BLUE-LIGHT INDUCED DEVELOPMENT OF CHLOROPLASTS IN ISOLATED SEEDLING ROOTS. PREFERENTIAL SYNTHESIS OF CHLOROPLAST RIBOSOMAL RNA SPECIES

Excised roots of pea seedlings (Pisum sativum var. “Alaska”) cultured in a synthetic medium under sterile conditions exhibit differentiation of functional chloroplasts from leucoplasts when irradiated with blue light (350–550 nm). This transition is a relatively slow process; nevertheless, the chloroplasts formed in blue light compare very well to leaf chloroplasts as far as microstructure and photosyn-thetic activities are concerned. Apparently certain activities of the apical meristem are mandatory in bringing about a transition from leucoplasts to chloroplasts in blue light. After short-time labelling with [^jH]uridine the synthesis of plastid ribosomal RNA (rRNA) was studied either during irradiation with blue and red light (600–700 nm), respectively, or in darkness. Polyacrylamide gel electrophoresis revealed that in blue light the synthesis of specific chloroplast rRNA species with molecular weights of 1.1 × 10⁶ and 0.56 × 10⁶ daltons is markedly stimulated. In contrast, in dark cultured roots these RNA species were synthesized to a limited extent only whereas the cytoplasmic rRNA species of 1.3 × 10⁶ and 0.7 × 10⁶ daltons molecular weight were preferentially formed. The same holds true for roots irradiated with red light.     

RELATIONSHIP BETWEEN LIGHT QUALITY DEPENDENCE AND IRRADIANCE ADAPTATION OF PHOTOSYNTHESIS IN Acetabularia mediterranea*

Characteristic differences in the light intensity curves of photosynthesis after growth of cells of Acetabularia mediterranea Lamour. (A. acetabulum (L.) Silva) in weak and strong white light were similar to those for red and blue light-treated cells, respectively. This indicated that responses to white light quantity and those to light quality might be causally related. Small differences in the thylakoid polypeptide composition of cells grown in high and low intensities of white light were not significant and thus did not help to clarify whether the adaptations to blue or red light, respectively, were the same. When the red to blue-light ratio was varied, keeping the total photon fluence rate constant, the photosynthetic capacity (red light saturated O₂-production) was dependent on blue light irradiance in a logarithmic fashion. The specific influence of red light was not detectable, indicating that only blue light was effective for light irradiance adaptation in Acetabularia. The situation was different, at least for a transient period, when adaptation to light irradiance was allowed to proceed from a low photosynthetic activity after preirradiation of the cells with prolonged red light. The effect of low white light irradiances was pronounced, causing a maximum increase of photosynthetic activity within 3 days. The response to blue light was enhanced as well, and a very low photon irradiance added to continuous red light caused a change of the same order as that produced by high irradiances of blue light alone. This elevated action of low intensity white and blue light is most likely due to increased metabolite supply derived from the degradation of starch enhanced by this light quality. Therefore, photosynthetic effectiveness in Acetabularia is regulated by the irradiance of blue light and by feedback via photosynthetic products.     

INTERACTION BETWEEN PHYTOCHROME AND THE BLUE LIGHT PHOTORECEPTOR SYSTEM IN Mougeotia*

Abstract— Face-to-profile chloroplast movement in Mougeotia was induced by sequences of strong blue and red short irradiations. This type of response occured only when blue light was applied prior to or simultaneously with red light, and far-red irradiation was necessary after the sequence to cancel the remaining gradient of the far-red absorbing form of phytochrome P_fr. The dependence of the response magnitude on blue and red light sequences was studied for a wide range of light durations and dark intervals. The relationship between the response and the dark interval points to the lack of direct coupling between phytochrome and blue-absorbing “cryptochrome”. It was postulated that a photoproduct having a life-time of2–3 min is formed by the blue-light-mediated reaction. This photoproduct interacts with phytochrome during its transformation or with its final P_fr form.     

