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1.
Photosystem II cyclic electron transport was investigated at low pH in spinach thylakoids and PS II preparations from the cyanobacteriumPhormidium laminosum. Variable fluorescence (Fv) quenching at a very low light intensity was examined as an indicator of cyclic electron flow. A progressive quenching of Fv was observed as the pH was lowered; however, this was shown to be mainly due to an inhibition of oxygen evolution. Cyclic electron flow in the uninhibited centres was estimated to occur at a rate comparable to or smaller than 1 mole O2 mg Chl–1 h–1 in the pH range 5.0 to 7.8.The quantum yeeld of oxygen production is known to decrease at low pH and has been taken to indicate cyclic electron flow (Crofts and Horton (1991) Biochim Biophys Acta 1058: 187–193). However, a direct all-or-none inhibition of oxygen production at low pH has also been reported (Meyer et al. (1989) Biochim Biophys Acta 974: 36–43). We have analysed the effects of light intensity on the rates of oxygen evolution in order to calculate U, the quantum yield of open and uninhibited centres. U was found to be constant over a broad pH range, and by using ferricyanide and phenyl-p-benzoquinone as electron acceptors the maximum possible rate of cyclic electron transport was equivalent to no more than 1 mole O2 mg Chl–1 h–1. The rate was no greater when the acceptor was adjusted to provide the most favourable conditions for cyclic flow. 相似文献
2.
We tested the two empirical models of the relationship between chlorophyll fluorescence and photosynthesis, previously published by Weis E and Berry JA 1987 (Biochim Biophys Acta 894: 198–208) and Genty B et al. 1989 (Biochim Biophys Acta 990: 87–92). These were applied to data from different species representing different states of light acclimation, to species with C3 or C4 photosynthesis, and to wild-type and a chlorophyll b-less chlorina mutant of barley. Photosynthesis measured as CO2-saturated O2 evolution and modulated fluorescence were simultaneously monitored over a range of photon flux densities. The quantum yields of O2 evolution (ØO2) were based on absorbed photons, and the fluorescence parameters for photochemical (qp) and non-photochemical (qN) quenching, as well as the ratio of variable fluorescence to maximum fluorescence during steady-state illumination (F'v/F'm), were determined. In accordance with the Weis and Berry model, most plants studied exhibited an approximately linear relationship between ØO2/qp (i.e., the yield of O2 evolution by open Photosystem II reaction centres) and qN, except for wild-type barley that showed a non-linear relationship. In contrast to the linear relationship reported by Genty et al. for qp×F'v/F'm (i.e., the quantum yield of Photosystem II electron transport) and ØCO2, we found a non-linear relationship between qp×F'v/F'm and ØO2 for all plants, except for the chlorina mutant of barley, which showed a largely linear relationship. The curvilinearity of wild-type barley deviated somewhat from that of other species tested. The non-linear part of the relationship was confined to low, limiting photon flux densities, whereas at higher light levels the relationship was linear. Photoinhibition did not change the overall shape of the relationship between qp×F'v/F'm and ØO2 except that the maximum values of the quantum yields of Photosystem II electron transport and photosynthetic O2 evolution decreased in proportion to the degree of photoinhibition. This implies that the quantum yield of Photosystem II electron transport under high light conditions may be similar for photoinhibited and non-inhibited plants. Based on our experimental results and theoretical analyses of photochemical and non-photochemical fluoresce quenching processes, we conclude that both models, although not universal for all plants, provide useful means for the prediction of photosynthesis from fluorescence parameters. However, we also discuss that conditions which alter one or more of the rate constants that determine the various fluorescence parameters, as well as differential light penetration in assays for oxygen evolution and fluorescence emission, may have direct effect on the relationships of the two models.Abbreviations F0 and F'0
fluorescence when all Photosystem II reaction centres are open in dark- and light-acclimated leaves, respectively
- Fm and F'm
fluorescence when all Photosystem II reaction centres are closed in dark and light, respectively
- Fv
variable fluorescence equal to Fm-F0
- Fs
steady state level of fluorescence in light
- F'v and F'm
variable (F'm-F'0) and maximum fluorescence under steady state light conditions
- HEPES
N-2-hydroxyethylpiperazine-N-2-ethane-sulphonic acid
- QA
the primary, stabile quinone acceptor of Photosystem II
- qN
non-photochemical quenching of fluorescence
- qp
photochemical quenching of fluorescence
- ØO2
quantum yield of CO2-saturated O2 evolution based on absorbed photons 相似文献
3.
