured as the exponential decay of the light-dark difference (D830) of the 830 nm transmission signal. D830 corresponding to maximum oxidisable P700 (D830_m) was obtained by applying white light flashes of different intensity and extrapolating the plot of the quantum yield Y vs. D830 to the axis of abscissae (Y->0). Maximum quantum yield of PS I at completely reduced P700 (Y_m) was obtained by extrapolating the same plot to the axis of ordinates (D830->0). Regulation of k_q, D830_m and Y_m under rate-limiting CO₂ and O₂ concentrations applied after air (21% O₂, 310 ppm CO₂) was investigated. The amplitude of the downregulation of k_q (photosynthetic control) was maximal when electron transport rate (ETR) was limited to about 3 nmol cm^–2 s^–1 and decreased when ETR was higher or lower. Downregulation did not occur in the absence of CO₂ and O₂. These gases acted only as substrates of ribulosebisphosphate carboxylase-oxygenase, no high-affinity reaction of O₂ leading to enhanced photosynthetic control (e.g. Mehler reaction) was detected. After the transition, D830_m at first decreased and then increased again, showing that the reduction of the PS I acceptor side disappeared as a result of the downregulation of k_q. The variation of Y_m had two reasons, PS I acceptor side reduction and variable excitation capture efficiency by P700. It is concluded that electron transport through PS I is coregulated by the rate of plastoquinol oxidation at Cyt b ₆ f, excitation capture efficiency by P700, and by acceptor side reduction.Abbreviations Cyt b ₆ f cytochrome b ₆ f complex - D830 difference of the 830 nm signal from the dark level - ETR electron transport rate - PAD photon absorption density nmol cm^–2 s^–1 - PFD incident photon flux density, nmol cm^–2 s^–1 - PS I Photosystem I - PS II Photosystem II - PQH₂ plastoquinol - P700 Photosystem I donor pigment - Y quantum yield of PS I electron transport, rel. un.     

Two sites of photoinhibition of the electron transfer in oxygen evolving and Tris-treated PS II membrane fragments from spinach

Photoinhibition was analyzed in O₂-evolving and in Tris-treated PS II membrane fragments by measuring flash-induced absorption changes at 830 nm reflecting the transient P680⁺ formation and oxygen evolution. Irradiation by visible light affects the PS II electron transfer at two different sites: a) photoinhibition of site I eliminates the capability to perform a stable charge separation between P680⁺ and Q_A ^- within the reaction center (RC) and b) photoinhibition of site II blocks the electron transfer from Y_Z to P680⁺. The quantum yield of site I photoinhibition (2–3×10^-7 inhibited RC/quantum) is independent of the functional integrity of the water oxidizing system. In contrast, the quantum yield of photoinhibition at site II depends strongly on the oxygen evolution capacity. In O₂-evolving samples, the quantum yield of site II photoinhibition is about 10^-7 inhibited RC/quantum. After selective elimination of the O₂-evolving capacity by Tris-treatment, the quantum yield of photoinhibition at site II depends on the light intensity. At low intensity (<3 W/m²), the quantum yield is 10^-4 inhibited RC/quantum (about 1000 times higher than in oxygen evolving samples). Based on these results it is inferred that the dominating deleterious effect of photoinhibition cannot be ascribed to an unique target site or a single mechanism because it depends on different experimental conditions (e.g., light intensity) and the functional status of the PS II complex.Abbreviations A₈₃₀ absorption change at 830 nm - P680 primary electron donor of PS II - PS II photosystem II - Mes 2(N-morpholino)ethansulfonic acid - Q_A, Q_B primary and secondary acceptors of PS II - DCIP 2,6-dichlorophenolindophenol - DPC 1,5-diphenylcarbohydrazide - FWHM fullwidth at half maximum - Ph-p-BQ phenyl-p-benzoquinone - PFR photon fluence rate - Pheo pheophytin - RC reaction center     

Consequences of LHC II deficiency for photosynthetic regulation in chlorina mutants of barley

