Photoinhibitory Light Research Articles

The chemical composition of Chaetoceros sp. (Bacillariophyceae) under different light conditions

Chaetoceros sp. growth with white, pure blue and mixed blue light in 10-1 semicontinuous culture with 50% daily dilutions was light-limited at photon flux densities of 199 μmol m −2 s −1. At the above light level the type of light did not cause variations in protein content, but the use of mixed blue (192.5 μmol m −2 s −1 of blue, and 6.5 of white, light) caused an important decrease in carbohydrates and an increase in lipids. Chlorophyll a was higher in blue than in white light. Increasing photon flux densities increased biomass production. In white light photoinhibition occurred earlier than in blue light ( >498 and 565 μmol m −2 s −1, respectively). The highest cell densities were in white light, but the highest yields in biomass (mg l −1) were in mixed blue, due to the higher mean cell weight. There were no differences in chlorophyll content. The proximate composition of the biomass produced at saturating photon fluxes did not change considerably in white light. Both types of blue caused an increase in proteins but the alternative carbon sinks were lipids in mixed blue and carbohydrates in pure blue light. Methionine was highest in mixed blue and there were no other important differences in amino acid profiles. 18C fatty acids were greater than 24% in pure blue and less than 5% in white and mixed blue. The unsaturate 18:1 (oleic acid) was more than 15% of the total fatty acids in pure blue, which might have important effects on the dietary value of the biomass produced.

Loss of the trans-thylakoid proton gradient is an early event during photoinhibitory illumination of chloroplast preparations

During photoinhibitory illumination of isolated thylakoids or intact chloroplasts from spinach, an initial increase of Photosystem I electron transport was observed while proton uptake associated with PMS-mediated cyclic electron transport was rapidly lost. The latter reaction was at least as light-sensitive as Photosystem II electron transport, normally considered to be the primary target for light stress. Thus under both moderate and extreme light stress, loss of the proton gradient associated with cyclic electron transport around Photosystem I was an early event. In accordance with this observation, photoinhibitory light very rapidly caused inactivation of cyclic photophosphorylation. There was no kinetic correlation between light-induced degradation of the D1 protein and collapse of the proton gradient. Notably, under anaerobic conditions when D1 protein degradation does not occur, loss of proton uptake still occurred. Low temperatures (3°C) provided partial protection against the photodamage, but a subsequent increase of the temperature to 25°C resulted in a total loss of the proton uptake in total darkness. The proton gradient could not be re-established by addition of DCCD. Moreover, there were no changes in the polypeptide composition of CF 1 or any impairment of the ATPase activity during photoinhibitory illumination. The mechanism of the light-induced loss of the proton gradient and its correlation to other effects of light stress at the molecular and physiological levels are discussed.

The protective effect of heat shock proteins against photoinhibition under heat shock in barley ( Hordeum vulgare)

Barley leaves, when analyzed by variable fluorescence measurements of PS II activity, have been found to possess a considerably higher resistance against photoinhibition under heat stress conditions after a pretreatment by heat shock. This preconditioning, protecting 2 h heat shock treatment could be applied either 24 h or immediately prior to the combination of heat- and photoinhibitory light stress at a light intensity of 1000 W m −2. Surprisingly, resistance against light stress under heat shock of heat-pretreated plants was even somewhat higher than in unshocked plants which were photoinhibited under normal growth temperature. The preformed, accumulated heat shock proteins (HSP) did not protect the leaves against photoinhibition under normal growth temperatures. Heat shock treatments under light intensities used for normal growth did not impair activity of PS II. Inhibition of protein synthesis by cycloheximide during the first heat shock abolished almost completely the protective effect of the conditioning heat shock. Plants pretreated with heat in the presence of chloramphenicol were as resistant against light stress during the first hour as were the controls which were heat treated in the absence of the drug; however, under prolonged conditions of light stress the chloramphenicol treated plantlets showed a rapid decline in PS II activity.

Role of dark respiration in photoinhibition of photosynthesis and its reactivation in the cyanobacterium Anacystis nidulans

Photoinhibition of photosynthesis and its reactivation was studied in the cyanobaterium A. nidulans in the presence of the respiratory inhibitor sodium azide, the uncouplers carbonyl cyanide p‐(trifluoromethoxy)‐phenylhydrazone (FCCP) and carbonyl cyanide m‐chlorophenylhydrazone (CCCP) and the photosystem I elicitor phenazine methosulphate (PMS). Inhibition of dark respiration by azide increased the susceptibility of the cyanobacterium to photoinhibition. Both FCCP and CCCP also remarkably affected the process of photoinhibition in A. nidulans. The PMS at lower photoinhibitory light intensity partially protected A. nidulans from photoinhibition. The recovery from photoinhibition in the presence of azide or FCCP was slow and normal photosynthesis could not be resumed even after a longer period of incubation under suitable reactivating condition. Thus dark respiration has a key function in the process of photoinhibition of photosynthesis and its reactivation in the cyanobacterium A. nidulans.

