Showing papers on "Photoinhibition published in 2005"

Molecular basis of photoprotection and control of photosynthetic light-harvesting

Abstract: In order to maximize their use of light energy in photosynthesis, plants have molecules that act as light-harvesting antennae, which collect light quanta and deliver them to the reaction centres, where energy conversion into a chemical form takes place. The functioning of the antenna responds to the extreme changes in the intensity of sunlight encountered in nature. In shade, light is efficiently harvested in photosynthesis. However, in full sunlight, much of the energy absorbed is not needed and there are vitally important switches to specific antenna states, which safely dissipate the excess energy as heat. This is essential for plant survival, because it provides protection against the potential photo-damage of the photosynthetic membrane. But whereas the features that establish high photosynthetic efficiency have been highlighted, almost nothing is known about the molecular nature of the dissipative states. Recently, the atomic structure of the major plant light-harvesting antenna protein, LHCII, has been determined by X-ray crystallography. Here we demonstrate that this is the structure of a dissipative state of LHCII. We present a spectroscopic analysis of this crystal form, and identify the specific changes in configuration of its pigment population that give LHCII the intrinsic capability to regulate energy flow. This provides a molecular basis for understanding the control of photosynthetic light-harvesting.

Handbook of Photosynthesis

Abstract: Preface Principles of Photosynthesis Mechanisms of Photosynthetic Oxygen Evolution and the Fundamental Hypotheses of Photosynthesis (Yuzeir Zeinalov) Thermoluminescence as a Tool in the study of Photosynthesis (Anil S. Bhagwat and Swapan K. Bhattachrjee) Biochemistry of Photosynthesis Chlorophyll Biosynthesis - A review (Benoit Schoefs and Martine Bertrand) Chloroplast Biogenesis 90, Probing the Relationship between Chlorophyll Biosynthetic Routes and the Topography of Chloroplast Biogenesis by Resonance Excitation Energy Transfer Determinations (Constantin A. Rebeiz, Karen J. Kopetz, and Vladimir L. Kolossov, USA) Protochlorophyllide Photoreduction - A review (Martine Bertrand and Benoit Schoefs) Formation and Demolition of Chloroplast during Leaf Ontogeny (Basanti Biswal) Role of Phosphorus in Photosynthetic Carbon Metabolism (Anna M. Rychter and I.M. Rao) Inhibition on Inactivation of Higher Plant Chloroplast Electron Transport (Rita Barr and Frederick L. Crane) Molecular Aspects of Photosynthesis: Photosystems, Photosynthetic Enzymes and Genes Photosystem I Structures and Functions (Teisuo Hiyama) Covalent Modification of Photosystem II Reaction-Center Polypeptides (Julian P. Whitelegge) Reactive Oxygen Species as Signaling Molecules Controlling Stress Adaptation in Plants (Tsanko Gechev, Ilya Gadjev, Stefan Dukiandjiev, and Ivan Minkov) Plastid Morphogenesis (Jan Hudak, Eliska Galova, and Lenka Zemanova) Plastid Proteases (Dennis E. Buetow) Supramolecular Organization of Water-Soluble Photosynthetic Enzymes Along the Thylakoid Membranes in Chloroplasts (Jayashree K. Sainis and Michael Melzer) Cytochrome C6 Genes in Cyanobacteria and Higher Plants (Kwok Ki Ho) Atmospheric and Environmental Factors Affecting Photosynthesis External and Internal Factors Responsible for Midday Depression of Photosynthesis (Da-Quan Xu and Yun-Kang Shen) Root Oxygen Depravation and the Reduction of Leaf Stomatal Aperture and Gas Exchange Effects on Photosynthesis (R.E. Sojka, H.D. Scott, and D.M. Oosterhuis) Rising Atmospheric CO2 and C4 Photosynthesis (Joseph