The light-harvesting chlorophyll a/b proteins associated with PS II (LHC II) are often considered to have a regulatory role in photosynthesis. The photosynthetic responses of four chlorina mutants of barley, which are deficient in LHC II to varying degrees, are examined to evaluate whether LHC II plays a regulatory role in photosynthesis. The efficiencies of light use for PS I and PS II photochemistry and for CO₂ assimilation in leaves of the mutants were monitored simultaneously over a wide range of photon flux densities of white light in the presence and absence of supplementary red light. It is demonstrated that the depletions of LHC II in these mutants results in a severe imbalance in the relative rates of excitation of PS I and PS II in favour of PS I, which cannot be alleviated by preferential excitation of PS II. Analyses of xanthophyll cycle pigments and fluorescence quenching in leaves of the mutants indicated that the major LHC II components are not required to facilitate the light-induced quenching associated with zeaxanthin formation. It is concluded that LHC II is important to balance the distribution of excitation energy between PS I and PS II populations over a wide range of photon flux densities. It appears that LHC II may also be important in determining the quantum efficiency of PS II photochemistry by reducing the rate of quenching of excitation energy in the PS II primary antennae.Abbreviations Fm, Fv maximal and variable fluorescence yields in a light adapted state - LHC II light harvesting chlorophyll a/b protein complex associated with PS II - q_p photochemical quenching - A₈₂₀ light-induced absorbance change at 820 nm - ø_PSI, ø_PSII relative quantum efficiencies of PS I and PS II photochemistry - øCO₂ quantum yield of CO₂ assimilation     

Photoinactivation of Photosystem II in leaves

Photoinactivation of Photosystem II (PS II), the light-induced loss of ability to evolve oxygen, inevitably occurs under any light environment in nature, counteracted by repair. Under certain conditions, the extent of photoinactivation of PS II depends on the photon exposure (light dosage, x), rather than the irradiance or duration of illumination per se, thus obeying the law of reciprocity of irradiance and duration of illumination, namely, that equal photon exposure produces an equal effect. If the probability of photoinactivation (p) of PS II is directly proportional to an increment in photon exposure (p = kΔx, where k is the probability per unit photon exposure), it can be deduced that the number of active PS II complexes decreases exponentially as a function of photon exposure: N = N_oexp(−kx). Further, since a photon exposure is usually achieved by varying the illumination time (t) at constant irradiance (I), N = N_oexp(−kI t), i.e., N decreases exponentially with time, with a rate coefficient of photoinactivation kI, where the product kI is obviously directly proportional to I. Given that N = N_oexp(−kx), the quantum yield of photoinactivation of PS II can be defined as −dN/dx = kN, which varies with the number of active PS II complexes remaining. Typically, the quantum yield of photoinactivation of PS II is ca. 0.1μmol PS II per mol photons at low photon exposure when repair is inhibited. That is, when about 10⁷ photons have been received by leaf tissue, one PS II complex is inactivated. Some species such as grapevine have a much lower quantum yield of photoinactivation of PS II, even at a chilling temperature. Examination of the longer-term time course of photoinactivation of PS II in capsicum leaves reveals that the decrease in N deviates from a single-exponential decay when the majority of the PS II complexes are inactivated in the absence of repair. This can be attributed to the formation of strong quenchers in severely-photoinactivated PS II complexes, able to dissipate excitation energy efficiently and to protect the remaining active neighbours against damage by light.     

Range of photosynthetic control of postillumination P700+ reduction rate in sunflower leaves

