The Susceptibility of Photosynthesis to Photoinhibition and the Capacity of Recovery in High and Low Light Grown Cyanobacteria, Anacystis nidulans

The susceptibility of photosynthesis to photoinhibition and the rate of its recovery were studied in the cyanobacterium Anacystis nidulans grown at a low (10 micromoles per square meter per second) and a high (120 micromoles per square meter per second) photosynthetically active radiation. The rate of light limited photosynthetic O₂ evolution was measured to determine levels of photoinhibition and rates of recovery. Studies of photoinhibition and recovery with and without the translation inhibitor streptomycin demonstrated the importance of a recovery process for the susceptibility of photosynthesis to photoinhibition. We concluded that the approximately 3 times lower susceptibility to photoinhibition of high light than of low light grown cells, significantly depended on high light grown cells having an approximately 3 times higher recovery capacity than low light grown cells. It is suggested that these differences in susceptibility to photoinhibition and recovery depends on high light grown cells having a higher turnover rate of photosystem II protein(s) that is(are) the primary site(s) of photodamage, than have low light grown cells. Furthermore, we demonstrated that photoinhibition of A. nidulans may occur under physiological light conditions without visible harm to the growth of the cell culture. The results give support for the hypotheses that the net photoinhibitory damage of photosystem II results from the balance between the photoinhibitory process and the operation of a recovery process; the capacity of the latter determining significant differences in the susceptibility of photosynthesis to photoinhibition of high and low light grown A. nidulans.     

Photoinhibition of photosynthesis and growth responses at different light levels in psbA gene mutants of the cyanobacterium Synechococcus

Photoinhibition of photosynthesis and growth responses at diffrent light levels (10, 120 and 250 μmol m⁻² s⁻¹) were studied in psbA gene mutants R2S2C3 ( psbAI gene present) and R2K1 ( psbAIIIpsbAIII genes present) of the cyanobacterium Synechococcus sp . PCC 7942 ( Anacystis nidulans R2). Mutant R2K1 (possessing form II of the D1 protein of photosystem II) was much more resistant to photoinhibition than the mutant R2S2C3 (possessing form I of the D1 protein). At moderate inhibitory light levels (100 to 300 μmol m⁻² s⁻¹) this was largely ascribed to an increased rsistance of the photosystem II reaction cetres possessing form II of the D1 protein. However, at higher light levels the higher resistance mutant R2K1 was assigned to a higher rate of photosystem II repair, i.e. turnover of the D1 protein. Moreover, our results support the hypothesis that photoinhibition of photosystem II and photoinhibitory induced quenching are due to separate processes. Results from growth experiments show that the R2K1 mutant has a slower growth rate than the R2S2C3 mutant but shows an increased survival under high light stress conditions. It is hypothesized that high resistance to photoinhibition, though allowing a better survival under high light, is not advantageous for optimal growth.     

Photoinhibition of Photosynthesis in Broken Chloroplasts as a Function of Electron Transfer Rates during Light Treatment

Photoinhibition was studied in osmotically broken chloroplasts isolated from spinach leaves (Spinacia oleracea L.). Both whole chain electron transport (measured as ferricyanide-dependent O₂ evolution in the presence of NH₄Cl) and photosystem II activity (measured as O₂ evolution in the presence of either silicomolybdate plus 3-(3,4-diphenyl)-1,1 dimethylurea or parabenzoquinone) showed similar decreases in activity in response to a photoinhibitory treatment (8 minutes of high light given in the absence of an electron acceptor other than O₂). Photosystem I activity was less affected. Photoinhibition of silicomolybdate reduction was largely reversible by an 8 minute dark incubation following the light treatment. Decreasing the O₂ concentration during photoinhibition below 2% increased photoinhibition of whole chain electron transport. Addition of superoxide dismutase to the reaction medium did not affect photoinhibition. Photoinhibition of both photosystem I and photosystem II activity increased as the rate of electron transfer during the treatment increased, and was largely prevented when 3-(3,4-diphenyl)-1,1-dimethylurea was present during the photoinhibition period. Noncyclic photophosphorylation was decreased as a consequence of whole chain electron transfer photoinhibition. Since diphenyl carbazide added after light treatment did not relieve photoinhibition of dichlorophenol indophenol reduction, we conclude that the site of inhibition is located within or near the photosystem II reaction center.     

