Quantum Yield Of Oxygen Evolution Research Articles

Single‐Metal Atoms and Ultra‐Small Clusters Manipulating Charge Carrier Migration in Polymeric Perylene Diimide for Efficient Photocatalytic Oxygen Production

AbstractLimited by the retarded charge carrier migration and the sluggish four‐electron reaction kinetics, it is still a great challenge for polymeric semiconductors to achieve efficient photocatalytic water oxidation. Herein, single Co atoms and ultra‐small CoOx clusters are simultaneously introduced into polymeric perylene diimide (PDI) through a facile impregnation‐calcination two‐step method. The obtained Co‐PDI exhibits excellent photocatalytic water oxidation activity under visible‐light irradiation, with an oxygen evolution rate reaching as high as 5.53 mmol h–1 g–1 (λ > 420 nm). The apparent quantum yield for oxygen evolution is determined to be 8.17% at 450 nm, and remains 0.77% at even longer visible light wavelength of 700 nm without redundant co‐catalysts, indicating that Co‐PDI may serve as an excellent oxygen evolution photocatalyst for water splitting. Theoretical calculations and experimental results demonstrate that single Co atoms act as the electron mediators connecting adjacent PDI layers to build directional channels for rapid charge transfer, while ultra‐small CoOx clusters as hole collectors and reaction sites to accelerate oxygen evolution reaction kinetics. The study presents a facile and reliable strategy to effectively activate polymeric semiconductors for efficient photocatalysis by rationally modulating atomic structures and active sites for promoted charge carrier transfer and surface reaction kinetics.

Light-Driven Water Oxidation with the Ir-blue Catalyst and the Ru(bpy)32+/S2O82- Cycle: Photogeneration of Active Dimers, Electron-Transfer Kinetics, and Light Synchronization for Oxygen Evolution with High Quantum Efficiency.

Light-driven water oxidation is achieved with the Ru(bpy)32+/S2O82- cycle employing the highly active Ir-blue water oxidation catalyst, namely, an IrIV,IV2(pyalc)2 μ-oxo-dimer [pyalc = 2-(2'-pyridyl)-2-propanoate]. Ir-blue is readily formed by stepwise oxidation of the monomeric Ir(III) precursor 1 by the photogenerated Ru(bpy)33+, with a quantum yield ϕ of up to 0.10. Transient absorption spectroscopy and kinetic evidence point to a stepwise mechanism, where the primary event occurs via a fast photoinduced electron transfer from 1 to Ru(bpy)33+, leading to the Ir(IV) monomer I1 (k1 ∼ 108 M-1 s-1). The competent Ir-blue catalyst is then obtained from I1 upon photooxidative loss of the Cp* ligand and dimerization. The Ir-blue catalyst is active in the Ru(bpy)32+/S2O82- light-driven water oxidation cycle, where it undergoes two fast photoinduced electron transfers to Ru(bpy)33+ [with kIr-blue = (3.00 ± 0.02) × 108 M-1 s-1 for the primary event, outperforming iridium oxide nanoparticles by ca. 2 orders of magnitude], leading to a IrV,V2 steady-state intermediate involved in O-O bond formation. The quantum yield for oxygen evolution depends on the photon flux, showing a saturation regime and reaching an impressive value of ϕ(O2) = 0.32 ± 0.01 (corresponding to a quantum efficiency of 64 ± 2%) at low irradiation intensity. This result highlights the key requirement of orchestrating the rate of the photochemical events with dark catalytic turnover.