The kinetics of the postillumination reduction of P700⁺ which reflects the rate constant for plastoquinol (PQH₂) oxidation was recorded in sunflower leaves at different photon absorption densities (PAD), CO₂ and O₂ concentrations. The P700 oxidation state was calculated from the leaf transmittance at 830 nm logged at 50 s intervals. The P700⁺ dark reduction kinetics were fitted with two exponents with time constants of 6.5 and about 45 ms at atmospheric CO₂ and O₂ concentrations. The time constant of the fast component, which is the major contributor to the linear electron transport rate (ETR), did not change over the range of PADs of 14.5 to 134 nmol cm^-2 s^-1 in 21% O₂, but it increased up to 40 ms under severe limitation of ETR at low O₂ and CO₂. The acceptor side of Photosystem I (PS I) became reduced in correlation with the downregulation of the PQH₂ oxidation rate constant. It is concluded that thylakoid pH-related downregulation of the PQH₂ oxidation rate constant (photosynthetic control) is not present under normal atmospheric conditions but appears under severe limitation of the availability of electron acceptors. The measured range of photosynthetic control fits with the maximum variation of ETR under natural stress in C₃ plants. Increasing the carboxylase/oxygenase specificity would lead to higher reduction of the PS I acceptor side under stress.Abbreviations Cyt b ₆ f cytochrome b ₆ f complex - C_w cell-wall CO₂ concentration, M - ETR electron transport rate - Fd ferredoxin - FNR ferredoxin-NADP reductase - FRL far-red light - PC plastocyanin - PAD photon absorption density nmol cm^-2 s^-1 - PFD photon flux density nmol cm^-2 s^-1 - PS I Photosystem I complex - PQ plastoquinon - PQH₂ plastoquinol - PS II Photosystem II complex - P700 Photosystem I donor pigment, reduced - S830 830 nm signal (D830, difference of S830 from the dark level) - WL white light - Y_l maximum quantum yield of PS I electron transport, rel. un     

Intense green light emission in Sr2ZnGe2O7:Mn2+ phosphors by the design of high symmetry melilite structure

A green phosphor Sr₂ZnGe₂O₇:Mn²⁺ with a melilite structure was prepared using a high-temperature solid-state reaction. When the 535 nm emission was monitored, the excitation spectrum of the Sr₂ZnGe₂O₇:Mn²⁺ was found to contain two excitation bands in the ultraviolet (UV) region. When excited by UV light, the sample shows bright green emission at 535 nm, which corresponds to the distinctive transition of Mn²⁺ (⁴T₁→⁶A₁). Moreover, the quantum efficiency of Sr₂ZnGe₂O₇:Mn²⁺ could reach 67.6%. Finally, a high-performance white-light-emitting diode (WLED) with a low correlated colour temperature of 4632 K and a high colour rendering index (CRI) of 92.3 were packaged by coating commercial blue and red phosphors with an optimized Sr₂ZnGe₂O₇:Mn²⁺ sample on a 310 nm UV chip. This indicated that Sr₂ZnGe₂O₇:Mn²⁺ has the potential application as a green component in the WLED lighting field.     

Anticancer drug released from near IR-activated prodrug overcomes spatiotemporal limits of singlet oxygen

Photodynamic therapy (PDT) is a cancer treatment modality where photosensitizer (PS) is activated by visible and near IR light to produce singlet oxygen (¹O₂). However, ¹O₂ has a short lifetime (<40 ns) and cannot diffuse (<20 nm) beyond the cell diameter (e.g., ∼1800 nm). Thus, ¹O₂ damage is both spatially and temporally limited and does not produce bystander effect. In a heterogeneous tumor, cells escaping ¹O₂ damage can regrow after PDT treatment. To overcome these limitations, we developed a prodrug concept (PS–L–D) composed of a photosensitizer (PS), an anti-cancer drug (D), and an ¹O₂-cleavable linker (L). Upon illumination of the prodrug, ¹O₂ is generated, which damages the tumor and also releases anticancer drug. The locally released drug could cause spatially broader and temporally sustained damage, killing the surviving cancer cells after the PDT damage. In our previous report, we presented the superior activity of our prodrug of CA4 (combretastatin A-4), Pc-(L-CA4)₂, compared to its non-cleavable analog, Pc-(NCL-CA4)₂, that produced only PDT effects. Here, we provide clear evidence demonstrating that the released anticancer drug, CA4, indeed damages the surviving cancer cells over and beyond the spatial and temporal limits of ¹O₂. In the limited light illumination experiment, cells in the entire well were killed due to the effect of released anti-cancer drug, whereas only a partial damage was observed in the pseudo-prodrug treated wells. A time-dependent cell survival study showed more cell death in the prodrug-treated cells due to the sustained damage by the released CA4. Cell cycle analysis and microscopic imaging data demonstrated the typical damage patterns by CA4 in the prodrug treated cells. A time-dependent histological study showed that prodrug-treated tumors lacked mitotic bodies, and the prodrug caused broader and sustained tumor size reduction compared to those seen in the tumors treated with the pseudo-prodrug. This data consistently support that the released CA4 overcomes the spatiotemporal limitations of ¹O₂, providing far superior antitumor effect.     

