OSCILLATIONS OF THE TRIGGERED LUMINESCENCES OF ISOLATED CHLOROPLASTS PREILLUMINATED BY SHORT FLASHES*

Abstract— Four types of triggered luminescence of isolated lettuce chloroplast (HCl-induced, methanol-induced, sodium benzoate-induced and T-jump-induced) were examined after preillumination by a series (from 1 to 10) of short flashes. Oscillations were observed in the luminescence peaks, with a period of four flashes. These oscillations had maxima after the second and the sixth flash, similar to those of delayed light emission. The maxima were shifted forward two flashes by 50 μ M hydroxylamine, as in oxygen evolution, and were abolished by 5 μ M DCMU, as for delayed light. These results may show that the mechanism of triggered luminescence is influenced directly by the oxidation states ( S ₁) on the donor side of photo-system II.     

COMPARAISON ENTRE L''EMISSION D''OXYGENE ET L''EMISSION DE LUMINESCENCE A LA SUITE D''UNE SERIE D''ECLAIRS SATURANTS

Abstract— Luminescence of Chlorella cells has been measured after illumination by a series of short saturating flashes.
The intensity of luminescence is strongly dependent upon the number of flashes; luminescence is minimum after a single flash and maximum after a double flash illumination.
If l /_n is the intensity of luminescence measured 0.24 sec after the nth flash, the series l _n shows oscillations as a function of n. The series l _n is very similar to the series y _n, in which y is the amount of oxygen evolved by the nth flash, the term l _n corresponding to the term y _n+1. To interpret the oscillations of the series y _n, different models of system II photochemical centers have been proposed, one by ourselves and the other one by KOK et al. In these models the electron donor Z of photosystem 11 is supposed to have 2 or 4 levels of oxidation, the more oxidized state being the precursor of oxygen. These different oxidation states of the donor Z allow storage of the photic energy in the system. The correlation between the series y _n and l _n shows that at least one of the oxidized forms of the donor Z is a substrate for the luminescence reaction.     

UN NOUVEAU MODELE DES CENTRES PHOTOCHIMIQUES DU SYSTEME II*

Abstract— The amount of oxygen evolved by Chlorella cells or by isolated chloroplasts has been measured after illumination by short saturating flashes. In all conditions, the amount of oxygen evolved by one flash is proportional to the fraction of the photochemical centers susceptible to produce oxygen. If dark adapted algae or chloroplasts are illuminated by a sene of flashes, no oxygen is produced by the first flash. This phenomenon is related to the activation process. If y_n is the amount of oxygen evolved by the nth flash of the sequence, it appears that the sene y_n shows large oscillations with a period 4. These oscillations are completely damped after 4–6 periods and the amount of oxygen evolved by a flash reaches a stationary value. For any value of n. the quantities y_n and y_n+2 are linked by a recurrent relation which is the same for Chlorella cells and spinach chloroplasts. No relation can be found between the terms y_n and y_(n+1). The mathematical properties of the series y_n can be understood if one admits that a two memory process is involved in the photochemical reaction. The results have been interpreted in terms of a new model of the System II photochemical centers. The main characteristics of this model are: (1) Each photochemical center includes two electron donors (Z) and one electron acceptor (Q). (2) The formation of one atom of oxygen requires a two quantum process corresponding to the transfer of two electrons from the same electron donor (first memory). (3) The photochemical center acts as a switch which connects alternately each donor to the acceptor (second memory). The switch process occurs after each photoact with an efficiency of about 85 per cent. Other arguments in favor of this model are obtained from studies of the rate of oxygen production at the onset of a weak illumination.     

AN OSCILLATING SYSTEM REGULATING DEVELOPMENT OF PLANTS

Phytochrome conversion shifts the developmental pattern of dark-grown bean (Phaseolus vulgaris) seedlings. Red light was found to initiate rapid oscillations in a system that links illumination with subsequent growth. A single 8 s flash of red light increased the average leaf weight measured 24 h later by 8%. When total illumination was kept constant but the interval between two 4 s flashes was varied, the resulting leaf weight increase was not uniform but depended on the length of the interval between flashes and showed a series of deep minima followed by sharp maxima. Weight increase at the maximum was 60% greater than at the minimum. The minimum-maximum transitions recurred every 45 s for at least 22 min. Four s of far red light interposed between the two red flashes abolished the oscillation. Temperature between 15 and 30°C had no significant influence on the period of the oscillation but the period varied directly with the duration of the initial red flash. A 2 s initial flash resulted in a 35 s period, while 40 s of red light caused a 2 min period. Oat (Avena sativa) and radish (Raphanus sativus) seedlings were found to possess similar oscillating mechanisms of growth regulation. The lack of pronounced temperature effects as well as our other findings, suggest that this oscillating system may participate in the time measuring as well as growth regulating mechanisms by which phytochrome controls circadian periodicity and development.     

