Effect of endophyte infection on chlorophyll a fluorescence in salinity stressed rice

We have earlier reported that the endophyte infection can enhance photosynthetic capacity and antioxidant enzyme activities in rice exposed to salinity stress. Now, the changes in primary photochemistry of photosystem (PS) II induced by Na₂CO₃ stress in endophyte-infected (E+) and endophyte-uninfected (E-) rice seedlings were studied using chlorophyll a fluorescence (OJIP-test). Performance indices (PI_ABS and PI_Total) of E- and E+ rice seedlings revealed the inhibitory effects of Na₂CO₃ on PS II connectivity (occurrence of an L-band), oxygen evolving complex (occurrence of a K-band), and on the J step of the induction curves, associated with an inhibition of electron transport from plastoquinone A (Q_A) to plastoquinone B (Q_B). In E+ seedlings, Na₂CO₃ effects on L and K bands were much smaller, or even negligible, and also there was no pronounced effect on the J step. Furthermore, the OJIP parameters indicated that 20 mM Na₂CO₃ had a greater influence on the photosystem (PS) II electron transport chain than did the 10 mM Na₂CO₃, and that changes were greater in E- than in E+. Endophyte infection was therefore deemed to enhance the photosynthetic mechanism of Oryza sativa exposed to salinity stress.     

Inhibition of photosynthetic oxygen evolution and electron transfer from the quinone acceptor Q<Subscript>A</Subscript><Superscript>−</Superscript> to Q<Subscript>B</Subscript> by iron deficiency

The effect of iron deficiency on photosynthetic electron transport in Photosystem II (PS II) was studied in leaves and thylakoid membranes of lettuce (Lactuca sativa, Romaine variety) plants. PS II electron transport was characterized by oxygen evolution and chlorophyll fluorescence parameters. Iron deficiency in the culture medium was shown to affect water oxidation and the advancement of the S-states. A decrease of maximal quantum yield of PS II and an increase of fluorescence intensity at step J and I of OJIP kinetics were also observed. Thermoluminescence measurements revealed that charge recombination between the quinone acceptor of PS II, Q_B, and the S₂ state of the Mn-cluster was strongly perturbed. Also the dark decay of Chl fluorescence after a single turnover white flash was greatly retarded indicating a slower rate of Q_A ⁻ reoxidation.     

Kinetic analyses of the OJIP chlorophyll fluorescence rise in thylakoid membranes

N,N,N′,N′-tetramethyl-p-phenylenediamine (TMPD) was previously used to study the kinetics of the OJIP chlorophyll fluorescence rise. The present study is an attempt to elucidate the origin of TMPD-induced delay and quenching of the I–P step of fluorescence rise. For this purpose, we analyzed the kinetics of OJIP rise in thylakoid membranes in which electron transport was modified using ascorbate, methyl viologen (MV), and 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone (DBMIB). In the absence of TMPD, the OJIP kinetics of fluorescence induction (FI) was not altered by ascorbate. However, ascorbate eliminated the I–P rise delay caused by high concentrations of TMPD. On the other hand, neither ascorbate nor DBMIB, which blocks the electron release from Photosystem II (PS II) at the cytochrome b₆/f complex, could prevent the quenching of I–P rise by TMPD. In control thylakoids, MV suppressed the I–P rise of FI by about 60. This latter effect was completely removed if the electron donation to MV was blocked by DBMIB unless TMPD was present. When TMPD intercepted the linear electron flow from PS II, re-oxidation of TMPD by photosystem I (PS I) and reduction of MV fully abolished the I–P rise. The above is in agreement with the fact that TMPD can act as an electron acceptor for PS II. With MV, the active light-driven uptake of O₂ during re-oxidation of TMPD by PS I contributes towards an early decline in the I–P step of the OJIP fluorescence rise.     

Exogenous glycine betaine and proline play a protective role in heat-stressed barley leaves (Hordeum vulgare L.): A chlorophyll a fluorescence study

Abstract

Effects of drought and exogenous glycine betaine and proline on Photosystem II (PSII) photochemistry were studied in barley leaves under heat stress induced by exposing them to 45°C for 10 min. Polyphasic fluorescence transient (OJIP) was used to evaluate PSII photochemistry in leaves treated with either glycine betaine or proline, combined or not with heat treatment. A distinct K step in the fluorescence transient OJIP appeared in control leaves, indicating an inactivation of the oxygen evolving complex (OEC). Drought stress and exogenous glycine betaine and proline modified the shape of the OJIP curve of leaves heated at 45°C and the K step was not as pronounced. Increased thermostability of PSII may be associated with the resistance of OEC and increased energy connectivity between PSII units. The thermostability of PSII was also reflected by a lower decrease in maximum quantum yield of primary photochemistry (?_Po = F _V/F _M) and performance index (PI). Exogenous application of glycine betaine or proline can play an important role in enhancing plant stress tolerance and may help reduce effects of environmental stresses.     