Photoinhibition of photosynthesis in vivo: Involvement of multiple sites in a photodamage process under CO 2- and O 2-free conditions

Intact Lemna gibba plants were illuminated by photoinhibitory light in air, in air minus O 2, in air minus CO 2, and in pure N 2. In pure N 2, the degree of photoinhibition increased 3–5-times compared with that in air. This high degree of photoinhibition is described as photodamage. Photodamage was found to constitute a syndrome, that is, it is due to inactivation of multiple sites. These sites include RC II component(s) from P680 to Q A; the Q B-site; and a component of PS I. In photodamage, the donor side of PS II and PS II excitation energy transfer remain unimpaired, but the size of the PS I antenna seems to decrease. Photodamage is distinguishable from photoinactivation. Photoinactivation occurred in air and could be attributed to inhibition of electron transport from Q − A to Q B. During photoinactivation the D1 protein of RC II became degraded faster than the detectable inhibition of Q B reduction. The photoinhibition-induced rise in F 0 occurred only during the process of photodamage but not during that of photoinactivation, and was a secondary event which arose as a consequence of photodamage. Atmospheric O 2 alleviated photodamage but increased photoinactivation. The light-induced D1 degradation and inhibition of Q A to Q B electron transfer were enhanced in vivo not only by O 2 but also by depletion of CO 2.

Photoinhibition of isolated mesophyll cells from cold‐hardened and non‐hardened winter rye

ABSTRACTCold‐hardened rye leaves have been shown to be more resistant to low temperature photoinhibition than non‐hardened rye leaves. Isolated mesophyll cells from winter rye (Secale cereale L. cv. Musketeer) were exposed to photoinhibitory light conditions to estimate the importance of leaf morphology and leaf optical properties in the resistance of cold‐hardened rye leaves to photoinhibition. Cold‐hardened rye cells showed more resistance to photoinhibition than non‐hardened rye cells when monitored with chlorophyll a variable to maximal fluorescence ratio (Fv/Fm). Thus, leaf morphology does not contribute to the resistance of cold‐hardened rye leaves to low temperature photoinhibition. However, cold‐hardened and non‐hardened rye cells showed a similar extent of photoinhibition when photsynthetic CO2 fixation rates were measured. They also showed the same capacity to recover from photoinhibition. During both photoinhibition and recovery, Fv/Fm and light limited CO2 fixation rates showed different kinetics. We propose that inactivation and subsequent reactivation during recovery of some light activated Calvin cycle enzymes explain the greater extent of photoinhibition of light limited CO2 fixation and its faster recovery compared to Fv/Fm kinetics during photoinhibition.

EFFECTS OF DESICCATION AND ILLUMINATION ON PHOTOSYNTHESIS AND PIGMENTATION OF AN EDAPHIC POPULATION OF TRENTEPOHLIA ODORATA (CHLOROPHYTA)1

ABSTRACTThe photosynthetic rates of Trentepohlia odorata (L.) Martius growing on wall surfaces in Singapore changed throughout the day with a maximum in midmorning and decreasing thereafter during the day. Optimum temperature for photosynthesis was 25° C. Different levels of air humidity also affected photosynthetic rates with low relative humidity reducing the rates and efficiency of photosynthesis. Our results suggested that T. odorata was able to maximize its rate of photosynthesis before photoinhibitory light levels were reached and that its growth might be dependent on high levels of atmospheric relative humidity, which may serve as a source of water supply for the alga.

Photoinhibition of hydroxylamine-extracted photosystem II membranes: studies of the mechanism.