SPECTRAL DEPENDENCE OF A SINGLE AND A SUBSEQUENT SECOND LIGHT PULSE INDUCING BARLEY LEAF UNFOLDING

The unfolding of etiolated barley leaves was induced by two short pulses of light separated by a dark interval. For certain wavelengths of light the effect of the “second light pulse was enhanced when the pulse was given after a 300–2000 s dark interval as compared with its effect when the two light pulses were given simultaneously. We investigated the spectral dependence of the effect of the first, inducing light flash and of the effect of the second Light flash given after a 500 s dark interval. The spectral actinity for the effect of the first flash showed phytochrome involvement. The spectral actinity for the effect of the second light flash, however, was shifted towards shorter wavelengths and the inductive action of red and far red light was attenuated.     

PHOTOTACTIC RESPONSE OF CHLAMYDOMON AS TO FLASHES OF LIGHT-I. RESPONSE OF CELL POPULATIONS

Abstract— Chlamydomonas reinhardi responds phototactically to a single, very short flash of blue light (6-4 μs). Net oriented response of a cell population is monitored photometrically, using the "population system" of Feinleib and Curry (1967). A single high-intensity flash elicits a small, but definite net movement away from the stimulus source. Repetitive flashing at low frequency (between 8 and 60 flashes per min) and at the same intensity elicits a prolonged response in the same direction. Net phototactic response to single or repetitive flashes varies with stimulus intensity in the same way as does response to continuous light (Feinleib and Curry, 1971b); response is positive at low intensity and negative at high intensity. These data indicate that at least some cells become oriented in response to a short flash. The occurrence of such a response has implications for the mechanism of phototactic orientation. If almost all the cells responded, one would assume that Chlamydomonas perceives light direction instantaneously by detecting an absorption gradient within the cell. Unequivocal interpretation of the short-flash response requires examination of the behavior of individual cells.     

COOPERATION OF CHARGES IN PHOTOSYNTHETIC O2 EVOLUTION–I. A LINEAR FOUR STEP MECHANISM

Abstract— Using isolated chloroplasts and techniques as described by Joliot and Joliot[6] we studied the evolution of O₂ in weak light and light flashes to analyze the interactions between light induced O₂ precursors and their decay in darkness. The following observations and conclusions are reported: 1. Light flashes always produce the same number of oxidizing equivalents either as precursor or as O₂. 2. The number of unstable precursor equivalents present during steady state photosynthesis is ∼ 1.2 per photochemical trapping center. 3. The cooperation of the four photochemically formed oxidizing equivalents occurs essentially in the individual reaction centers and the final O₂ evolution step is a one quantum process. 4. The data are compatible with a linear four step mechanism in which a trapping center, or an associated catalyst, ( S ) successively accumulates four + charges. The S ⁴⁺ state produces O₂ and returns to the ground state S ₀. 5. Besides S ₀ also the first oxidized state S ⁺ is stable in the dark, the two higher states, S²⁺ and S³⁺ are not. 6. The relaxation times of some of the photooxidation steps were estimated. The fastest reaction, presumably S *₁← S ₂, has a (first) half time ≤ 200 μsec. The S *₂ state and probably also the S *₀ state are processed somewhat more slowly (˜ 300–400 μsec).     

PHOTOCONVERSION OF PHYTOCHROME IN VIVO STUDIED BY DOUBLE FLASH IRRADIATION IN Mougeotia AND Avena

Abstract— The kinetics of phytochrome phototransformation from the red-absorbing form (P_r) to the far-red-absorbing form (P_fr) in vivo at 22°C were studied using a double flash apparatus with 1-ms flashes. Photoconversion by simultaneous flashes of red light saturates at a low P_fr level, indicating the possible attainment of a photoequilibrium between the excitation of P_r and the photoreversion of intermediates in the course of the I-ms flashes. At saturation energy, simultaneous flashes resulted in about 50% as much P_fr as was produced by saturating irradiation with 5 s red light. Intermediates of the phototransformation pathway were analysed by separating two red or a red and a far-red flash by variable dark intervals. In both plants phototransformation intermediates with half-lives < 1 ms occur, but they are too short-lived to characterize by our method. The subsequent intermediates have half-lives of about 7 ms and 150 ms in A vena , 2 ms and 10 ms in Mougeotia. The conversion from P_r to P_fr seems to be completed 1 s after the red flash in Avena. In the alga Mougeotia , P_fr formation seems to be finished within only 50 ms after the inducing red flash. The kinetics obtained from physiological and spectrophotometric experiments with Avena mesocotyls are almost identical. These observations indicate that the physiological response corresponds directly to the amount of P_fr produced and not to phototransformation intermediates or "cycling" between P_r and P_fr.     