C.V. Vu) Influence of High Light Intensity on Photosynthesis: Photoinhibition and Energy Dissipation (Robert Carpentier) Development of Functional Thylakoid Membranes: Regulation by Light and Hormones (Peter Nyitrai) Photosynthetic Pathways in Various Crop Plants Photosynthetic Carbon Assimilation of C3, C4 and CAM Pathways (Anil S. Bhagwat) Photosynthesis in Non-Typical C4 Species (Maria Valeria Lara and Carlos Santiago Andreo) Photosynthesis in Lower and Monocellular Plants Regulation of Phycobilisome Biosynthesis and Degradation in Cyanobacteria (Johannes Geiselmann, Jean Houmard, and Benoit Schoefs) Photosynthesis in Higher Plants Short-Term and Long-Term Regulation of Photosynthesis During Leaf Development (Dan Stessman, Martin Spalding, and Steven Rodermel) Recent Advances in Chloroplast Development in Higher Plants (Ilia D. Denev, Galina T. Yahubian, and Ivan N. Minkov) Photosynthesis in Different Plant Parts Photosynthesis in Leaf, Stem, Flower, and Fruit (Abdul Wahid, and Ejaz Rasul) Photosynthesis and Plant/Crop Productivity and Photosynthetic Products Photosynthetic Plant Productivity (Lubomir Natr and David W. Lawlor) Photosynthates Formation and Partitioning in Crop Plants (Alberto A. Iglesias and F.E. Podesta) Photosynthesis and Plant Genetics Crop Radiation Use Efficiency - Avenue for Genetic Improvement (G.V. Subbarao, O. Ito, and W.L. Berry) Physiological Perspectives on Improving Crop Adaptation to Drought - Justification for a Systemic Compnent-Based Approach (G.V. Subbarao, O. Ito, R. Serraj, J.H. Crouch, S. Tobita, K. Okada, C.T. Hash, R. Ortiz, and W.L. Berry) Photosynthetic Activity Measurements and Analysis of Photosynthetic Pigments Whole-Plant CO2 Exchange as a Non-Invasive Tool for Measuring Growth (Evangelos D. Leonardos, and Bernard Grodzinski) Approaches to Measuring Plant Photosynthetic Activity (Elena Masarovicova and Katarina Kralova) Analysis of Photosynthetic Pigments: An Update (Martine Bertrand, Jose L. Garrido, and Benoit Schoefs) Photosynthesis and Its Relationship with other Plant Physiological Processes Photosynthesis, Respiration, and the Limits to Growth (Bruce N. Smith, Heidi A. Summers, Emily A. Keller, and Tonya Thygerson) Nitrogen Assimilation and Carbon Metabolism (Alberto A. Iglesias, Maria J. Estrella, and Fernando Pieckenstain) Leaf Senescence (Agnieszka Mostowska) Photosynthesis Under Environmental Stress Conditions Photosynthesis in Plants under Stressful Conditions (Rama Shanker Dubey) Photosynthetic Response of Green Plants to Environmental Stress: Inhibition of Photosynthesis and Adaptational Mechanisms (Basanti Biswal) Salt and Drought Stress Effects on Photosynthesis, Enzyme Cohesion and High Turn Over Metabolite Shuttling, Essential for Functioning of Pathways, Is Impaired by Changes in Cytosolic Water Potential (B, Huchzermeyer and H.W. Koyro) Photosynthetic Carbon Metabolism of Crops under Salt Stress (Bruria Heuer) Photosynthesis under Drought Stress (Habib-ur-Rahman Athar and Muhammad Ashraf) Role of Plant Growth Regulators in Stomatal Limitation to Photosynthesis During Water Stress (Jana Pospisilova and Ian C. Dodd) Adverse Effects of UV-B Light on the Structure and Function of the Photosynthetic Apparatus (Imre Vass, Andras Szilard, and Cosmin Sicora) Heavy Metal Toxicity Induced Alterations in Photosynthetic Metabolism in Plants (Shruti Mishra and R.S, Dubey) Effects of Heavy Metals on Chlorophyll-Protein Complexes in Higher Plants: Causes and Consequences (Eva Sarvari) Photosynthesis in the Past, Present, and Future The Origin and Evolution of C4 Photosynthesis (Bruce N. Smith)

Vitamin E Protects against Photoinhibition and Photooxidative Stress in Arabidopsis thaliana