Effects of elevated ozone on chlorophyll <Emphasis Type="Italic">a</Emphasis> fluorescence in symptomatic and asymptomatic leaves of two tomato genotypes

Two different genotypes of Lycopersicon esculentum Mill. (cv. Cuor di Bue, O₃-sensitive and line 93.1033/1, O₃-resistant) were treated with a single dose of ozone (150 mm³ m⁻³ for 3 h). The PS 2 activity was examined by measurements of chlorophyll a fluorescence on symptomatic and asymptomatic leaves. Symptoms were evident on the 4^th leaves from the bottom, in both genotypes, while the 2^nd leaves of the line 93.1033/1 were asymptomatic. In these leaves, the net photosynthetic rate (P_N) did not change even if the F_v/F_m ratio significantly decreased. A strong reduction in P_N, mostly due to the stomatal closure, was observed in Cuor di Bue. The non photochemical quenching coefficient (q_NP) and the degree of PS 2 reaction centres closure (1-q_P) were higher, while the quantum efficiency of PS 2 photochemistry (Φ_PS2) and quantum efficiency of excitation energy capture (Φ_exc.) were lower in O₃ treated leaves of both genotypes. The limitation of photosynthesis was shown also by a decrease in the parameter %P, which diminished compared to controls in both genotypes. The response of the two genotypes for the energy fraction dissipated as thermal energy in the PS 2 antennae (%D) was similar. The fraction of %P remained lower during the recovery in symptomatic leaves of the resistant line as compared to the controls, whereas %X, which represents the amount of light energy that is not utilized in photochemistry or dissipated in the PS 2 antennae, significantly rose in the asymptomatic leaves of this line and in both the leaves of Cuor di Bue. From data obtained we concluded that ozone affected the plants independently on the appearance of visible symptoms of injury because the leaves without visible symptoms of both the genotypes were negatively influenced.     

Effects of different irradiances on the photosynthetic process during ex-vitro acclimation of Anoectochilus plantlets

Six months old in vitro-grown Anoectochilus formosanus plantlets were transferred to ex-vitro acclimation under low irradiance, LI [60 μmol(photon) m⁻² s⁻¹], intermediate irradiance, II [180 μmol(photon) m⁻² s⁻¹], and high irradiance, HI [300 μmol(photon) m⁻² s⁻¹] for 30 d. Imposition of II led to a significant increase of chlorophyll (Chl) b content, rates of net photosynthesis (P _N) and transpiration (E), stomatal conductance (g _s), electron transfer rate (ETR), quantum yield of electron transport from water through photosystem 2 (Φ_PS2), and activity of ribulose-1,5-bisphosphate carboxylase/ oxygenase (RuBPCO, EC 4.1.1.39). This indicates that Anoectochilus was better acclimated at II compared to LI treatment. On the other hand, HI acclimation led to a significant reduction of Chl a and b, P _N, E, g _s, photochemical quenching, dark-adapted quantum efficiency of open PS2 centres (F_v/F_m), probability of an absorbed photon reaching an open PS2 reaction centre (F_v′/F_m′), ETR, Φ_PS2, and energy efficiency of CO₂ fixation (Φ_CO2/Φ_PS2). This indicates that HI treatment considerably exceeded the photo-protective capacity and Anoectochilus suffered HI induced damage to the photosynthetic apparatus. Imposition of HI significantly increased the contents of antheraxanthin and zeaxanthin (ZEA), non-photochemical quenching, and conversion of violaxanthin to ZEA. Thus Anoectochilus modifies its system to dissipate excess excitation energy and to protect the photosynthetic machinery.     

