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1.
Artificial electron donors to photosystem II provide an important means for characterizing the newly discovered site of energy coupling near photosystem II. However, water oxidation must be completely abolished, without harming the phosphorylation mechanism, for these donor reactions and the associated phosphorylation to withstand rigorous quantitative analysis. In this paper we have demonstrated that treatment of chloroplasts with hydroxylamine plus EDTA at pH 7.5 in the presence of Mg 2+ followed by washing to remove the amine is a highly reliable technique for this purpose. The decline of the Hill reaction and the coupled phosphorylation during the treatment were carefully followed. No change in the efficiency of phosphorylation (P/e 2 1.0-1.1) was observed until the reactions became immeasurable. Photosystem I-dependent reactions, such as the transfer of electrons from diaminodurene or reduced 2,6-dichlorophenolindophenol to methylviologen, and the associated phosphorylation were totally unaffected. It is clear that the hydroxylamine treatment is highly specific, with no adverse effect on the mechanism of phosphorylation itself. Benzidine photooxidation via both photosystems II and I in hydroxylamine-treated chloroplasts (electron acceptor, methylviologen; assayed as O 2 uptake) supports phosphorylation with the same efficiency as that observed for the normal Hill reaction (P/e 2 = 1.1). An apparent P/e 2 ratio of 0.6 was computed for the photooxidation of ascorbate. 相似文献
2.
Addition of 1m ascorbate to isolated chloroplasts with methyl viologen (MV) as electron acceptor trebled the rate of oxygen uptake and decreased the ratio to a third of that with no ascorbate present. These effects of ascorbate were reversed by superoxide dismutase (SOD), which in the absence of ascorbate had little effect on O 2 uptake or ratio. A chloroplast-associated SOD activity equivalent to 500 units/mg chlorophyll was detected. The effects of ascorbate and SOD on O 2 uptake were similar in both coupled and uncoupled chloroplasts. The results are consistent with the hypothesis that ascorbate stimulates O 2 uptake by reduction of superoxide, which is formed by autoxidation of the added electron acceptor (MV), and which dismutates in the absence of ascorbate. Ascorbate does not seem to stimulate O 2 uptake by replacing water as the photosystem II donor. 相似文献
3.
A Photosystem I submembrane fraction isolated from spinach was used to study the mechanism of heat-stress stimulation of oxygen uptake by the photosystem. Various artificial electron donors were shown to generate electron transport reactions with various degrees of thermally induced stimulation. A strong stimulation was observed with durohydroquinone as electron donor with a maximal effect at 50 °C. The degree of stimulation obtained was independent from the redox potential of the electron donors and from their oxidation site because the enzyme superoxide dismutase fully inhibited the stimulation. Instead, it is proposed that thermal stress causes the release of membrane bound superoxide dismutase from the thylakoids thus allowing the reduced form of electron donors with specific properties to reduce O 2
– radicals to H 2O 2 besides the usual disproportionation of O 2
– into O 2 and H 2O 2.Abbreviations: PS
photosystem
- DCIP
2,6-dichlorophenolindophenol
- MV
methylviologen
- TMPD
N,N,N,N-tetramethylphenylenediamine
- SOD
superoxide dismutase
- Chl
chlorophyll
- DQ
duroquinone
- DAD
N,N,N,N-tetramethyl-1,4-benzenediamine
- PMS
5-methylphenazium methyl sulfate
- PC
plastocyanin 相似文献
4.
Isolated maize bundle sheath chloroplasts showed substantial rates of noncyclic photophosphorylation. A typical rate of phosphorylation coupled to whole-chain electron transport (methylviologen or ferricyanide as acceptor) was 60 μmol per hour per milligram chlorophyll) with a coupling efficiency (P/e 2) of 0.6. Partial electron transport reactions driven by photosystem I or II supported phosphorylation with P/e 2 values of 0.2 to 0.3. Thus, two sites of phosphorylation seem to be associated with the photosynthetic chain in much the same way as in spinach chloroplasts. 相似文献
5.