The effects of photosystem II (PSII) exogenous electron donors and acceptors on the kinetics of weak light photoinhibition of NH2OH/EDTA-extracted spinach PSII membranes were examined. Under aerobic conditions, Mn2+ (approximately 1 Mn/reaction center; Km approximately 400 nM) inhibited photoinactivation and approximately 1 Mn/reaction center plus 100 microM NH2NH2 gave almost complete protection. In the absence of electron donors, strict anaerobiosis greatly inhibited photoinactivation even in the presence of an electron acceptor. Under aerobic conditions, the addition of electron acceptors (FeCN, DCIP), oxyradical scavengers, or superoxide dismutase strongly suppressed rates of photodamages. Increase in the concentrations of superoxide above those produced by illuminated NH2OH/EDTA-photosystem II membranes increased the rates of damage in the light but gave no damage in the dark. Scavengers of hydroxyl radicals and singlet oxygen did not suppress the rates of aerobic photoinhibition. These findings, along with others, lead us to conclude that photodamage of the secondary donors of the PSII reaction center occurs by two mechanisms: (1) a rapid superoxide and tyrosine YZ+ dependent process and (2) a slower process in which P680+/Chl+ catalyze the damages.

Structural changes and lateral redistribution of photosystem II during donor side photoinhibition of thylakoids.

The structural and topological stability of thylakoid components under photoinhibitory conditions (4,500 microE.m-2.s-1 white light) was studied on Mn depleted thylakoids isolated from spinach leaves. After various exposures to photoinhibitory light, the chlorophyll-protein complexes of both photosystems I and II were separated by sucrose gradient centrifugation and analysed by Western blotting, using a set of polyclonals raised against various apoproteins of the photosynthetic apparatus. A series of events occurring during donor side photoinhibition are described for photosystem II, including: (a) lowering of the oligomerization state of the photosystem II core; (b) cleavage of 32-kD protein D1 at specific sites; (c) dissociation of chlorophyll-protein CP43 from the photosystem II core; and (d) migration of damaged photosystem II components from the grana to the stroma lamellae. A tentative scheme for the succession of these events is illustrated. Some effects of photoinhibition on photosystem I are also reported involving dissociation of antenna chlorophyll-proteins LHCI from the photosystem I reaction center.

Light-induced degradation of D2 protein in isolated photosystem II reaction center complex

When isolated photosystem II reaction centers from spinach are exposed to photoinhibitory light in the presence of an electron acceptor, breakdown products of the D2 protein at 28, 25, 23, 18, 9, 5 and 4.5 kDa are detected by immunoblotting with a monospecific anti-D2 polyclonal antibody. In a time—course experiment the 23 and 4.5 kDa fragments show a transient appearance, whilst the others are photoaccumulated. The regions of the D2 protein containing the cleavage sites for the 28 and 18 kDa photoinduced fragments have been identified. Significant degradation of D2 takes place only in the presence of an electron acceptor, and breakdown of the protein is partially prevented by serine-type protease inhibitors.

Thermoluminescence Investigation of Photoinhibition in the Green Alga, Chlamydobotrys stellata and in Pisum sativum L. Leaves

During photoinhibitory light treatment of the green alga, Chlamydobotrys stellata and Pisum sativum leaves the amplitude of the B thermoluminescence band (S2OB - charge recombination) decreased faster than that of the Q band (S2QA - charge recombination). Consistently, in the alga cells the electron transport rate from water to oxidized diaminodurene (electron acceptor after Q B) was also diminished faster than that measured from water to phenyl- p-benzoquinone (electron acceptor after QA). These observations suggest that in intact photosynthetizing organisms at high light intensities on the acceptor side of photosystem II photoinhibition is initiated at the binding site of the secondary quinone acceptor, QB.

Photoinhibition of Photosynthesis and its Recovery in Red Algae

AbstractIn different marine red algae (Chondrus crispus, Delesseria sanguinea, Membranoptera alata, Phycodrys rubens, Phyllophora truncata, Polyneura hilliae) photoinhibition of photosynthesis has been investigated by means of both fluorescence and oxygen measurements. Measurements of absolute oxygen production show that photoinhibition causes a decline in the initial slope and in the rate of bending of the fluence rate‐response curve (i.e. the photosynthetic efficiency at non‐saturating fluence rates), as well as a decline in the photosynthetic capacity (Pm) at saturating fluence rates. Fluorescence data (Fv/Fm) were consistent with the results of oxygen measurements. Under excessive light photoinhibition protects photosynthesis against photo‐damage in red algae. However, an increase in the initial fluorescence (Fo) after photoinhibitory treatment indicates that it could not prevent photodamage entirely.Action spectra of photoinhibition demonstrate that the main photoinhibition site in Polyneura hiliae is PS II, because far red light absorbed by PS I was ineffective. The strong increase of Fo in the blue wavelength range and the slight and partial recovery in weak blue light indicate that blue light especially causes photodamage.Recovery of photosynthesis requires dim white light conditions. Experiments with monochromatic light also show a wavelength dependence of recovery. Moreover, the recovery of photosynthesis after a photoinhibitory treatment is strongly temperature dependent, indicating participation of enzymatic processes. The comparison of fluorescence and oxygen measurement of the recovery shows different results in some species. The rate of oxygen production in red control light increased immediately after photoinhibited algae were exposed to weak light conditions. Surprisingly, the ratio of variable to maximum fluorescence (Fv/Fm) of Phyllophora truncata and the maximum fluorescence (Fm) of Polyneura hilliae show first a delay of the recovery under weak light conditions. Thus, in recovery experiments fluorescence and oxygen data are not quite consistent.