CHARGE TRANSFER FROM PHOTOSYSTEM I TO THE CYTOCHROME b/f COMPLEX: DIFFUSION AND MEMBRANE LATERAL HETEROGENEITY

Abstract— We discuss here the minimum requirements for diffusion of a charge carrier between appressed and stroma-exposed membrane regions of chloroplasts based on recent models of the thylakoid membrane and flash-induced kinetic data. We have investigated the kinetics of the transfer of a positive charge from photosystem I to the cytochrome b/f complex in spinach chloroplasts by measuring the light-induced oxidation of cytochrome f. The rate and extent of cytochrome f oxidation were measured spectrophotometrically using either long actinic flashes that induced several turnovers of photosystem I or short actinic flashes that induced a single turnover of photosystem I. In the long actinic flashes, in the electron transfer reaction from water to methyl viologen, we observed the rapid oxidation of all of the cytochrome f present in the membrane. The half-time of the oxidation was 1.0 ± 0.1 ms. The total amount of the cytochrome was determined by chemical difference spectra to be one molecule of cytochrome f per 650 – 30 chlorophyll molecules. Using short actinic flashes we studied the photosystem I-driven electron transfer reaction from duroquinol to methyl viologen in the presence of the inhibitor 5-n-undecyl-6-hydroxy-4,7-dioxobenzothiazole. Under these conditions a single turnover flash induced the oxidation of 62 ± 5% of cytochrome f with a half-time of 240 ± 30 μs. An Arrhenius plot of the temperature dependence of the cytochrome f oxidation rate revealed an activation energy between 16 and 21 kJ/mol, a value consistent with a diffusion-controlled reaction. These kinetic data are considered in the context of two models of the thylakoid membrane.     

INDUCTION OF SEED GERMINATION IN Lactuca sativa L. BY NANOSECOND DYE LASER FLASHES*,†

Abstract— A 15 ns, tunable dye laser was used to induce germination of the photoblastic seeds of Lactuca sativa. One red laser flash in the range from 620 to 690 nm was sufficient to increase germination significantly above the dark level. Repeated flashes, however, were necessary to saturate the physiological response. The wavelength dependence for induction of germination differed for single and repetitive flashes. After saturating far-red irradiation, the effect of single-flash induction was a function primarily of the absorption spectrum of P_r. In addition, the establishment within the lifetime of a flash of a photochromic system between the red absorbing form of phytochrome (P_r) and the sum of photoreversible intermediate forms (ΣI₇₀₀) contributes to this wavelength dependence at high fluence rates. This photochromic system is assumed to be shifted significantly toward P_r by wavelengths 660 nm. Similarly, a strong double-flash effect, which is seen as an increase in effectiveness when a given total fluence is provided by two consecutive flashes rather than by one flash only, is restricted to those wavelengths that considerably shift the photochromic system P_r?ΣI₇₀₀ toward P_r. Finally, the saturation level produced by a series of laser flashes depends, additionally, on absorption by P_fr.     

SENSITIZED PHOTOOXYGENATION ACCORDING TO TYPE I MECHANISM (RADICAL MECHANISM) — PART II. FLASH PHOTOLYSIS EXPERIMENTS

Abstract— In order to gain further insight into the sensitized photooxygenation of the system thionine, allylthiourea, and oxygen, the influence of the leucothionine, which is formed during the photoreaction, was studied by flash photolysis. In the presence of leucothionine, additional thionine (Λ_obs= 598 nm) is reformed; i.e., leucothionine is oxidized to thionine by way of a semithionine intermediate (Λ_bs= 770 nm). This additional semithionine formation due to leucothionine is complete by 30 μsec after the flash. By varying the leucothionine concentration, the flash intensity and the pH it can be shown that the agent which oxidizes leucothionine to semithionine is identical to the agent which transforms semithionine to thionine.     