The time course of photoinactivation of photosystem II in leaves revisited

Since photosystem II (PS II) performs the demanding function of water oxidation using light energy, it is susceptible to photoinactivation during photosynthesis. The time course of photoinactivation of PS II yields useful information about the process. Depending on how PS II function is assayed, however, the time course seems to differ. Here, we revisit this problem by using two additional assays: (1) the quantum yield of oxygen evolution in limiting, continuous light and (2) the flash-induced cumulative delivery of PS II electrons to the oxidized primary donor (P700(+)) in PS I measured as a 'P700 kinetics area'. The P700 kinetics area is based on the fact that the two photosystems function in series: when P700 is completely photo-oxidized by a flash added to continuous far-red light, electrons delivered from PS II to PS I by the flash tend to re-reduce P700(+) transiently to an extent depending on the PS II functionality, while the far-red light photo-oxidizes P700 back to the steady-state concentration. The quantum yield of oxygen evolution in limiting, continuous light indeed decreased in a way that deviated from a single-negative exponential. However, measurement of the quantum yield of oxygen in limiting light may be complicated by changes in mitochondrial respiration between darkness and limiting light. Similarly, an assay based on chlorophyll fluorescence may be complicated by the varying depth in leaf tissue from which the signal is detected after progressive photoinactivation of PS II. On the other hand, the P700 kinetics area appears to be a reasonable assay, which is a measure of functional PS II in the whole leaf tissue and independent of changes in mitochondrial respiration. The P700 kinetics area decreased in a single-negative exponential fashion during progressive photoinactivation of PS II in a number of plant species, at least at functional PS II contents ≥6% of the initial value, in agreement with the conclusion of Sarvikas et al. (Photosynth Res 103:7-17, 2010). That is, the single-negative-exponential time course does not provide evidence for photoprotection of functional PS II complexes by photoinactivated, connected neighbours.

Electron transfer kinetics in water splitting dye-sensitized solar cells based on core–shell oxide electrodes

Photoelectrochemical water splitting occurs in a dye-sensitized solar cell when a [Ru(bpy)3]2+-based dye covalently links a porous TiO2 anode film to IrO2 x nH2O nanoparticles. The quantum yield for oxygen evolution is low because of rapid back electron transfer between TiO2 and the oxidized dye, which occurs on a timescale of hundreds of microseconds, When iodide is added as an electron donor, the photocurrent increases, confirming that the initial charge injection efficiency is high. When the porous TiO2 film is coated with a 1-2 nm thick layer of ZrO2 or Nb2O5, both the charge injection rate and back electron transfer rate decrease. The efficiency of the cell increases and then decreases with increasing film thickness, consistent with the trends in charge injection and recombination rates. The current efficiency for oxygen evolution, measured electrochemically in a generator-collector geometry, is close to 100%. The factors that lead to polarization of the photoanode and possible ways to re-design the system for higher efficiency are discussed.

The multiple roles of light-harvesting chlorophyll a/ b-protein complexes define structure and optimize function of Arabidopsis chloroplasts: A study using two chlorophyll b-less mutants

The multiple roles of light-harvesting chlorophyll a/ b-protein complexes in the structure and function of Arabidopsis chloroplasts were investigated using two chlorophyll b-less mutants grown under metal halide lamps with a significant far-red component. In ch1-3, all six light-harvesting proteins of photosystem (PS) II were greatly decreased; in ch1-3lhcb5, Lhcb5 was completely absent while the other five proteins were further decreased. The thylakoids of ch1-3 were less negatively-charged than the wild type, and those of ch1-3lhcb5 were even less so. Despite the expected weaker electrostatic repulsion, however, thylakoids in leaves of the mutants were not well stacked, an effect we attribute to lower van der Waals attraction, lower electrostatic attraction between opposite charges, and the absence or instability of PSII supercomplexes and peripheral light-harvesting trimers. The quantum yield of oxygen evolution in leaves decreased from 0.109 (wild type) to 0.087 ( ch1-3) and 0.081 ( ch1-3lhcb5) O 2 (photon absorbed) − 1 ; we attribute this decrease to an excessive spillover from PSII to PSI, a limited PSII antenna, and increased light-independent thermal dissipation in PSII in the mutants. Destabilization of the donor side of PSII, indicated by slower electron donation to the redox-active tyrosine Y Z in ch1-3, probably enhanced PSII susceptibility to photoinactivation, increased the non-functional PSII complexes in vivo, and further inactivated PSII complexes in vitro. The evolution of chlorophyll b-containing chloroplasts seems to fine-tune oxygenic photosynthesis.