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

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

Improving the photosynthetic H2-productivity of the green alga Chlorella fusca by physiologically directed O2 avoidance and ammonium stimulation

Low production rates and sensitivity to O₂ are two major obstacles which prevent the technical exploitation of the ability of green algae to produce H₂ from water. Both problems were addressed in the present work. The inhibitory effect of O₂ on the hydrogen photoproduction of the green alga Chlorella fusca could be minimized by using algal cells which had not yet fully restored their oxygen evolving capacities after an artificially induced chloroplast de/regeneration cycle (de-/regreening). The H₂ photoproductivity peaked after 30 h of greening light while the O₂ evolution at this time reached only 59% of its normal capacity. The H₂PP yields could be further increased if NH₄Cl was added to the reaction medium at the beginning of the anaerobic preincubation period. No stimulatory effect was observed when NH₄Cl was added just before illumination, i.e. at the end of the 5-h-preincubation period. It is assumed that NH₄Cl inhibited the photosynthetic reduction of nitrite, which competed with hydrogen photoproduction indirectly by feedback repression of the NO ₂ ^- /NO ₃ ^- -reductive system. The impacts of the given results on an optimized H₂-production in green algae based on photosynthesis are discussed.Abbreviations H₂PP H₂ photoproduction - H₂ase hydrogenase - DA dark adaptation - LRG light regreening - DCMU 3-(3,4-dichlorophenyl)-l, 1-dimethylurea - Dit sodium dithionite - HEPES N-2-hydroxyethylpiperazin-N-2-ethan-sulfonic acid - PS I/II photosystem I/II     

Inhibition of photosynthesis in current-year needles of unfertilized and fertilized Norway spruce [Picea abies (L.) Karst.] during autumn and early winter

Inhibition of photosynthesis was followed during autumn and early winter in current-year sun and shade needles of unfertilized and fertilized Norway spruce [Picea abies (L.) Karst.] by simultaneous measurements of photosynthetic O₂ evolution and chlorophyll a fluorescence at 20 °C. The CO₂-saturated rate of O₂ evolution was generally higher in sun needles of fertilized trees than in those of unfertilized trees over a wide range of incident photon flux densities (PFDs). Furthermore, the maximum photo-chemical efficiency of photosystem (PS) II, as indicated by the ratio of variable to maximum fluorescence (F_V/F_M) was generally higher for sun needles of fertilized trees. The depression of f_v/f_m during frost periods was more pronounced in sun needles than in shade needles, indicating that winter inhibition in Norway spruce is strongly light-dependent. However, the inhibition of the rate of O₂ evolution at high PFDs in needles of fertilized trees during early winter was partly independent of the light regime experienced by those needles in the field, which appeared to result in a pronounced decrease in the proportion of oxidized PS II reaction centres in shade needles. A nearly identical linear relationship between the quantum yield of PS II electron transport determined by chlorophyll fluorescence and the quantum yield of O₂ evolution (gross rate of O₂ evolution/PFD) was obtained for the investigated types of needles during autumn and early winter. Except for shade needles of fertilized trees, this appeared to be largely achieved by adjustments in thermal energy dissipation within PS II.     

High optical-throughput spectroscopic singlet oxygen and photosensitizer luminescence dosimeter for monitoring of photodynamic therapy

We report a high light-throughput spectroscopic dosimeter system that is able to noninvasively measure luminescence signals of singlet oxygen (¹O₂) produced during photodynamic therapy (PDT) using a CW (continuous wave) light source. The system is based on a compact, fiber-coupled, high collection efficiency spectrometer (>50% transmittance) designed to maximize optical throughput but with sufficient spectral resolution (~7 nm). This is adequate to detect ¹O₂ phosphorescence in the presence of strong luminescence background in vivo. This system provides simultaneous acquisition of multiple spectral data points, allowing for more accurate determination of luminescence baseline via spectral fitting and thus the extraction of ¹O₂ phosphorescence signal based solely on spectroscopic decomposition, without the need for time-gating. Simultaneous collection of photons at different wavelengths improves the quantum efficiency of the system when compared to sequential spectral measurements such as filter-wheel or tunable-filter based systems. A prototype system was tested during in vivo PDT tumor regression experiments using benzoporphyrin derivative (BPD) photosensitizer. It was found that the treatment efficacy (tumor growth inhibition rate) correlated more strongly with ¹O₂ phosphorescence than with PS fluorescence. These results indicate that this high photon-collection efficiency spectrometer instrument may offer a viable option for real-time ¹O₂ dosimetry during PDT treatment using CW light.     