NH 2OH-treated, non-water oxidizing chloroplasts are shown to be capable of oxidizing ferrocyanide and I ? via Photosystem II at appreciable rates (? 200 μequiv/h per mg chlorophyll). Using methylviologen as electron acceptor, ferrocyanide oxidation can be measured as O 2 uptake, as ferricyanide formation, or as H + consumption (2 Fe 2+ + 2H + + O 2 → 2 Fe 3+ + H 2O 2). I ? oxidation can be measured as methylviologen-mediated O 2 uptake, or spectrophotometrically, using ferricyanide as electron acceptor. The oxidation product I 2 is re-reduced, as it is formed, by unknown reducing substances in the reaction system.The rate-saturating concentrations of these donors are very high: 30 mM with ferricyanide and 15 mM with I ?. Relatively lipophilic Photosystem II donors such as catechol, benzidine and p-aminophenol saturate the photooxidation rate at much lower concentrations (< 0.5 mM). It thus seems that the oxidation of hydrophilic reductants such as ferricyanide and I ? is limited by permeability barriers. Very likely the site of Photosystem II oxidation is embedded in the thylakoid membrane or is situated on the inner surface of the membrane.The efficiency of phosphorylation (P/e 2) is 0.5 to 0.6 with ferrocyanide and about 0.5 with I ?. In contrast the P/e 2 ratio is 1.0 to 1.2 when water, catechol, p-aminophenol or benzidine serves as electron donor. These differences imply that only one of two phosphorylation sites operate when ferrocyanide and I ? are oxidized. Ferrocyanide and I ? are also chemically distinct from other Photosystem II donors in that their oxidation does not involve proton release. It is suggested that the mechanism of energy conservation associated with Photosystem II may be only operative when the removal of electrons from the donor results in release of protons (i.e. with water, hydroquinones, phenylamines, etc.). 相似文献
6.
A comparative study of H 2 photoproduction by chloroplasts and solubilized chlorophyll was performed in the presence of hydrogenase preparations of Clostridium butyricum. The photoproduction of H 2 by chloroplasts in the absence of exogenous electron donors, and with irreversibly oxidized dithiothreitol and cysteine, is thought to be limited by a cyclic transport of electrons wherein methylviologen short-circuits the electron transport in photosystem I. The efficiency of H 2 photoproduction by chloroplasts with ascorbate and NADPH is limited by a back reaction between light-reduced methylviologen and the oxidized electron donors. The use of a combination of electron donors (dithiothreitol and ascorbate), providing anaerobiosis without damage to chloroplasts, makes it possible to avoid consumption of reduced methylviologen for the reduction of oxidized electron donors and to exclude the short-circuiting of electron transfer. Under these conditions, photoproduction of H 2 was observed to occur with a rate of 350 to 400 micromoles H 2 per milligram chlorophyll per hour. In this case, the full electron-transferring capability of photosystem I (measured by irreversible photoreduction of methyl red or O 2) is used to produce H 2. 相似文献
7.
1. The Photosystem I-mediated transfer of electrons from diaminodurene, diaminotoluene and reduced 2,6-dichlorophenolindophenol to methylviologen is optimal at pH 8–8.5, where phosphorylation is also maximal. In the presence of superoxide dismutase, the efficiency of phosphorylation rises from ? 0.1 at pH 6.5 to 0.6–0.7 at pH 8–8.5, regardless of the exogenous electron donor used.2. The apparent Km (at pH 8.1) for diaminodurene is 6·10 ?4 M and for diaminotoluene is 1.2·10 ?3 M. The concentrations of diaminodurene and diaminotoluene required to saturate the electron transport processes are > 2 mM and > 5 mM, respectively. At these higher electron donor concentrations the rates of electron transport are markedly increased by phosphorylation (1.5-fold) or by uncoupling conditions (2-fold).3. Kinetic analysis of the transfer of electrons from reduced 2,6-dichlorophenolindophenol (DCIPH 2) to methylviologen indicates that two reactions with very different apparent Km values for DCIPH 2 are involved. The rates of electron flux through both pathways are increased by phosphorylation or uncoupling conditions although only one of the pathways is coupled to ATP formation. No similar complications are observed when diaminodurene or diaminotoluene serves as the electron donor.4. In the diaminodurene → methylviologen reaction, ATP formation and that part of the electron transport dependent upon ATP formation are partially inhibited by the energy transfer inhibitor HgCl 2. This partial inhibition of ATP formation rises to about 50% at less than 1 atom of mercury per 20 molecules of chlorophyll, then does not further increase until very much higher levels of mercury are added.5. It is suggested that exogenous electron donors such as diaminodurene, diaminotoluene and DCIPH 2 can substitute for an endogenous electron carrier in donating electrons to cytochrome f via the mercury-sensitive coupling site (Site I) located on the main electron-transporting chain. If this is so, there would seem to be no reason for postulating yet another coupling site on a side branch of the electron transport chain in order to account for cyclic photophosphorylation. 相似文献
8.