Photoinhibition and its wavelength dependence in the cyanobacterium Anabaena variabilis

In the cyanobacterium Anabaena variabilis the dependence of photoinhibition on fluence rate, duration and wavelength of irradiation were studied by measurements of oxygen production and fluorescence emission spectra. The analysis of the photosynthetic activity revealed that photoinhibition affects exclusively photosystem II (PS II), whereas photosystem I (PS I) remained largely unimpaired. Furthermore, PS II fluorescence emission decreased much faster in bleached than in unbleached controls.Studying the wavelength dependence of photoinhibition it was found that only radiation between 520 and 680 nm causes photoinhibition. This is about the same range of wavelengths which causes photobleaching. Fluorescence emission spectra of samples exposed to high fluence rates of 582 and 662 nm, respectively, essentially agree with those samples exposed to high fluence rates of white light, whereas the fluorescence emission spectra of samples exposed to blue light resemble those exposed to dim white light.NaN₃, a substance which prevents photobleaching, inhibits the photosynthetic O₂ production of Anabaena and, hence, enhances the photoinhibitory effect.     

Characterization of a spinach photosystem II core preparation isolated by a simplified method

A photosystem II core complex from spinach exhibiting high rates of electron transport was obtained rapidly and in high yield by treatment of a Tris-extracted, O2-evolving photosystem II preparation with the detergent dodecyl-beta-D-maltoside. The core complex was essentially free of light-harvesting chlorophyll-protein and photosystem I polypeptides, and was highly enriched in the polypeptides associated with the photosystem II reaction center (45 and 49 kDa), cytochrome b559, and three polypeptides in the region 32-34 kDa. The photosystem II core complex contained two chlorophyll-proteins which had a slightly higher apparent molecular mass than CPa-1 and CPa-2. Additionally, a high-molecular-mass chlorophyll-protein complex termed CPa* was observed, which exhibited a low fluorescence yield when illuminated with ultraviolet light. This observation suggests that CPa* contains a functionally efficient quencher of chlorophyll fluorescence, possibly P680.     

A chloroplast-targeted heat shock protein 70 (HSP70) contributes to the photoprotection and repair of photosystem II during and after photoinhibition.

Dark-grown Chlamydomonas reinhardtii cultures that were illuminated at low fluence rates before exposure to high-light conditions exhibited a faster rate of recovery from photoinhibition than did dark-grown cells that were directly exposed to photoinhibitory conditions. This pretreatment has been shown to induce the expression of several nuclear heat shock protein 70 (HSP70) genes, including HSP70B, encoding a chloroplast-localized chaperone. To investigate a possible role of plastidic HSP70B in photoprotection and repair of photosystem II, which is the major target of photoinhibition, we have constructed strains overexpressing or underexpressing HSP70B. The effect of light stress on photosystem II in nuclear transformants harboring HSP70B in the sense or antisense orientation was monitored by measuring variable fluorescence, flash-induced charge separation, and relative amounts of various photosystem II polypeptides. Underexpression of HSP70B caused an increased light sensitivity of photosystem II, whereas overexpression of HSP70B had a protective effect. Furthermore, the reactivation of photosystem II after photoinhibition was enhanced in the HSP70B-overexpressing strain when compared with the wild type, both in the presence or absence of synthesis of chloroplast-encoded proteins. Therefore, HSP70B may participate in vivo both in the molecular protection of the photosystem II reaction centers during photoinhibition and in the process of photosystem II repair.     