Photoinhibition of photosynthesis: effect of O2 and selective excitation of the photosystems in intact Lemna gibba plants

Intact Lemna gibba plants were photoinhibited under anaerobic conditions on illumination with monochromatic light which selectively excited the photosystems. Photoinhibition was less when PS 1 was excited and greatest when mainly PS 2 was excited, which suggests that PS 2 was most damaged by photoinhibition induced in complete absence of O₂ and CO₂.The illumination of plants with monochromatic light exciting PS 1, at different O₂ concentrations (in CO₂ deficient conditions), showed that PS 1 photoinhibition was increased at the low O₂ concentrations. The damage to PS 1 was more evident at 2% O₂ than at the higher O₂ concentrations.CO₂ as well as O₂ at atmospheric concentration, (air), was necessary for complete protection of the plant from photoinhibition when both photosystems were excited either separately or together.Abbreviations I irradiance, photon fluence rate - PCO photosynthetic carbon oxidation cycle - PCR photosynthetic carbon reduction cycle - PS 1 photosystem 1 - PS 2 photosystem 2     

CYTOCHROME f LOSS IN ASTAXANTHIN-ACCUMULATING RED CELLS OF HAEMATOCOCCUS PLUVIALIS (CHLOROPHYCEAE): COMPARISON OF PHOTOSYNTHETIC ACTIVITY, PHOTOSYNTHETIC ENZYMES, AND THYLAKOID MEMBRANE POLYPEPTIDES IN RED AND GREEN CELLS

Photosynthetic activity, chloroplast enzymes, and poly-peptides were compared in green and red (ketocarotenoid-containing) cultures of the microalga Haematococcus pluvialis Flotow. Green cultures, grown at 80 μmol pho-tons.m^-2. s^-1 in an acetate-containing medium, had a mean generation time of 27 h. Ketocarotenoid accumulation was induced by transfer of green cultures to PO₄-deficient medium and exposure to 250 μmol photons.m^-2. s^-1. Under these conditions, there was no increase in cell number, and the cultures turned red. Relative amounts of enzymes and thylakoid polypeptides in red and green cells were ascertained by immunoprobing with standardization on a chlorophyll (Chl) basis. In red cultures, the level of cytochrome f was greatly decreased (< 1% of green cell level), which is expected to greatly impair the linear electron flow from photosystem (PS) II to PS I. Also, the levels of apoproteins in red cells, namely, of CPI, D2, CP47, LHC I, and ribulose-1, 5-bisphosphate carboxylase were reduced to 15, 18, 29, 48, and 80%, respectively, of those in green cells. Only adenosine triphosphate syn-thase exhibited no significant change in the two types of cultures. The respiration rate of red cultures was much higher (100 μmoles O₂. mg Chl^-1.h^-1) than that of green cells (16 μmoles O₂. mg Chl^-1.h^-1). Conversely, net O₂ evolution (at P_max in green cultures was 80 μmoles O₂. mg Chl^-1.h^-1 but was —40 μmoles O₂. mg Chl^-1.h^-1 in red cultures. PS II activity was demonstrated in broken cells of both green and red cultures, showing activity of 40 and 15 μmoles DCPIP-mg Chl^-1.h^-1 (with DPC as electron donor), respectively. In contrast, PS I activity measured by the Mehler reaction showed that red rather than green cells had a greater activity (64 vs. 46 μmoles O₂. mg Chl^-1.h^-1, respectively). Thus, in spite of the decline of O₂ evolution in red cells, the photosystems were still functional. We postulate that the decline of O₂, evolution in red cells is largely attributable to an increase in the respiration rate and the impairment of linear electron flow from PS II to PS I and, to some extent, to a decrease in components of the photosystems.     