BLUE AND WHITE LIGHT EFFECTS ON CHLOROPLAST DEVELOPMENT IN A SOYBEAN MUTANT

Phenotypic difference for chloroplast development between the normal green (CL1) and the Cy₉y₉ soybean mutant was observed when the plants were grown under 18W m^?2 white or blue light. Under these conditions the mutant soybean accumulated less Chi b, neoxanthin, carotene and less total pigment than the CL1 genotype. Chloroplasts of the Cy₉y₉ line were deficient in the LHP complex relative to that of chloroplasts from the normal soybean. Specific differences were noted between chloroplasts from plants grown under blue and white light. Accumulations of a 34 kD (PSII) and a 16–17 kD (PSI) membrane polypeptide were decreased by blue light in both soybean genotypes. Blue light induced a greater accumulation of a 32 kD (PSII) polypeptide than white light. Blue light reduced granal thylakoid stacking and increased the proportion of stroma thylakoids compared to those that developed under white light. PSI electron transport activity was stimulated by the blue light treatment more than that of PSII.     

INVOLVEMENT OF THYLAKOID MEMBRANE-DEPENDENT PHOTOSENSITIZATION IN PHOTOINHIBITION OF THE CALVIN CYCLE ACTIVITY IN SPINACH CHLOROPLASTS

Photoinhibition of the light-regulated key enzymes of the photosynthetic carbon reduction (PCR) cycle was investigated using chloroplasts isolated from spinach leaves. Light quality dependence of the light-induced activity change (activation or inactivation) of key PCR enzymes in situ demonstrated that, while light activation is promoted mainly by red light (Λ.> 600 nm), inactivation takes place largely in the region of blue light (Λ < 500 nm). Inactivation was suppressed by a lipid soluble singlet oxygen (¹O_2,¹Δ_g) quencher. When “stromal protein” was subjected to a severe photoinhibitory treatment, no significant loss of activity was observed for any PCR enzyme assayed. However, the inclusion of thylakoids in the photolysis system resulted in a substantial inactivation of the enzymes; this inactivation was significantly diminished in the presence of imidazole and enhanced to some extent by a partial deuteration of medium. In contrast, superoxide dismutase did not exert any effect. The blue light-induced inactivation of the enzymes was remarkably decreased in the presence of thylakoids whose Fe-S centers were destroyed. The results obtained in this study suggest that photoinactivation of the PCR enzymes in situ is mediated mainly by ¹O₂, which is photoproduced primarily by the Fe-S centers of thylakoids and diffuses into the stroma.     

ORIENTATION MOVEMENTS OF CHLOROPLASTS IN Vallisneria EPIDERMAL CELLS: DIFFERENT EFFECTS OF LIGHT AT LOW- and HIGH-FLUENCE RATE*

Abstract— Epidermal cells of Vallisneria gigantea have a large central vacuole which is surrounded by a thin layer of cytoplasm. The chloroplasts are distributed over all six cytoplasmic layers of an approximate cuboid. In low-intensity light, the accumulation of chloroplasts in the side facing the outer periclinal wall (the P side) continues for several hours. Red light (650 nm) shows the highest effect and induces such an accumulation even at a fluence rate of only 0.02 W/m². In response to high-intensity light, the chloroplasts move to the sides that face the anticlinal walls (the A sides) within a few tens of minutes. Blue light (450 nm) is most effective in inducing this movement. At a fluence rate of 1.51 W/m², the reaction is induced in only half of the specimens. Neither red nor blue light can induce any orientation movement in the presence of 100 μg/ml of cytochalasin B. The chloroplast movements in the P side have been examined with a time-lapse video system. When cells, in which the chloroplast accumulation has been completed after red-light irradiation, are subsequently irradiated with blue light, the rapid movement of chloroplasts to A sides is induced. However, a considerable number of chloroplasts remains in the center of the P side. The same is true of cells in which the chloroplasts have not accumulated in the P side because of cytochalasin B treatment during red-light irradiation, when such cells are irradiated with blue light after removal of the drug. Some anchoring mechanism seems to work in low-intensity light to render the chloroplasts immobile in the P side.     