On the role of photosynthesis in the nitrate-dependent induction of the alternative oxidase inChlamydomonas reinhardtii

In the green alga Chlamydomonas reinhardtii Dangeard a shift in nitrogen source from ammonium to nitrate results in the rapid induction of the mitochondrial alternative oxidase (AOX). It has been hypothesized that this induction compensates for the increased generation of ATP in the chloroplast. Based upon light response curves of oxygen evolution, a shift from ammonium to nitrate resulted in a significant increase in both apparent quantum yield of oxygen evolution and photosynthetic capacity. Changes in chlorophyll a fluorescence including a decrease in both excitation pressure and nonphotochemical quenching were also observed over a 12 h nitrate shift. To investigate whether changes in chloroplast ATP generation were behind the nitrate-dependent induction of AOX, transcript and protein accumulation were monitored in cells grown under high, moderate and low irradiances. No major differences in Aox1 gene expression or AOX accumulation were found among the different light regimes. These data do not support the hypothesis that an increase in AOX within the mitochondrion in response to nitrate is tied to changes in photosynthesis occurring within the chloroplast.

Changes in the pigment patterns and the photosynthetic activity during a light-induced cell cycle of the green alga Scenedesmus armatus

The photosynthetic oxygen evolution as well as the chlorophyll and carotenoid patterns were studied during the light phase (14 h) of the Scenedesmus armatus cell cycle. The alga was synchronised by the light/dark regime (14/10 h). In this publication, the term “cell cycle” refers to this period of light only. The oxygen evolution measured by a Clark-type electrode and expressed per cell, gradually increased from the beginning of the cell cycle, reaching its maximum at 12 h and then slowly declined towards the end of the cell cycle. This pattern reflects the final reproductive events of the cell cycle consisting of the third mitotic division, chloroplast and protoplast fission, followed by the formation of the autospores. When the same amount of oxygen was expressed per chlorophyll a content, we observed a rapid increase just after the onset of light, which reached a maximum at the third hour, after which, this slowly declined until the end of the cell cycle. A similar pattern for the relative quantum yield of oxygen evolution was obtained when a photobaric component of a photoacoustic signal was analysed by the photoacoustic spectroscopy method. The most abundant carotenoid was lutein. Much smaller amounts of α-carotene, β-carotene, loroxanthin, violoxanthin and neoxanthin were noted; traces of zeaxanthin and antheraxanthin were also identified. The photosynthetic efficiency and the ratio of lutein/α-carotene followed the same patterns during the cell cycle and similar relationships were also observed in the ratio changes of violoxanthin/β-carotene and violoxanthin/neoxanthin. The most photosynthetic-efficient cells contained the highest level of lutein, and had a much lower violoxanthin content. The content of neoxanthin and β-carotene found was lower, with both pigments still being present in similar amounts. These results suggest that the molecular organisation of LHC IIb mainly determined the photosynthetic efficiency of algae during its light-induced cell cycle.

Relations between electron transport rates determined by pulse amplitude modulated chlorophyll fluorescence and oxygen evolution in macroalgae under different light conditions.