Contrasting behavior of higher plant photosystem I and II antenna systems during acclimation

In this work we analyzed the photosynthetic apparatus in Arabidopsis thaliana plants acclimated to different light intensity and temperature conditions. Plants showed the ability to acclimate into different environments and avoid photoinhibition. When grown in high light, plants had a faster activation rate for energy dissipation (qE). This ability was correlated to higher accumulation levels of a specific photosystem II subunit, PsbS. The photosystem II antenna size was also regulated according to light exposure; smaller antenna size was observed in high light-acclimated plants with respect to low light plants. Different antenna polypeptides did not behave similarly, and Lhcb1, Lchb2, and Lhcb6 (CP24) are shown to undergo major levels of regulation, whereas Lhcb4 and Lhcb5 (CP29 and CP26) maintained their stoichiometry with respect to the reaction center in all growth conditions. The effect of acclimation on photosystem I antenna was different; in fact, the stoichiometry of any Lhca antenna proteins with respect to photosystem I core complex was not affected by growth conditions. Despite this stability in antenna stoichiometry, photosystem I light harvesting function was shown to be regulated through different mechanisms like the control of photosystem I to photosystem II ratio and the association or dissociation of Lhcb polypeptides to photosystem I.     

Effect of moderate and high light on photosystem II function in Arabidopsis thaliana depleted in digalactosyl-diacylglycerol

The response of the heat-sensitive dgd1-2 and dgd1-3 Arabidopsis mutants depleted in the galactolipid DGDG to photoinhibition of chloroplasts photosystem II was studied to verify if there is a relationship between heat stress vulnerability due to depletion in DGDG and the susceptibility to photoinhibitory damage. Non-photochemical quenching (NPQ) is known to dissipate excessive absorbed light energy as heat to protect plants against photodamage. The main component of NPQ is dependent of the transthylakoid pH gradient and is modulated by zeaxanthin (Zx) synthesis. These processes together with chlorophyll fluorescence induction were used to characterize the response of the genotypes. The mutants were more sensitive to photoinhibition to a small extent but this was more severe for dgd1-3 especially at high light intensity. It was deduced that DGDG was not a main factor to influence photoinhibition but other lipid components could affect PSII sensitivity towards photoinhibition in relation to the physical properties of the thylakoid membrane. This article is part of a Special Issue entitled: Photosynthesis Research for Sustainability: from Natural to Artificial.     

Phosphorylation of chloroplast membrane proteins partially protects against photoinhibition

Thylakoids isolated from peas (Pisum sativum cv. Kelvedon Wonder) and phosphorylated by incubation with ATP have been compared with non-phosphorylated thylakoids in their sensitivity to photoinhibition by exposure to illumination in vitro. Assays of the kinetics of fluorescence induction at 20° C and the fluorescence emission spectra at-196° C indicate a proportionally larger decrease in fluorescence as a result of photoinhibitory treatment of non-phosphorylated compared with phosphorylated thylakoids. It is concluded that protein phosphorylation can afford partial protection to thylakoids exposed to photoinhibitory conditions.Abbreviations and symbols DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - F ₀ Level of chlorophyll fluorescence when photosystem 2 traps are open - F _m Level of chlorphyll fluorescence when photosystem 2 traps are closed - P Maximum level of fluorescence reached in the absence of DCMU - PSI (II) photosystem I(II)     

Photoinhibition of photosynthesis: effect on chlorophyll fluorescence at 77K in intact leaves and in chloroplast membranes of Nerium oleander

The effect of exposing intact leaves and isolated chloroplast membranes of Nerium oleander L. to excessive light levels under otherwise favorable conditions was followed by measuring photosynthetic CO₂ uptake, electron transport and low-temperature (77K=-196°C) fluorescence kinetics. Photoinhibition, as manifested by a reduced rate and photon (quantum) yield of photosynthesis and a reduced electron transport rate, was accompanied by marked changes in fluorescence characteristics of the exposed upper leaf surface while there was little effect on the shaded lower surface. The most prominent effect of photoinhibitory treatment of leaves and chloroplasts was a strong quenching of the variable fluorescence emission at 692 nm (F_v,692) while the instantaneous fluorescence (F_o,692) was slightly increased. The maximum and the variable fluorescence at 734 nm were also reduced but not as much as F_M,692 and F_v,692. The results support the view that photoinhibition involves an inactivation of the primary photochemistry of photosystem II by damaging the reaction-center complex. In intact leaves photoinhibition increased with increased light level, increased exposure time, and with decreased temperature. Increased CO₂ pressure or decreased O₂ pressure provided no protection against photoinhibition. With isolated chloroplasts, inhibition of photosystem II occurred even under essentially anaerobic conditions. Measurements of fluorescence characteristics at 77K provides a simple, rapid, sensitive and reproducible method for assessing photoinhibitory injury to leaves. The method should prove especially useful in studies of the occurrence of photoinhibition in nature and of interactive effects between high light levels and major environmental stress factors.Abbreviations and symbols PFD photon flux area density - PSI, PSII photosystem I, II - F_M, F_O, F_V maximum, instantaneous, variable fluorescence emission C.I.W.-D.P.B. Publication No. 773     