The dominance of diatoms in turbulent waters suggests special adaptations to the wide fluctuations in light intensity that
phytoplankton must cope with in such an environment. Our recent demonstration of the unusually effective photoprotection by
the xanthophyll cycle in diatoms [Lavaud et al. (2002) Plant Physiol 129 (3) (in press)] also revealed that failure of this
protection led to inactivation of oxygen evolution, but not to the expected photoinhibition. Photo-oxidative damage might
be prevented by an electron transfer cycle around Photosystem II (PS II). The induction of such a cycle at high light intensity
was verified by measurements of the flash number dependence of oxygen production in a series of single-turnover flashes. After
a few minutes of saturating illumination, the oxygen flash yields are temporarily decreased. The deficit in oxygen production
amounts to at most 3 electrons per PS II, but continues to reappear with a half time of 2 min in the dark until the total
pool of reducing equivalents accumulated during the illumination has been consumed by (chloro)respiration. This is attributed
to an electron transfer pathway from the plastoquinone pool or the acceptor side of PS II to the donor side of PS II that
is insignificant at limiting light intensity but is accelerated to milliseconds at excess light intensity. Partial filling
of the 3-equivalents capacity of the cyclic electron transfer path in PS II may prevent both acceptor-side photoinhibition
in oxygen-evolving PS II and donor-side photoinhibition when the oxygen-evolving complex is temporarily inactivated.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
4.
The oxygen production of dark-adapted Photosystem II upon illumination by a series of single-turnover flashes shows a damped
period four oscillation with flash number. The damping is attributed to `misses' resulting from a statistical probability
that a reaction center fails to produce a stable charge separation after a saturating flash. The origin of misses is of interest
because its probable dependence on flash number, in principle, affects the quantitative interpretation of all measurements
on phenomena associated with the period four oscillation. We show that the kinetics of chlorophyll fluorescence yield transients
induced by a flash series can be used to estimate the relative amplitudes of the miss probability on each flash. It is concluded
that a major part of the misses must be caused by failure of the reduction of the oxidized primary electron donor chlorophyll
P680+ by the secondary donor tyrosine YZ before the charge separation is lost by recombination. The probability of this failure is found to increase with the oxidation
state of the oxygen-evolving complex: more than half of it occurs upon charge separation in the S3 state, which is attributed to the presence of YZ
ox S2 in Boltzmann equilibrium with YZS3.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
5.
Kruk Jerzy Burda Kveta Schmid Georg H. Radunz Alfons Strzałka Kazimierz 《Photosynthesis research》1998,58(2):203-209
We have found that in petroleum-ether extracted tobacco thylakoids, plastoquinone A (PQ-A) and plastoquinone C (PQ-C) had similar efficiency in restoration of oxygen-evolving activity, while plastoquinone B (PQ-B), which is a fatty acid ester of PQ-C, was about 50% less effective. This indicates that apart from PQ-A, PQ-C and to a smaller extent PQ-B may function as electron acceptors of Photosystem II (PS II). The DCMU inhibition curves for PQ-C and PQ-B were biphasic and an initial slow decline was followed by a sharp decrease in oxygen evolution yield with a 50% inhibition (I50) at 0.25 M DCMU. In the case of PQ-A (I50 = 0.20 M DCMU), the activity decreased gradually without the sharp transition. The corresponding inhibition curve for unextracted thylakoids, where all the native prenylquinones are present, shows an intermediate shape between PQ-A and PQ-C but with a higher I50, equal to 0.32 M, suggesting that the contribution of PQ-C as an electron acceptor of Photosystem II might be significant in thylakoid membranes with natural prenyllipid composition. -Tocopherol quinone showed no activity in the restoration of oxygen evolution in extracted thylakoids, indicating that it cannot accept electrons from PS II. The fatty acid composition of PQ-B isolated from maple leaves showed a high degree of saturated fatty acids like myristic and palmitic acid, and its unique composition indicates that it is a natural component of the thylakoid membrane. 相似文献
6.
