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1.
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
2.
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
3.
Using intact and osmotically ruptured chloroplasts, ratios ofcoupling between deposition of protons in the intrathylakoidspace and light-dependent transport of electrons from waterto an external acceptor were determined. The data indicate couplingbetween proton and electron transport at a ratio of H +/e=3 withmethylviologen as electron acceptor in thylakoids and with nitriteas electron acceptor in intact chloroplasts. With ferricyanideas electron acceptor in thylakoids, values close to H +/e=2 wereobserved. Evidence is discussed that H +/e=3 is a fixed valuein intact chloroplasts at levels of thylakoid energization sufficientfor supporting effective carbon assimilation. In the presence of methylviologen and ascorbate, the minimumquantum requirement of oxygen uptake by thylakoids was about2.7 quanta of 675 nm light per O 2 indicating an e/O 2 ratio of1.33. In the absence of ascorbate, and with KCN present in additionto methylviologen, e/O 2 ratios up to 4 were observed. The minimumquantum requirement of oxygen evolution by thylakoids in thepresence of ferricyanide and by intact chloroplasts in the presenceof nitrite was about 8 quanta/O 2. (Received May 1, 1995; Accepted October 2, 1995) 相似文献
4.
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
5.
The effect of pretreatment with ultraviolet-B (UV-B) light (280-320 nanometers) on the enzymatic conversion of the diepoxyxanthophyll violaxanthin to the epoxy-free zeaxanthin occurring in thylakoid membranes was investigated. When isolated chloroplasts of pea ( Pisum sativum) were exposed to UV-B, a biologically effective fluence of 7000 joules per square meter caused about 50% inhibition of the activity of the violaxanthin deepoxidase, measured as the first order rate constant of the absorbance change at 505 nanometers. The dose requirement for the inhibition of the deepoxidase in intact leaves, however, was about 2 orders of magnitude higher. The inhibition of the rate constant was observed for both the dark deepoxidation at pH 5, and for the light-driven deepoxidation induced by the lumen acidification due to electron transport from H 2O to methylviologen or due to a photosystem I partial reaction with duroquinol as the electron donor. The availability of violaxanthin was not directly affected by UV-B radiation, as shown for UV-B-treated chloroplasts by the final extent of the 505 nanometer change measured in the dark at pH 5 or by the partial photosystem I reaction. A significant decrease in the violaxanthin availability was observed when lumen acidification was caused by electron transport from H 2O to methylviologen. That effect was probably caused by the wellknown UV-B inhibition of photosystem II with a subsequent decreased ability to reduce the plastoquinone pool, the redox state of which is believed to regulate the final amount of converted violaxanthin. 相似文献
6.
I. Isolated intact chloroplasts: Photosystem II, but not photosystem I, of the electron transport chain is rapidly photoinactivated even by very low intensities of red light when no large proton gradient can be formed and the electron transport chain becomes over-reduced in the absence of oxygen and other reducable substrates. Electron acceptors including oxygen provide protection against photoinactivation. Nevertheless, photosystem II is rapidly, and photosystem I more slowly, photoinactivated by high intensities of red light when oxygen is the only electron acceptor available. Increased damage is observed at increased oxygen concentrations although catalase is added to destroy H 2O 2 formed during oxygen reduction in the Mehler reaction. Photoinactivation can be decreased, but not prevented by ascorbate which reduces hydrogen peroxide inside the chloroplasts and increases coupled electron flow. II. Leaves: Simple measurements of chlorophyll fluorescence permit assessment of damage to photosystem II after exposure of leaves to high intensity illumination. In contrast to isolated chloroplasts, chloroplasts suffer more damage in situ at reduced than at elevated oxygen concentrations. The difference in the responses is due to photorespiration which is active in leaves, but not in isolated chloroplasts. After photosynthesis and photorespiration are inhibited by feeding glyceraldehyde to leaves, photoinactivation is markedly increased, although oxygen reduction in the Mehler reaction is not affected by glyceraldehyde. In the presence of reduced CO 2 levels, photorespiratory reactions, but not the Mehler reaction, can prevent the overreduction of the electron transport chain. Over-reduction indicates ineffective control of photosystem II activity. Effective control is needed for protection of the electron transport chain against photoinactivation. It is suggested to be made possible by coupled cyclic electron flow around photosystem I which is facilitated by the redox poising resulting from the interplay between photorespiratory carbohydrate oxidation and the refixation of evolved CO 2. 相似文献
7.
