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1.
The structural-functional organization of higher plant chloroplasts has been investigated in relation to the particular light conditions during plant growth. (1) Light intensity variations during growth caused changes in the ratio, in the light-saturated uncoupled rates of electron transport to a Hill oxidant and in the distribution of the chloroplast volume between the membrane and stroma phases. (2) Light quality differences during growth had an effect on the PS II/PS I reaction center ratio and on the chloroplast membrane phase differentiation into grana and stroma thylakoids. Plants grown under far-red-enriched (680–710 nm) illumination contained higher (20–25%) amounts of PS II and simultaneously lower (20–25%) amounts of PS I reaction centers. They also showed a higher grana density along with thicker grana stacks in their chloroplasts. (3) The size of the light-harvesting antenna pool of PS II centers was estimated from the fluorescence time course of 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea-poisoned chloroplasts and was found to be fairly constant (±10%) in spite of the variable PS II/PS I reaction center ratio. The results are compatible with the hypothesis that the structural entities of grana facilitated the centralization and relative concentration increase of a certain group of PS II reaction centers. 相似文献
2.
The I-D transient in the chlorophyll fluorescence induction curve (Kautsky effect) is investigated in the view of recently discovered rapid changes in energy distribution between the two photosystems (Schreiber, U. and Vidaver, W., FEBS Lett., in the press). Fluorescence induction curves differ appreciably depending on whether measured at λ < 690 nm, originating in pigment system II, or at λ > 715 nm, which is in part from pigment system I. The differences occur as well in the rapid part of the induction curve (O-I-D-P) as in the slower P-S decay. Most significant changes in energy distribution are indicated in the region of the I-D dip, being induced by appropriate preillumination. The effect is studied by (a) comparing the individual fluorescence time courses at λ < 690 nm and λ > 715, (b) plotting F < 690 vs. F > 715 and (c) recording time courses of ratios. In (a) the I and D characteristics are delayed at F > 715 relative to F < 690, which is accompanied by periods close to I and D, where the two emissions follow inverse courses. In (b) the I-D dip corresponds to a loop. And in (c) it is shown that a rapid ratio decay, reflecting increasing excitation of System I pigments, is initiated before the I-D dip. These data indicate that the I-D transient is caused by a rapid switch of energy distribution in favor of System I and the resulting stimulation of Q reoxidation via the electron transport chain. It is suggested that as with the slow fluorescence transients the rapid also can be understood as a composite of two different changes, (1) direct changes resulting from a switch in energy distribution, which are inverse for F < 690 and F > 715, and (2) indirect changes due to stimulated Q reduction or Q oxidation, which are parallel for both emissions. The rapid ratio decay, correlated to I-D, persists and is even stimulated in the presence of electron transport inhibitors. This and the speed of the phenomenon make it improbable that the rapid energy distribution changes are affected by an ion flux-induced mechanism. It is proposed that the electrical field across the thylakoid membrane is involved in the energy switch mechanism. 相似文献
3.
4.
When the photosystem II quinone acceptor complex has been singly reduced to the state QAQ?B, there is a 22 s half-time back-reaction of Q?B with an oxidized photosystem II donor (S2), directly measured here for the first time. From the back-reaction kinetics with and without inhibitors, kinetic and equilibrium parameters have been estimated. We suggest that the state QAQ?B of the complex is formed by a second-order reaction of vacant reaction centers in the state Q?A with plastoquinone from the pool, and discuss the physico-chemical parameters involved. 相似文献
5.
(1) In isolated chloroplasts (class B) electron flow is controlled mainly by the intrathylakoid pH (pHin). A decrease in pHin due to the light-driven injection of protons inside the thylakoid leads to the retardation of electron flow between two photosystems. This effect can be abolished by uncouplers or under photophosphorylation conditions (addition of Mg2+-ADP with Pi); Mg2+-ATP does not influence the steady-state rate of electron flow, (2) The steady-state pH difference, ΔpH, across the thylakoid membrane was estimated from quantitative analysis of the rate of P-700+ reduction. In chloroplasts, without adding Mg2+-ADP, ΔpH increases from 1.6 to 3.2 as the external pH rises from 6 to 9.5. Under the photophosphorylation conditions, ΔpH decreases showing a minimum at the external pH 7.5 (). (3) The value of photosynthetic control, K, measured as the ratio of the steady-state rates of P-700+ reduction in the presence of Mg2+-ADP (with Pi) and without adding Mg2+-ADP is dependent on external pH variations, showing a maximum value of at pHout 7.5. This pH dependence coincides with that of the ADP-stimulated ΔpH decrease. (4) Experiments with spin labels provide evidence that the light-induced changes in the thylakoid membrane are sensitive to the addition of uncouplers and are affected only slightly by the addition of Mg2+-ADP and Pi. 相似文献
6.