Noncyclic electron transport to ferricyanide and photophosphorylation as well as the methylviologen mediated aerobic and anaerobic photophosphorylation with dichlorophenolindophenol-ascorbate as the electron donor of photosystem I were measured during the development of high-light and low-light adapted leaves of Sinapis alba. Anaerobic methylviologen-catalyzed phosphorylation is more than twice as high as aerobic phosphorylation. The difference between the rates of aerobic and anaerobic phosphorylation is sensitive to dibromothymoquinone. Thus, under anaerobic conditions, methylviologen mediates a cyclic phosphorylation including plastoquinone. All photochemical activities of high-light chloroplasts are about twice as high as that of low-light chloroplasts and show a permanent decline with increasing plant age. The lower activities of low-light chloroplasts correlate with a decrease of electron transport components, such as cytochrome f. This indicates that the number of electron transport chains is decreased under low-light conditions and more chlorophyll molecules interact with one electrontransport chain.Abbreviations Asc
ascorbate
- Chl
chlorophyll a+b
- DBMIB
2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone
- DCMU
3-(dichlorophenyl)-1,1-dimethylurea
- DCPIP
dichlorophenolindophenol
- HL
high light
- LL
low light
- MV
methylviologen
- PhAR
photosynthetically active radiation
- PS
photosystem 相似文献
9.
The Photosystem I-dependent transport of electrons from diaminodurene to methylviologen is linear with reaction time and supports a constant rate of phosphorylation. However, if the diaminodurene is not kept fully reduced by the presence of excess ascorbate, the oxidized diaminodurene accumulates and begins to compete with the methylviologen as the electron acceptor. Thus, although the rate of ATP formation remains unchanged, an increasing proportion of the electron transport becomes cyclic and hence unmeasured. This leads to a rapid increase in the apparent efficiency of phosphorylation which is misleading.In contrast, it is known that the oxidized form of 3,3′-diaminobenzidine polymerizes to form an insoluble substance which should not be available to serve as an electron acceptor. However, 3,3′-diaminobenzidine is not a satisfactory donor of electrons in Photosystem I reactions for two reasons: the rate of electron transport quickly falls with reaction time and the oxidized form of 3,3′-diaminobenzidine seems to be an exceptionally efficient electron acceptor near the beginning of the period of illumination when it is presumably not yet polymerized. Thus in the first 2–3 sec of illumination when the reaction is still rapid much of the electron transport is cyclic and therefore unmeasured, especially in the absence of excess ascorbate. This cycling of electrons, which leads to an inflated apparent efficiency ( ), is particularly pronounced at low donor concentrations.When cyclic electron transport is avoided by the use of ascorbate or by the selection of appropriate reaction times, both diaminodurene and 3,3′-diaminobenzidine support phosphorylation with an efficiency which is approximately half of the efficiency exhibited by the overall Hill reaction. The same is true when 2,5-diaminotoluene, tetrachlorohydroquinone, 4,5-dimethyl- o-phenylenediamine, and reduced 2,6-dichloroindophenol serve as electron donors. With these six substances, the phosphorylation efficiences were 0.57 ± 0.1 molecules of ATP formed for each pair of electrons transferred ( ). In the same chloroplasts preparations, the transport of electrons from water to methylviologen-supported phosphorylation with a of 1.2. 相似文献
10.