Externally added quercetin (100 micromolar) was oxidized by intact spinach chloroplasts at a rate of 30 micromoles per mg chlorophyll per hour in the presence of 100 micromolar H 2O 2. The oxidation rate was increased by about 20% in a hypotonic reaction mixture. The thylakoid fraction also oxidized the flavonol in the presence of H 2O 2, and the rate was about 25% of that by intact chloroplasts. The oxidation of quercetin was inhibited by KCN and NaN 3. Ascorbate, which permeates slowly across chloroplast envelope, only slightly suppressed the initial rate of quercetin oxidation by intact chloroplasts, while the oxidation by ruptured chloroplasts was suppressed by ascorbate by about 60%. Quercetin glycosides, quercitrin and rutin, were also oxidized by chloroplasts in the presence of H 2O 2. These results suggest that flavonols are oxidized by peroxidase-like activity in chloroplasts and that externally added flavonols can permeate into the stroma through the envelope of intact chloroplasts. 相似文献
8.
The photoproduction of hydrogen by 2-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)-inhibited chloroplasts from ascorbate under anaerobic conditions was studied in the pH range 5.0 to 7.5 using methyl viologen (MV), N,N,N′,N′-tetramethyl-P-phenylenediamine (TMPD), and excess hydrogenase from Desulfovibrio desulfuricans. (a) At neutral and basic pHs, the photoreduction of MV, which reacted back with photoxidized ascorbate (dehydroascorbate [DHASC]), and the rates of H 2 photoproduction were very low. The slow H 2 photoproduction was explained by the reversible reduction of MV by the photoproduced H 2 (catalyzed by hydrogenase) and its reoxidation by DHASC resulting in H 2 uptake. (b) At pH 5.2, relatively high initial rates of H 2 photoproduction were obtained, which were comparable to the rates of O 2 consumption at pH 5.2 by photosystem I (catalyzed by photoreduced MV). However, accumulation of photoreduced MV under anaerobic conditions was not detected. In the presence of high concentrations of protons, the H 2 uptake by DHASC was very slow because the equilibrium concentration of H 2-reduced MV was very small, thus allowing H 2 evolution mediated by photoreduced MV to compete with the back reaction with DHASC. (c) The continuous accumulation of DHASC, which was generated together with H 2, gradually slowed the H 2 evolution until it stopped after about 3 hours. At high concentrations, DHASC was able to compete with the coupling of photoreduced MV to hydrogenase and H 2 evolution. (d) Dithiothreitol (DTT) reduced the DHASC and consequently competed with the back reaction of the photoreduced and H 2-reduced MV with DHASC. DTT thus prolonged the time period of H 2 photoproduction from ascorbate and abolished the dependence of its rate on pH in the range of 5.2 to 7.5 (e) A study of H 2 uptake by chemically oxidized ascorbate (in the dark) showed that MV and hydrogenase were both required to catalyze electron transfer from H 2 to DHASC. TMPD prevented this H 2 consumption by DHASC (in a chloroplast reaction mixture containing MV and hydrogenase). Illumination restored the H 2 uptake presumably by generating reduced MV which activated the hydrogenase. 相似文献
9.
