Selective Photoinhibition of Photosystem I in Isolated Thylakoid Membranes from Cucumber and Spinach

The site of photoinhibition at low temperatures in leaves ofa chilling-sensitive plant, cucumber, is photosystem I [Terashimaet al. (1994) Planta 193: 300]. As described herein, selectivephotoinhibition of PSI can also be induced in isolated thylakoidmembranes in vitro. Inhibition was observed both at chillingtemperatures and at 25°C, and not only in the thylakoidmembranes isolated from cucumber, but also in those isolatedfrom a chilling-tolerant plant, spinach. Comparison of theseobservations in vitro to the earlier results in vivo indicatesthat (1) photoinhibition of PSI is a universal phenomenon; (2)a mechanism exists to protect PSI in vivo; and (3) the protectivemechanism is chilling-sensitive in cucumber. The chilling-sensitivecomponent seems to be lost during the isolation of thylakoidmembranes. Very weak light (10–20µmol m^-2 s^-1) wassufficient to cause the inhibition of PSI. About 80% of theoxygen-evolving activity by PSII was maintained even after theactivity of PSI had decreased by more than 70%. This is thefirst report of the selective photoinhibition of PSI in vitro. (Received March 1, 1995; Accepted April 26, 1995)     

Protective Action of Spermine and Spermidine against Photoinhibition of Photosystem I in Isolated Thylakoid Membranes

The photo-stability of photosystem I (PSI) is of high importance for the photosynthetic processes. For this reason, we studied the protective action of two biogenic polyamines (PAs) spermine (Spm) and spermidine (Spd) on PSI activity in isolated thylakoid membranes subjected to photoinhibition. Our results show that pre-loading thylakoid membranes with Spm and Spd reduced considerably the inhibition of O₂ uptake rates, P700 photooxidation and the accumulation of superoxide anions (O₂ ⁻) induced by light stress. Spm seems to be more effective than Spd in preserving PSI photo-stability. The correlation of the extent of PSI protection, photosystem II (PSII) inhibition and O₂ ⁻ generation with increasing Spm doses revealed that PSI photo-protection is assumed by two mechanisms depending on the PAs concentration. Given their antioxidant character, PAs scavenge directly the O₂ ⁻ generated in thylakoid membranes at physiological concentration (1 mM). However, for non-physiological concentration, the ability of PAs to protect PSI is due to their inhibitory effect on PSII electron transfer.     

Photosystem II Regulation and Dynamics of the Chloroplast D1 Protein in Arabidopsis Leaves during Photosynthesis and Photoinhibition

Arabidopsis thaliana leaves were examined in short-term (1 h) and long-term (10 h) irradiance experiments involving growth, saturating and excess light. Changes in photosynthetic and chlorophyll fluorescence parameters and in populations of functional photosystem II (PSII) centers were independently measured. Xanthophyll pigments, 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)-binding sites, the amounts of D1 protein, and the rates of D1 protein synthesis were determined. These comprehensive studies revealed that under growth or light-saturating conditions, photosynthetic parameters remained largely unaltered. Photoprotection occurred at light saturation indicated by a dark-reversible increase in non-photochemical quenching accompanied by a 5-fold increase in antheraxanthin and zeaxanthin. No consistent change in the concentrations of functional PSII centers, DCMU-binding sites, or D1 protein pool size occurred. D1 protein synthesis was rapid. In excess irradiance, quantum yield of O2 evolution and the efficiency of PSII were reduced, associated with a 15- to 20-fold increase in antheraxanthin and zeaxanthin and a sustained increase in nonphotochemical quenching. A decrease in functional PSII center concentration occurred, followed by a decline in the concentration of D1 protein; the latter, however, was not matched by a decrease in DCMU-binding sites. In the most extreme treatments, DCMU-binding site concentration remained 2 times greater than the concentration of D1 protein recognized by antibodies. D1 protein synthesis rates remained unaltered at excess irradiances.     