An overview is presented of secondary electron transfer at the electron donor side of Photosystem II, at which ultimately two water molecules are oxidized to molecular oxygen, and the central role of manganese in catalyzing this process is discussed. A powerful technique for the analysis of manganese redox changes in the water-oxidizing mechanism is the measurement of ultraviolet absorbance changes, induced by single-turnover light flashes on dark-adapted PS II preparations. Various interpretations of these ultraviolet absorbance changes have been proposed. Here it is shown that these changes are due to a single spectral component, which presumably is caused by the oxidation of Mn(III) to Mn(IV), and which oscillates with a sequence +1, +1, +1, –3 during the so-called S0 S1 S2 S3 S0 redox transitions of the oxygen-evolving complex. This interpretation seems to be consistent with the results obtained with other techniques, such as those on the multiline EPR signal, the intervalence Mn(III)-Mn(IV) transition in the infrared, and EXAFS studies. The dark distribution of the S states and its modification by high pH and by the addition of low concentrations of certain water analogues are discussed. Finally, the patterns of proton release and of electrochromic absorbance changes, possibly reflecting the change of charge in the oxygen-evolving system, are discussed. It is concluded that nonstoichiometric patterns must be considered, and that the net electrical charge of the system probably is the highest in state S2 and the lowest in state S1. 相似文献
7.
A model is presented describing the relationship between chlorophyll fluorescence quenching and photoinhibition of Photosystem (PS) II-dependent electron transport in chloroplasts. The model is based on the hypothesis that excess light creates a population of inhibited PS II units in the thylakoids. Those units are supposed to posses photochemically inactive reaction centers which convert excitation energy to heat and thereby quench variable fluorescence. If predominant photoinhibition of PS II and cooperativity in energy transfer between inhibited and active units are presumed, a quasi-linear correlation between PS II activity and the ratio of variable to maximum fluorescence, FVFM, is obtained. However, the simulation does not result in an inherent linearity of the relationship between quantum yield of PS II and FVFM ratio. The model is used to fit experimental data on photoinhibited isolated chloroplasts. Results are discussed in view of current hypotheses of photoinhibition.Abbreviations FM
maximum total fluorescence
- F0
initial fluorescence
- FV
maximum variable fluorescence
- PS
Photosystem
- QA, QB
primary and secondary electron acceptors of Photosystem II 相似文献
8.
Many of the core proteins in Photosystem II (PS II) undergo reversible phosphorylation. It is known that protein phosphorylation
controls the repair cycle of Photosystem II. However, it is not known how protein phosphorylation affects the partial electron
transport reactions in PS II. Here we have applied variable fluorescence measurements and EPR spectroscopy to probe the status
of the quinone acceptors, the Mn cluster and other electron transfer components in PS II with controlled levels of protein
phosphorylation. Protein phosphorylation was induced in vivo by varying illumination regimes. The phosphorylation level of the D1 protein varied from 10 to 58% in PS II membranes isolated
from pre-illuminated spinach leaves. The oxygen evolution and QA
− to QB(QB
−) electron transfer measured by flash-induced fluorescence decay remained similar in all samples studied. Similar measurements
in the presence of DCMU, which reports on the status of the donor side in PS II, also indicated that the integrity of the
oxygen-evolving complex was preserved in PS II with different levels of D1 protein phosphorylation. With EPR spectroscopy
we examined individual redox cofactors in PS II. Both the maximal amplitude of the charge separation reaction (measured as
photo-accumulated pheophytin−) and the EPR signal from the QA
− Fe2+ complex were unaffected by the phosphorylation of the D1 protein, indicating that the acceptor side of PS II was not modified.
Also the shape of the S2 state multiline signal was similar, suggesting that the structure of the Mn-cluster in Photosystem II did not change. However,
the amplitude of the S2 multiline signal was reduced by 35% in PS II, where 58% of the D1 protein was phosphorylated, as compared to the S2 multiline in PS II, where only 10% of the D1 protein was phosphorylated. In addition, the fraction of low potential Cyt b
559 was twice as high in phosphorylated PS II. Implications from these findings, were precise quantification of D1 protein phosphorylation
is, for the first time, combined with high-resolution biophysical measurements, are discussed.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
9.