The relationship between O(2)-based gross photosynthesis (GP) and in vivo chlorophyll fluorescence of Photosystem II-based electron transport rate (ETR) as well as the relationship between effective quantum yield of fluorescence (Phi(PSII)) and quantum yield of oxygen evolution (Phi(O_2)) were examined in the green algae Ulva rotundata and Ulva olivascens and the red alga Porphyra leucosticta collected from the field and incubated for 3 days at 100 mumol m(-2) s(-1) in nutrient enriched seawater. Maximal GP was twice as high in Ulva species than that measured in P. leucosticta. In all species ETR was saturated at much higher irradiance than GP. The initial slope of ETR versus absorbed irradiance was higher than that of GP versus absorbed irradiance. Only under absorbed irradiances below saturation or at values of GP <2 mumol O(2) m(-2) s(-1) a linear relationship was observed. In the linear phase, calculated O(2) evolved /ETR molar ratios were closed to the theoretical value of 0.25 in Ulva species. In P. leucosticta, the estimated GP was associated to the estimated ETR only at high irradiances. ETR was determined under white light, red light emitting by diodes and solar radiation. In Ulva species the maximal ETR was reached under red light and solar radiation whereas in P. leucosticta the maximal ETR was reached under white light and minimal under red light. These results are in agreement with the known action spectra for photosynthesis in these species. In the case of P. leucosticta, GP and ETR were additionally determined under saturating irradiance in algae pre-incubated for one week under white light at different irradiances and at white light (100 mumol m(-2) s(-1)) enriched with far-red light. GP and growth rate increased at a growth irradiance of 500 mumol m(-2) s(-1) becoming photoinhibited at higher irradiances, while ETR increased when algae were exposed to the highest growth irradiance applied (2000 mumol m(-2) s(-1)). The calculated O(2) evolved /ETR molar ratios were close to the theoretical value of 0.25 when algae were pre-incubated under 500-1000 mumol m(-2) s(-1). The enrichment by FR light provoked a decrease in both GP and ETR and an increase of nonphotochemical quenching although the irradiance of PAR was maintained at a constant level. In addition to C assimilation, other electron sinks, such as nitrogen assimilation, affected the GP-ETR relationship. The slopes of GP versus ETR or Phi(PSII) versus Phi(O_2) were lower in the algae with the highest N assimilation capacity, estimated as nitrate reductase activity and internal nitrogen contents, i.e., Ulva rotundata and Porphyra leucosticta, than that observed in U. olivascens. The possible mechanisms to explain this discrepancy between GP and ETR are discussed.

Alterations of the oxygen-evolving apparatus induced by a 305Arg --> 305Ser mutation in the CP43 protein of photosystem II from Synechocystis sp. PCC 6803 under chloride-limiting conditions.

The psbC gene encodes CP43, a component of Photosystem II (PSII) in higher plants, algae, and cyanobacteria. Previous work demonstrated that alteration of an arginine residue occurring at position 305 to serine produced a strain (R305S) with altered PSII activity (Knoepfle, N., Bricker, T. M., and Putnam-Evans, C. (1999) Biochemistry 38, 1582-1588). This strain grew at wild-type rates in complete BG-11 media (480 microM chloride) and evolved oxygen at rates that were 60-70% of the observed wild-type rates. The R305S strain assembled approximately 70-80% of the functional PSII centers contained in the control strain, and these PSII centers were very sensitive to photoinactivation at high light intensities. We recently observed that the R305S mutant exhibited a pronounced chloride effect. When this mutant was grown in media depleted of chloride (30 microM chloride), it exhibited a severely reduced photoautotrophic growth rate. The effect of chloride depletion on the growth rate of the mutant was reversed by the addition of 480 microM bromide to the chloride-depleted BG-11 media. Oxygen evolution rates for the mutant were further depressed to about 22% of that observed in control cells under chloride-limiting conditions. Addition of bromide restored these rates to those observed under chloride-sufficient conditions. The mutant exhibited a significantly lower relative quantum yield for oxygen evolution than did the control strain, and this was exacerbated under chloride-limiting conditions. Fluorescence yield measurements indicated that both the mutant and the control strains assembled fewer PSII reaction centers under chloride-limiting conditions. The reaction centers assembled by the mutant exhibited an enhanced sensitivity to photoinactivation under chloride-limiting conditions, with a t(1/2) of photoinactivation of 2.6 min under chloride-limiting conditions as compared to a t(1/2) of 4.7 min under normal growth conditions. The mutant also exhibited an enhanced stability of its S(2) state and increased number of centers in the S(1) state following dark incubation. These results indicate that the mutant R305S exhibits a defect in its ability to utilize chloride in support of efficient oxygen evolution in PSII. This is the first mutant of this type described in the CP43 protein.