Recovery of photosynthesis and phtosystem II fluorescence in Chlamydomonas reinhardtii after exposure to three levels of high light

Recovery from 60 min of photoinhibitory treatment at photosynthetic photon flux densities of 500, 1400 and 2200 μMmol m^?2 s^? was followed in cells of the green alga Chlamydomonas reinhardtii grown at 125 μMmol m^?2 s^?1. These light treatments represent photoregulation, moderate photoinhibition and strong photoinhibition, respectively. Treatment in photoregulatory light resulted in an increased maximal rate of oxygen evolution (P_max) and an increased quantum yield (Φ), but a 15% decrease in F_v/F_M. Treatment at moderately photoinhibitory light resulted in a 30% decrease in F_v/F_M and an approximately equal decrease in Φ. Recovery in dim light restored F_v/F_M within 15 and 45 min after high light treatment at 500 and 1400 μMmol m^?2 s^?1, respectively. Convexity (Θ), a measure of the extent of co-limitation between PS II turnover and whole-chain electron transport, and Φ approached, but did not reach the control level during recovery after exposure to 1400 μMmol m^?2 s^?1, whereas P_max increased above the control. Treatment at 2200 μMmol m^?2 s^?1 resulted in a strong reduction of the modeled parameters Φ, Θ and P_max. Subsequent recovery was initially rapid but the rate decreased, and a complete recovery was not reached within 120 min. Based on the results, it is hypothesized that exposure to high light results in two phenomena. The first, expressed at all three light intensities, involves redistribution within the different aspects of PS II heterogeneity rather than a photoinhibitory destruction of PS II reaction centers. The second, most strongly expressed at 2200 μmol m^?2 s^?1, is a physical damage to PS II shown as an almost total loss of PS II_α and PS II Q_B-reducing centers. Thus recovery displayed two phase, the first was rapid and the only visible phase in algae exposed to 500 and 1400 μmol m^?2 s^?1. The second phase was slow and visible only in the later part of recovery in cells exposed to 2200 μmol m^?2 s^?1.     

Differential Control of Xanthophylls and Light-Induced Stress Proteins, as Opposed to Light-Harvesting Chlorophyll a/b Proteins, during Photosynthetic Acclimation of Barley Leaves to Light Irradiance

Barley (Hordeum vulgare L.) plants were grown at different photon flux densities ranging from 100 to 1800 μmol m⁻² s⁻¹ in air and/or in atmospheres with reduced levels of O₂ and CO₂. Low O₂ and CO₂ partial pressures allowed plants to grow under high photosystem II (PSII) excitation pressure, estimated in vivo by chlorophyll fluorescence measurements, at moderate photon flux densities. The xanthophyll-cycle pigments, the early light-inducible proteins, and their mRNA accumulated with increasing PSII excitation pressure irrespective of the way high excitation pressure was obtained (high-light irradiance or decreased CO₂ and O₂ availability). These findings indicate that the reduction state of electron transport chain components could be involved in light sensing for the regulation of nuclear-encoded chloroplast gene expression. In contrast, no correlation was found between the reduction state of PSII and various indicators of the PSII light-harvesting system, such as the chlorophyll a-to-b ratio, the abundance of the major pigment-protein complex of PSII (LHCII), the mRNA level of LHCII, the light-saturation curve of O₂ evolution, and the induced chlorophyll-fluorescence rise. We conclude that the chlorophyll antenna size of PSII is not governed by the redox state of PSII in higher plants and, consequently, regulation of early light-inducible protein synthesis is different from that of LHCII.     