Genotypic difference in response of peroxidase and superoxide dismutase isozymes and activities to salt stress in barley

Difference in isozymes and activities of peroxidase (POD) and superoxide dismutase (SOD) in two barley (Hordeum vulgare L.) genotypes differing in salt tolerance (Gebeina, tolerant; Quzhou, sensitive) was investigated using a hydroponic experiment. The activities of both enzymes were significantly increased when the plants of the two barley genotypes were exposed to salt stress, with salt-tolerant genotype being generally higher than the sensitive one. The variation in the POD and SOD isozymes was dependent on barley genotype, salt level and exposure time. When the plants were exposed to salt stress for 10 days, two new POD isozymes were found, R _m0.26 (R _m, relative mobility of enzyme to dye) in Gebeina and R _m0.45 in Quzhou. Both isozymes disappeared after 20 days of salt stress, but R _m0.26 appeared again 30 days after the stress. Two new SOD isozymes of R _m0.19 and R _m0.46 were found in Gebeina when exposed to NaCl for 10 days, but only R _m0.46 in Quzhou. As the time of salt stress extended, more new SOD isozymes were detected, R _m0.35 in both genotypes in all different salt treatments and R _m0.48 in Gebeina under 200 mM NaCl stress. At 30 days after the stress, all the new SOD isozymes disappeared except for R _m0.48 in Gebeina under 200 mM NaCl stress. The results suggest that the increased POD and SOD activities could be partly due to the formation of some new isozymes and the tolerant variety had better ability to form new isozymes to overcome salt stress.     

Photosynthetic electron transport activity in heat-treated barley leaves: The role of internal alternative electron donors to photosystem II

Electron transport processes were investigated in barley leaves in which the oxygen-evolution was fully inhibited by a heat pulse (48 °C, 40 s). Under these circumstances, the K peak (∼ F_400 μs) appears in the chl a fluorescence (OJIP) transient reflecting partial Q_A reduction, which is due to a stable charge separation resulting from the donation of one electron by tyrozine Z. Following the K peak additional fluorescence increase (indicating Q_A⁻ accumulation) occurs in the 0.2-2 s time range. Using simultaneous chl a fluorescence and 820 nm transmission measurements it is demonstrated that this Q_A⁻ accumulation is due to naturally occurring alternative electron sources that donate electrons to the donor side of photosystem II. Chl a fluorescence data obtained with 5-ms light pulses (double flashes spaced 2.3-500 ms apart, and trains of several hundred flashes spaced by 100 or 200 ms) show that the electron donation occurs from a large pool with t_1/2 ∼ 30 ms. This alternative electron donor is most probably ascorbate.     

The influence of salinity on cell ultrastructures and photosynthetic apparatus of barley genotypes differing in salt stress tolerance

A hydroponic experiment was conducted to elucidate the difference in growth and cell ultrastructure between Tibetan wild and cultivated barley genotypes under moderate (150 mM NaCl) and high (300 mM NaCl) salt stress. The growth of three barley genotypes was reduced significantly under salt stress, but the wild barley XZ16 (tolerant) was less affected relative to cultivated barley Yerong (moderate tolerant) and Gairdner (sensitive). Meanwhile, XZ16 had lower Na⁺ and higher K⁺ concentrations in leaves than other two genotypes. In terms of photosynthetic and chlorophyll fluorescence parameters, salt stress reduced maximal photochemical efficiency (F _v/F _m), net photosynthetic rate (Pn), stomatal conductance (Gs), and intracellular CO₂ concentration (Ci). XZ16 showed relatively smaller reduction in comparison with the two cultivated barley genotypes. The observation of transmission electron microscopy found that fundamental cell ultrastructure changes happened in both leaves and roots of all barley genotypes under salt NaCl stress, with chloroplasts being most changed. Moreover, obvious difference could be detected among the three genotypes in the damage of cell ultrastructure under salt stress, with XZ16 and Gairdner being least and most affected, respectively. It may be concluded that high salt tolerance in XZ16 is attributed to less Na⁺ accumulation and K⁺ reduction in leaves, more slight damage in cell ultrastructure, which in turn caused less influence on chloroplast function and photosynthesis.     