Abstract: Vitamin E is considered a major antioxidant in biomembranes, but little evidence exists for this function in plants under photooxidative stress. Leaf discs of two vitamin E mutants, a tocopherol cyclase mutant (vte1) and a homogentisate phytyl transferase mutant (vte2), were exposed to high light stress at low temperature, which resulted in bleaching and lipid photodestruction. However, this was not observed in whole plants exposed to long-term high light stress, unless the stress conditions were extreme (very low temperature and very high light), suggesting compensatory mechanisms for vitamin E deficiency under physiological conditions. We identified two such mechanisms: nonphotochemical energy dissipation (NPQ) in photosystem II (PSII) and synthesis of zeaxanthin. Inhibition of NPQ in the double mutant vte1 npq4 led to a marked photoinhibition of PSII, suggesting protection of PSII by tocopherols. vte1 plants accumulated more zeaxanthin in high light than the wild type, and inhibiting zeaxanthin synthesis in the vte1 npq1 double mutant resulted in PSII photoinhibition accompanied by extensive oxidation of lipids and pigments. The single mutants npq1, npq4, vte2, and vte1 showed little sensitivity to the stress treatments. We conclude that, in cooperation with the xanthophyll cycle, vitamin E fulfills at least two different functions in chloroplasts at the two major sites of singlet oxygen production: preserving PSII from photoinactivation and protecting membrane lipids from photooxidation.

Photosystem II core phosphorylation and photosynthetic acclimation require two different protein kinases

Abstract: Illumination changes elicit modifications of thylakoid proteins and reorganization of the photosynthetic machinery. This involves, in the short term, phosphorylation of photosystem II (PSII) and light-harvesting (LHCII) proteins. PSII phosphorylation is thought to be relevant for PSII turnover, whereas LHCII phosphorylation is associated with the relocation of LHCII and the redistribution of excitation energy (state transitions) between photosystems. In the long term, imbalances in energy distribution between photosystems are counteracted by adjusting photosystem stoichiometry. In the green alga Chlamydomonas and the plant Arabidopsis, state transitions require the orthologous protein kinases STT7 and STN7, respectively. Here we show that in Arabidopsis a second protein kinase, STN8, is required for the quantitative phosphorylation of PSII core proteins. However, PSII activity under high-intensity light is affected only slightly in stn8 mutants, and D1 turnover is indistinguishable from the wild type, implying that reversible protein phosphorylation is not essential for PSII repair. Acclimation to changes in light quality is defective in stn7 but not in stn8 mutants, indicating that short-term and long-term photosynthetic adaptations are coupled. Therefore the phosphorylation of LHCII, or of an unknown substrate of STN7, is also crucial for the control of photosynthetic gene expression.

Is photoinhibition of zooxanthellae photosynthesis the primary cause of thermal bleaching in corals

Abstract: The bleaching of corals in response to increases in temperature has resulted in significant coral reef degradation in many tropical marine ecosystems. This bleaching has frequently been attributed to photoinhibition of photosynthetic electron transport and the consequent photodamage to photosystem II (PSII) and the production of damaging reactive oxygen species (ROS) in the zooxanthellae (Symbiodinium spp.). However, these events may be because of perturbations of other processes occurring within the zooxanthellae or the host cells, and consequently constitute only secondary responses to temperature increase. The processes involved with the onset of photoinhibition of electron transport, photodamage to PSII and pigment bleaching in coral zooxanthellae are reviewed. Consideration is given to how increases in temperature might lead to perturbations of metabolic processes in the zooxanthellae and/or their host cells, which could trigger events leading to bleaching. It is concluded that production of ROS by the thylakoid photosynthetic apparatus in the zooxanthellae plays a major role in the onset of bleaching resulting from photoinhibition of photosynthesis, although it is not clear which particular ROS are involved. It is suggested that hydrogen peroxide generated in the zooxanthellae may have a signalling role in triggering the mechanisms that result in expulsion of zooxanthellae from corals.

Two-step mechanism of photodamage to photosystem II: step 1 occurs at the oxygen-evolving complex and step 2 occurs at the photochemical reaction center.