CHLOROPLAST PIGMENTS and THEIR BIOSYNTHESIS IN RELATION TO LIGHT INTENSITY*

Abstract— Depending on the light intensity that they received during growth, radish seedlings altered not only the pigment and quinone composition of the thylakoid membrane but also the chloroplast ultrastructure. In strong light, sun chloroplasts of radish were very similar to those from sun leaves of beech trees, while those developed under under dim light possessed a typical shade chloroplast. Radish shade chloroplasts contained a higher chlorophyll content and a higher concentration of xanthophylls resulting in a lower xanthophyll to carotene ratio as compared to sun chloroplasts. Chloroplasts from radish grown in strong light showed a much higher activity in their terpenoid metabolism than plastids from shade plants. Chlorophylls and carotenoids which are involved in the absorption of light and the transfer of energy during photosynthesis were labeled by [³H]-mevalonate to a much higher degree in plastids from sun leaves as compared to plastids from shade leaves. This shows that in strong light where pigments are continuously broken down and resynthesized in order to maintain photosynthesis, chlorophylls and carotenoids exhibit a much higher turnover rate than the pigments of shade plants.     

FIBER-OPTIC PLANT TISSUES: SPECTRAL DEPENDENCE IN DARK-GROWN AND GREEN TISSUES*

Abstract— The fiber-optic properties of etiolated plant tissues can be used to detect and characterize pigment absorption in vivo. Transmission spectra of light guided through several monocot and dicot etiolated tissues show a decreasing red/far red ratio with increasing tissue length. Absorption bands attributable both to vacuolar pigments such as anthocyanins and to chloroplast pigments lead to the conclusion that the guided light passes through both vacuole and cytoplasm. As etiolated tissue becomes green under white light treatment, the red/far red ratio also changes, the nature of the change depending upon the tissue involved. The blue/red ratio also changes both with increasing length of etiolated tissue and during the greening process, with the changes again dependent on the tissue involved. The spectral dependence of the light-guiding phenomenon in dark grown and green plants may have implications for physiological responses mediated by phytochrome.     

LIGHT-ABSORPTION AND ELECTRON-TRANSPORT BALANCE BETWEEN PHOTOSYSTEM II AND PHOTOSYSTEM I IN SPINACH CHLOROPLASTS

Abstract— The distribution of absorbed light and the turnover of electrons by the two photosystems in spinach chloroplasts was investigated. This was implemented upon quantitation of photochemical reaction centers, chlorophyll antenna size and composition of each photosystem (PS), and rate of light absorption in situ. In spinach chloroplasts, the photosystem stoichiometry was PSIIJPSII_α/PSII_β/PSI= 1.3/0.4/1.0. The number (N) of chlorophyll (a+b) molecules associated with each PS was N(PSII_α)/N(PSII_β)/N(PSI)=230/100/200, i.e. about 65% of all Chl is associated with PSII and about 35% with PSI. Light absorption by PSII in vivo is selectively attenuated at the molecular, membrane and leaf levels, (a) The rate of light absorption by PSII was only 0.85 that of PSI because of the lower rate of light absorption by Chl b as compared to Chl a (approximately 80% of all Chl b in the chloroplast is associated with PSII). (b) The exclusive localization of PSII_α in the membrane of the grana partition regions and of PSI in intergrana lamellae resulted in a differential “sieve effect” or “flattening of absorbance” by the photosystems in the two membrane regions. Due to this phenomenon, the rate of light absorption by PSII was lower than that of PSI by 15-20%. (c) Selective filtering of sunlight through the spinach leaf results in a substantial distortion of the effective absorbance spectra and concomitant attenuation of light absorption by the two photosystems. Such attenuation was greater for PSII than for PSI because the latter benefits from light absorption in the 700-730 nm region. It is concluded that, in spite of its stoichiometric excess in spinach chloroplasts, light absorption by PSII is not greater than that by PSI due to the different molecular composition of the two light-harvesting antenna systems, due to the localization of PSII in the grana, and also because of the light transmission properties through the leaf. The elevated PSII/PSI reaction center ratio of 1.7 and the association of 65% of all Chl with PSII help to counter the multilevel attenuation of light absorption by PSII and ensure a balanced PSII/PSI electron turnover ratio of about 1:1.     