The Stromal Chloroplast Deg7 Protease Participates in the Repair of Photosystem II after Photoinhibition in Arabidopsis

Light is the ultimate source of energy for photosynthesis; however, excessive light leads to photooxidative damage and hence reduced photosynthetic efficiency, especially when combined with other abiotic stresses. Although the photosystem II (PSII) reaction center D1 protein is the primary target of photooxidative damage, other PSII core proteins are also damaged and degraded. However, it is still largely unknown whether degradation of D1 and other PSII proteins involves previously uncharacterized proteases. Here, we show that Deg7 is peripherally associated with the stromal side of the thylakoid membranes and that Deg7 interacts directly with PSII. Our results show that Deg7 is involved in the primary cleavage of photodamaged D1, D2, CP47, and CP43 and that this activity is essential for its function in PSII repair. The double mutants deg5 deg7 and deg8 deg7 showed no obvious phenotypic differences under normal growth conditions, but additive effects were observed under high light. These results suggest that Deg proteases on both the stromal and luminal sides of the thylakoid membranes are important for the efficient PSII repair in Arabidopsis (Arabidopsis thaliana).Chloroplasts of higher plants carry out one of the most important biochemical reactions: the capture of light energy and its conversion into chemical energy. Although light is the ultimate source of energy for photosynthesis, it can also be harmful to plants. Light-induced loss of photosynthetic efficiency, which is generally termed as photoinhibition, limits plant growth and lowers productivity, especially when combined with other abiotic stresses.The main target of photoinhibition is PSII, which catalyzes the light-dependent water oxidation concomitantly with oxygen production (for review, see Prasil et al., 1992; Aro et al., 1993; Adir et al., 2003). In higher plants, PSII consists of more than 20 subunits, including the reaction center D1 and D2 proteins, cytochrome (Cyt) b₅₅₉, the light-harvesting chlorophyll a-binding proteins CP47 and CP43, the oxygen-evolving 33-kD protein (PsbO), and several low molecular mass proteins (Nelson and Yocum, 2006). The PSII reaction center D1 protein has been identified among PSII proteins as the primary target of light-induced damage (Kyle et al., 1984; Mattoo et al., 1984; Ohad et al., 1984; Adir et al., 1990), but several studies have shown that the D2, CP47, and CP43 proteins are degraded under photoinhibitory conditions (Schuster et al., 1988; Yamamoto and Akasaka, 1995; Jansen et al., 1999; Adir et al., 2003). Moreover, several small PSII subunits, such as PsbH, PsbW, and Cyt b₅₅₉, were also found to be frequently replaced within PSII (Hagman et al., 1997; Ortega et al., 1999; Bergantino et al., 2003). Evidence for the involvement of two families of proteases, FtsH and Deg, in the degradation of the D1 protein in thylakoids of higher plants has been recently described (Lindahl et al., 1996, 2000; Bailey et al., 2002; Sakamoto et al., 2003; Silva et al., 2003; Kapri-Pardes et al., 2007; Sun et al., 2007a, 2007b). However, it is still largely unknown whether degradation of D1 and other PSII proteins involves previously uncharacterized proteases.DegP (or HtrA) proteases were initially identified based on the fact that they are required for the survival of Escherichia coli at high temperatures and for the degradation of abnormal periplasmic proteins (Lipinska et al., 1988; Strauch and Beckwith, 1988). DegP is an ATP-independent Ser endopeptidase, and it contains a trypsin-like protease domain at the N terminus, followed by two PDZ domains (Gottesman, 1996; Pallen and Wren, 1997; Clausen et al., 2002). PDZ domains appear to be important for complex assembly and substrate binding through three