Salt stress effects on the photosynthetic electron transport chain in two chickpea lines differing in their salt stress tolerance

The main objective of this study was to evaluate the effects of salt stress on the photosynthetic electron transport chain using two chickpea lines (Cicer arietinum L.) differing in their salt stress tolerance at the germination stage (AKN 87 and AKN 290). Two weeks after sowing, seedlings were exposed to salt stress for 2 weeks and irrigated with 200 ml of 200 mM NaCl every 2 days. The polyphasic OJIP fluorescence transient and the 820-nm transmission kinetics (photosystem I) were used to evaluate the effects of salt stress on the functionality of the photosynthetic electron transport chain. It was observed that a signature for salt stress was a combination of a higher J step (V_J), a smaller IP amplitude, and little or no effect on the primary quantum yield of PSII (φ_Po). We observed for AKN 290 a shorter leaf life cycle, which may represent a mechanism to cope with salt stress. For severely salt-stressed leaves, an inhibition of electron flow between the PQ pool and P700 was found. The data also suggest that the properties of electron flow beyond PSI are affected by salt stress.     

Photosystem 2 is more tolerant to high temperature in apple (<Emphasis Type="Italic">Malus domestica</Emphasis> Borkh.) leaves than in fruit peel

Tolerance of photosystem 2 (PS2) to high temperature in apple (Malus domestica Borkh. cv. Cortland) leaves and peel was investigated by chlorophyll a fluorescence (OJIP) transient after exposure to 25 (control), 40, 42, 44, and 46 °C in the dark for 30 min. The positive L-step was more pronounced in a peel than in leaves when exposed to 44 °C. Heat-induced K-step became less pronounced in leaves than in peel when exposed to 42 °C or higher temperature. Leaves had negative L-and K-steps relative to the peel. The decrease of oxygen-evolving complex (OEC) by heat stress was higher in the peel than in the leaves. OJIP transient from the 46 °C treated peel could not reach the maximum fluorescence (F_m). The striking thermoeffect was the big decrease in the relative variable fluorescence at 30 ms (V_I), especially in the leaves. Compared with the peel, the leaves had less decreased maximum PS2 quantum efficiency (F_v/F_m), photochemical rate constant (K_P), F_m and performance index (PI) on absorption basis (PI_abs) and less increased minimum fluorescence (F₀) and non-photochemical rate constant (K_N), but more increased reduction of end acceptors at PS1 electron acceptor side per cross section (RE₀/CS₀) and per reaction center (RE₀/RC₀), quantum yield of electron transport from Q_A ⁻ to the end acceptors (ϕ _R0) and total PI (PI_abs,total) when exposed to 44 °C. In conclusion, PS2 is more thermally labile than PS1. The reduction of PS2 activity by heat stress primarily results from an inactivation of OEC. PS2 was more tolerant to high temperature in the leaves than in the peel.     

Probing of photosynthetic reactions in four phytoplanktonic algae with a PEA fluorometer

High-resolution light-induced kinetics of chlorophyll fluorescence (OJIP transients) were recorded and analyzed in cultures of diatoms (Thalassiosira weissflogii, Chaetoceros mulleri) and dinoflagellates (Amphidinium carterae, Prorocentrum minimum). Fluorescence transients showed the rapid exponential initial rise from the point O indicating low connectivity between PS II units and high absorption cross-section of PS II antenna. Dark-adapted dinoflagellates revealed capability to maintain the PS I-mediated re-oxidation of the PQ pool at the exposure to strong actinic light that may lead to the underestimation of F _M value. In OJIP transients recorded in phytoplanktonic algae the fluorescence yield at the point O exceeded F _O level because Q_A has been already partly reduced at 50 μs after the illumination onset. PEA was also employed to study the recovery of photosynthetic reactions in T. weissflogii during incubation of nitrogen starved cells in N-replete medium. N limitation caused the impairment of electron transport between Q_A and PQs, accumulation of closed PS II centers, and the reduced ability to generate transmembrane ΔpH upon illumination, almost fully restored during the recovery period. The recovered cells showed much higher values of NPQ than control ones suggesting maximization of photoprotection mechanisms in the population with a ‘stress history.’     