Photosynthesis Assessment in Microphytobenthos Using Conventional and Imaging Pulse Amplitude Modulation Fluorometry

Imaging pulse amplitude modulated (Imaging‐PAM) fluorometry is a breakthrough in the study of spatial heterogeneity of photosynthetic assemblages. However, Imaging and conventional PAM uses a different technology, making comparisons between these techniques doubtful. Thereby, photosynthetic processes were comparatively assessed using conventional (Junior PAM and PAM 101) and Imaging‐PAM on intertidal microphytobenthos (MPB; mud and sand) and on cork oak leaves. Lower values of α (initial slope of the rETR, relative photosynthetic electron transport rate) vs E (incident photosynthetic active radiation) curve), ETR_max (maximum relative ETR), E_k (light saturation parameter) and F_v/F_m (maximum quantum efficiency of photosystem II of dark‐adapted samples) were obtained using the Imaging‐PAM. The level of discrepancy between conventional and Imaging‐PAM systems was dependent on the type of sample, being more pronounced for MPB muddy sediments. This may be explained by differences in the depth integration of the fluorescence signal related to the thickness of the photosynthetic layer and in the light attenuation coefficients of downwelling irradiance. An additional relevant parameter is the taxonomic composition of the MPB, as cyanobacteria present in sandy sediments rendered different results with red and blue excitation light fluorometers. These findings emphasize the caution needed when interpreting chlorophyll fluorescence data of MPB communities.     

BLUE LIGHT EFFECT ON CHLOROPHYLL FORMATION IN CHLORELLA PROTOTHECOIDES

The effectiveness of monochromatic light on chlorophyll formation over a range of 420–80 nm was examined in the regreening cells of Chlorella protothecoides in the presence of chloro-phenyl dimethylurea (CMU). An action spectrum showed two maxima with a minimum near 450 nm. At the most effective wavelength examined (444 nm), 0.2 W. m ^-2 was sufficient for half saturation. The activity of 5-aminolevulinic acid (ALA) production was examined in the greening cells under irradiation with white, blue and red light. The activity was always limited by availability of substrate, especially in the case where the greening cells were incubated with cycloheximide or transferred to darkness. Decay of the activity in these cases was delayed by provision of organic compounds such as glycine, pyruvate or glucose. The effectiveness of blue light on ALA* production observed in the presence of CMU was inferred to be brought about by the increased availability of endogenous substrates.     

ACTION SPECTRA FOR LIGHT-INDUCED DE-EPOXIDATION AND EPOXIDATION OF XANTHOPHYLLS IN SPINACH LEAF*

Abstract— The action spectra for violaxanthin de-epoxidation and zeaxanthin epoxidation in New Zealand spinach leaf segments, Tetragonia expansa, were determined at equal incident quanta of 2·0 × 10¹⁵ quanta cm^-2 sec^-1. Precise action spectra were not obtained due to variable leaf activity. The de-epoxidation action spectrum had major peaks at approximately 480 and 648 nm. Blue light was slightly more effective than red light and little activity was observed beyond 700 nm. The epoxidation action spectrum showed major peaks at around 440 and 670 nm. Blue light was more effective than red light and light beyond 700 nm showed definite activity. The net result of de-epoxidation and epoxidation is a cyclic scheme, the violaxanthin cycle, which consumes O₂ and photoproducts. The action spectra indicate that the violaxanthin cycle is more active in blue than in red light and therefore could account for O₂ uptake stimulated by blue light. However, the violaxanthin cycle is not the pathway for O₂ uptake by photosynthetic system 1. It was suggested that the violaxanthin cycle may function as a pathway for the consumption of excess photoproducts generated in blue light or the conversion of these photo-products to other forms of energy.     