In isolated barley chloroplasts, the presence of 2 millimolar ZnSO 4 inhibits the electron transport activity of photosystem II, as measured by photoreduction of dichlorophenolindophenol, O 2 evolution, and chlorophyll a fluorescence. The inhibition of photosystem II activity can be restored by the addition of the electron donor hydroxylamine or diphenylcarbazide, but not by benzidine and MnCl 2. These observations suggest that Zn inhibits electron flow at the oxidizing side of photosystem II at a site prior to the electron donating site(s) of hydroxylamine and diphenylcarbazide. No inhibition of photosystem I-dependent electron transport by 3 millimolar ZnSO 4 is observed. However, with concentrations of ZnSO 4 above 5 millimolar, photosystem I activity is partially inactivated. Washing Zn 2+-treated chloroplasts partially restores the O 2-evolving activity. 相似文献
11.
Effects of batatasins I, III and V, phenolic growth inhibitors occuring in dormant bulbils of Dioscorea batatas Decne., on photosynthetic reactions of chloroplasts from spinach ( Spinacia oleracea L.) and on respiration of mitochondria from potatoes ( Solanum tuberosum L.) were investigated. In chloroplasts, the batatasins effectively inhibited CO 2-dependent oxygen evolution and electron flow from water to acceptors such as dichlorophenolindophenol, ferricyanide and methylviologen. Photosystem-I dependent electron transport from ascorbate to oxygen was stimulated. The proton conductivity of thylakoid membranes was increased and phosphorylation was uncoupled from electron transport. Inhibition of electron transport with water as electron donor appeared to precede uncoupling. In mitochondrial, batatasin I did not much inhibit succinate-dependent O 2 uptake in the absence of ADP, but caused strong inhibition in the presence of ADP. Batatasins III and V inhibited oxygen uptake irrespective of the presence or absence of ADP. Inhibition of chloroplast and mitochondrial reactions by batatasins was shown to be reversible.Abbrevations B-I
batatasin I, 6-hydroxy-2,4,7-trimethoxyphenanthrene
- B-III
batatasin III, 3,3-dihydroxy-5-methoxybibenzyl
- B-V
batatasin V, 2-hydroxy-3,4,5-trimethoxybibenzyl
- Chl
chlorophyll
- MV
methylviologen
- DCPIP
2,6-dichlorophenol-indophenol
- DCMU
3-(3,4-dichlorophenyl)-1,1-dimethylurea
- PVP
polyvinylpyrrolidone 相似文献
12.
High rates of hydrogen photoproduction are obtained when glutaraldehyde-fixed Photosystem I-enriched vesicles (Photosystem II-depleted) are added to hydrogenase-containing cells of Proteus mirabilis in the presence of the mediator methylviologen and a suitable electron donating system. This donor system includes ascorbate, dithioerythritol (DTE) and the mediator tetramethylphenylene-diamine (TMPD) and reduces the photosynthetic electron transfer chain at the level of plastocyanin. Both DTE and ascorbate are required for hydrogen photoproduction, DTE being the ultimate electron donor and ascorbate only having a catalytic function. Whereas the aerobic photoreduction of methylviologen is similar in the presence of DTE, ascorbate or both, under anaerobic conditions only combination of both compounds results in a high and stable amount of reduced methylviologen that can be utilized by the hydrogenase. It is concluded that oxidation reactions of reduced methylviologen, competing with the hydrogenase, rather than methylviologen photoreduction, limit hydrogen photoproduction in the presence of either DTE or ascorbate. These oxidation reactions are suggested to involve back reactions to the oxidized form(s) of ascorbate and DTE but backflow to the photosynthetic electron transfer chain (i.e. cyclic electron transfer) can not be excluded.Abbreviations Tes
N-tris (hydroxymethyl) methyl-2-aminoethanesulfonic acid
- DTE
1,4-dithioerythritol
- TMPD, N,N,N
N-tetramethyl-p-phenylenediamine
- DCMU
3-(3, 4-dichlorophenyl)-1, 1,-dimethylureum
- EDAC
1-ethyl-3-(3-dimethylaminopropyl)-carbodiimide
- DNP-INT
2-iodo-6-isopropyl-3-methyl-2, 4, 4-trinitrodiphenyl ether
- DBMIB
2,5-dibromo-3-methyl-6-isopropyl-benzoquinone
- PS
photosystem
- Chl
chlorophyll 相似文献
13.