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

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

Effects of salt stress on photosystem II efficiency and CO2 assimilation of two Syrian barley landraces

The photosynthetic activity of two Syrian barley landraces, Arabi (A.) Aswad and A. Abiad, grown under 120 mM NaCl, was studied, using gas exchange and chlorophyll (Chl) a fluorescence transient (OJIP) measurements. Salt treatment of barley seedlings decreased both the rates of photosynthesis and photosystem II (PSII) activity, as evaluated from chlorophyll fluorescence data. However, the noted decrease was dependent on the duration of the salt treatment and the barley cultivar. Several parameters (e.g., light absorption flux per cross section of leaf; time to reach maximum chlorophyll a fluorescence intensity; plastoquinone pool size; yield of heat loss; rate of reaction center closure; and the so-called Performance Index), calculated and inferred from Chl fluorescence measurements, and related to PSII activity, were affected after 24 h of salt application, but these changes were much more pronounced after 7 days of salt treatment. Similar changes were found for measured gas exchange parameters: CO₂ uptake (photosynthetic) rate and stomatal conductance. The photosynthetic apparatus of the cultivar variety (c.v.) Arabi Aswad was found to be much more tolerant to salt treatment, compared with c.v. Arabi Abiad. After 7 days of salt treatment, the latter showed a very high value of the initial (minimal) fluorescence (F_o) and then essentially almost flat fluorescence transient curve; this result may be due to several causes that include structural changes as well as changes in the rate constants of different dissipative processes. The parameters that were most affected, by salt treatment, were: the time needed to reach the maximal chlorophyll fluorescence (F_m), and the inferred oxygen evolving complex activity (F_v/F_o, where F_v, is F_m − F_o), and the calculated Performance Index (PI_ABS) that depends on the efficiency and the yield of energy transfer and primary photochemistry. We suggest that the early reactions of the photosynthetic apparatus of barley plants could play a key role in their tolerance to salt stress. Further, we found that the first stage of salinity effect on photosynthesis of barley plants is related to stomatal conductance limitation rather than to PSII activity reduction. Thus, on the basis of our results on the two barley landraces, we recommend the use of a combination of gas exchange measurements along with the analysis of the OJIP fluorescence transient for the detection of salt stress-induced changes in plants.     

Photophysical processes of energy conversion in thylakoid membranes of <Emphasis Type="Italic">Chlamydomonas reinhardtii</Emphasis> mutants D1-R323H,D1-R323D,and D1-R323L

Chlamydomonas reinhardtii mutants D1-R323H, D1-R323D, and D1-R323L showed elevated chlorophyll fluorescence yields, which increased with decline of oxygen evolving capacity. The extra step K ascribed to the disturbance of electron transport at the donor side of PS II was observed in OJIP kinetics measured in mutants with a PEA fluorometer. Fluorescence decay kinetics were recorded and analyzed in a pseudo-wild type (pWt) and in mutants of C. reinhardtii with a Becker and Hickl single photon counting system in pico- to nanosecond time range. The kinetics curves were fitted by three exponentials. The first one (rapid, with lifetime about 300 ps) reflects energy migration from antenna complex to the reaction center (RC) of photosystem II (PS II); the second component (600–700 ps) has been assigned to an electron transfer from P680 to Q_A, while the third one (slow, 3 ns) assumingly originates from charge recombination in the radical pair [P680^+• Pheo^−•] and/or from antenna complexes energetically disconnected from RC II. Mutants showed reduced contribution of the first component, whereas the yield of the second component increased due to slowing down of the electron transport to Q_A. The mutant D1-R323L with completely inactive oxygen evolving complex did not reveal rapid component at all, while its kinetics was approximated by two slow components with lifetimes of about 2 and 3 ns. These may be due to two reasons: a) disconnection between antennae complexes and RC II, and b) recombination in a radical pair [P680^+• Pheo^−•] under restricted electron transport to Q_A. The data obtained suggest that disturbance of oxygen evolving function in mutants may induce an upshift of the midpoint redox potential of Q_A/Q_A⁻ couple causing limitation of electron transport at the acceptor side of PS II.     