LIGHT REQUIREMENTS FOR THE ENHANCED SYNTHESIS OF A PLASTID mRNA DURING SPIRODELA GREENING

A plastid mRNA (5 × 10⁵ mol wt) appears as a burst 3 h after white light greening of steady state dark grown plants of Spirodela oligorrhiza. In this species, chlorophyll synthesis begins after 12 h. The light requirement is different from the pulse of far-red reversible red light required to abolish the lag of chlorophyll synthesis in many species, including Spirodela. Continuous high energy far-red is not stimulatory. When the illumination is not continued throughout the time of incorporation, the stimulation is minimal. Low energy blue and red light are stimulatory, and green and far-red light are ineffectual. Blue light was > 5 times as effective as red light at many dose levels. Illumination with 3 × 10¹⁷ quanta/m²/s (50pEm/cm²/s) blue light at 476 nm gave about half maximum stimulation.     

IDENTITY OF THE PHOTORECEPTORS FOR THE PERCEPTION OF IRRADIANCE IN PHOTOSYNTHETIC LIGHT ACCLIMATION IN TOMATO*

The photoreceptors involved in the photosynthetic acclimation of tomato (Lycopersicon esculentum Mill.) to increased irradiance were investigated. Plants were transferred from 100 p.mol m^?2 s^?1 cool white fluorescent light to higher irradiances of white light or white light supplemented with blue, red, green or yellow light. In these experiements light of all wavelengths tested was capable of causing acclimation as measured by the rate of light-saturated photosynthesis. It was concluded that the photosynthetic system rather than the blue-absorbing photoreceptor or phytochrome system acts as the photoreceptor for increased irradiance. No acclimation was observed in response to increased CO₂ levels, but increasing light integral at a constant irradiance was effective in bringing about acclimation. We conclude that acclimation is a response to increased photosynthetic light capture rather than increased photosynthetic carbon fixation, and involves a photon counting mechanism.     

THE STIMULATING EFFECT OF A COLD, DARK PRETREATMENT ON THE ETIOPLAST/CHLOROPLAST TRANSFORMATION OF ANGIOSPERMS–II. SYNERGISM BETWEEN RED-LIGHT and COLD PRETREATMENT ON THE CO2- and O2-GAS-EXCHANGE OF WHEAT LEAVES

Abstract— Recently we reported on the stimulating effect of a cold, dark pretreatment on the processes of greening of dark-grown angiosperms under continuous white light (Schönbohm et al., 1988; Physiol. Plant. 72 ,541–546). These effects could be nullified by a subsequent second dark phase (25°C) which precedes the white light period. Our analysis was now focused on the effect of cold, dark pretreatments (with or without red-preirradiation) on the gas exchange (O₂; CO₂) of etiolated primary leaves of wheat during a subsequent white light period. The following results were obtained: (1) The net-O₂-consumption under continuous white light decreased much more quickly after a cold than after a warm pretreatment. (2) This effect was enhanced by red-preirradiation. (3) The O₂-compensation point was reached more quickly during the de-etiolation period when the leaves had been subjected to a cold instead to a warm pretreatment. (4) The “cold-effect” could be nullified by a subsequent warm, dark pretreatment (in this material the compensation point could not be reached within 8 h). (5) Cold treated material which had also been exposed to red light showed during the first 30 min of de-etiolation an extremely strong ^“out-burst” of CO₂. No correlation was apparent between the O₂-uptake and this high CO₂-release. (6) A cold, dark pretreatment induces a decrease of CO₂-release during the second half of the de-etiolation period. This effect could be nullified by a secondary warm, dark treatment given after the cold, dark pretreatment. Our experiments indicate that red-preirradiation and cold, dark pretreatment stimulate chlorophyll synthesis (Schonbohm et al., 1988) and also photosynthetic O₂-evolution and CO₂-uptake. We also assume that this pretreatment causes CO₂ release that is neither directly related to the respiratory electron transport chain nor to the photorespiration.     