The interaction between plastocyanin and photosystem I is inefficient in transgenic Arabidopsis plants lacking the PSI-N subunit of photosystem I.

The PSI-N subunit of photosystem I (PSI) is restricted to higher plants and is the only subunit located entirely in the thylakoid lumen. The role of the PSI-N subunit in the PSI complex was investigated in transgenic Arabidopsis plants which were generated using antisense and co-suppression strategies. Several lines without detectable levels of PSI-N were identified. The plants lacking PSI-N assembled a functional PSI complex and were capable of photoautotrophic growth. When grown on agar media for several weeks the plants became chlorotic and developed significantly more slowly. However, under optimal growth conditions, the plants without PSI-N were visually indistinguishable from the wild-type although several photosynthetic parameters were affected. In the transformants, the second-order rate constant for electron transfer from plastocyanin to P700+, the oxidized reaction centre of PSI, was only 55% of the wild-type value, and steady-state NADP+ reduction was decreased to a similar extent. Quantum yield of oxygen evolution and PSII photochemistry were about 10% lower than in the wild-type at leaf level. Photochemical fluorescence quenching was lowered to a similar extent. Thus, the 40-50% lower activity of PSI at the molecular level was much less significant at the whole-plant level. This was partly explained by a 17% increase in PSI content in the plants lacking PSI-N.

A photoacoustic spectrometer for measuring heat dissipation and oxygen quantum yield at the microscopic level within leaf tissues

A photoacoustic instrument has been developed to measure photothermal signals and oxygen evolution at the microscopic level within leaves and leaf cross sections. The device utilizes lines of light, obtained from laser diodes, that are focussed on the sample and permit measurements with a spatial resolution of 40 μm. In cross sections of spinach leaves (700 μm thick) the profile for the photothermal signals is bell shaped and coincides closely with the chlorophyll content of the tissues. By contrast, the profile for oxygen evolution is relatively flat, suggesting uniform capacity for oxygen evolution in the mesophyll. Maximum values for relative quantum yield for oxygen evolution are found in palisade tissues near the adaxial leaf surface. The photoacoustic device shows that it is possible to collect thermal and oxygen signals from microscopic leaf samples and opens a new avenue for studies of leaf structure—function.

Acclimation of shade-developed leaves on saplings exposed to late-season canopy gaps.

We hypothesized that photoinhibition of shade-developed leaves of deciduous hardwood saplings would limit their ability to acclimate photosynthetically to increased irradiance, and we predicted that shade-tolerant sugar maple (Acer saccharum Marsh.) would be more susceptible to photoinhibition than intermediately shade-tolerant red oak (Quercus rubra L.). After four weeks in a canopy gap, photosynthetic rates of shade-developed leaves of both species had increased in response to the increase in irradiance, although final acclimation was more complete in red oak. However, photoinhibition occurred in both species, as indicated by short-term reductions in maximum rates of net photosynthesis and the quantum yield of oxygen evolution, and longer-term reductions in the efficiency of excitation energy capture by open photosystem II (PSII) reaction centers (dark-adapted F(v)/F(m)) and the quantum yield of PSII in the light (phi(PSII)). The magnitude and duration of this decrease were greater in sugar maple than in red oak, suggesting greater susceptibility to photoinhibition in sugar maple. Photoinhibition may have resulted from photodamage, but it may also have involved sustained rates of photoprotective energy dissipation (especially in red oak). Photosynthetic acclimation also appeared to be linked to an ability to increase leaf nitrogen content. Limited photosynthetic acclimation in shade-developed sugar maple leaves may reflect a trade-off between shade-tolerance and rapid acclimation to a canopy gap.