Abstract: Under strong light, photosystem II (PSII) of oxygenic photosynthetic organisms is inactivated, and this phenomenon is called photoinhibition. In a widely accepted model, photoinhibition is induced by excess light energy, which is absorbed by chlorophyll but not utilized in photosynthesis. Using monochromatic light from the Okazaki Large Spectrograph and thylakoid membranes from Thermosynechococcus elongatus, we observed that UV and blue light inactivated the oxygen-evolving complex much faster than the photochemical reaction center of PSII. These observations suggested that the light-induced damage was associated with a UV- and blue light-absorbing center in the oxygen-evolving complex of PSII. The action spectrum of the primary event in photodamage to PSII revealed the strong effects of UV and blue light and differed considerably from the absorption spectra of chlorophyll and thylakoid membranes. By contrast to the photoinduced inactivation of the oxygen-evolving complex in untreated thylakoid membranes, red light efficiently induced inactivation of the PSII reaction center in Tris-treated thylakoid membranes, and the action spectrum resembled the absorption spectrum of chlorophyll. Our observations suggest that photodamage to PSII occurs in two steps. Step 1 is the light-induced inactivation of the oxygen-evolving complex. Step 2, occurring after step 1 is complete, is the inactivation of the PSII reaction center by light absorbed by chlorophyll. We confirmed our model by illumination of untreated thylakoid membranes with blue and UV light, which inactivated the oxygen-evolving complex, and then with red light, which inactivated the photochemical reaction center.

Functional role of anthocyanins in high-light winter leaves of the evergreen herb Galax urceolata.

Abstract: Summary • High-light leaves of the evergreen herb Galax urceolata exhibit a striking color change from green to red during winter months due to anthocyanin synthesis in outermost mesophyll cells. Here we investigate three possible functions of this color change. •T o test the hypothesis that anthocyanins function as light attenuators, maximum photosystem II efficiency ( F v /F m ) of red and green leaves was measured during and after exposure to wavelengths either strongly or poorly absorbed by anthocyanin. To determine whether anthocyanins elevate radical-scavenging capacity, antioxidant activity of red and green leaves was assessed using the α , α -diphenyl- β -picrylhydrazyl assay. Nonstructural carbohydrate levels were analyzed to test the hypothesis that anthocyanins function as a carbon sink. • Declines in F v /F m under white and green light were significantly greater for green than red leaves, but were comparable under red light. Anthocyanin content positively correlated with antioxidant activity. Although levels of anthocyanins did not appear to be related to nonstructural carbohydrate concentration, high levels of sugars may be necessary for their photoinduction. • Results suggest that anthocyanins function as light attenuators and may also contribute to the antioxidant pool in winter leaves.

Systematic Analysis of the Relation of Electron Transport and ATP Synthesis to the Photodamage and Repair of Photosystem II in Synechocystis

Abstract: The photosynthetic machinery and, in particular, the photosystem II (PSII) complex are susceptible to strong light, and the effects of strong light are referred to as photodamage or photoinhibition. In living organisms, photodamaged PSII is rapidly repaired and, as a result, the extent of photoinhibition represents a balance between rates of photodamage and the repair of PSII. In this study, we examined the roles of electron transport and ATP synthesis in these two processes by monitoring them separately and systematically in the cyanobacterium Synechocystis sp. PCC 6803. We found that the rate of photodamage, which was proportional to light intensity, was unaffected by inhibition of the electron transport in PSII, by acceleration of electron transport in PSI, and by inhibition of ATP synthesis. By contrast, the rate of repair was reduced upon inhibition of the synthesis of ATP either via PSI or PSII. Northern blotting and radiolabeling analysis with [35S]Met revealed that synthesis of the D1 protein was enhanced by the synthesis of ATP. Our observations suggest that ATP synthesis might regulate the repair of PSII, in particular, at the level of translation of the psbA genes for the precursor to the D1 protein, whereas neither electron transport nor the synthesis of ATP affects the extent of photodamage.

The effects of salt stress on photosynthetic electron transport and thylakoid membrane proteins in the cyanobacterium Spirulina platensis.