or four residues in the C terminus of their target proteins (Doyle et al., 1996; Harris and Lim, 2001). DegP switches between chaperone and protease functions in a temperature-dependent manner. The chaperone function dominates at low temperatures, and DegP becomes proteolytically active at elevated temperatures (Spiess et al., 1999). Crystal structures of different members of the DegP protein family (Krojer et al., 2002; Li et al., 2002; Kim et al., 2003; Wilken et al., 2004) have revealed the structure-function relationship of these PDZ-containing proteases. Trimeric DegP is the functional unit, and the hexameric DegP is formed via the staggered association of trimers (Clausen et al., 2002; Kim and Kim, 2005). At normal growth temperatures, the active site of the protease is located within the chamber of hexameric DegP, which is not accessible to the substrates. However, at high temperatures, conformational changes induce the activation of the protease function (Krojer et al., 2002). Recent studies have shed light on the substrate binding-induced formation of larger oligomeric complexes of DegP (Jiang et al., 2008; Krojer et al., 2008).In Arabidopsis (Arabidopsis thaliana), 16 genes coding for DegP-like proteases have been identified, and at least seven gene products are predicted to be located in chloroplasts (Kieselbach and Funk, 2003; Huesgen et al., 2005; Adam et al., 2006; Sakamoto, 2006; Kato and Sakamoto, 2009). Based on proteomic data, four Deg proteases have been shown to be localized to the chloroplast (Peltier et al., 2002; Schubert et al., 2002) and functionally characterized. Deg1, Deg5, and Deg8 are located in thylakoid lumen, and Deg2 is peripherally associated with the stromal side of thylakoid membranes (Itzhaki et al., 1998; Haußühl et al., 2001; Sun et al., 2007a). Recombinant DegP1, now renamed Deg1, has been shown to be proteolytically active toward thylakoid lumen proteins such as plastocyanin and PsbO of PSII in vitro (Chassin et al., 2002). A 5.2-kD C-terminal fragment of the D1 protein was detected in vitro after incubation of recombinant Deg1 with inside-out thylakoid membranes. In transgenic plants with reduced levels of Deg1, fewer of its 16- and 5.2-kD degradation products were observed (Kapri-Pardes et al., 2007). Deg5 and Deg8 form a dodecameric complex in the thylakoid lumen, and recombinant Deg8 is able to degrade the photodamaged D1 protein of PSII in an in vitro assay (Sun et al., 2007a). The 16-kD N-terminal degradation fragment of the D1 protein was detected in wild-type plants but not in a deg5 deg8 double mutant after high-light treatment. The deg5 deg8 double mutant showed increased sensitivity to high light and high temperature in terms of growth and PSII activity compared with the single mutants deg5 and deg8, suggesting that Deg5 and Deg8 have overlapping functions in the primary cleavage of the CD loop of the D1 protein (Sun et al., 2007a, 2007b). In vitro analysis has demonstrated that recombinant stroma-localized Deg2 was also shown to be involved in the primary cleavage of the DE loop of the D1 protein (Haußühl et al., 2001). However, analysis of a mutant lacking Deg2 suggested that Deg2 may not be involved in D1 degradation in vivo (Huesgen et al., 2006).Here, we have expressed and purified a recombinant DegP protease, His-Deg7. In vitro experiments showed that His-Deg7 is proteolytically active toward the PSII proteins D1, D2, CP43, and CP47. In vivo analyses of a deg7 mutant revealed that the mutant is more sensitive to high light stress than the wild-type plants. We demonstrated that Deg7 is a chloroplast stroma protein associated with the thylakoid membranes and that it interacts with PSII, which suggests that it can cleave the stroma-exposed region of substrate proteins. Our results also provide evidence that Deg7 is important for maintaining PSII function.     