The variation of the rate of cyclic electron transport around Photosystem I (PS I) during photosynthetic induction was investigated
by illuminating dark-adapted spinach leaf discs with red + far-red actinic light for a varied duration, followed by abruptly
turning off the light. The post-illumination re-reduction kinetics of P700+, the oxidized form of the photoactive chlorophyll of the reaction centre of PS I (normalized to the total P700 content),
was well described by the sum of three negative exponential terms. The analysis gave a light-induced total electron flux from
which the linear electron flux through PS II and PS I could be subtracted, yielding a cyclic electron flux. Our results show
that the cyclic electron flux was small in the very early phase of photosynthetic induction, rose to a maximum at about 30 s
of illumination, and declined subsequently to <10% of the total electron flux in the steady state. Further, this cyclic electron
flow, largely responsible for the fast and intermediate exponential decays, was sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethyl
urea, suggesting an important role of redox poising of the cyclic components for optimal function. Significantly, our results
demonstrate that analysis of the post-illumination re-reduction kinetics of P700+ allows the quantification of the cyclic electron flux in intact leaves by a relatively straightforward method. 相似文献
10.
11.
Simultaneous fluorescence and photoacoustic measurements have been used to study the effects of metal ions (copper, lead, and mercury) during dark incubation of thylakoid membranes. The values of the chlorophyll fluorescence parameters Fo (initial fluorescence yield with the reaction centers in the open state), Fm (maximal fluorescence yield), Ft (steady state fluorescence yield) and the calculated parameters, o (maximal quantum yield of Photosystem II photochemistry) and t (actual quantum yield of Photosystem II photochemistry), strongly decreased in the presence of the metal ions coinciding with an increase in the non-photochemical deexcitation rate constant k(N). It was observed that photosynthetic energy storage measured by photoacoustic spectroscopy also decreased but a large portion of energy storage remained unaffected even at the highest metal ion concentrations used. A maximal inhibition of photosyntheti c energy storage of 80% and 50% was obtained with Hg2+ and Cu2+-treated thylakoids, respectively, while energy storage was insensitive to Pb2+. The results are consistent with the known predominant inhibition of the donor side of Photosystem II by the metal ions. The insensitive portion of energy storage is attributed to the possible recurrence of cyclic electron transport around Photosystem II that would depend on the extent of inhibition produced on the acceptor side by the metal ion used. 相似文献
12.
Kevin Oxborough Ladislav Nedbal Roger A. Chylla John Whitmarsh 《Photosynthesis research》1996,48(1-2):247-254
In dark-adapted spinach leaves approximately one third of the Photosystem II (PS II) reaction centers are impaired in their ability to transfer electrons to Photosystem I. Although these inactive PS II centers are capable of reducing the primary quinone acceptor, QA, oxidation of QA
– occurs approximately 1000 times more slowly than at active centers. Previous studies based on dark-adapted leaves show that minimal energy transfer occurs from inactive centers to active centers, indicating that the quantum yield of photosynthesis could be significantly impaired by the presence of inactive centers. The objective of the work described here was to determine the performance of inactive PS II centers in light-adapted leaves. Measurements of PS II activity within leaves did not indicate any increase in the concentration of active PS II centers during light treatments between 10 s and 5 min, showing that inactive centers are not converted to active centers during light treatment. Light-induced modification of inactive PS II centers did occur, however, such that 75% of these centers were unable to sustain stable charge separation. In addition, the maximum yield of chlorophyll fluorescence associated with inactive PS II centers decreased substantially, despite the lack of any overall quenching of the maximum fluorescence yield. The effect of light treatment on inactive centers was reversed in the dark within 10–20 mins. These results indicate that illumination changes inactive PS II centers into a form that quenches fluorescence, but does not allow stable charge separation across the photosynthetic membrane. One possibility is that inactive centers are converted into centers that quench fluorescence by formation of a radical, such as reduced pheophytin or oxidized P680. Alternatively, it is possible that inactive PS II centers are modified such that absorbed excitation energy is dissipated thermally, through electron cycling at the reaction center.Abbreviations A518
absorbance change at 518 nm, reflecting the formation of an electric field across the thylakoid membrane
- AFL1
amplitude of the fast (<100 ms) phase of A518 induced by the first of two saturating, single-turnover flashes spaced 30 ms apart
- AFL2
amplitude of the fast (<100 ms) phase of A518 induced by the second of two saturating, single-turnover flashes spaced 50 ms apart
- DCBQ
2,6-dichloro-p-benzoquinone
- Fo
yield of chlorophyll fluorescence when QA is fully oxidized
- Fm
yield of chlorophyll fluorescence when QA is fully reduced
- Fx
yield of chlorophyll fluorescence when QA is fully reduced at inactive PS II centers, but fully oxidized at active PS II centers
- Pheo
pheophytin
- P680
the primary donor of Photosystem II
- PPFD
photosynthetic photon flux density
- QA
Primary quinone acceptor of PS II
- QB
secondary quinone acceptor of PS II 相似文献
13.