Abstract: The response of Spirulina (Arthrospira) platensis to high salt stress was investigated by incubating the cells in light of moderate intensity in the presence of 0.8 M NaCl. NaCl caused a decrease in photosystem II (PSII) mediated oxygen evolution activity and increase in photosystem I (PSI) activity and the amount of P700. Similarly maximal efficiency of PSII (Fv/Fm) and variable fluorescence (Fv/Fo) were also declined in salt-stressed cells. Western blot analysis reveal that the inhibition in PSII activity is due to a 40 % loss of a thylakoid membrane protein, known as D1, which is located in PSII reaction center. NaCl treatment of cells also resulted in the alterations of other thylakoid membrane proteins: most prominently, a dramatic diminishment of the 47-kDa chlorophyll protein (CP) and 94-kDa protein, and accumulation of a 17-kDa protein band were observed in SDS-PAGE. The changes in 47-kDa and 94-kDa proteins lead to the decreased energy transfer from light harvesting antenna to PSII, which was accompanied by alterations in the chlorophyll fluorescence emission spectra of whole cells and isolated thylakoids. Therefore we conclude that salt stress has various effects on photosynthetic electron transport activities due to the marked alterations in the composition of thylakoid membrane proteins.

Shade tolerance, photoinhibition sensitivity and phenotypic plasticity of Ilex aquifolium in continental Mediterranean sites

Abstract: Shade tolerance, plastic phenotypic response to light and sensitivity to photoinhibition were studied in holly (Ilex aquifolium L.) seedlings transported from the field to a greenhouse and in adult trees in the field. All plants were growing in, or originated from, continental Mediterranean sites in central Spain. Seedlings tolerated moderate but not deep shade. Mortality was high and growth reduced in 1% sunlight. Survival was maximal in 12% sunlight and minimal in full sunlight, although the relative growth rate of the seedlings surviving in high light was similar to that of plants in moderate shade. Maximum photochemical efficiency at predawn was significantly lower in sun plants than in shade plants in the field, revealing chronic photoinhibition that was most pronounced in winter. Plasticity in response to available light varied according to the variable studied, being low for photosynthetic capacity and stomatal conductance, and high for specific leaf area, root:shoot ratio and leaf area ratio, particularly in seedlings. Differences in water relations and hydraulic features between sun and shade plants in the field were marginal. High water potential at the turgor loss point of field-grown plants suggested that holly is sensitive to drought during both the seedling and the adult stage. Low relative growth rates in both high and low light with low physiological plasticity in response to light indicate the existence of a stress-tolerance mechanism. We conclude that holly is a facultative understory plant in areas of oceanic and relatively mild climate, but an obligate understory plant in dry continental areas such as the study site. The impact of abandonment of traditional management practices and climate change on these Mediterranean populations is discussed.

Effects of winter cold stress on photosynthesis and photochemical efficiency of PSII of the Mediterranean Cistus albidus L. and Quercus ilex L.

Abstract: This study examined the photosynthetic and growth performances of potted plants of Cistus albidus L. and Quercus ilex L. submitted either to natural Mediterranean winter conditions or to mild greenhouse conditions. Plants grown outdoors exhibited lower light and CO2-saturated CO2 assimilation rates (Asat) and apparent quantum yield (Φi) than those indoors. Until mid-winter, C. albidus had higher Asat than Q. ilex, but differences disappeared after a period of severe cold. Maximal photochemical efficiency of PSII (Fv/Fm) measured predawn was higher in C. albidus than in Q. ilex, and decreased throughout the season in outdoor plants. Fv/Fm also decreased at light saturation (Asat) in both species. Fv/Fm was correlated with photosynthetic capacity and efficiency (quantum yield), but the resulting regression slopes were different between the two species. At the physiological level, C. albidus seemed to cope better with cold stress than Q. ilex. However, winter stress induced reduction of leaf absorptance, increased leaf mass per area, extensive leaf damage and high plant mortality in C. albidus. This suggests that the high performance of C. albidus leaves is not likely to be maintained for long periods of cold stress, and may therefore depend on continuous leaf replacement. Quercus ilex showed a conservative behaviour, with low net assimilation rates but greater leaf and plant survival than C. albidus.