Dark Respiration Protects Photosynthesis Against Photoinhibition in Mesophyll Protoplasts of Pea (Pisum sativum)

The optimal light intensity required for photosynthesis by mesophyll protoplasts of pea (Pisum sativum) is about 1250 microeinsteins per square meter per second. On exposure to supra-optimal light intensity (2500 microeinsteins per square meter per second) for 10 min, the protoplasts lost 30 to 40% of their photosynthetic capacity. Illumination with normal light intensity (1250 microeinsteins per square meter per second) for 10 min enhanced the rate of dark respiration in protoplasts. On the other hand, when protoplasts were exposed to photoinhibitory light, their dark respiration also was markedly reduced along with photosynthesis. The extent of photoinhibition was increased when protoplasts were incubated with even low concentrations of classic respiratory inhibitors: 1 micromolar antimycin A, 1 micromolar sodium azide, and 1 microgram per milliliter oligomycin. At these concentrations, the test inhibitors had very little or no effect directly on the process of photosynthetic oxygen evolution. The promotion of photoinhibition by inhibitors of oxidative electron transport (antimycin A, sodium azide) and phosphorylation (oligomycin) was much more pronounced than that by inhibitors of glycolysis and tricarboxylic acid cycle (sodium fluoride and sodium malonate, respectively). We suggest that the oxidative electron transport and phosphorylation in mitochondria play an important role in protecting the protoplasts against photoinhibition of photosynthesis. Our results also demonstrate that protoplasts offer an additional experimental system for studies on photoinhibition.

Inhibition of Water Splitting Increases the Susceptibility of Photosystem II to Photoinhibition

Photosystem II (PSII)-enriched membrane particles were isolated from peas (Pisum sativum L.) and treated in several different ways to inhibit the water oxidation reactions, but not reaction center function itself, as judged by the light-induced rate of reduction of 2,6-dichlorophenol indophenol with and without the artificial electron donor, diphenyl carbazide. It was shown that such treatments increased the susceptibility of the PSII-enriched membranes to photoinhibition. This trend was further observed if 2,6-dichlorophenol indophenol was present during the illumination with photoinhibitory light. On the other hand, protection against the enhanced photoinhibition was found when the water-splitting activity was reconstituted or when the artificial electron donor diphenyl carbazide was present during the preillumination. The results indicate that irreversible photodamage occurred within the PSII reaction center as a consequence of illumination with strong light and that the rate of this damage was enhanced under conditions that are expected to give rise to a photoaccumulation of oxidizing species such as P680(+) on the donor side of PSII. This mechanism of photoinhibitory damage occurred under both aerobic and anaerobic conditions.

Recovery of photosynthesis and phtosystem II fluorescence in Chlamydomonas reinhardtii after exposure to three levels of high light

Recovery from 60 min of photoinhibitory treatment at photosynthetic photon flux densities of 500, 1400 and 2200 μMmol m−2 s− was followed in cells of the green alga Chlamydomonas reinhardtii grown at 125 μMmol m−2 s−1. These light treatments represent photoregulation, moderate photoinhibition and strong photoinhibition, respectively. Treatment in photoregulatory light resulted in an increased maximal rate of oxygen evolution (Pmax) and an increased quantum yield (Φ), but a 15% decrease in Fv/FM. Treatment at moderately photoinhibitory light resulted in a 30% decrease in Fv/FM and an approximately equal decrease in Φ. Recovery in dim light restored Fv/FM within 15 and 45 min after high light treatment at 500 and 1400 μMmol m−2 s−1, respectively. Convexity (Θ), a measure of the extent of co‐limitation between PS II turnover and whole‐chain electron transport, and Φ approached, but did not reach the control level during recovery after exposure to 1400 μMmol m−2 s−1, whereas Pmax increased above the control. Treatment at 2200 μMmol m−2 s−1 resulted in a strong reduction of the modeled parameters Φ, Θ and Pmax. Subsequent recovery was initially rapid but the rate decreased, and a complete recovery was not reached within 120 min. Based on the results, it is hypothesized that exposure to high light results in two phenomena. The first, expressed at all three light intensities, involves redistribution within the different aspects of PS II heterogeneity rather than a photoinhibitory destruction of PS II reaction centers. The second, most strongly expressed at 2200 μmol m−2 s−1, is a physical damage to PS II shown as an almost total loss of PS IIα and PS II QB‐reducing centers. Thus recovery displayed two phase, the first was rapid and the only visible phase in algae exposed to 500 and 1400 μmol m−2 s−1. The second phase was slow and visible only in the later part of recovery in cells exposed to 2200 μmol m−2 s−1.