PHOTOCHEMISTRY OF THE REACTION CENTERS OF SYSTEM II UNDER REPETITIVE FLASH GROUP EXCITATION IN ISOLATED CHLOROPLASTS

Abstract— Absorption changes induced in isolated chloroplasts by excitation with repetitive flash groups have been measured at 690 nm, indicating the photochemical turnover of chlorophyll-a_II (Chl-αn), and at 480 nm and 513 nm respectively, reflecting via electrochromic effect the formation of a transmembrane electric field. The data are compared with measurements of oxygen evolution. In chloroplasts with practically fully intact oxygen evolving capacity it was found: 1. The initial amplitude of the 690 nm absorption change induced by the second flash as a function of the time t_v between the first and second flash of a group increases with a half life of about 35 µs. On the other hand, the average oxygen yield due to the second flash as a function of the time t_v rises with a half life of about 600 µs (and a kinetics in the ms-range of minor extent), confirming the data of Vater et al. (1968). 2. Under far red background illumination, where contributions due to PS I in the µs-range can be excluded, the difference spectrum in the red of the absorption changes induced by the first flash corresponds with that of the absorption changes induced by the second flash fired 200 µ after the first flash. 3. The pattern of the absorption changes at 690 nm induced by repetitive double flash groups at t_v= 200 µs does not markedly change in normal chloroplasts by the presence of DBMIB?. Similar 690 nm absorption changes occur in trypsin treated chloroplasts, independent of the presence of DCMU. 4. The fast regeneration in the µs-range of Chl-a_n is also observed in the third flash of a triple flash group at a time t_v= 200 µs between the flashes of the group. 5. The initial amplitudes of the absorption changes with a decay kinetics slower than 100 µs induced by the second flash at 480 nm and 513 nm, respectively, as a function of the time t, between the first and second flash of a group, are characterized by a recovery half-time of about 600 µs, confirming earlier measurements at 520 nm (Witt and Zickler, 1974). On the basis of these results it is inferred that there does exist a photoreaction of Chl-α_n., with an electron acceptor, referred to as X_a, other than the ‘primary’ plastoquinone acceptor X320, if X320 persists in its reduced state. Under conditions of X320 being in the reduced state, this photochemical reaction was shown to be highly dissipative with respect to charging up the watersplitting enzyme system Y. Furthermore, this Chl-a_n-photoreaction with X_a does not lead to a vectorial transmembrane charge separation, which is stable for more than a few microseconds. Different models for the functional and structural organization of PS II are discussed.     

Flash-induced blocking of the high-affinity manganese-binding site in photosystem II by iron cations: dependence on the dark interval between flashes and binary oscillations of fluorescence yield

Incubation of Fe(II) cations with Mn-depleted PSII membranes (PSII(-Mn)) under weak continuous light is accompanied by blocking of the high-affinity, Mn-binding (HAZ) site with ferric cations (Semin, B.K. et al. Biochemistry 2002, 41, 5854-5864). In this study we investigated the blocking yield under single-turnover flash conditions. The flash-probe fluorescence method was used to estimate the blocking efficiency. We found that the yield of blocking increases with flash number and reaches 50% after 7 flashes. When the dark interval between the flashes (Delta t) was varied, we found that the percentage of blocking decreases at Delta t < 100 ms (t 1/2, 4-10 ms). No inhibition of the blocking yield was found at longer time intervals (as with photoactivation). This result shows the necessity of a dark rearrangement during the blocking process (the dual-site hypothesis described in the text) and indicates the formation of a binuclear iron center. During the blocking experiments, we found a binary oscillation of the Fmax elicited during a train of flashes. The oscillations were observed only in the presence of Fe(II) cations or other electron donors (including Mn(II)) but not in the presence of Ca2+. Chelators had no effect on the oscillations. Our results indicate that the oscillations are due to processes on the acceptor side of PSII and to the appearance of "acceptor X" after odd flashes. Acceptor X is reduced by QA- at very high rate (<2 ms), is not sensitive to DCMU, and is rather stable in the dark (t l/2 approximately 2 min). These properties are similar to those of nonheme Fe(III) (Fe(III)NHI). When Fe(II)NHI was oxidized with ferricyanide (Fe(CN)6), the fluorescence decay kinetics and yield of fluorescence were identical to those observed when the sample was exposed to 1 flash prior to the fluorescence measurement. We suggest that acceptor X is Fe(III)NHI, oxidized by the semiquinone form of QB-. This is similar to the mechanism of "reduction-induced oxidation of Fe(II)NHI" by exogenous quinones reported in the literature. We suggest that involvement of QB- in the oxidation of Fe(II)NHI in PSII(-Mn) membranes is due to the modification of the QB-binding site and increase of its redox potential resulting from extraction of the functional Mn cluster.     