Photoinhibition of Photosynthesis: Role of Carotenoids in Photoprotection of Chloroplast Constituents

Exposure of plants to irradiation, in excess to saturate photosynthesis, leads to reduction in photosynthetic capacity without any change in bulk pigment content. This effect is known as photoinhibition. Photoinhibition is followed by destruction of carotenoids (Cars), bleaching of chlorophylls (Chls), and increased lipid peroxidation due to formation of reactive oxygen species if the excess irradiance exposure continues. Photoinhibition of photosystem 2 (PS2) in vivo is often a photoprotective strategy rather than a damaging process. For sustainable maintenance of chloroplast function under high irradiance, the plants develop various photoprotective strategies. Cars perform essential photoprotective roles in chloroplasts by quenching the triplet Chl and scavenging singlet oxygen and other reactive oxygen species. Recently photoprotective role of xanthophylls (zeaxanthin) for dissipation of excess excitation energy under irradiance stress has been emphasised. The inter-conversion of violaxanthin (Vx) into zeaxanthin (Zx) in the light-harvesting complexes (LHC) serves to regulate photon harvesting and subsequent energy dissipation. De-epoxidation of Vx to Zx leads to changes in structure and properties of these xanthophylls which brings about significant structural changes in the LHC complex. This ultimately results in (1) direct quenching of Chl fluorescence by singlet-singlet energy transfer from Chl to Zx, (2) trans-thylakoid membrane mediated, pH-dependent indirect quenching of Chl fluorescence. Apart from these, other processes such as early light-inducible proteins, D1 turnover, and several enzymatic defence mechanisms, operate in the chloroplasts, either for tolerance or to neutralise the harmful effect of high irradiance.     

A chloroplast membrane lacking Photosystem II. Thylakoid stacking in the absence of the Photosystem II particle

The polypeptide composition and membrane structure of a variegated mutant of tobacco have been investigated. The pale green mutant leaf regions contain chloroplasts in which the amount of membrane stacking has been reduced (although not totally eliminated). The mutant membranes are almost totally deficient in Photosystem II when compared to wild-type chloroplast membranes, but still show near-normal levels of Photosystem I activity. The pattern of membrane polypeptides separated by sodium dodecyl sulfate-polyacrylamide gel electrophoresis shows several differences between mutant and wild-type membranes, although the major chlorophyll-protein complexes described in many other plant species are present in both mutant and wild-type samples. Freeze-fracture analysis of the internal structure of these photosynthetic membranes shows that the Photosystem II-deficient membranes lack the characteristic large particle associated with the E fracture face of the thylakoid. These membranes also lack a tetramer-like particle visible on the inner (ES) surface of the membrane. The other characteristics of the photosynthetic membrane, including the small particles observed on the P fracture faces in both stacked and unstacked regions, and the characteristic changes in the background matrix of the E fracture face which accompany thylakoid stacking, are unaltered in the mutant. From these and other observations we conclude that the large (EF and ES) particle represents an amalgam of many components comprising the Photosystem II reaction complex, that the absence of one or more of its components may prevent the structure from assembling, and that in its absence, Photosystem II activity cannot be observed.     

The Molecular Mechanism of Transport by the Mitochondrial ADP/ATP Carrier

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Heterogeneity and Photoinhibition of Photosystem II Studied with Thermoluminescence

Thermoluminescence (TL) signals were recorded from grana stacks, margins, and stroma lamellae from fractionated, dark-adapted thylakoid membranes of spinach (Spinacia oleracea L.) in the absence and in the presence of 2,6-dichlorphenylindophenol (DCMU). In the absence of DCMU, the TL signal from grana fractions consisted of a homogenous B-band, which originates from recombination of the semi-quinone Q_B⁻ with the S₂ state of the water-splitting complex and reflects active photosystem II (PSII). In the presence of DCMU, the B-band was replaced by the Q-band, which originates from an S₂Q_A⁻ recombination. Margin fractions mainly showed two TL-bands, the B- and C-bands, at approximately 50°C in the absence of DCMU, and Q- and C-bands in the presence of DCMU. The C-band is ascribed to a Tyr_D⁺-Q_A⁻ recombination. In the absence of DCMU, the fractions of stromal lamellae mainly gave rise to a TL emission at 42°C. The intensity of this band was independent of the number of excitation flashes and was shifted to higher temperatures (52°C) after the addition of DCMU. Based on these observations, this band was considered to be a C-band. After photoinhibitory light treatment of uncoupled thylakoid membranes, the TL intensities of the B- and Q-bands decreased, whereas the intensity at 45°C (C-band) slightly increased. It is proposed that the 42 to 52°C band that was observed in marginal and stromal lamellae and in photoinhibited thylakoid membranes reflects inactive PSII centers that are assumed to be equivalent to inactive PSII Q_B-nonreducing centers.     