Psb29, a conserved 22-kD protein, functions in the biogenesis of Photosystem II complexes in Synechocystis and Arabidopsis.

Abstract: Photosystem II (PSII), the enzyme responsible for photosynthetic oxygen evolution, is a rapidly turned over membrane protein complex. However, the factors that regulate biogenesis of PSII are poorly defined. Previous proteomic analysis of the PSII preparations from the cyanobacterium Synechocystis sp PCC 6803 detected a novel protein, Psb29 (Sll1414), homologs of which are found in all cyanobacteria and vascular plants with sequenced genomes. Deletion of psb29 in Synechocystis 6803 results in slower growth rates under high light intensities, increased light sensitivity, and lower PSII efficiency, without affecting the PSII core electron transfer activities. A T-DNA insertion line in the PSB29 gene in Arabidopsis thaliana displays a phenotype similar to that of the Synechocystis mutant. This plant mutant grows slowly and exhibits variegated leaves, and its PSII activity is light sensitive. Low temperature fluorescence emission spectroscopy of both cyanobacterial and plant mutants shows an increase in the proportion of uncoupled proximal antennae in PSII as a function of increasing growth light intensities. The similar phenotypes observed in both plant and cyanobacterial mutants demonstrate that the function of Psb29 has been conserved throughout the evolution of oxygenic photosynthetic organisms and suggest a role for the Psb29 protein in the biogenesis of PSII.

Coral photobiology studied with a new imaging pulse amplitude modulated fluorometer

Abstract: A new high-resolution imaging fluorometer (Imaging-PAM) was used to identify heterogeneity of photosynthetic activity across the surface of corals. Three species were examined: Acropora nobilis Dana (branching), Goniastrea australiensis Edwards & Haime (massive), and Pavona decussata Dana (plate). Images of fluorescence parameters (F, F m ', effective quantum yield, optimal quantum yield, electron transport rate, relative photosynthetic rate, and non-photochemical quenching) allowed heterogeneity to be detected in terms of position on colony and indicated that the photosynthetic activity of polyp and coenosarc tissues responded differently to changing light for all three species. The Imaging-PAM offers a special routine, with which images of PAR absorption (absorptivity) are obtained. In this way, for the first time it has become possible to derive images of the relative photosynthesis rate. Polyps had a lower PAR absorptivity than coenosarc tissue for A. nobilis and P. decussata, whereas G. australiensis showed the opposite pattern. Acropora nobilis showed heterogeneity along the longitudinal axis of the branch, which could be differentiated from the effect of variations in illumination across the rugose and curved surface. Diel changes were apparent and influenced the longitudinal heterogeneity along the A. nobilis branch. Images were also obtained showing the degree of photoinhibition caused by high-light stress across a coral surface at a hitherto unobtainable level of resolution.

Photoinhibition of photosynthesis is reduced by water flow in the reef-building coral Acropora digitifera

Abstract: While photosynthesis of symbiotic algae is essential for reef-building corals, excess irra- diance inhibits photosynthesis through photoinhibition, which can lead to coral bleaching under ele- vated temperature conditions. Here we show that water flow reduces photoinhibition of in hospite endosymbionts in the coral Acropora digitifera. Diurnal monitoring of chlorophyll fluorescence, under 2 different flow regimes (<3 and 20 cm s -1 flow rates) in an outdoor aquarium, showed reduced photoinhibition, but only under moderate flow conditions (20 cm s -1 ). Experimental (laboratory) measurements, on time scales ranging from minutes to hours, showed that flow-mediated reductions in photoinhibition occurred not by enhancing recovery of the damaged photosystem, but rather through inducing differential photodamage. Moreover, experiments involving sequential light oscil- lations (500/20 and 1000/20 µmol photons m -2 s -1 ) at 3 flow regimes, <3, 10, and 20 cm s -1 , on a time scale ranging from hours to days, revealed water-velocity-dependent reductions of dynamic photo- inhibition. These results, on time scales ranging from minutes to weeks, confirm that reduced water flow amplifies photodamage of algal photosynthesis under strong irradiance, which in turn affects coral tolerance to strong irradiance and temperature.