1. Using a phosphoroscope, delayed luminescence and prompt chlorophyll fluorescence from isolated chloroplasts have been compared during the induction period.2. Two distinct decay components of delayed luminescence were measured a “fast” component (from ≈1 ms to ≈6 ms) and a “slow” component (at ≈6 ms).3. The fast luminescence component often did not correlate with the fluorescence changes while the slow component significantly changed its intensity during the induction period in a manner which could usually be linearly correlated with variable portion of the fluorescence yield change.4. This correlation was evident after preillumination with far-red light or after allowing a considerable time for dark relaxation.5. The close relationship between the slow luminescence component and variable fluorescence yield was observed with a large range of light intensities and also in the presence of 3(3,4-dichlorophenyl)-1,1-dimethylurea which considerably changes the fluorescence induction kinetics.6. Valinomycin and other antibiotics reduced the amplitude of the 6 ms (slow) luminescence without affecting its relation with the fluorescence induction suggesting possibly that a constant electrical gradient exist in the dark or formed very rapidly in the light, which effects the emission intensity.7. Changes in salt levels of suspending media equally affected the amplitude of both delayed luminescence and variable fluorescence under conditions when the reduction of Q is maximal and constant.8. The results are discussed in terms of several models. It is concluded that the model of independent Photosystem II units together with photosynthetic back reaction concept is incompatible with the data. Other alternative models (the “lake” model and photosynthetic back reaction; recombination of charges in the antenna chlorophyll; the “W” hypothesis) were in closer agreement with the results. 相似文献
7.
Redox titrations of the flash-induced formation of C550 (a linear indicator of Q?) were performed between pH 5.9 and 8.3 in Chlamydomonas Photosystem II particles lacking the secondary electron acceptor, B. One-third of the reaction centers show a pH-dependent midpoint potential (Em,7.5) = ? 30 mV) for redox couple , which varies by ?60 mV/pH unit. Two-thirds of the centers show a pH-independent midpoint potential (Emm = + 10 mV) for this couple. The elevated pH-independent Em suggests that in the latter centers the environment of Q has been modified such as to stabilize the semiquinone anion, Q?. The midpoint potentials of the centers having a pH-dependent Em are within 20 mV of those observed in chloroplasts having a secondary electron acceptor. It appears therefore that the secondary electron acceptor exerts little influence on the Em of . An EPR signal at g 1.82 has recently been attributed to a semiquinone-iron complex which comprises Q?. The similar redox behavior reported here for C550 and reported by others (Evans, M.C.W., Nugent, J.H.A., Tilling, L.A. and Atkinson, Y.E. (1982) FEBS Lett. 145, 176–178) for the g 1.82 signal in similar Photosystem II particles confirm the assignment of this EPR signal to Q?. At below ?200 mV, illumination of the Photosystem II particles produces an accumulation of reduced pheophytin (Ph?). At ?420 mV Ph? appears with a quantum yield of 0.006–0.01 which in this material implies a lifetime of 30–100 ns for the radical pair P-680+Ph?. 相似文献
8.
Yung-Sing Li 《BBA》1975,376(1):180-188
Chloroplast fluorescence was excited by a weak measuring beam. A time-separated actinic light was used to modify the redox states of Q which in turn induced a change in the fluorescence yield. In salt-depleted chloroplasts, fluorescence saturated at a low actinic light intensity. CaCl2 increased the “variable” fluorescence as well as the rate of ferricyanide-Hill reaction. With Tris-washed chloroplasts, Photosystem II donor couple, phenylenediamine and ascorbate, did not increase the fluorescence to a large extent without the presence of CaCl2. It is suggested that salt-depletion inactivates the Photosystem II reaction center of chloroplasts. 相似文献
9.