Carbon Dioxide-Induced Oscillations in Fluorescence and Photosynthesis: Role of Thylakoid Membrane Energization in Regulation of Photosystem II Activity

The response of CO₂ fixation to a sudden increase in ambient CO₂ concentration has been investigated in intact leaf tissue from spinach (Spinacia oleracea) using a dual channel infrared gas analyzer. Simultaneous with these measurements, changes in fluorescence emission associated with a weak, modulated measuring beam were recorded. Application of brief (2-3 seconds) dark intervals enabled estimation of the dark fluorescence level (F_o) under both steady state and transient conditions. The degree of suppression of F_o level fluorescence in the light was strongly correlated with nonphotochemical quenching under all conditions. During CO₂-induced oscillations in photosynthesis under 2% O₂ the changes in nonphotochemical quenching anticipate changes in the rate of uptake of CO₂. At such low levels of O₂ and constant illumination, changes in the relative quantum efficiency of open photosystem II units were estimated as the ratio of the rate of CO₂ uptake and the photochemical quenching coefficient. Under the same conditions the relative quantum efficiency of photosystem II was found to vary inversely with the degree of nonphotochemical quenching. The relationship between changes in the rate of CO₂ uptake: photochemical quenching coefficient and nonphotochemical quenching was altered somewhat when the same experiment was conducted under 20% O₂. The results suggest that electron transport coupled to reduction of O₂ occurs to varying degrees with time during oscillations, especially when ambient O₂ concentrations are high.     

Identification and Characterization of ATP/ADP Isopentenyltransferases (ATP/ADP PpIPTs) Genes in Peach

ATP/ADP isopentenyltransferase (IPTs) genes encode key enzymes involved in cytokinin synthesis. In this study, the functions of ATP/ADP PpIPTs in peach were investigated. According to the genome sequence, we have found and verified that there are four members of this gene family in peach, namely, PpIPT1, PpIPT3, PpIPT5, and PpIPT7. Overexpression of each of these genes in Arabidopsis resulted in increased levels of cytokinins in the transgenic plants, confirming their roles in cytokinin synthesis. Numerous altered phenotypes were observed in the transgenic plants, including vigorous growth and enhanced salt resistance. ATP/ADP PpIPTs were expressed in tissues throughout the plant, but the expression patterns differed between the genes. Only PpIPT3 was upregulated within 2 h after the application of nitrate to N-deprived peach seedlings, and the increase was resistant to pre-treatment of a specific nitrate metabolism inhibitor. Results showed that ATP/ADP PpIPT expression levels decreased significantly in pulp within 2 weeks after flowering and remained low. However, pulp cytokinin levels were quite high during this time. Only PpIPT5 in seed increased significantly within 2 weeks after flowering, which was consistent with cytokinin levels during early fruit development, suggesting that PpIPT5 in seed is the key gene for cytokinin biosynthesis during early fruit development. ATP/ADP PpIPT expression also increased significantly during later fruit development in seed.