BASF 13.338 INDUCED CHANGES IN STRUCTURE and FUNCTION OF THE PHOTOSYNTHETIC APPARATUS OF WHEAT SEEDLINGS

Abstract— Growing wheat seedlings in the presence of BASF 13.338 [4-chloro-5-dimethylamino-2-phenyl-3(2H)pyridazinone], a PS II inhibitor of the pyridazinone group, brought about notable changes in the structure and functioning of photosynthetic apparatus. In BASF 13.338 treated plants, there was a decrease in the ratio of Chi a/Chl b, an increase in xanthophyll/carotene ratio and an increase in the content of Cyt b 559 (HP + LP). Chl/p700 ratio increased when measured with the isolated chloroplasts but not with the isolated PS I particles of the treated plants. The SDS-PAGE pattern of chloroplast preparations showed an increase in the CPII/CP I ratio. The F685/F740 ratio in the emission spectrum of chloroplasts at -196°C increased. The difference absorption spectrum of chloroplasts between the control and the treated plants showed a relative increase of a chlorophyll component with a peak absorption at 676 nm and a relative decrease of a chlorophyll component with a peak absorption at 692 nm for the treated plants. The excitation spectra of these chloroplast preparations were similar. Chloroplasts from the treated plants exhibited a greater degree of grana stacking as measured by the chlorophyll content in the 10 K pellet. The rate of electron transfer through photosystem II at saturating light intensity in chloroplast thylakoids isolated from the treated plants increased (by 50%) optimally at treatment of 125 μM BASF 13.338 as compared to the control. This increase was accompanied by an increase in (a) I₅₀ value of DCMU inhibition of photosystem II electron transfer; (b) the relative quantum yield of photosystem II electron transfer; (c) the magnitude of C550 absorbance change; and (d) the rate of carotenoid photobleaching. These observations were interpreted in terms of preferential synthesis of photosystem II in the treated plants. The rate of electron transfer through photosystems I and through the whole chain (H₂O → methyl viologen) also increased, due to an additional effect of BASF 13.338, namely, an increase in the rate of electron transfer through the rate limiting step (between plastoquinol and cytochrome f). This was linked to an enhanced level of functional cytochrome f. The increase in the overall rate of electron transfer occurred in spite of a decrease in the content of photosystem I relative to photosystem II. Treatment with higher concentrations (> 125 μM) of BASF 13.338 caused a further increase in the level of cytochrome f, but the rate of electron transfer was no greater than in the control. This was due to an inhibition of electron transfer at several sites in the chain.     

Optically Matched Semiconductor Quantum Dots Improve Photophosphorylation Performed by Chloroplasts

A natural–artificial hybrid system was constructed to enhance photophosphorylation. The system comprises chloroplasts modified with optically matched quantum dots (chloroplast–QD) with a large Stokes shift. The QDs possess a unique optical property and transform ultraviolet light into available and highly effective red light for use by chloroplasts. This favorable feature enables photosystem II contained within the hybrid system to split more water and produce more protons than chloroplasts would otherwise do on their own. Consequently, a larger proton gradient is generated and photophosphorylation is improved. At optimal efficiency activity increased by up to 2.3 times compared to pristine chloroplasts. Importantly, the degree of overlap between emission of the QDs and absorption of chloroplasts exerts a strong influence on the photophosphorylation efficiency. The chloroplast–QD hybrid presents an efficient solar energy conversion route, which involves a rational combination of a natural system and an artificial light‐harvesting nanomaterial.     

A LINEAR DICHROISM STUDY USING CHLOROPLASTS OF VARIOUS STRUCTURE and PIGMENT COMPOSITION

Abstract— The orientation of pigments was studied in maize chloroplasts of different granum content and pigment composition. Comparison of the linear dichroism (LD)t of granal and agranal chloroplasts exhibited differences in the chl a forms, Ca 670 and Ca 691. The chl b content of the chloroplasts appeared to have little effect on the LD spectra.
In the LD spectra of carotenoid deficient mutant chloroplasts, the red band corresponding to oriented Ca 678 was considerably smaller than in those of normal chloroplasts. LD spectra of lycopenic chloroplasts revealed a band at 740 nm not present in the spectra of normal chloroplasts. Various signals of carotenoids observed in the LD spectra of carotenoid deficient mutant chloroplasts imply that the different carotenoids can be oriented in a different manner in the photosynthetic membrane.     