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

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

Stacking small segments of the 1D chromosome of bread wheat containing major gluten quality genes into durum wheat: transfer strategy and breeding prospects

Using a chromosome engineering strategy, we previously developed two durum wheat recombinant lines, each containing on chromosome 1A a short segment of bread wheat chromosome 1D with either the Glu-D1 (PL line) or the Gli-D1/Glu-D3 (PS line) genes. Since PL and PS transfers produced substantial but different effects on durum gluten properties, in the present work stacking of their 1DS and 1DL segments into the same chromosome 1A was undertaken to investigate their combined effect in durum wheat and to potentially widen the spectrum of the crop end-uses. Development of genetic and genomic in situ hybridization (GISH)-based physical maps of PS and PL recombinant arms facilitated selection of carriers of the double-recombinant (PS?+?PL) 1A chromosome among F₂ progeny from the PS?×?PL cross. With the 1DS transfer spanning the terminal 17% of recombinant 1AS, and the interstitial 1DL segment occupying 16% of recombinant 1AL, PS?+?PL chromosomes were generated by pairing events occurred with a 68.4% frequency in 1A regions shared by parental chromosomes. Homozygous double-recombinant F₃ plants exhibited no significant differences for relevant agronomic traits compared to sib lines possessing either one or no 1D segment. Among quality parameters preliminarily assessed, SDS sedimentation values increased by 12% in PS and by over 32 and 38% in PL and PS?+?PL lines, respectively, compared to null controls. As a whole, the novel recombinant genotype offers good prospects for direct exploitation in breeding, and hence for an effective contribution to the enhancement of the crop value.     

Cyclic electron flow around PSI monitored by afterglow luminescence in leaves of maize inbred lines (<Emphasis Type="Italic">Zea mays</Emphasis> L.): correlation with chilling tolerance

Maize (Zea mays L.) inbred lines of contrasting chilling sensitivity (three tolerant, three sensitive lines) were acclimated to 280 mol photons m^–2 s^–1 white light at a 17°C sub-optimal temperature. They showed no symptoms of photoinhibition, despite slight changes in photosystem II (PSII) fluorescence and thermoluminescence properties in two tolerant lines. A luminescence afterglow emission [Bertsch and Azzi (1965) Biochim Biophys Acta 94:15–26], inducible by a far-red (FR) illumination of unfrozen leaf discs, was detected either as a bounce in decay kinetics at constant temperatures or as a sharp thermoluminescence afterglow band at about 45°C, in dark-adapted leaves. This band reflects the induction by warming of an electron pathway from stromal reductants to plastoquinones and to the Q_B secondary acceptor of PSII, resulting in a luminescence-emitting charge recombination in the fraction of centres that were initially in the S_2/3Q_B non-luminescent state. A 5-h exposure of plants to growth chamber light shifted this luminescence emission towards shorter times and lower temperatures for several hours in the three chilling-tolerant lines. This downshift was not observed, or only transiently, in the three sensitive lines. In darkness, the downshifted afterglow band relaxed within hours to resume its dark-adapted location, similar for all maize lines. A faster dark re-reduction of P700⁺ oxidized by FR light (monitored by 820-nm absorbance) and an increase of photochemical energy storage under FR excitation (determined by photoacoustic spectroscopy) confirmed that a cyclic pathway induced by white actinic light remained activated for several hours in the tolerant maize lines.     

Photosynthetic response of clusterbean chloroplasts to UV-B radiation: Energy imbalance and loss in redox homeostasis between QA and QB of photosystem II

The effects of ultraviolet-B (UV-B: 280-320 nm) radiation on the photosynthetic pigments, primary photochemical reactions of thylakoids and the rate of carbon assimilation (P_n) in the cotyledons of clusterbean (Cyamopsis tetragonoloba) seedlings have been examined. The radiation induces an imbalance between the energy absorbed through the photophysical process of photosystem (PS) II and the energy consumed for carbon assimilation. Decline in the primary photochemistry of PS II induced by UV-B in the background of relatively stable P_n, has been implicated in the creation of the energy imbalance_. The radiation induced damage of PS II hinders the flow of electron from Q_A to Q_B resulting in a loss in the redox homeostasis between the Q_A to Q_B leading to an accumulation of Q_A⁻. The accumulation of Q_A⁻ generates an excitation pressure that diminishes the PS II-mediated O₂ evolution, maximal photochemical potential (F_v/F_m) and PS II quantum yield (Φ_{PS II}). While UV-B radiation inactivates the carotenoid-mediated protective mechanisms, the accumulation of flavonoids seems to have a small role in protecting the photosynthetic apparatus from UV-B onslaught. The failure of protective mechanisms makes PS II further vulnerable to the radiation and facilitates the accumulation of malondialdehyde (MDA) indicating the involvement of reactive oxygen species (ROS) metabolism in UV-B-induced damage of photosynthetic apparatus of clusterbean cotyledons.     