     

The ADP/ATP Carrier and Its Relationship to Oxidative Phosphorylation in Ancestral Protist Trypanosoma brucei

The highly conserved ADP/ATP carrier (AAC) is a key energetic link between the mitochondrial (mt) and cytosolic compartments of all aerobic eukaryotic cells, as it exchanges the ATP generated inside the organelle for the cytosolic ADP. Trypanosoma brucei, a parasitic protist of medical and veterinary importance, possesses a single functional AAC protein (TbAAC) that is related to the human and yeast ADP/ATP carriers. However, unlike previous studies performed with these model organisms, this study showed that TbAAC is most likely not a stable component of either the respiratory supercomplex III+IV or the ATP synthasome but rather functions as a physically separate entity in this highly diverged eukaryote. Therefore, TbAAC RNA interference (RNAi) ablation in the insect stage of T. brucei does not impair the activity or arrangement of the respiratory chain complexes. Nevertheless, RNAi silencing of TbAAC caused a severe growth defect that coincides with a significant reduction of mt ATP synthesis by both substrate and oxidative phosphorylation. Furthermore, TbAAC downregulation resulted in a decreased level of cytosolic ATP, a higher mt membrane potential, an elevated amount of reactive oxygen species, and a reduced consumption of oxygen in the mitochondria. Interestingly, while TbAAC has previously been demonstrated to serve as the sole ADP/ATP carrier for ADP influx into the mitochondria, our data suggest that a second carrier for ATP influx may be present and active in the T. brucei mitochondrion. Overall, this study provides more insight into the delicate balance of the functional relationship between TbAAC and the oxidative phosphorylation (OXPHOS) pathway in an early diverged eukaryote.     

Activation of a Reserve Pool of Photosystem II in Chlamydomonas reinhardtii Counteracts Photoinhibition

The effect of strong irradiance (2000 micromole photons per square meter per second) on PSII heterogeneity in intact cells of Chlamydomonas reinhardtii was investigated. Low light (LL, 15 micromole photons per square meter per second) grown C. reinhardtii are photoinhibited upon exposure to strong irradiance, and the loss of photosynthetic functioning is due to damage to PSII. Under physiological growth conditions, PSII is distributed into two pools. The large antenna size (PSII_α) centers account for about 70% of all PSII in the thylakoid membrane and are responsible for plastoquinone reduction (Q_b-reducing centers). The smaller antenna (PSII_β) account for the remainder of PSII and exist in a state not yet able to photoreduce plastoquinone (Q_b-nonreducing centers). The exposure of C. reinhardtii cells to 60 minutes of strong irradiance disabled about half of the primary charge separation between P680 and pheophytin. The PSII_β content remained the same or slightly increased during strong-irradiance treatment, whereas the photochemical activity of PSII_α decreased by 80%. Analysis of fluorescence induction transients displayed by intact cells indicated that strong irradiance led to a conversion of PSII_β from a Q_b-nonreducing to a Q_b-reducing state. Parallel measurements of the rate of oxygen evolution revealed that photosynthetic electron transport was maintained at high rates, despite the loss of activity by a majority of PSII_α. The results suggest that PSII_β in C. reinhardtii may serve as a reserve pool of PSII that augments photosynthetic electron-transport rates during exposure to strong irradiance and partially compensates for the adverse effect of photoinhibition on PSII_α.     

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.     

High-Chloride Concentrations Abolish the Binding of Adenine Nucleotides in the Mitochondrial ADP/ATP Carrier Family

The ADP/ATP carrier (AAC) is a very effective membrane protein that mediates the exchange of ADP and ATP across the mitochondrial membrane. In vivo transport measurements on the AAC overexpressed in Escherichia coli demonstrate that this process can be severely inhibited by high-chloride concentrations. Molecular-dynamics simulations reveal a strong modification of the topology of the local electric field related to the number of chloride ions inside the cavity. Halide ions are shown to shield the positive charges lining the internal cavity of the carrier by accurate targeting of key basic residues. These specific amino acids are highly conserved as highlighted by the analysis of multiple AAC sequences. These results strongly suggest that the chloride concentration acts as an electrostatic lock for the mitochondrial AAC family, thereby preventing adenine nucleotides from reaching their dedicated binding sites.     