14.
Curtis W. Hoganson Demetrios F. Ghanotakis Gerald T. Babcock Charles F. Yocum 《Photosynthesis research》1989,22(3):285-293
Manganese in the oxygen-evolving complex is a physiological electron donor to Photosystem II. PS II depleted of manganese may oxidize exogenous reductants including benzidine and Mn2+. Using flash photolysis with electron spin resonance detection, we examined the room-temperature reaction kinetics of these reductants with Yz
+, the tyrosine radical formed in PS II membranes under illumination. Kinetics were measured with membranes that did or did not contain the 33 kDa extrinsic polypeptide of PS II, whose presence had no effect on the reaction kinetics with either reductant. The rate of Yz
+ reduction by benzidine was a linear function of benzidine concentration. The rate of Yz
+ reduction by Mn2+ at pH 6 increased linearly at low Mn2+ concentrations and reached a maximum at the Mn2+ concentrations equal to several times the reaction center concentration. The rate was inhibited by K+, Ca2+ and Mg2+. These data are described by a model in which negative charge on the membrane causes a local increase in the cation concentration. The rate of Yz
+ reduction at pH 7.5 was biphasic with a fast 400 s phase that suggests binding of Mn2+ near Yz
+ at a site that may be one of the native manganese binding sites.Abbreviations PS II
Photosystem II
- YD
tyrosine residue in Photosystem II that gives rise to the stable Signal II EPR spectrum
- Yz
tyrosine residue in Photosystem II that mediates electron transfer between the reaction center chlorophyll and the site of water oxidation
- ESR
electron spin resonance
- DPC
diphenylcarbazide
- DCIP
dichlorophenolindophenol 相似文献
15.
The inhibitory mechanism of Cu(II) on the Photosystem II electron transport from higher plants 总被引:1,自引:0,他引:1
In a previous paper, we reported that Cu(II) inhibited the photosynthetic electron transfer at the level of the pheophytin-QA-Fe domain of the Photosystem II reaction center. In this paper we characterize the underlying mechanism of Cu(II) inhibition. Cu(II)-inhibition effect was more sensitive with high pH values. Double-reciprocal plot of the inhibition of oxygen evolution by Cu(II) is shown and its corresponding inhibition constant, Ki, was calculated. Inhibition by Cu(II) was non-competitive with respect to 2,6-dichlorobenzoquinone and 3-(3,4-dichlorophenyl)-1,1-dimethylurea and competitive with respect to protons. The non-competitive inhibition indicates that the Cu(II)-binding site is different from that of the 2,6-dichlorobenzoquinone electron acceptor and 3-(3,4-dichlorophenyl)-1,1-dimethylurea sites, the QB niche. On the other hand, the competitive inhibition with respect to protons may indicate that Cu(II) interacts with an essential amino acid group(s) that can be protonated or deprotonated in the inhibitory-binding site.Abbreviations BSA
bovine seroalbumin
- Chl
chlorophyll
- DCBQ
2,6-dichlorobenzoquinone
- DCMU
3-(3,4-dichlorophenyl)-1,1-dimethylurea
- MES
2-(N-morpholino)-ethanesulphonic acid
- Pheo
pheophytin
- QA
primary quinone acceptor
- QB
secondary quinone acceptor
- PS
Photosystem
- RC
reaction center
- Tricine
N-[Tris(hydroxymethyl)-methyl]-glycine 相似文献
16.