Artificial photosynthesis by using chloroplasts from spinach adsorbed on a nanocrystalline TiO2 electrode for photovoltaic conversion

Photovoltaic conversion has been achieved by use of chloroplasts (photosynthetic organs) from spinach adsorbed on a nanocrystalline TiO₂ film on an indium tin oxide (ITO) glass electrode (chloroplast/TiO₂ electrode). The shape of the absorption spectrum of the chloroplast/TiO₂ electrode is almost the same that of a dispersion of the chloroplasts. Absorption maxima of the chloroplast/TiO₂ electrode observed at 430, 475, and 670 nm were attributed to carotenoid and chlorophyll molecules, suggesting that chloroplasts have been adsorbed by the nanocrystalline TiO₂ film on the ITO electrode. The photocurrent responses of chloroplast/TiO₂ electrodes were measured by using a solution of 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile as redox electrolyte in the presence or absence of water and 100 mW cm^?2 irradiation. The photocurrent of the chloroplast/TiO₂ electrode was increased by adding water to the redox electrolyte. The photocurrent responses of chloroplast/TiO₂ electrodes irradiated with monochromatic light (680 nm, the absorption band of photosystem II complexed with evolved oxygen) were measured by use of a solution of 0.1 M tetrabutylammonium hexafluorophosphate in acetonitrile as redox electrolyte in the presence or absence of water. A chloroplast/TiO₂ electrode photocurrent was observed only when the redox electrolyte containing water was used, indicating that the oxygen evolved from water by photosystem II in chloroplasts adsorbed by a nanocrystalline TiO₂ film on an ITO electrode irradiated at 680 nm is reduced to water by the catalytic activity of the platinum electrode. The maximum incident photon-to-current conversion efficiency (IPCE) was 0.8 % on irradiation at 670 nm.     

CINETIQUE DES ECHANGES D'OXYGENE ET DE LA FLUORESCENCE DES CHLOROPLASTES ISOLES

Abstract Oxygen evolution and fluorescence have been studied with isolated chloroplasts illuminated, in the absence of Hill reagents, by flashes or continuous light. As in whole cells, at least two substances are involved in the primary process leading to the oxygen evolution. The first, called E, probably is the photochemical “complex” of System II. After a long period of darkness, E is not active. It is activated in two steps. Step one is a photochemical reaction, induced by a quantum of light absorbed by pigment-system II, which results in the production of E in a reduced state. Step two is a dark oxidation of the reduced E by the second substance, A. The oxidized E can then enter the normal photochemical cycle of system II. Reduced E might alternatively be oxidized by oxygen, this reaction being responsible for a very rapid and brief light-induced oxygen uptake. Substance A is measured by the oxygen burst and is present in the chloroplasts at the approximate ratio of 1 molecule for 70 molecules of total chlorophyll while E is at the ratio of about 1/2800. This gives a E over A value of 1/35 which is much smaller than the one found in whoe cells (ca. 1/10). This independent behavior of E and A suggests that chloroplast extraction destroys some photochemical centers without having a direct impact on A, which might diffuse from one center to another. Besides the brief light-induced oxygen uptake above mentioned, there is another one which is related to System I functioning. The kinetics of the oxygen evolution and of the fluorescence have been compared. During the activation process of the oxygen evolving ability, rate of oxygen evolution and fluorescence yield increase in a parallel way. After the maximum velocity of the oxygen burst is reached (i.e. after activation), the fluorescence yield keeps growing up until the steady-state is attained (with an intermediary plateau), whereas the rate of oxygen emission slows down. The time-course curves of fluorescence obtained with inactivated or activated chloroplasts are essentially different in that the initial yield is higher in the latter case.