Zeaxanthin Synthesis, Energy Dissipation, and Photoprotection of Photosystem II at Chilling Temperatures

When leaves of a mangrove, Rhizophora mangle, were exposed to an excess of light at chilling temperatures, synthesis of zeaxanthin through violaxanthin de-epoxidation as well as nonphotochemical fluorescence quenching were markedly reduced. The results suggest a protective role of energy dissipation against the adverse effects of high light and chilling temperatures: leaves of R. mangle that had been preilluminated in 2% O₂, 0% CO₂ at low photon flux density and showed a high level of zeaxanthin, and leaves that had been kept in the dark and contained no zeaxanthin, were both exposed to high light and chilling temperatures (5°C leaf temperature) in air and then held under control conditions in low light in air at 25°C. Measurements of chlorophyll a fluorescence at room temperature showed that the photochemical efficiency of PSII and the yield of maximum fluorescence of the preilluminated leaf recovered completely within 1 to 3 hours under the control conditions. In contrast, the fluorescence responses of the predarkened leaf in high light at 5°C did not recover at all. During a dark/light transient in 2% O₂, 0% CO₂ in low light at 5°C, nonphotochemical fluorescence quenching increased linearly with an increase in the zeaxanthin content in leaves of R. mangle. In soybean (Glycine max) leaves, which contained a background level of zeaxanthin in the dark, a similar treatment with excess light induced a level of nonphotochemical fluorescence quenching that was not paralleled by an increase in the zeaxanthin content.     

Photoinhibition in vivo of Photosystem II reactions during developmentof the photosystems of wheat seedlings

Photoinhibition of O₂ evolution and reactions leading to millisecond-delayed light emission (ms-DLE) of chlorophyll by illumination of leaves with excess white light were investigated in wheat seedlings greened for different times in a special chamber with constant conditions (20°C; CO₂ and humidity). A sharp reduction in initial and steady state rates of O₂ evolution and in the intensity of different components of ms-DLE under excess light on the stage of lag-phase of chlorophyll biosynthesis (4–6h of greening) were observed. An increasing stability of the oxygen-evolving process and ms-DLE of chlorophyll during formation of the thylakoid membrane photosystems (12–24 h of greening) was shown. Rifampicin did not influence the stability of oxygen evolution whereas cycloheximide led to the intensification of photoinhibition of the initial and steady-state rates of oxygen evolution under the inhibitory light action. The early stages of photosystems formation during short time of greening of etiolated seedlings were more sensitive to the action of inhibitory light, possibly due to a weak interaction of the oxygen-evolving system components and connection with reaction centers of Photosystem II.     

水淹对水芹叶片结构和光系统Ⅱ光抑制的影响

通过探讨在水淹条件下水芹（Oenanthe javanica）叶片结构的变化以及出水对其光系统II功能和光抑制的影响,阐明水芹光合机构在水淹条件下及出水后死亡的可能原因。结果表明：水淹条件下新生沉水功能叶光系统Ⅱ（PSⅡ）最大光化学效率（Fv/Fm）、电子传递活性与对照叶片差异很小,但水淹使气生功能叶的Fv/Fm显著降低;植株总生物量呈负增长趋势;活体弱光条件下,沉水叶出水后2小时叶片相对含水量（RWC）和Fv/Fm无显著变化;中等光强和强光条件下其RWC和Fv/Fm迅速降低;离体条件下,5小时的中等光强对沉水叶的Fv/Fm影响不显著,在随后的弱光下能恢复到出水时的初始状态;强光能使沉水叶的Fv/Fm大幅降低,且弱光下不能恢复到出水时的初始水平;在解剖结构上,水芹沉水叶的叶片总厚度、上下表皮厚度和气孔大小都显著低于气生叶,而且沉水叶没有明显的栅栏组织分化,但是沉水叶上表皮的气孔密度显著高于气生叶。研究结果表明,水淹使水芹原气生叶PSⅡ功能迅速衰退,但对新生沉水叶片影响很小。水芹植株出水后,沉水叶片结构变化使其在光下保水能力下降,而强光导致了光合机构的光抑制和反应中心失活。田间条件下两者共同作用则加剧了对叶片光合机构的破坏,进而致使其死亡。