THE IN VITRO PHOTOCHEMISTRY OF BIOLOGICAL MOLECULES-III. ABSORPTION SPECTRA, LIFETIMES AND RATES OF OXYGEN QUENCHING OF THE TRIPLET STATES OF β-CAROTENE, RETINAL AND RELATED POLYENES

Abstract— The triplet-triplet absorption spectra of six polyenes have been characterised using flash photolysis, in the presence of anthracene as sensitizer, and pulse radiolysis, in the absence of a sensitizer. The polyenes include several which contain carbonyl groups whose triplet states, unlike retinal , could not be detected unsensitized by flash photolysis. The triplet lifetimes appear to be a function of the number of conjugated double bonds, and vary between 7 and 14 μ sec. In general, the longer the polyene, the shorter the lifetime. An empirical linear relation was found between the frequencies of the polyene triplet-triplet absorption maxima, and the frequencies of the corresponding ground singlet-singlet maxima. The rate constants for quenching by oxygen of nine polyene triplet states were determined to lie in the range 2–7 × 10⁹ M ^-1 sec^-1. The possible mechanisms for oxygen quenching of triplet states are discussed and analogies between the results for oxygen quenching of polyenes and of polyacenes are drawn. The rate constant for oxygen quenching of all- trans -β-carotene triplet was the same in benzene and hexane.     

STUDIES OF SYSTEM II PHOTOCENTERS BY COMPARATIVE MEASUREMENTS OF LUMINESCENCE,FLUORESCENCE, AND OXYGEN EMISSION*

Abstract— New results are presented on the emission of oxygen by algae and chloroplasts illuminated by a sequence of short saturating flashes. These results favor the four-state hypothesis of Kok and co-workers, in which formation of oxygen requires the accumulation of four oxidants produced by four successive photoreactions. Deactivation of the more oxidized precursor states in the dark is studied under different conditions of preillumination. Our results suggest that both a one step and a two step mechanism of deactivation exist. In order to understand the biological significance of Kok's parameter α—the fraction of photochemical centers unable to react on each flash (“misses”)-we study reoxidation of acceptor Q after one flash by fluorescence techniques. It appears that a fraction of Q^- is reoxidized by a back reaction which cancels the effect of the preilluminating flash and is probably responsible for the misses. The results of some luminescence experiments are also reported. These experiments demonstrate that delayed emission of light is associated with the deactivation of states S₂ and S₃. It is possible that excitons produced by deactivation can be reabsorbed by active photochemical centers, which can modify considerably the deactivation process.     

THE QUANTUM EFFICIENCY OF DYE-INDUCED PHOTOELECTRIC EFFECTS IN BILAYER MEMBRANES

Abstract— Voltage transients are generated across lipid bilayer membranes by light flashes as a result of photophysical processes in sorbed dyes which displace electrical charges. A theory is presented which indicates that: (i) the fraction of sorbed dye which displaces charge from one flash can be determined by the fractional reduction in the photovoltage amplitude resulting from a second and identical flash, providing the second flash occurs before dye excited by the first flash returns to its equilibrium condition. (ii) The photoeffect quantum efficiency can be determined from the fraction of dye displacing charge, the light intensity and the dyes' optical absorption cross section. Apparatus constraints required different experimental procedures for dyes with different excited state life times, which are discussed. Experimental results are presented for an azo dye, 3,3'-bis(α-(trimethyiammonium)methyl)azobenzenebromide (Bis-Q), three carbocyanine dyes in the series 3,3'-dimethyl-2,2'-oxacarbocyanine-iodide (diO-C₁-3), an amino-pyridinium dye, 4-( p -(dimethyl-amino)styryl)-1-rnethyl-pyridinium-iodide (di-1-ASP), and a xanthene dye, 2',4',5',7'-tetraiodofluorescein (erythrosin), the sodium salt of which is known as F, D and C red number 3. The dyes' optical absorption cross section values are uncertain owing to solvent and orientational effects in membranes. Photoeffect quantum efficiency values obtained by calculating optical absorption cross sections from the dyes' molar extinction coefficients in aqueous solutions are: Bis-Q (0.08), diO-C₁-3 (0.31), diO-C₂-3 (0.22), diO-C₅-3 (0.08), di-l-ASP (0.3) and erythrosin (0.39).     