Summary The apoplast of barley and oat leaves contained superoxide dismutase (SOD), catalase, ascorbate peroxidase, dehydroascorbate reductase, monodehydroascorbate reductase, and glutathione reductase activities. The activities of these enzymes in the apoplastic extracts were greatly modified 24 h after inoculation with the biotrophic fungal pathogen Blumeria graminis. The quantum efficiency of photosystem II, which is related to photosynthetic electron transport flux, was comparable in inoculated and healthy leaves during this period. Apoplastic soluble acid invertase activity was also modified in inoculated leaves. Inoculation-dependent increases in apoplastic SOD activity were observed in all lines. Major bands of SOD activity, observed in apoplastic protein extracts by activity staining of gels following isoelectric focusing, were similar to those observed in whole leaves but two additional minor bands were found in the apoplastic fraction. The apoplastic extracts contained substantial amounts of dehydroascorbate (DHA) but little or no glutathione (GSH). Biotic stress decreased apoplastic ascorbate and DHA but increased apoplastic GSH in resistant lines. The antioxidant cycle enzymes may function to remove apoplastic H 2O 2 with ascorbate and GSH derived from the cytoplasm. DHA and oxidized glutathione may be reduced in the apoplast or returned to the cytosol for rereduction.Abbreviations AA
reduced ascorbate
- APX
ascorbate peroxidase
- DHA
dehydroascorbate (oxidised ascorbate)
- DHAR
dehydroascorbate reductase
- G6PDH
glucose-6-phosphate dehydrogenase
- GSH
reduced glutathione
- GSSG
glutathione disulphide
- GR
glutathione reductase
- MDHA
monodehydroascorbate
- MDHAR
monodehydroascorbate reductase
- SOD
superoxide dismutase 相似文献
14.
A reconstituted spinach chloroplast system containing thylakoids, stroma and 0.1 mM NADPH supported O 2 evolution in the presence of oxidised glutathione (GSSG). The properties of the reaction were consistent with light-coupled GSSG-reductase activity involving H 2O as eventual electron donor. The reconstituted system also supported dehydroascorbate-dependent O 2 evolution in the presence of 0.6 mM reduced glutathione (GSH) and 0.1 mM NADPH with the concomitant production of ascorbate. The GSSG could replace GSH in which case the production of GSH preceded the accumulation of ascorbate. The data are consistent with the light-dependent reduction of dehydroascorbate using H 2O as eventual electron donor via the sequence H 2O→NADP→GSSG→dehydroascorbate. Approximately 30% of the GSH-dehydrogenase activity of spinach leaf protoplasts is localised in chloroplasts: this could not be attributed to contamination of chloroplasts by activity from the extrachloroplast compartment. Washed intact chloroplasts supported the uptake of ascorbate but the uptake mechanism had a very low affinity for ascorbate (K m approximately 20 mM). The rate of uptake of ascorbate was less than the rate of light-dependent reduction of dehydroascorbate and too slow to account for the rate of H 2O 2 reduction by washed intact chloroplasts. 相似文献
15.
The artificial electron donor compounds p-phenylenediamine (PD), N, N, N′, N′-tetramethyl-p-phenylenediamine (TMPD), and 2,6-dichlorophenol-indophenol (DCPIP) restored the Hill reaction and photophosphorylation in chloroplasts that had been inhibited by washing with 0.8 m tris (hydroxymethyl) aminomethane (tris) buffer, pH 8.0. The tris-wash treatment inhibited the electron transport chain between water and photosystem II and electron donation occurred between the site of inhibition and photosystem II. Photoreduction of nicotinamide adenine dinucleotide phosphate (NADP) supported by 33 μ m PD plus 330 μ m ascorbate was largely inhibited by 1 μ m 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) while that supported by 33 μ m TMPD or DCPIP plus ascorbate was relatively insensitive to DCMU. Experiments with the tris-washed chloroplasts indicated that electron donors preferentially donate electrons to photosystem II but in the presence of DCMU the donors (with the exception of PD at low concentrations) could also supply electrons after the DCMU block. The PD-supported photoreduction of NADP showed the relative inefficiency in far-red light characteristic of chloroplast reactions requiring photosystem II. With phosphorylating systems involving electron donors at low concentrations (33 μ m donor plus 330 μ m ascorbate) photophosphorylation, which occurred with P/e 2 ratios approaching unity, was completely inhibited by DCMU but with higher concentrations of the donor systems, photophosphorylation was only partially inhibited. 相似文献
16.