Sensitivity of Laminariales zoospores from Helgoland (North Sea) to ultraviolet and photosynthetically active radiation: implications for depth distribution and seasonal reproduction

Abstract: Depth distribution of kelp species in Helgoland (North Sea) is characterized by occurrence of Laminaria digitata in the upper sublittoral, whereas L . saccharina and L . hyperborea dominate the mid and lower sublittoral region. Laminaria digitata is fertile in summer whereas both other species are fertile in autumn/winter. To determine the light sensitivity of the propagules, zoospores of L. digitata , L . saccharina and L . hyperborea were exposed in the laboratory to different exposure times of photosynthetically active radiation (PAR; 400‐700 nm), PAR + UVA radiation (UVAR; 320‐400 nm) and PAR + UVAR + UVB radiation (UVBR; 280‐320 nm). Optimum quantum yield of PSII and DNA damage were measured after exposure. Subsequently, recovery of photosynthetic efficiency and DNA damage repair, as well as germination rate were measured after 2 and 3 d cultivation in dim white light. Photosynthetic efficiency of all species was photoinhibited already at 20 m mol photons m - 2 s - 1 PAR, whereas UV radiation (UVR) had a significant additional effect on photoinhibition. Recovery of the PSII function was observed in all species but not in spores exposed to irradiation longer than 4 h of PAR + UVA + UVB and 8 h of PAR + UVA. The amount of UVB-induced DNA damage measured as cyclobutane‐pyrimidine dimers (CPDs) increased with exposure time and highest damage was detected in the spores of lower subtidal L . hyperborea relative to the other two species. Significant removal of CPDs indicating repair of DNA damage was observed in all species after 2 d in low white light especially in the spores of upper subtidal L . digitata . Therefore, efficient DNA damage repair and recovery of PSII damage contributed to the germination success but not in spores exposed to 16 h of UVBR. UV absorption of zoospore suspension in L . digitata is based both on the absorption by the zoospores itself as well as by exudates in the medium. In contrast, the absorption of the zoospore suspension in L . saccharina and L . hyperborea is based predominantly on the absorption by the exudates in the medium. This study indicates that UVR sensitivity of zoospores is related to the seasonal zoospore production as well as the vertical distribution pattern of the large sporophytes.

Growth, photosynthesis and stress indicators in young rosewood plants (Aniba rosaeodora Ducke) under different light intensities

Abstract: Aniba rosaeodora is an Amazonian tree species that belongs to the family Lauraceae. Due to intense exploitation for extraction of essential oils (mainly linalol), A. rosaeodora is now considered an endangered species. On the other hand, there is little information about its ecophysiology which would be useful to support future forest planting programs. Hence, the effect of different light intensities on the growth and photosynthetic characteristics of young plants of A. rosaeodora was studied. Nine-month-old plants were subjected to four light treatments (T1= 10 a 250 µmol.m-2.s-1 / control; T2=500 to 800, T3=700 to 1000 and T4=1300 to 1800 µmol.m-2.s-1 / full sunlight). Allometric variables, gas exchange, contents of pigments and chlorophyll a fluorescence were analysed. As to the relative growth rates, it was found that plants of A. rosaeodora showed higher biomass accumulation when grown under intermediary irradiance conditions (T2). The best photosynthetic performance was achieved under conditions of T3. When growth was correlated with photosynthesis, it was found that plants under treatments T2 and T3 presented better responses in comparison with the lowest (T1) and highest (T4) light extremes. The highest pigment contents were obtained for plants in the shade (T1) and the lowest for those exposed to full sunlight (T4). The photochemical efficiency of photosystem II (Fv/Fm) was found that only plants in the shade treatment (T1) presented no stress from high irradiance. These findings suggest that both treatments (T1 and T4) altered the function of the A. rosaeodora plants, inhibiting photosynthesis and growth. Plants of A. rosaeodora developed photo-protection mechanisms under full sunlight. However, the species presented better photosynthetic response and biomass gain under intermediary irradiance conditions, displaying relative physiological plasticity, during the seedling phase.