Complementary chromatic adaptation alters photosynthetic strategies in the cyanobacterium Calothrix.

The cyanobacterium Calothrix sp. strain PCC 7601 drastically changes phycobiliprotein composition and colour in response to light quality, through complementary chromatic adaptation (CCA). Red light promotes phycocyanin-II and inhibits phycoerythrin synthesis, while green light has the opposite effect, through changes in transcription regulated by a putative green/red photoreceptor(s). The effects of CCA on photosynthesis were characterized by measuring oxygen evolution and chlorophyll fluorescence parameters. Cells fully acclimated to either red or green light achieve a similar photosynthetic quantum yield of oxygen evolution (light-use efficiency). Shifting acclimated cells from green to red or from red to green light caused similar 40% drops in photosynthetic quantum yield. Therefore, full CCA significantly increases light use efficiency, which is of great importance under light-limited growth. Cells growing under red light are in state I, with very low PS II to PS I energy transfer, since red light is absorbed both by phycocyanin in the phycobilisome/PS II supracomplex and by PS I chlorophyll. Cells growing under green light are in state II, with high transfer of excitation energy from the phycobilisome/PS II supracomplex to PS I. This transfer allows green light captured by phycoerythrin to ultimately drive both PS I and PS II photochemistry.

D1 exchange and the Photosystem II repair cycle in the cyanobacterium Synechococcus

Photoinhibition in cyanobacteria results when photodynamic inactivation of the Photosystem II (PS II) D1 protein exceeds new D1 synthesis, largely without the additional down-regulation processes found in plants. Furthermore, unlike plants, the cyanobacterium Synechococcus sp. PCC 7942 contains genes coding for two different forms of the D1 protein, D1:1 and D1:2. Cells containing only D1:1 have a lower quantum yield of oxygen evolution and generally higher non-photochemical quenching than do cells containing only D1:2. D1:1 is the predominant form in acclimated cells, but is transiently replaced by D1:2 after a shift to excess excitation, primarily as a result of changes in transcription. The exchange is triggered not by changes in light per se, but by treatments which cause fractional PS II reaction centre closure, including excess light, low temperature and partial inhibition of electron transport by DCMU. In contrast, DBMIB or heat shock treatments do not cause PS II closure and do not drive the exchange, even though overall electron transport is inhibited. After a temperature drop, high levels of nascent D1:2 accumulate in a novel form, D1:2 ∗, approximately 3 kDa larger than the normal D1:2 polypeptide. D1:2 ∗ is not phosphorylated, does not appear to contribute to PS II function, and is probably an inactive precursor whose processing is retarded by chilling. As the cells acclimate to the lower temperature D1:2 ∗ is cleared in parallel with full recovery of PS II activity. Interestingly, D1:2 ∗ does not accumulate upon chilling in a mutant expressing only D1:2, apparently processing in this strain continues without an acclimation lag.

Comparison of Maize Inbred Lines Differing in Low Temperature Tolerance: Effect of Acclimation at Suboptimal Temperature on Chloroplast Functioning