Biochemical characteristics of thylakoid membranes in chloroplasts of dark-grown pine cotyledons

Japanese black pine (Pinus thunbergii) cotyledons were found to synthesize chlorophylls in complete darkness during germination, although the synthesis was not as great as that in the light. The compositions of thylakoid components in plastids of cotyledons grown in the dark and light were compared using sodium dodecyl sulfate-polyacrylamide gel electrophoresis patterns of polypeptides and spectroscopic determination of membrane redox components. All thylakoid membrane proteins found in preparations from light-grown cotyledons were also present in preparations from dark-grown cotyledons. However, levels of photosystem I, photosystem II, cytochrome b_[ill]/f, and light-harvesting chlorophyll-protein complexes in dark-grown cotyledons were only one-fourth of those in light-grown cotyledons, on a fresh weight basis. These results suggest that the low abundance of thylakoid components in dark-grown cotyledons is associated with the limited supply of chlorophyll needed to assemble the two photosystem complexes and the light-harvesting chlorophyll-protein complex.     

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 photosystem II in vitro: Spectral and kinetic analyses

Two different preparations of photosystem II (PSII) (BBY-type membrane fragments and PSII core complexes) were isolated from 14-day-old pea seedlings (Pisum sativum L.) and used for spectral and kinetic study of photobleaching of chlorophyll (Chl) and amino acids under photoinhibitory conditions. A short-term (2–4 min) illumination of PSII preparations with high-intensity red light (λ > 610 nm, 800 W/m²) resulted in irreversible photobleaching of Chl at 672 and 682 nm under conditions of both acceptor- and donor-side photoinhibition. At longer illumination exposures (> 10 min) the photobleaching maximum at 682 nm was predominant. The calculated kinetic constants for Chl photobleaching in both absorption bands at temperatures of 20 and 4°C had similar values under different photoinhibitory conditions. The shape of action spectrum for Chl photooxidation indicates that photoinhibition of PSII was sensitized by two spectral forms of Chl with absorption maxima at 670 and 680 nm. The photobleaching of amino acids in PSII membrane fragments was only observed during acceptor-side photoinhibition and displayed the photobleaching peaks at 220 and 274 nm. The photogeneration of superoxide anion radical during donor-side photoinhibition was 4–6 times larger than during acceptor-side photoinhibition. Nevertheless, the kinetics of Chl and amino acid photobleaching in PSII preparations showed no appreciable differences. The activation energies for Chl photooxidation were estimated around 3.5 and 9 kcal/mol during acceptor- and donor-side photoinhibition, respectively, providing evidence for the involvement of biochemical stages in PSII photoinhibition. Based on the data obtained, it is proposed that the antenna Chl, rather than Chl of the reaction center, is the sensitizer for both acceptor- and donor-side photoinhibition of PSII in vitro.     

Adaptation of the thylakoid membranes of pea chloroplasts to light intensities. I. Study on the distribution of chlorophyll-protein complexes

The effect of light intensity (16 h white light and 8 h dark) during growth of pea plants at 20°C on the chlorophyll composition and on the relative distribution of chlorophyll amongst the various chlorophyll-protein of pea thylakoids was studied. The chl a/chl b ratios increased from 2.1 to 3.2 as light intensity during growth varied from 10 to 840 Em^-2 s^-1. This function can be described by two straight lines intersecting at a transition point of approximately 200 Em^-2 s^-1. Similar discontinuities in the responses were observed in the changes in the relative distribution of chlorophyll amongst the various chlorophyll-protein complexes. This demonstrates that the chl a/chl b ratio of the various thylakoids is a good indicator of changes in the relative distribution of chlorophyll. As the chl a/chl b ratio decreased, the amount of chlorophyll associated with photosystem I complexes decreased, that with photosystem II core reaction centre complex was halved, and that with the main chl a/b-proteins of the light-harvesting complex was markedly increased.Abbreviations chl chlorophyll - PS photosystem - SDS sodium dodecyl sulphate - Tricine N-tris (hydroxymethyl) methylglycine     