Unifying model for the photoinactivation of Photosystem II in vivo under steady-state photosynthesis

We present a unifying mechanism for photoinhibition based on current obsevations from in vivo studies rather than from in vitro studies with isolated thylakoids or PS II membranes. In vitro studies have limited relevance for in vivo photoinhibition because very high light is used with photon exposures rarely encountered in nature, and most of the multiple, interacting, protective strategies of PS II regulation in living cells are not functional. It is now established that the photoinactivation of Photosystem II in vivo is a probability and light-dosage event which depends on the photons absorbed and not the irradiance per se. As the reciprocity law is obeyed and target theory analysis strongly suggests that only one photon is required, we propose that a single dominant molecular mechanism occurs in vivo with one photon inactivating PS II under limiting, saturating or sustained high light. Two mechanisms have been proposed for photoinhibition under high light, acceptor-side and donor-side photoinhibition [see Aro et al. (1994) Biochim Biophys Acta 1143: 113–134], and another mechanism for very low light, the low-light syndrome [Keren et al. (1995) J Biol Chem 270: 806–814]. Based on the exciton-radical pair equilibrium model of exciton dynamics, we propose a unifying mechanism for the photoinactivation of PS II in vivo under steady-state photosynthesis that depends on the generation and maintenance of increased concentrations of the primary radical pair, P680⁺Pheo_–, and the different ways charge recombination is regulated under varying environmental conditions [Anderson et al. (1997) Physiol Plant 100: 214–223]. We suggest that the primary cause of damage to D1 protein is P680⁺, rather than singlet O₂ formed from triplet P680, or other reactive oxygen species.     

Photoinhibition of photosynthesis in chilled potato leaves is not correlated with a loss of Photosystem-II activity

When 23 °C-grown potato leaves (Solanum tuberosum L.) were irradiated at 23 °C with a strong white light, photosynthetic electron transport and Photosystem-II (PS II) activity were inhibited in parallel. When the light treatment was given at a low temperature of 3 °C, the photoinhibition of photosynthesis was considerably enhanced, as expected. Surprisingly, no such stimulation of photoinhibition was observed with respect to the PS II function. A detailed functional analysis of the photosynthetic apparatus, using in-vivo fluorescence, absorbance, oxygen and photoacoustic measurements, and artificial electron donors/acceptors, showed a pronounced alteration of PS I activity during light stress at low temperature. More precisely, it was observed that both the pool of photooxidizeable reaction center pigment (P₇₀₀) of PS I and the efficiency of PS I to oxidize P₇₀₀ were dramatically reduced. Loss of P₇₀₀ activity was shown to be essentially dependent on atmospheric O₂ and to require a continued flow of electrons from PS II, suggesting the involvement of the superoxide anion radical which is produced by the interaction of O₂ and the photosynthetic electron-transfer chain through the Mehler reaction. Mass spectrometric measurements of O₂ exchange by potato leaves under strong illumination did not reveal, however, any stimulation of the Mehler reaction at low temperature, thus leading to the conclusion that O₂ toxicity mainly resulted from a chilling-induced inhibition of the scavenging system for O₂-radicals. Support for this interpretation was provided by the light response of potato leaves infiltrated with an inhibitor (diethyldithiocarbamate) of the chloroplastic Cu-Zn superoxide dismutase. It was indeed possible to simulate the differential inhibition of the PS II photochemical activity and the linear electron transport observed during light stress at low temperature by illuminating at 23 °C diethyldithiocarbamate-poisoned leaves. The experimental data presented here suggests that (i) the previously reported resistance of PS I to photoinhibition damage in-vivo is not an intrinsic property of PS I but results from efficient protective systems against O₂ toxicity, (ii) PS I is photoinhibited in chilled potato leaf due to the inactivation of this PS I defence system and (iii) PS I is more sensitive to superoxide anion radicals than PS II.Abbreviations PS - Photosystem - E - Emerson enhancement - _open ^p and ^P maximal and actual quantum yields of PS II photochemistry - DDC - diethyldithiocarbamate - Q_A and Q_B - primary and secondary (quinone) electron acceptors of PS II - P₆₈₀ and P₇₀₀ - reaction center pigments of PS II and PS I, respectively - SOD - superoxide dismutase