Disulfiram (tetraethylthiuram disulfide), a metal chelator, inhibits photosynthetic electron transport in broken chloroplasts. A major site of inhibition is detected on the electron-acceptor side of photosystem II between Q A, the first plastoquinone electron-acceptor, and the second plastoquinone electron-acceptor, Q B. This site of inhibition is shown by a severalfold increase in the half-time of Q −A oxidation, as monitored by the decay of the variable chlorophyll a flourescence after an actinic flash. Another site of inhibition is detected in the functioning of the reaction center of photosystem II; disulfiram is observed to quench the room temperature variable chlorophyll a fluorescence, as well as the intensity of the 695 nm peak, relative to the 685 nm peak, in the chlorophyll a fluorescence spectrum at 77 K. Electron transport from H 2O to the photosystem II electron-acceptor silicomolybdate is also inhibited. Disulfiram does not inhibit electron flow before the site(s) of donation by exogenous electron donors to photosystem II, and no inhibition is detected in the partial reactions associated with photosystem I. 相似文献
10.
The light-dependent quenching of 9-aminoacridine fluorescence was used to monitor the state of the transthylakoid proton gradient in illuminated intact chloroplasts in the presence or absence of external electron acceptors. The absence of appreciable light-dependent fluorescence quenching under anaerobic conditions indicated inhibition of coupled electron transport in the absence of external electron acceptors. Oxygen relieved this inhibition. However, when DCMU inhibited excessive reduction of the plastoquinone pool in the absence of oxygen, coupled cyclic electron transport supported the formation of a transthylakoid proton gradient even under anaerobiosis. This proton gradient collapsed in the presence of oxygen. Under aerobic conditions, and when KCN inhibited ribulose bisphosphate carboxylase and ascorbate peroxidase, fluorescence quenching indicated the formation of a transthylakoid proton gradient which was larger with oxygen in the Mehler reaction as electron acceptor than with methylviologen at similar rates of linear electron transport. Apparently, cyclic electron transport occured simultaneously with linear electron transport, when oxygen was available as electron acceptor, but not when methylviologen accepted electrons from Photosystem I. The ratio of cyclic to linear electron transport could be increased by low concentrations of DCMU. This shows that even under aerobic conditions cyclic electron transport is limited in isolated intact chloroplasts by excessive reduction of electron carriers. In fact, P 700 in the reaction center of Photosystem I remained reduced in illuminated isolated chloroplasts under conditions which resulted in extensive oxidation of P 700 in leaves. This shows that regulation of Photosystem II activity is less effective in isolated chloroplasts than in leaves. Assuming that a Q-cycle supports a H +/e ratio of 3 during slow linear electron transport, vectorial proton transport coupled to Photosystem I-dependent cyclic electron flow could be calculated. The highest calculated rate of Photosystem I-dependent proton transport, which was not yet light-saturated, was 330 mol protons (mg chlorophyll h) –1 in intact chloroplasts. If H +/e is not three but two proton transfer is not 330 but 220 mol (mg Chl H) –1. Differences in the regulation of cyclic electron transport in isolated chloroplasts and in leaves are discussed. 相似文献
11.
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
12.
The inactivated O 2-evolving center of Tris-washed chloroplasts was reactivated by DCPIP-treatment and photoreactivation in the presence of Mn 2+, Ca 2+, DTT and weak light. Many electron donors (Asc and reduced DCPIP, etc.) were found to be suitable substitutes for DTT. By studying the anaerobic inhibition of the reactivation, the electron acceptors O 2, NADP +, etc. were also found to be essential factors in photoreactivation. Weak light stimulated the chloroplast electron transport from the above-mentioned electron donors to the electron acceptor and effected the photoreactivation. More than 280 electrons were transported to NADP + in the anaerobic photoreactivation of one unit of an O 2-evolving center with 400 Chl. Electron transport in the reactivation was inhibited by omitting DTT or Mn 2+ ion, and by adding DCMU. The photoreactivated chloroplasts incorporated about 2 Mn by 400 Chl. Omission of DTT in the reactivation caused chloroplasts in the weak light to bind large amounts of excess Mn.Abbreviations Asc
ascorbate
- Chl
chlorophyll
- DCPIP
2, 6-dichlorophenol indophenol
- DPC
diphenyl carbazide
- DTT
dithiothreitol
- Fd
ferredoxin
- STN
a chloroplast preparation medium, containing 0.4 M sucrose, 0.05 M Tris-Cl and 0.01 M NaCl (pH 7.8 and 8.0)
- TMPD
tetramethyl-p-phenylenediamine 相似文献
13.