Probing the events of photoinhibition by altering electron-transport activity and light-harvesting capacity in chloroplast thylakoids

Abstract. The effect of photoinhibition on the activity of photosystem II (PSII) in spinach chloroplasts was investigated. Direct light-induced absorbance change measurements at 320 nm (Δ A ₃₂₀) provided a measure of the PSII charge separation reaction and revealed that photoinhibition prevented the stable photoreduction of the primary quinone acceptor Q_A. Sensitivity to photoinhibition was substantially enhanced by treatment of thylakoids with NH₂OH which extracts manganese from the H₂O-splitting enzyme and prevents electron donation to the reaction centre. Incubation with 3-(3,4,-dichlorophenyl)-1,1-dimethylurea (DCMU) during light exposure did not affect the extent of photoinhibitory damage. The chlorophyll (Chl) b -less chlorina (2 mutant of barley displayed a significantly smaller light-harvesting antenna size of PSII (about 20% of that in wild type chloroplasts) and, simultaneously, a lower sensitivity to photoinhibition. These observations suggest that photoinhibition depends on the amount of light absorbed by PSII and that the process of photoinhibition is accelerated when electron donation to the reaction centre is prevented. It is postulated that the probability of photoinhibition is greater when excitation energy is trapped by P680⁺, the oxidized form of the PSII reaction centre. The results are discussed in terms of the D1/D2 heterodimer which contains the functional PSII components P680, pheophytin, Q_A and Q_B.     

Correlation of iron content, spectral forms of chlorophyll and chlorophyll-proteins in iron deficient cucumber (Cucumis sativus)

Leaves and chloroplast suspensions of severely and slightly iron deficient cucumber ( Cucumis sativus L.) plants were characterized by low-temperature fluorescence emission spectroscopy and Deriphat polyacrylamide gel electrophoresis. The emission spectra of the chloroplast suspensions were resolved into Gaussian components and those changes induced by iron deficiency were related to the variations in the chlorophyll-protein pattern. The symptoms described with these methods were also correlated with the iron content of the leaves. It was concluded that the lack of physiologically active iron caused a relative decrease of photosystem I (PSI) and light harvesting complex I (LHCI), together with the long wavelength fluorescence, especially the 740 nm Gaussian component, and. to a much lesser extent, of the photosystem II (PSII) core complexes (relative increase of 685, 695 nm components). However, the relative decrease in the amount of light harvesting complex II (LHCII) was followed by a relative increase in its fluorescence band at 680 nm, showing that energy transfer from LHCII to core complex II (CCII) was partly disturbed. Thus iron deficiency affected the photosynthetic apparatus in a complex way: it decreased the synthesis of chlorophylls (Chls) and influenced the expression and assembly of Chl-binding proteins.     

Relationship between photoinhibition of photosynthesis, D1 protein turnover and chloroplast structure: effects of protein synthesis inhibitors

Irradiation of Spinach oleracea intact leaf tissue and of mesophyll protoplasts of Valerianella locusta at 20° C with strong light resulted in severe (40–80%) inhibition of photosynthesis, measured as photosystem II electron transport activity in isolated thylakoids or as fluorescence parameter F_V/F_M on intact leaf disks. No net degradation of the D1 protein of photosystem II was seen under these conditions. However, in the presence of streptomycin, an inhibitor of chloroplast protein synthesis, net D1 degradation (up to about 80%) did occur with a half-time of 4–6h, and photoinhibition was enhanced. Thylakoid ultrastructure remained stable during photoinhibition, even when substantial degradation of D1 took place in the presence of streptomycin. When leaf disks were irradiated at 2°C, streptomycin did not influence the degree of photoinhibition, and net Dl degradation did not occur. These results suggest that in excess (photoinhibitory) light at 20°C, turnover (coordinated degradation and synthesis) of D1 diminished the degree of photoinhibition. The observed photoinhibition is thought to be due to the accumulation of inactive photosystem II reaction centres still containing D1. In the presence of streptomycin, the Dl protein was degraded (probably in the previously inactivated centres), but restoration of active centres via D1 synthesis was blocked, leading to more severe photoinhibition. Low temperature (2°C), by restricting both degradation and resynthesis of D1, favoured the accumulation of inactive centres. Streptomycin and chloramphenicol (another inhibitor of chloroplast protein synthesis) were tested for side-effects on photosynthesis. Strong inhibitory effects of chloramphenicol, but much less severe effects of streptomycin were observed.     