Thylakoid membrane energization and swelling in photoinhibited Chlamydomonas cells is prevented in mutants unable to perform cyclic electron flow

Photoinhibition of Photosystem II in unicellular algae in vivo is accompanied by thylakoid membrane energization and generation of a relatively high ΔpH as demonstrated by (14)C-methylamine uptake in intact cells. Presence of ammonium ions in the medium causes extensive swelling of the thylakoid membranes in photoinhibited Chlamydomonas reinhardtii but not in Scenedesmus obliquus wild type and LF-1 mutant cells. The rise in ΔpH and the related thylakoid swelling do not occur at light intensities which do not induce photoinhibition. The rise in ΔpH and membrane energization are not induced by photoinhibitory light in C. reinhardtii mutant cells possessing an active Photosystem II but lacking cytochrome b6/f, plastocyanin or Photosystem I activity and thus being unable to perform cyclic electron flow around Photosystem I. In these mutants the light-induced turnover of the D1 protein of Reaction Center II is considerably reduced. The high light-dependent rise in ΔpH is induced in the LF-1 mutant of Scenedesmus which can not oxidize water but otherwise possesses an active Reaction Center II indicating that PS II-linear electron flow activity and reduction of plastoquinone are not required for this process. Based on these results we conclude that photoinhibition of Photosystem II activates cyclic electron flow around Photosystem I which is responsible for the high membrane energization and ΔpH rise in cells exposed to excessive light intensities.

Photoinhibition of photosynthesis represents a mechanism for the long-term regulation of photosystem II

The obligate shade plant, Tradescantia albiflora Kunth grown at 50 μmol photons · m(-2) s(-1) and Pisum sativum L. acclimated to two photon fluence rates, 50 and 300 μmol · m(-2) · s(-1), were exposed to photoinhibitory light conditions of 1700 μmol · m(-2) · s(-1) for 4 h at 22° C. Photosynthesis was assayed by measurement of CO2-saturated O2 evolution, and photosystem II (PSII) was assayed using modulated chlorophyll fluorescence and flash-yield determinations of functional reaction centres. Tradescantia was most sensitive to photoinhibition, while pea grown at 300 μmol · m(-2) · s(-1) was most resistant, with pea grown at 50 μmol · m(-2) · s(-1) showing an intermediate sensitivity. A very good correlation was found between the decrease of functional PSII reaction centres and both the inhibition of photosynthesis and PSII photochemistry. Photoinhibition caused a decline in the maximum quantum yield for PSII electron transport as determined by the product of photochemical quenching (qp) and the yield of open PSII reaction centres as given by the steady-state fluorescence ratio, F'vF'm, according to Genty et al. (1989, Biochim. Biophys. Acta 990, 81-92). The decrease in the quantum yield for PSII electron transport was fully accounted for by a decrease in F'vF'm, since qp at a given photon fluence rate was similar for photoinhibited and noninhibited plants. Under lightsaturating conditions, the quantum yield of PSII electron transport was similar in photoinhibited and noninhibited plants. The data give support for the view that photoinhibition of the reaction centres of PSII represents a stable, long-term, down-regulation of photochemistry, which occurs in plants under sustained high-light conditions, and replaces part of the regulation usually exerted by the transthylakoid ΔpH gradient. Furthermore, by investigating the susceptibility of differently lightacclimated sun and shade species to photoinhibition in relation to qp, i.e. the fraction of open-to-closed PSII reaction centres, we also show that irrespective of light acclimation, plants become susceptible to photoinhibition when the majority of their PSII reaction centres are still open (i.e. primary quinone acceptor oxidized). Photoinhibition appears to be an unavoidable consequence of PSII function when light causes sustained closure of more than 40% of PSII reaction centres.