10.
Michael R. Wasielewski Douglas G. Johnson Govindjee Christopher Preston Michael Seibert 《Photosynthesis research》1989,22(1):89-99
We have measured the rate constant for the formation of the oxidized chlorophyll a electron donor (P680+) and the reduced electron acceptor pheophytin a
– (Pheo a
–) following excitation of isolated Photosystem II reaction centers (PS II RC) at 15 K. This PS II RC complex consists of D1, D2, and cytochrome b-559 proteins and was prepared by a procedure which stabilizes the protein complex. Transient absorption difference spectra were measured from 450–840 nm as a function of time with 500fs resolution following 610 nm laser excitation. The formation of P680+-Pheo a
– is indicated by the appearance of a band due to P680+ at 820 nm and corresponding absorbance changes at 490, 515 and 546 nm due to the formation of Pheo a
–. The appearance of the 490 nm and 820 nm bands is monoexponenital with =1.4±0.2 ps. Treatment of the PS II RC with sodium dithionite and methyl viologen followed by exposure to laser excitation results in accumulation of Pheo a
–. Laser excitation of these prereduced RCs at 15 K results in formation of a transient absorption spectrum assigned to 1*P680. We observe wavelength-dependent kinetics for the recovery of the transient bleach of the Qy absorption bands of the pigments in both untreated and pre-reduced PS II RCs at 15K. This result is attributed to an energy transfer process within the PS II RC at low temperature that is not connected with charge separation.Abbreviations PS I
Photosystem I
- PS II
Photosystem II
- RC
reaction center
- P680
primary electron donor in Photosystem II
- Chl a
chlorophyll a
- Pheo a
pheophytin a 相似文献
11.
Strong externally applied electrical field pulses are known to stimulate delayed luminescence from preilluminated blebs (hypotonically swollen vesicles originating from thylakoid membranes of broken chloroplasts) by up to 3 orders of magnitude. This phenomenon is known as electrophotoluminescence. Previous analysis showed the kinetics of the electrophotoluminescence to be biphasic, displaying a rapid (R) phase which decays towards a slower one (S) (Ellenson, J.L. and Sauer, K. (1976) Photochem. Photobiol. 23, 113–123). We demonstrate that these two components represent different processes. At low pH, a good kinetic separation is obtained between the two phases, which become distinct, with the S phase manifesting also an initial rise period. Under these conditions, it is possible to estimate separately the approximate rise times of the two phases. It is shown that the R and S components have a different dependence on the pH and on the time between the actinic flash and onset of the field. The field dependence is also different, with the S phase requiring a lower threshold field than R. From these observations, it is concluded that the R and S luminescence components are formed by different precursors. The difference in behaviour of the two phases during formation of the bleb indicates that the precursors of the R and S phases belong to different parts of the bleb. We suggest that R precursors are located in the wall of the swollen thylakoid and S precursors in the membrane formations which are attached to this wall. 相似文献
12.
To investigate the effects of a membrane potential on excitation trapping and charge separation in Photosystem II we have studied the chlorophyll fluorescence yield in osmotically swollen chloroplasts subjected to electrical field pulses. Significant effects were observed only in those membrane regions where a large membrane potential opposing the photochemical charge separation was built up. When the fluorescence yield was low, close to F0, a much higher yield, up to Fmax, was observed during the presence of the membrane potential. This is explained by an inhibition by the electrical field of electron transfer to the quinone acceptor Q, resulting in a decreased trapping of excitations. A field pulse applied when the fluorescence yield was high, Q and the donor side being in the reduced state, had the opposite effect: the fluorescence was quenched nearly to F0. This field-induced fluorescence quenching is ascribed to reversed electron transfer from Q? to the intermediate acceptor, pheophytin. Its field strength dependence suggests that the midpoint potential difference between pheophytin and Q is at most about 300 mV. Even then it must be assumed that electron transfer between pheophytin and Q spans 90% of the potential difference across the membrane. 相似文献
13.