Decrease of Fluorescence Intensity After the K Step in Chlorophyll a Fluorescence Induction is Suppressed by Electron Acceptors and Donors to Photosystem 2

Chlorophyll a fluorescence induction measured by a fluorometer with a high temperature stressed plant material shows a new K step which is a clear peak due to fast fluorescence rise and subsequent decrease of fluorescence intensity. We focused on an explanation of the decrease of fluorescence after the K step using artificial electron acceptors and donors to photosystem 2 (PS2). Addition of the artificial electron acceptors or donors suppressed the decrease of fluorescence after the K step. We suggest that the decrease mainly reflects (by more than 81 %) an energy loss process in the reaction centre of PS2 which is most probably a nonradiative charge recombination between P680⁺ (oxidised primary electron donor in PS2) and a negative charge stored on either Pheo⁻ or Q_A ⁻ (reduced primary electron acceptor of PS2 and reduced primary quinone electron acceptor of PS2, respectively). We suggest that the energy loss process is only possible when the inhibition of both the donor and the acceptor sides of PS2 occurs. This revised version was published online in August 2006 with corrections to the Cover Date.     

Photoprotective function of photorespiration in Reaumuria soongorica during different levels of drought stress in natural high irradiance

Diurnal patterns of gas exchange and chlorophyll (Chl) fluorescence parameters of photosystem 2 (PS2) as well as H₂O₂ content were analyzed in Reaumuria soongorica (Pall.) Maxim., a perennial semi-shrub. The rate of photorespiration was estimated by combined measurement of gas exchange and Chl fluorescence. The rate of photorespiration increased with the increasing drought stress (DS). The ratio of carboxylation electron flow to oxygenation electron flow (J_c/J_o) and the maximal photochemical efficiency of PS2 (variable to maximum fluorescence ratio, F_v/F_m) decreased with the increasing DS. F_v/F_m in isonicotinic acid hydrazide (INH)-sprayed plants was lower than that in normal plants under moderate DS, but no significant difference was observed under severe DS. H₂O₂ content in INH-sprayed plants was significantly lower than that in normal plants under severe DS. Taken together, photorespiration in R. soongorica consumed excess electrons and protected photosynthetic apparatus under moderate DS, whereas it accelerated H₂O₂ accumulation markedly and induced the leaf abscission under severe DS.     

Acceptor and donor-side interactions of phenolic inhibitors in Photosystem II

Certain phenolic compounds represent a distinct class of Photosystem (PS) II Q_B site inhibitors. In this paper, we report a detailed study of the effects of 2,4,6-trinitrophenol (TNP) and other phenolic inhibitors, bromoxynil and dinoseb, on PS II energetics. In intact PS II, phenolic inhibitors bound to only 90-95% of Q_B sites even at saturating concentrations. The remaining PS II reaction centers (5-10%) showed modified Q_A to Q_B electron transfer but were sensitive to urea/triazine inhibitors. The binding of phenolic inhibitors was 30- to 300-fold slower than the urea/triazine class of Q_B site inhibitors, DCMU and atrazine. In the sensitive centers, the S₂Q_A⁻ state was 10-fold less stable in the presence of phenolic inhibitors than the urea/triazine herbicides. In addition, the binding affinity of phenolic herbicides was decreased 10-fold in the S₂Q_A⁻ state than the S₁Q_A state. However, removal of the oxygen-evolving complex (OEC) and associated extrinsic polypeptides by hydroxylamine (HA) washing abolished the slow binding kinetics as well as the destabilizing effects on the charge-separated state. The S₂-multiline electron paramagnetic resonance (EPR) signal and the ‘split’ EPR signal, originating from the S₂Y_Z state showed no significant changes upon binding of phenolic inhibitors at the Q_B site. We thus propose a working model where Q_A redox potential is lowered by short-range conformational changes induced by phenolic inhibitor binding at the Q_B niche. Long-range effects of HA-washing eliminate this interaction, possibly by allowing more flexibility in the Q_B site.