Acclimation to optimal or suboptimal temperature may influence photosynthetic properties of different maize genotypes in distinct ways. In this study, leaf growth and chloroplast functioning of the second leaves of Penjalinan, an inbred line used in warm tropical regions (CS) and Z7, an inbred line adapted to cool temperate regions (CT), acclimated at near optimal (24/22°C) and suboptimal (15/13°C) temperature, were compared. When acclimated at 24/22°C, there was little difference between CT and CS in pigment content, in photosynthetic capacity, measured as the maximum quantum yield of oxygen evolution under light limiting conditions (ΦO2) and the maximum rate of oxygen evolution at light saturation (Pmax), both at 24 and 14°C, or in quantum yield of PSII electron transport (Φe) at decreasing temperature. When acclimated at 15/13°C, differences increased or changed in favour of CT, especially when measured at suboptimal temperature (14°C). CS was more sensitive to light independent chilling compared to CT, as indicated by a sharper decrease in time of the maximal rise of chlorophyll fluorescence induction (FR). A decrease in FR was accompanied by a smaller decrease in light use efficiency, measured as the ratio of variable to maximal fluorescence (Fv/Fm) of leaf sections kept at 0°C in the dark. When acclimated at 15/13°C, differences in FR between CT and CS markedly increased compared to leaves acclimated at 24/22°C. Following a chilling period of 24 h, a faster and more complete recovery of FR at room temperature in the dark was found in leaves acclimated at 15/13°C. CS was also more sensitive to photoinhibition than CT, as indicated by a sharper decrease with time in the potential quantum yield of PSII (Fv/Fm) when leaf disks were subjected to high light intensity (600 μmol m-2s-1) at low temperature (0-2°C). Compared to leaves acclimated at 24/22°C, differences between CT and CS increased when acclimated at 15/13°C. Following a photoinhibitory treatment of 2·5 h, the fastest rates of recovery were found in leaves acclimated at 15/13°C. However, differences in Fv/Fm between CT and CS were still evident after 20 h of recovery.

Zeaxanthin Dependent and Zeaxanthin Independent Changes in Nonphotochemical Energy Dissipation

The influence of zeaxanthin on high-energy-state quenching (qE) of room temperature chlorophyll fluorescence and on the quantum yield of oxygen evolution(Ф O 2 ) has been studied with isolated spinach thylakoids. A set of three different pretreatments was tested which all led to thylakoids with high contents of zeaxanthin. Depending on the presence of light the pretreatments differed strongly with respect to their effect on nonphotochemical fluorescence quenching. Independently from the extent of changes in nonphotochemical energy dissipation in the membrane energized state as reflected by nonphotochemical quenching the light pretreatments caused also changes in nonphotochemical energy dissipation in the nonenergized state as measured by the quantum yield of oxygen evolution (Ф O 2 ) in uncoupled thylakoids. Zeaxanthin was not involved in the decrease of Ф O 2 .

Developmental Variability of Photooxidative Stress Tolerance in Paraquat-Resistant Conyza.

Paraquat-resistant hairy fleabane (Conyza bonariensis L. Cronq.) has been extensively studied, with some contention. A single, dominant gene pleiotropically controls levels of oxidant-detoxifying enzymes and tolerance to many photooxidants, to photoinhibition, and possibly to other stresses. The weed forms a rosette on humid short days and flowers in dry long days and, thus, needs plasticity to photooxidant stresses. In a series of four experiments over 20 months, the resistant and susceptible biotypes were cultured in constant 10-h low-light short days at 25[deg]C. Resistance was measured as recovery from paraquat. The concentration required to achieve 50% inhibition of the resistant biotype was about 30 times that of the susceptible one just after germination, increased to >300 times that of the susceptibles at 10 weeks of growth, and then decreased to 20-fold, remaining constant except for a brief increase while bolting. Resistance increased when plants were induced to flower by long days. The levels of plastid superoxide dismutase and of glutathione reductase were generally highest in resistant plants compared to those of the susceptibles at the times of highest paraquat resistance, but they were imperceptibly different from the susceptible type at the times of lower paraquat resistance. Photoinhibition tolerance measured as quantum yield of oxygen evolution at ambient temperatures was highest when the relative amounts of enzymes were highest in the resistant biotype. Resistance to photoinhibition was not detected by chlorophyll a fluorescence. Enzyme levels, photoinhibition tolerance, and paraquat resistance all increased during flowering in both biotypes. Imperceptibly small increases in enzyme levels would be needed for 20-fold resistance, based on the moderate enzyme increases correlated with 300-fold resistance. Thus, it is feasible that either these enzymes play a role in the first line of defense against photooxidants, or another, yet unknown mechanism(s) facilitate(s) the lower level of resistance, or the enzymes and unknown mechanisms act together.