Redox titration of the electrochromic carotenoid band shift, detected at 50 μs after a saturating actinic flash, in spinach chloroplasts, shows that only one electron acceptor in Photosystem II participates in a transmembrane primary electron transfer. This species, the primary quinone acceptor, Q, shows only one midpoint potential (Em,7.5) of approx. 0 V and is undoubtedly equivalent to the fluorescence quencher, QH. A second titration wave is observed at low potential () and at greater than 3 ms after a saturating actinic flash. This wave has an action spectrum different from that of Photosystem II centers containing Q and could arise from a secondary but not primary electron transfer. A low-potential fluorescence quencher is observed in chloroplasts which largely disappears in a single saturating flash at ? 185 mV and which does not participate in a transmembrane electron transfer. This low-potential quencher (probably equivalent to fluorescence quencher, QL) and Q are altogether different species. Redox titration of C550 shows that if electron acceptor Qβ is indeed characterized by an Em,7 of + 120 mV, then this acceptor does not give rise to a C550 signal upon reduction and does not participate in a transmembrane electron transfer. This titration also shows that C550 is not associated with QL. 相似文献
14.
The effect of divalent cations on the primary photoconversion kinetics of chloroplast Photosystems (PS) I and II was investigated by absorbance difference spectrophotometry in the ultraviolet (ΔA320) and red (ΔA700) regions and by fluorescence at room temperature. Three main chlorophyll (Chl) a fluorescence emission components were identified. Addition of 5 mM MgCl2 to unstacked chloroplasts caused a 5–7-fold increase in Fvα, the variable fluorescence yield controlled by the α-centers. The fluorescence yield Fvβ controlled by the β-centers and the nonvariable fluorescence yield F0 were only slightly changed by the treatment. The absolute number of α- and β-centers remained unchanged and independent of divalent cations. The rate constants Kα, Kβ and KP-700 determined from the photoconversion kinetics of Qα, Qβ and P-700 were also unchanged by divalent cations, suggesting a constancy of the respective absorption cross-sections. Evidence is presented that the Mg2+ effect on Chl a fluorescence is not due simply to unstacking. Conclusion: (1) In the absence of divalent cations from the chloroplast suspending medium, the variable fluorescence yield is not complementary to the rate of PS II photochemistry. (2) A spillover of excitation from PS II to PS I in the absence of Mg2+ cannot account for the 7-fold lowering of the variable fluorescence yield Fvα at room temperature. The results are discussed in view of a model of excitation transfer and fluorescence emission in the pigment bed of PS IIα and PS IIβ. 相似文献
15.
Peter Horton 《BBA》1981,635(1):105-110
The effect of alteration of redox potential on the kinetics of fluorescence induction in pea chloroplasts has been investigated. Potentiometric titration of the initial (Fi) level of fluorescence recorded upon shutter opening gave a two component curve, with Em(7) at ?20 mV and ?275 mV, almost, identical to results obtained using continuous low intensity illumination (Horton, P. and Croze, E. (1979) Biochim. Biophys. Acta 545, 188–201). The slow or tail phase of induction observed in the presence of DCMU can be eliminated by poising the redox potential at approx. 0 to +50 mV. At this potential Fi was increased by less than 10% and the higher potential quencher described above was only marginally reduced. The disappearance of the slow phase titrated as an n = 1 component with an Em(7) of +120 mV. Therefore it seems unlikely that the slow phase of fluorescence induction is due to photoreduction of the ?20 mV quencher. These results are discussed with reference to current ideas concerning heterogeneity on the acceptor side of Photosystem II. 相似文献
16.
17.
Reversible phosphorylation and turnover of the D1 protein in vivo were studied under low-temperature photoinhibition of pumpkin leaves and under subsequent recovery at low light at 4 °C or 23 °C. The inactivation of PS II and photodamage to D1 were not enhanced during low-temperature photoinhibition when compared to that at room temperature. The PS II repair cycle, however, was completely blocked at 4 °C at the level of D1 degradation. Both the recovery of the photochemical activity of PS II and the degradation of the damaged D1 protein at low light at 23 °C were delayed about 1 hour after low-temperature photoinhibition, suggesting that in addition to the decrease in catalytic turnover of the enzyme, the protease was specifically inactivated in vivo at low temperature. The effect of low temperature on the other regulatory enzymes of PS II repair, protein kinase and phosphatase [Rintamäki et al. (1996) J Biol Chem 271: 14870-14875] was variable. The D1 protein kinase was operational at low temperature while dephosphorylation of the D1 protein seemed to be completely inhibited during low temperature treatment. Under subsequent recovery conditions at low light and 23 °C, the high phosphorylation level of D1 was sustained in leaf discs photoinhibited at low temperature, despite the recovery of the phosphatase activity. This high phosphorylation level of D1 was due to the persistently active kinase. The D1 kinase, previously shown to get activated by reduction of plastoquinone, was, however, found to be maximally active already at relatively low redox state of the plastoquinone pool. We suggest that phosphorylation of PS II centers increases the stability of PS II complexes and concomitantly improves their survival under stress conditions. 相似文献
18.