1. 1. The mechanism of the photooxidation of ascorbate and of Mn2+ by isolated chloroplasts was reinvestigated. 2. 2. Our results suggest that ascorbate or Mn2+ oxidation is the result of the Photosystem I-mediated production of the radical superoxide, and that neither ascorbate nor Mn2+ compete with water as electron donors to Photosystem II nor affect the rate of electron transport through the two photosystems: The radical superoxide is formed as a result of the autooxidation of the reduced forms of low potential electron acceptors, such as methylviologen, diquat, napthaquinone, or ferredoxin. 3. 3. In the absence of ascorbate or Mn2+ the superoxide formed dismutases either spontaneously or enzymatically producing O2 and H2O2. In the presence of ascorbate or Mn2+, however, the superoxide is reduced to H2O2 with no formation of O2. Consequently, in the absence of reducing compounds, in the reaction H2O to low potential acceptor one O2 (net) is taken up per four electrons transported where as in the presence of ascorbate, Mn2+ or other suitable reductants up to three molecules O2 can be taken up per four electrons transported. 4. 4. This interpretation is supported by the following observations: (a) in a chloroplast-free model system containing NADPH and ferredoxin-NADP reductase, methylviologen can be reduced to a free radical which is autooxidizable in the presence of O2; the addition of ascorbate or Mn2+ to this system results in a two fold stimulation of O2 uptake, with no stimulation of NADPH oxidation. The stimulation of O2 uptake is inhibited by the enzyme superoxide dismutase; (b) the stimulation of light-dependent O2 uptake in the system H2O → methylviologen in chloroplasts is likewise inhibited by the enzyme superoxide dismutase. 5. 5. In Class II chloroplasts in the system H2O → NADP upon the addition of ascorbate or Mn2+ an apparent inhibition of O2 evolution is observed. This is explained by the interaction of these reductants with the superoxide formed by the autooxidation of ferredoxin, a reaction which proceeds simultaneously with the photoreduction of NADP. Such an effect usually does not occur in Class I chloroplasts in which the enzyme superoxide dismutase is presumably more active than in Class II chloroplasts. 6. 6. It is proposed that since in the Photosystem I-mediated reaction from reduced 2,4-dichlorophenolindophenol to such low potential electron acceptor as methylviologen, superoxide is formed and results in the oxidation of the ascorbate present in the system, the ratio ATP/2e in this system (when the rate of electron flow is based on the rate of O2 uptake) should be revised in the upward direction.
Abbreviations: DCMU, 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea; HEPES, hydroxyethyl-piperazineethanesulfonic acid; MES, (N-morpholino)ethanesulfonic acid; DCIP, 2,4-dichlorophenol-indophenol 相似文献
17.
Cold acclimation of Scots pine did not affect the susceptibility of photosynthesis to photoinhibition. Cold acclimation did however cause a suppression of the rate of CO 2 uptake, and at given light and temperature conditions a larger fraction of the photosystem II reaction centres were closed in cold-acclimated than in nonacclimated pine. Therefore, when assayed at the level of photosystem II reaction centres, i.e. in relation to the degree of photosystem closure, cold acclimation caused a significant increase in resistance to photoinhibition; at given levels of photosystem II closure the resistance to photoinhibition was higher after cold acclimation. This was particularly evident in measurements at 20° C. The amounts and activities of the majority of analyzed active oxygen scavengers were higher after cold acclimation. We suggest that this increase in protective enzymes and compounds, particularly Superoxide dismutase, ascorbate peroxidase, glutathione reductase and ascorbate of the chloroplasts, enables Scots pine to avoid excessive photoinhibition of photosynthesis despite partial suppression of photosynthesis upon cold acclimation. An increased capacity for light-induced de-epoxidation of violaxanthin to zeaxanthin upon cold acclimation may also be of significance.Abbreviations APX
ascorbate peroxidase
- DHA
dehydroascorbate
- DHAR
dehydroascorbate reductase
- Fm
maximal fluorescence when all reaction centres are closed
- Fv/Fm
maximum photochemical yield of PSII
- GR
glutathione reductase
- GSH
reduced glutathione
- Je
rate of photosynthetic electron transport
- MDAR
monodehydroascorbate reductase
- q N
nonphotochemical quenching of fluorescence
- q P
photochemical quenching of fluorescence
- SOD
superoxide dismutase
This work was supported by the Swedish Natural Science Research Council and the National Natural Science Foundation of China. 相似文献
18.