Radiation inactivation technique was employed to determine the functional size of photosynthetic electron transport chain of spinach chloroplasts. The functional size for photosystem I+II (H 2O to methylviologen) was 623 ± 37 kilodaltons; for photosystem II (H 2O to dimethylquinone/ferricyanide), 174 ± 11 kilodaltons; and for photosystem I (reduced diaminodurene to methylviologen), 190 ± 11 kilodaltons. The difference between 364 ± 22 (the sum of 174 ± 11 and 190 ± 11) kilodaltons and 623 ± 37 kilodaltons is partially explained to be due to the presence of two molecules of cytochrome b6/f complex of 280 kilodaltons. The molecular mass for other partial reactions of photosynthetic electron flow, also measured by radiation inactivation, is reported. The molecular mass obtained by this technique is compared with that determined by other conventional biochemical methods. A working hypothesis for the composition, stoichiometry, and organization of polypeptides for photosynthetic electron transport chain is proposed. 相似文献
14.
The rate of Hill reaction can be measured accurately as O 2 uptake (the Mehler reaction) if a rapidly autoxidizable electron acceptor (e.g., methylviologen) is used. However, when an artificial electron donor-ascorbate couple (or ascorbate alone) replaces the natural donor, water, the rate of O 2 consumption is no longer a reliable measure of the electron flux, because superoxide radical reactions contribute to O 2 uptake. Such radical reactions, however, can be suppressed by adding enough superoxide dismutase to the reaction mixture. Indeed in all of the photosystem I- and photosystem II-donor reactions tested (except with benzidine which was tested without ascorbate added), the O 2 uptake was inhibited by 30 to 50% by the addition of superoxide dismutase. The rate of phosphorylation was totally unaffected by the enzyme. The reasessment of the phosphorylation efficiencies thus made by the use of superoxide dismutase led us to the following conclusions. The phosphorylation efficiency associated with the transfer of electrons from a donor to methlylviologen (than to O 2) through both photosystems II and I is practically independent of the donor used—catechol, benzidine, p-aminophenol, dicyanohydroquinone, or water. The P/e 2 ratio is 1.0 ± 0.1. Only ascorbate gives a slightly lower value (P/e 2 = 0.9). (NH 2OH-treated, non-water-splitting chloroplasts were used for reactions with these artificial donors.) The phosphorylation efficiency associated with DCMU-insensitive, photosystem I-mediated transfer of electrons from a donor to methylviologen (then to O 2) is again largely independent of the donor used, such as diaminodurene, diaminotoluene, and reduced 2,6-dichlorphenol-indophenol. The P/e 2 ratio is 0.6 ± 0.08. 相似文献
15.
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
16.
Intact mesophyll and bundle sheath chloroplasts wee isolated from the NADP-malic enzyme type C 4 plants maize, sorghum (monocots), and Flaveria trinervia (dicot) using enzymic digestion and mechanical isolation techniques. Bundle sheath chloroplasts of this C 4 subgroup tend to be agranal and were previously reported to be deficient in photosystem II activity. However, following injection of intact bundle sheath chloroplasts into hypotonic medium, thylakoids had high Hill reaction activity, similar to that of mesophyll chloroplasts with the Hill oxidants dichlorophenolindophenol, p-benzoquinone, and ferricyanide (approximately 200 to 300 micromoles O 2 evolved per mg chlorophyll per hour). In comparison to that of mesophyll chloroplasts, the Hill reaction activity of bundle sheath chloroplasts of maize and sorghum was labile and lost activity during assay. Bundle sheath chloroplasts of maize also exhibited some capacity for 3-phosphoglycerate dependent O 2 evolution (29 to 58 micromoles O 2 evolved per milligram chlorophyll per hour). Both the mesophyll and bundle sheath chloroplasts were equally effective in light dependent scavenging of hydrogen peroxide. The results suggest that both chloroplast types have noncyclic electron transport and the enzymology to reduce hydrogen peroxide to water. The activities of ascorbate peroxidase from these chloroplast types was consistent with their capacity to scavenge hydrogen peroxide. 相似文献
17.