Estimates of thylakoid electron transport rates (Je) from chlorophyll fluorometry are often used in combination with leaf gas exchange measurements to provide detailed information
about photosynthetic activity of leaves in situ. Estimating Je requires accurate determination of the quantum efficiency of Photosystem II (ΦP), which in turn requires momentary light saturation of the Photosystem II light harvesting complex to induce the maximum
fluorescence signal (FM′). In practice, full saturation is often difficult to achieve, especially when incident photosynthetic photon flux density
(Q) is high and energy is effectively dissipated by non-photochemical quenching. In the present work, a method for estimating
the true FM′ under high Q was developed, using multiple light pulses of varying intensity (Q′). The form of the expected relationship
between the apparent FM′ and Q′ was derived from theoretical considerations. This allowed the true FM′ at infinite Q′ to be estimated from linear regression. Using a commercially available leaf gas exchange/ chlorophyll fluorescence
measurement system, Je was compared to gross photosynthetic CO2 assimilation (AG) under conditions where the relationship between Je and AG was expected to be linear. Both in C4 leaves (Zea mays) in ambient air and also in C3 leaves (Gossypium hirsutum) under non-photorespiratory conditions the apparent ratio between Je and AG declined at high Q when ΦP was calculated from FM′ measured simply using the highest available saturating pulse intensity. When FM′ was determined using the multiple pulse / linear regression technique, the expected relationship between Je and AG at high Q was restored, indicating that the ΦP estimate was improved. This method of determining FM′ should prove useful for verifying when saturating pulse intensities are sufficient, and for accurately determining ΦP when they are not.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
17.
pH-dependent inactivation of Photosystem (PS) II and related quenching of chlorophyll-a-fluorescence have been investigated in isolated thylakoids and PS II-particles and related to calcium release at the donor side of PS II. The capacity of oxygen evolution (measured under light saturation) decreases when the pH is high and the pH in the thylakoid lumen decreases below 5.5. Oxygen evolution recovers upon uncoupling. The pH-response of inactivation can be described by a 1 H+-transition with an apparent pK-value of about 4.7. The yield of variable fluorescence decreases in parallel to the inactivation of oxygen evolution. pH-dependent quenching requires light and can be inhibited by DCMU. In PS II-particles, inactivation is accompanied by a reversible release of Ca2+-ions (one Ca2+ released per 200 Chl). In isolated thylakoids, where a pH was created by ATP-hydrolysis, both inactivation of oxygen evolution (and related fluorescence quenching) by internal acidification and the recovery of that inactivation can be suppressed by calcium-channel blockers. In the presence of the Ca2+-ionophore A23187, recovery of Chl-fluorescence (after relaxation of the pH) is stimulated by external Ca2+ and retarded by EGTA. As shown previously (Krieger and Weis 1993), inactivation of oxygen evolution at low pH is accompanied by an upward shift of the midpoint redox-potential, Em, of QA. Here, we show that in isolated PS II particles the pH-dependent redox-shift (about 160 mV, as measured from redox titration of Chl-fluorescence) is suppressed by Ca2+-channel blockers and DCMU. When a redox potential of –80 to –120mV was established in a suspension of isolated thylakoids, the primary quinone acceptor, QA, was largely reduced in presence of a pH (created by ATP-hydrolysis) but oxidized in presence of an uncoupler. Ca2+-binding at the lumen side seems to control redox processes at the lumen- and stroma-side of PS II. We discuss Ca2+-release to be involved in the physiological process of high energy quenching. 相似文献
18.