P. Mehta A. Jajoo S. Mathur S. I. Allakhverdiev S. Bharti 《Biochemistry. Biokhimii?a》2009,74(6):620-624
The effect of high salt concentration on photosystem II (PS II) electron transport rates and chlorophyll a fluorescence induction kinetics was investigated in coupled and uncoupled spinach thylakoid membranes. With increase in salt
concentration, the rates of electron transport mediated by PS II and the F
v/F
m ratio were affected more in uncoupled thylakoids as compared to coupled thylakoid membranes. The uncoupled thylakoid membranes
seemed to behave like coupled thylakoid membranes at high NaCl concentration (∼1 M). On increasing the salt concentration,
the uncoupler was found to be less effective and Na+ probably worked as a coupling enhancer or uncoupling suppressor. We suggest that positive charge of Na+ mimics the function of positive charge of H+ in the thylakoid lumen in causing coupled state. The function of NaCl (monovalent cation) could be carried out by even lower
concentration of Ca2+ (divalent cation) or Al3+ (trivalent cation). We conclude that this function of NaCl as coupling enhancer is not specific, and in general a positive
charge is required for causing coupling in uncoupled thylakoid membranes.
Published in Russian in Biokhimiya, 2009, Vol. 74, No. 6, pp. 761–767. 相似文献
19.
1. Photosystem I particles enriched in P-700 prepared by Triton X-100 treatment of chloroplasts show a light-induced increase in fluorescence yield of more than 100% in the presence of dithionite but not in its absence.2. Steady state light maintains the P-700, of these particles, in the oxidised state when ascorbate is present but in the presence of dithionite only a transient oxidation occurs.3. EPR data show that, in these particles, the primary electron acceptor (X) is maintained in the reduced state by light at room temperature only when the dithionite is also present. In contrast, the secondary electron acceptors are reduced in the dark by dithionite.4. Fluorescence emission and excitation spectra and fluorescence lifetime measurements for the constant and variable fluorescence indicate a heterogeneity of the chlorophyll in these particles.5. It is concluded that the variable fluorescence comes from those chlorophylls which can transfer their energy to the reaction centre and that the states PX and P+X are more effective quenchers of chlorophyll fluorescence than PX?, where P is P-700. 相似文献
20.
Addition of ATP to chloroplasts causes a reversible 25–30% decrease in chlorophyll fluorescence. This quenching is light-dependent, uncoupler insensitive but inhibited by DCMU and electron acceptors and has a half-time of 3 minutes. Electron donors to Photosystem I can not overcome the inhibitory effect of DCMU, suggesting that light activation depends on the reduced state of plastoquinone. Fluorescence emission spectra recorded at ?196°C indicate that ATP treatment increases the amount of excitation energy transferred to Photosystem I. Examination of fluorescence induction curves indicate that ATP treatment decreases both the initial (Fo) and variable (Fv) fluorescence such that the ratio of Fv to the maximum (Fm) yield is unchanged. The initial sigmoidal phase of induction is slowed down by ATP treatment and is quenched 3-fold more than the exponential slow phase, the rate of which is unchanged. A plot of Fv against area above the induction curve was identical plus or minus ATP. Thus ATP treatment can alter quantal distribution between Photosystems II and I without altering Photosystem II-Photosystem II interaction. The effect of ATP strongly resembles in its properties the phosphorylation of the light-harvesting complex by a light activated, ATP-dependent protein kinase found in chloroplast membranes and could be the basis of physiological mechanisms which contribute to slow fluorescence quenching in vivo and regulate excitation energy distribution between Photosystem I and II. It is suggested that the sensor for this regulation is the redox state of plastoquinone. 相似文献