Role of the two PsaE isoforms on O2 reduction at photosystem I in Arabidopsis thaliana

Leaves of Arabidopsis thaliana plants grown in short days (8 h light) generate more reactive oxygen species in the light than leaves of plants grown in long days (16 h light). The importance of the two PsaE isoforms of photosystem I, PsaE1 and PsaE2, for O₂ reduction was studied in plants grown under these different growth regimes. In short day conditions a mutant affected in the amount of PsaE1 (psae1-1) reduced more efficiently O₂ than a mutant lacking PsaE2 (psae2-1) as shown by spin trapping EPR spectroscopy on leaves and by following the kinetics of P700⁺ reduction in isolated photosystem I. In short day conditions higher O₂ reduction protected photosystem II against photoinhibition in psae1-1. In contrast in long day conditions the presence of PsaE1 was clearly beneficial for photosynthetic electron transport and for the stability of the photosynthetic apparatus under photoinhibitory conditions. We conclude that the two PsaE isoforms have distinct functions and we propose that O₂ reduction at photosystem I is beneficial for the plant under certain environmental conditions.     

A critical role for the Var2 FtsH homologue of Arabidopsis thaliana in the photosystem II repair cycle in vivo.

Using a var2-2 mutant of Arabidopsis thaliana, which lacks a homologue of the zinc-metalloprotease, FtsH, we demonstrate that this protease is required for the efficient turnover of the D1 polypeptide of photosystem II and protection against photoinhibition in vivo. We show that var2-2 leaves are much more susceptible to light-induced photosystem II photoinhibition than wild-type leaves. Furthermore, the rate of photosystem II photoinhibition in untreated var2-2 leaves is equivalent to that of var2-2 and wild-type leaves, which have been treated with lincomycin, an inhibitor of the photosystem II repair cycle at the level of D1 synthesis. This is in contrast to untreated wild-type leaves, which show a much slower rate of photosystem II photoinhibition due to an efficient photosystem II repair cycle. The recovery of var2-2 leaves from photosystem II photoinhibition is also impaired relative to wild-type. Using Western blot analysis in the presence of lincomycin we show that the D1 polypeptide remains stable in leaves of the var2-2 mutant under photoinhibitory conditions that lead to D1 degradation in wild-type leaves and that the abundance of DegP2 is not affected by the var2-2 mutation. We conclude, therefore, that the Var2 FtsH homologue is required for the cleavage of the D1 polypeptide in vivo. In addition, we identify a conserved lumenal domain in Var2 that is unique to FtsH homologues from oxygenic phototrophs.     

Kinetic correlation of recovery from photoinhibition and zeaxanthin epoxidation

The generation of non-photochemical fluorescence quenching under photoinhibitory illumination and its relaxation under subsequent low light illumination in leaves from intermittent-light-grown pea (Pisum sativum L.) plants (IML-plants) has been investigated. In parallel, we studied (i) the activity of the xanthophyll cycle with emphasis on zeaxanthin formation and reconversion to violaxanthin and (ii) the degradation rate of D1 protein. In comparison to control plants grown in continuous light, IML-plants were much more susceptible to photoinhibition as determined from the increase of slowly (halftimes > 20 min) relaxing quenching (qI) of variable chlorophyll fluorescence. The relaxation (recovery) kinetics of qI (under weak light) in both types of plant depended on the photon flux density, temperature and duration of pre-illumination. The recovery time generally increased with an increasing degree of qI. In IML-plants, relaxation of qI was kinetically closely related to the epoxidation of zeaxanthin. At high degrees of photosystem II inhibition the kinetics resembled those of D1 degradation. The results are discussed in terms of the mechanisms of photosystem II inactivation in vivo.