Theoretical assessment of alternative mechanisms for non-photochemical quenching of PS II fluorescence in barley leaves

The components of non-photochemical chlorophyll fluorescence quenching (qN) in barley leaves have been quantified by a combination of relaxation kinetics analysis and 77 K fluorescence measurements (Walters RG and Horton P 1991). Analysis of the behaviour of chlorophyll fluorescence parameters and oxygen evolution at low light (when only state transitions - measured as qNt - are present) and at high light (when only photoinhibition - measured as qNi - is increasing) showed that the parameter qNt represents quenching processes located in the antenna and that qNi measures quenching processes located in the reaction centre but which operate significantly only when those centres are closed. The theoretical predictions of a variety of models describing possible mechanisms for high-energy-state quenching, measured as the residual quenching, qNe, were then tested against the experimental data for both fluorescence quenching and quantum yield of oxygen evolution. Only one model was found to agree with these data, one in which antennae exist in two states, efficient in either energy transfer or energy dissipation, and in which those photosynthetic units in a dissipative state are unable to exchange energy with non-dissipative units.

Characterization of drought resistance in a wild relative of wheat, Triticum kotschyi

Wild relatives of wheat have served as a genetic source for economically useful traits. A better understanding of the mechanisms underlying such traits may be useful in the genetic transfer and selection processes. Research was undertaken to compare the effects of controlled water stress on photosynthetic parameters in Triticum kotschyi, a drought resistant wild wheat and Triticum aestivum cv. Lakhish, a drought sensitive wheat cultivar. During stress development, the leaf water potential decreased at a slower rate, and the quantum yield of oxygen evolution, measured photoacoustically in vivo, decreased to a smaller extent in the drought resistant wild wheat than in the wheat cultivar. The decrease in quantum yield at water potentials from -0.9 Mpa down to -2.3 Mpa was not accompanied by damage to PS II reaction centers as there was no change in variable fluorescence. Below -2.3 Mpa the fluorescence yield of both species decreased indicating loss of intrinsic efficiency of PS II. The osmotic potential of cell sap was found to decrease at the same rate in both species at high hydration states. Proline accumulated to a much greater extent in the wild wheat as compared to the cultivated wheat as a result of water stress. Drought resistance was also examined in relation to thylakoid membrane fluidity measured by fluorescence polarization. Thylakoid membrane fluidity was fully maintained in the wild wheat, but decreased substantially in the wheat cultivar, at equal tissue water potentials below -1.9 Mpa. One mechanism for maintaining the higher quantum yield of oxygen evolution during severe stress (at water potentials below -1.9 Mpa), may involve the greater stability of thylakoid membrane fluidity in the wild wheat.

Sulfite inhibition of photochemical activity of intact pea leaves

Sulfite treatment of pea leaf disks in light caused a significant decrease in the relative quantum yield of photosynthetic oxygen evolution and energy storage (ES) as measured by photoacoustic (PA) spectroscopy. The inhibition was concentration dependent and was less in darkness than in light, indicating light-dependent inhibitory site(s) on the photosynthetic electron transport chain. Further, in darksulfite-treated leaves, the energy storage was more affected than the relative quantum yield of oxygen evolution, suggesting that photophosphorylation and/or cyclic electron transport around PS I are sites of sulfite action in darkness. The Rfd values, the ratio of fluorescence decrease (fd) to the steady-state fluorescence (fs), decreased significantly in leaves treated with sulfite in light but were not affected in dark-treated ones, confirming the photoacoustic observations. Similarly, the ratio of variable fluorescence (Fv) to maximum fluorescence (Fm), a measure of PS II photochemical efficiency, was affected by sulfite treatment in light and not changed by treatment in darkness. An attempt was made to explain the mechanism of sulfite action on photosynthetic electron transport in light and in darkness.