Spinach ( Spinacia oleracea L. cv. Clermont) leaves grown in open-top chambers and exposed to three different concentrations of ozone were measured for gas exchange, chlorophyll a fluorescence, antioxidant systems, and lipid peroxidation at the end of growing season. High O 3 concentration reduced F v/F m, indicating that the efficiency in the energy conversion of photosystem 2 (PS2) was altered. The rate of non-cyclic electron transport rate and the capacity to reduce the quinone pool were also affected. The development of non-photochemical quenching was not high enough to decrease the photon excess in the PS2. The limitation of photosynthetic activity was probably correlated with stomata closure and with an increase in intercellular CO 2 concentration. Under oxidative stress, superoxide dismutase (SOD) activity was stimulated in parallel with lipid peroxidation. We did not find any differences in the ascorbate (AsA) pool and ascorbate peroxidase (APX) or glutathione reductase (GR) activities between air qualities. Small, but similar responses were observed in spinach leaves exposed to ambient ozone concentration. 相似文献
19.
NH 2OH-treated, non-water-splitting chloroplasts can oxidize H 2O 2 to O 2 through Photosystem II at substantial rates (100–250 μequiv · h ?1 · mg ?1 chlorophyll with 5 mM H 2O 2) using 2,5-dimethyl- p-benzoquinone as an electron acceptor in the presence of the plastoquinone antagonist dibromothymoquinone. This H 2O 2 → Photosystem II → dimethylquinone reaction supports phosphorylation with a ratio of 0.25–0.35 and proton uptake with values of 0.67 (pH 8)–0.85 (pH 6). These are close to the value of 0.3–0.38 and the values of 0.7–0.93 found in parallel experiments for the H 2O → Photosystem II → dimethylquinone reaction in untreated chloroplasts. Semi-quantitative data are also presented which show that the donor → Photosystem II → dibromothymoquinone (→O 2) reaction can support phosphorylation when the donor used is a proton-releasing reductant (benzidine, catechol) but not when it is a non-proton carrier (I ?, ferrocyanide). 相似文献
20.
Bundle sheath and mesophyll chloroplasts from Zea mays showed comparable rates of O 2 evolution, which amounted to about half of the rate observed in spinach ( Spinacia oleracea) chloroplasts. Ratios of 4.5, 4.6, and 6.2 Mn2+ atoms per 400 chlorophylls were observed in mesophyll, bundle sheath, and spinach chloroplasts, respectively. These ratios roughly correspond to the observed O2 evolution rates. Rates of electron transport from water to methylviologen (photosystem I and II) in both types of corn chloroplasts were about one-third that in spinach. Compared to spinach, transport rates from reduced diaminodurene to methylviologen (photosystem I) were about one-third and greater than one-half in mesophyll and bundle sheath material, respectively. In both types of corn chloroplasts, electron flow from photosystem II to P700 was abnormal. This observation, together with the low rates of all activities, suggests that damage occurred during isolation. Such damage may limit the quantitative significance of observations made with these materials (including the following data). Measurements of flash yields of O2 evolution or O2 uptake showed that the size of the photosynthetic unit was the same in photosystems I and II and in all three types of chloroplasts (about 400 chlorophylls per equivalent). Similarity of the photochemical cross-section of the two photosystems in the three preparations was also found in optical experiments: that is the half-times of the fluorescence rise in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) (photosystem II) and of the photooxidation of P700 (photosystem I). The ratio of P700 to chlorophyll appeared to be about 2-fold higher in bundle sheath chloroplasts than in the other materials (1/200 versus 1/400). 相似文献
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