Small Light-Harvesting Antenna Does Not Protect from Photoinhibition

High-light-induced decrease in photosystem II (PSII) electron transfer activity was studied in high- and low-light-grown pumpkin (Cucurbita pepo L.) plants in vivo and in vitro. The PSII light-harvesting antenna of the low-light leaves was estimated to be twice as big as that of the high-light leaves. The low-light leaves were more susceptible to photoinhibition in vivo. However, thylakoids isolated from these two plant materials were equally sensitive to photoinhibition when illuminated in the absence of external electron acceptors. Only the intensity of the photoinhibitory light and the chlorophyll concentration of the sample, not the size of the light-harvesting antenna, determined the rate of PSII photoinhibition in vitro. Because excitation of the reaction center and not only the antenna chlorophylls is a prerequisite for photoinhibition of PSII activity, independence of photoinhibition on antenna size provides support for the hypothesis (Schatz EH, Brock H, Holzwarth AR [1988] Biophys J 54: 397-405) that the excitations of the antenna chlorophylls are in equilibrium with the excitations of the reaction centers. Better tolerance of the high-light leaves in vivo was due to a more active repair process and more powerful protective mechanisms, including photosynthesis. Apparently, some protective mechanism of the high-light-grown plants is at least partially active at low temperature. The protective mechanisms do not appear to function in vitro.

Photoinhibition and recovery of photosynthesis in intact barley leaves at 5 and 20°C

Photoinhibition of photosynthesis and its recovery were studied in intact barley (Hordeum vuigare L. cv. Gunilla) leaves grown in a controlled environment by exposing them to two temperatures, 5 and 20°C, and a range of photon flux densities in excess of that during growth. Additionally, photoinhibtion was examined in the presence of chloramphenicol (CAP, an inhibitor of chloroplast protein synthesis) and of 3‐(3,4‐dichlorophenyl)‐1,1‐dimethylurea (DCMU). Susceptibility to photoinhibition was much higher at 5 than at 20°C. Furthermore, at 20°C. CAP exacerbated photoinhibition strongly, whereas CAP had little additional effect (10%) at 5°C. These results support the model that net photoinhibition is the difference between the inactivation and repair of photosystem II (PSII); i.e. the degradation and synthesis of the reaction centre protein, Dl. Furthermore, the steady‐state extent of photoinhibition was strongly dependent on temperature and the results indicated this was manifested through the effects of temperature on the repair process of PSII. We propose that the continuous repair of PS II at 20°C conferred at least some protection from photoinhibition. At 5°C the repair process was largely inhibited, with increased photoinhibition as a consequence. However, we suggest where repair is inhibited by low temperature, some protection is alternatively conferred by the photoinhibited reaction centres. Providing they are not degraded, such centres could still dissipate excitation energy non‐radiatively, thereby conferring protection of remaining photochemically active centres under steady‐state conditions.A fraction of PS II centres were capable of resisting photoinhibition when the repair process was inhibited by CAP. This is discussed in relation to PS II heterogeneity. Furthermore, the repair process was not apparently activated within 3 h when barley leaves were transferred to photoinhibitory light conditions at 20°C.

Changes in the organization of Photosystem II following light-induced D 1-protein degradation

The composition and organization of photosystem II was studied in thylakoid membranes and subfractions which had been subjected to photoinhibitory light conditions. The results show that D 1-protein degradation can occur in vitro leading to a 50–60% loss of the protein. Apart from the D 2-protein, which shows a limited decline, there was no loss of any other photosystem II proteins. The D 1-protein degradation induced several changes in the organization of the photosystem II complex. Using inside-out thylakoid vesicles we demonstrate that concomitant with the D 1-protein degradation there is a release of the extrinsic 33, 23 and 16 kDa proteins from the inner thylakoid surface into the lumenal space. In addition, there is a release of four manganese ions per D 1-protein degraded. The correlation between the D 1-protein degradation and the release of manganese is also seen in inside-out thylakoid vesicles that have been CaCl 2-washed to remove the three extrinsic proteins prior to photoinhibitory illumination. Subfractionation of thylakoids subsequent to photoinhibitory treatment suggests a migration of certain photosystem II subunits from the appressed to the non-appressed thylakoid regions following D 1-protein degradation. The photosystem II subunits showed an individual migration behaviour, suggesting a disassembly of the photosystem II core. Our data suggest that repair of photodamaged photosystem II involves, apart from reinsertion of new D 1-proteins, reassembly of the photosystem II complex including lateral movement of proteins between the two thylakoid regions and religation of the manganese cluster.