The photoreduction and dark reoxidation of Qα and Qβ, the primary electron acceptors of Photosystems (PS) IIα and IIβ, respectively, in the presence of 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea (DCMU) were studied in tobacco chloroplasts by means of fluorescence and absorbance measurements. The magnitude of a correction for an absorbance change by the oxidizing side of PS II needed in our previous study of the quantum yield of Q reduction (Biochim. Biophys. Acta 635 (1981), 111–120) has been determined. The absorbance change occurs in PS IIα mainly. The maximum fluorescence yield was found to be the same as in the mutant Su/su, which has a 3-fold higher reaction center concentration and a lower PS IIα to PS IIβ ratio. The kinetics of the light-induced fluorescence increase were measured after various pretreatments and the corresponding kinetics of the integrated fluorescence deficit were analyzed into their α and β components. From the results the contribution to the minimum fluorescence level, the degree of energy transfer between units, and the quantum efficiency of Q reduction were calculated for both types of PS II. This led to the following conclusions. The absence of energy between PS IIβ antennae is confirmed. Fluorescence quenching in PS IIα was adequately described by the matrix model, except for a decrease in the energy transfer between units during photoreduction of Qα, possibly due to the formation of ‘islets’ of closed centers. PS II reaction centers in which Q is reduced do not significantly quench fluorescence. The ratio of variable to maximum fluorescence, 0.77 in PS IIα and 0.92 in PS IIβ, multiplied by the fraction of Q remaining in the reduced state after one saturating flash, 0.88 in PS IIα and greater than 0.95 in PS IIβ, leads to a net quantum efficiency of Q reduction in the presence of DCMU and NH2OH of 0.68 in PS IIα and about 0.90 in PS IIβ. These values are in good agreement with the measured overall quantum efficiency of Q reduction. 相似文献
19.
Ulrich Heber Spidola Neimanis Katharina Siebke Gerald Schönknecht Eva Katona 《Photosynthesis research》1992,34(3):433-447
Chlorophyll fluorescence, light scattering, the electrochromic shift P515 and levels of some photosynthetic intermediates were measured in illuminated leaves. Oxygen and CO2 concentrations in the gas phase were varied in order to obtain information on control of Photosystem II activity under conditions such as produced by water stress, when stomatal closure restricts access of CO2 to the photosynthetic apparatus. Light scattering and energy-dependent fluorescence quenching indicated a high level of chloroplast energization under high intensity illumination even when linear electron transport was curtailed in CO2-free air or in 1% oxygen with 35 ll-1 CO2. Calculations of the phosphorylation potential based on measurements of phosphoglycerate, dihydroxyacetone phosphate and NADP revealed ratios of intrathylakoid to extrathylakoid proton concentrations, which were only somewhat higher in air containing 35 l l-1 CO2 than in CO2-free air or 1% oxygen/35 l l-1 CO2. Anaerobic conditions prevented appreciable chloroplast energization. Acceptor-limitation of electron flow resulted in a high reduction level of the electron transport chain, which is characterized by decreased oxidation of P700, not only under anaerobic conditions, but also in air, when CO2 was absent, and in 1% oxygen, when the CO2 concentration was reduced to 35 ll-1. Efficient control of electron transport was indicated by the photoaccumulation of P700
+ at or close to the CO2 compensation point in air. It is proposed to require the interplay between photorespiratory and photosynthetic electron flows, electron flow to oxygen and cyclic electron flow. The field-indicating electrochromic shift (P515) measured as a rapid absorption decrease on switching the light off followed closely the extent of photoaccumulation of P700
+ in the light.Abbreviations F, F0, F0, FM, FM
chlorophyll fluorescence levels
- GA
glyceraldehyde
- P515
field indicating rapid absorption change peaking at 522 nm
- QA
primary quinone acceptor in Photosystem II
- QN
non-photochemical quenching of chlorophyll fluorescence
- Qq
photochemical quenching of chlorophyll fluorescence 相似文献
20.
The risetime of EPR signal IIvf (S IIvf) has been measured in oxygen-evolving Photosystem II particles from spinach chloroplasts at pH 6.0. The EPR signal shows an instrument-limited rise upon induction (). These data are consistent with a model where the species Z responsible for S IIvf is the immediate electron donor to P-680+ in spinach chloroplasts. A new, faster decay component of S IIvf has also been detected in these experiments. 相似文献