Light-dependent H 2 evolution from dithiothreitol as electron donor was observed with cell-free preparations of anaerobically adapted and from spinach chloroplasts mixed with hydrogenase. NADH substituted for dithiothreitol as electron donor only in the preparation. Dibromothymoquinone, an antagonist of plastoquinone, selectively inhibited H 2 photoevolution from NADH. These results are interpreted as indicating that 3-(3,4-dichlorophenyl)-1,1-dimethyl urea insensitive H 2 photoevolution by algae containing hydrogenase is due to the capability of NADH to reduce plastoquinone in the electron transport chain, and to evolve H 2 by a low redox potential carrier of photosystem I. 相似文献
18.
Sulfur deprivation of algal cultures selectively and partially inactivates photosystem II (PSII)-catalyzed O 2 evolution, induces anaerobiosis and hydrogenase expression, and results in sustained H 2 photoproduction for several days. We show that re-addition of limiting amounts of sulfate (1–10 μM final concentration) to the cultures during the H 2-production phase temporarily reactivates PSII photochemical and O 2-evolution activity and re-establishes higher rates of electron transport through the photosynthetic electron transport chain. The reactivation of PSII occurs by de novo D1 protein synthesis, but does not result in the re-establishment of aerobic conditions in the reactor, detectable by dissolved-O 2 sensors. However, concomitant H 2 photoproduction is inhibited, possibly due to excessive intra-cellular levels of photosynthetically-evolved O 2. The partial recovery of electron transport rates correlates with the re-oxidation of the plastoquinone (PQ) pool, as observed by pulse-amplitude modulated (PAM) and fluorescence-induction measurements. These results show that the presence of a more oxidized PQ pool releases some of the down-regulation of electron transport caused by the anaerobic conditions. 相似文献
19.
A fifteen minute incubation of spinach chloroplasts with the divalent Ca 2+ chelator, EGTA, in concentrations 50–250 μM, inhibits electron transport through both photosystems. All photosystem II partial reactions, including indophenol, ferricyanide and the DCMU-insensitive silicomolybdate reduction are inhibited from 70–100%. The photosystem II donor reaction, diphenyl carbazide → indophenol, is also inhibited, indicating that the inhibition site comes after the Mn 2+ site, and that the first Ca 2+ effect noted (site II) is not on the water oxidation enzyme, as is commonly assumed, but between the Mn 2+ site and plastoquinone A pool. The other photosystem II effect of EGTA (Ca 2+ site I), occurs in the region between plastoquinone A and P700 in the electron transport chain of chloroplasts. About 50% inhibition of the reaction ascorbate + TMPD → methyl viologen is given by incubation with 200 μM EGTA for 15 min. Ca 2+ site II activity can be restored with 20 mM CaCl 2. Ca 2+ site I responds to Ca 2+ and plastocyanin added jointly. More than 90% activity in the ascorbate + TMPD → methylviologen reaction can be restored. Various ways in which Ca 2+ ions could affect chloroplast structure and function are discussed. Since EGTA is more likely to penetrate chloroplast membranes than EDTA, which is known to remove CF 1, the coupling factor, from chloroplast membranes, and since Mg 2+ ions are ineffective in restoring activity, it is concluded that Ca 2+ may function in the electron transport chain of chloroplasts in a hitherto unsuspected manner. 相似文献
20.
Three seedling lethal mutants of Zea mays with impaired photosynthesis are described. These recessive mutants were selected on the basis of high chlorophyll fluorescence. They have normal chlorophyll pigmentation but are unable to fix CO 2 fully. Evidence is presented from fluorescence characteristics of isolated chloroplasts that both photosystem I and II mutants were isolated. Using conventional measures of photosynthetic electron transport, we suggest that the photosystem I mutant has limited ability to reduce NADP. The other two mutants are clearly blocked in photosystem II, one possibly lacking the primary electron acceptor. 相似文献
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