RHYTHMIC OSCILLATION OF CYCLIC 3',5'AMP AND -GMP CONCENTRATION AND STIMULATION OF FLOWERING BY CYCLIC 3',5'-GMP IN Lemna paucicostata 381

Abstract— Rhythmic oscillation of the concentration of cyclic 3',5'-AMP and -GMP in a short day plant, Lemna paucicostata 381 in continuous darkness was detected after 2 cycles of 12 h dark and 12 h light entrainment. Cyclic 3',5'-AMP and -GMP, extracted from whole plant showed parallel oscillations in their concentrations for initial 36 h in continuous darkness and the oscillation in the concentration of cyclic 3',5'-AMP was roughly circadian. Their concentrations decreased during the initial 12 h (subjective night) and increased during 12 to 28 h. Exogenous addition of 2 μ.M of cyclic 3',5'-GMP or the dibutyryl derivative of it stimulated floral induction by 20 to 30%, when the plants were grown under 12 h light and 12 h dark regime. Cyclic 3',5'-AMP or the dibutyryl derivative of it showed little effect on flowering.     

STOICHIOMETRY and KINETICS OF PROTON RELEASE UPON PHOTOSYNTHETIC WATER OXIDATION

Abstract— We studied the magnitude and the rise kinetics of proton release into the interior of thylakoids by flash spectrophotometty with neutral red as pH indicator. Excitation of dark-adapted thylakoids by a series of between 4 and 11 flashes produced a complex pattern of proton release into the thylakoid lumen. Proton release upon each flash was time resolved.
A slow component of proton release oscillated weakly in magnitude with period of two as function of flash number. It exhibited a half-rise time of approximately 20 ms from the very first flash on, and it was abolished by inhibitors of plastohydroquinone oxidation. This component was attributed to the oxidation of plastohydroquinone by PS I via the Cytb₆/f complex.
Additionally, rapid and multiphasic proton release was observed with half-rise times of less than 2 ms which exhibited a pronounced and damped oscillation with period of four as function of flash number. This rapid proton release was attributed to water oxidation. A detailed kinetic analysis suggested that proton release occurred with the following stoichiometry and with the following half-rise times during the transitions S₁ S_i+1 of water oxidation: 1 H⁺(250 μs, S₀→₁): 0 H⁺(S₁→ S₂):1 H⁺(200 μs, S₂→S₃):2 H⁺(1.2 ms, S₃→ S₄→ S₀) . Proton release and proton rebinding upon oxidation and reduction of the intermediate electron carrier Z, respectively, may have influenced the kinetics of the respective proton yields but not the stoichiometric pattern.     

THE LIGHT REQUIREMENT OF ACID-BASE TRANSITION INDUCED LUMINESCENCE OF CHLOROPLASTS

Abstract— The luminescence that occurs when chloroplasts are taken from an acid environment to a basic one is shown to be dependent on prior illumination of the chloroplasts. The relation between the light absorbed and luminescence is given by the following equation L = L _max(1-e ^al where L and L _max are the light emitted and maximum light emission at high flash energy, respectively, J quanta absorbed per chlorlphyll molecule, and α a constant with a value of approximately 200 chlorophyll molecules per quanta absorbed. The action spectrum of the luminescence is consistent with that of photosystem II. The metastable state formed during illumination decays in the dark via a temperature dependent second order process.     

Energy transfer between photosynthetic units analyzed by flash oxygen yield vs. flash intensity

Abstract— Relative yield of O₂ (Y) was measured in Chlorella pyrenoidosa in response to varied intensity (l) of single 10μsec flashes on a constant low background of 710 nm light. Analysis is based on the proposition that the photochemical event leading to O₂ evolution occurs at a reaction center or trap which requires a time much longer than the flash for regeneration by dark reactions. Hence O₂, flash yield measures the number of traps ‘killed’ and allows treatment in terms of target theory. Data for Y vs. l were analyzed by computer fitting to four models. The first three models supposed that each unit (aggregate of light-harvesting pigment molecules) contains one, two, and three traps, respectively, and allows no transfer of excitation energy out of the unit. The last model supposed only one trap per unit and a probability of transfer out of a unit with closed trap. Among the first three models, the data best fit the one with two traps per unit. A slightly better fit for two traps per unit was obtained by introducing a trapping efficiency less than unity. An equally good fit was also obtained with the model of the Joliots with a probability of 0.3 that excitation energy in a unit with closed trap is transferred to another unit. Uncertainties in analysis arose from the necessity of treating maximum flash yield as an estimated parameter and by the possible inhomogeneity in units and traps.