Photosynthesis and photoinhibition in two xerophytic shrubs during drought

Abstract: Seasonal changes in water relations, net photosynthetic rate (P(N)), and fluorescence of chlorophyll (Chl) a of two perennial C(3) deciduous shrubs, Ipomoea carnea and Jatropha gossypifolia, growing in a thorn scrub in Venezuela were studied in order to establish the possible occurrence of photoinhibition during dry season and determine whether changes in photochemical activity of photosystem 2 (PS2) may explain variations of PN in these species. Leaf water potential (psi) decreased from -0.2 to -2.1 MPa during drought in both species. The PN decreased with y in T carnea and J. gossypifolia by 64 and 74%, respectively. Carboxylation efficiency (CE) decreased by more than 50 and 70% in I. carnea and J gossypifolia, respectively. In I. carnea, relative stomatal limitation (L(s)) increased by 17% and mesophyll limitation (L(m)) by 65% during drought, while in J. gossypifolia Ls decreased by 27% and L. increased by 51%. Drought caused a reduction in quantum yield of PS2 (theta(PS2)) in both species. Drought affected the capacity of energy dissipation of leaves, judging from the changes in the photochemical (q(P)) and non-photochemical quenching (NPQ) coefficients. Photo-inhibition during drought in I. carnea and J gossypifolia was evidenced in the field by a drop in the maximum quantum yield of PS2 (F(v)/F(m)) below 0.8 and also by non-coordinated changes in phi(PS2) and quantum yield of non-photochemical excitation quenching (Y(n)). Total soluble protein content on an area basis increased with psi but the ribulose-1,5-bisphosphate carboxylase/oxygenase content remained unchanged. A reduction of total Chl content with drought was observed. Hence in the species studied photoinhibition occurred, which imposed an important limitation on carbon assimilation during drought.

Developmental and light effects on the accumulation of FtsH protease in Arabidopsis chloroplasts – implications for thylakoid formation and photosystem II maintenance

Abstract: The chloroplast ATP-dependent metalloprotease FtsH is involved in the degradation of unassembled proteins, the repair of photosystem II (PSII) from photoinhibition, and, apparently, the formation of thylakoids. In Arabidopsis, it is encoded by a family of 12 genes. However, the products of only four of them, FtsH1, 2, 5 and 8, have been found in chloroplasts to date. Mutations in two of these, FtsH2 and 5, demonstrate a visible phenotype of variegated leaves, with the phenotype of the FtsH2 mutant being more pronounced. Moreover, the degree of variegation appears to be dependent on developmental stage and environmental factors, suggesting an intricate relationship between the different gene products. To explore this, developmental and light effects on the accumulation of FtsH protease were studied in wild-type (WT) and FtsH2-mutant plants. Whereas cotyledons of the mutant were indistinguishable from those of the WT, the first true leaves were almost completely white. Subsequent leaves contained increasing proportions of green sectors. Analysis of the mRNA of the four FtsH genes, in cotyledons, first and second leaves of WT and mutant plants, revealed that: (i) transcript level increases during development, and (ii) transcript level in the mutant is higher than in the WT. FtsH protein level in the mutant was ca. 50% of that found in the WT, whereas the levels of other thylakoid proteins were the same. In individual leaves, the level of FtsH protein increased during development as well. Exposure of seedlings to different light intensities did not affect the degree of variegation, suggesting that it is due to a defect in chloroplast development rather than photobleaching. Examination of FtsH protein during exposure to high light revealed a decrease in its level, concomitant with a decrease in PSII potential, suggesting that the kinetics of photoinhibition reflects not only photodamage to PSII and induction of protective mechanisms, but also a decrease in repair capacity due to a reduction in the level of FtsH protease.

Functional in situ evaluation of photosynthesis-protecting carotenoids in mutants of the cyanobacterium Synechocystis PCC6803

Abstract: Cyanobacteria possess different carotenoids as scavengers of reactive oxygen species. In Synechocystis PCC6803, zeaxanthin, echinenone, β-carotene and myxoxanthophyll are synthesized. By disruption of the ketolase and hydroxylase genes, it was possible to obtain mutants devoid of either zeaxanthin, echinenone, or a combination of both carotenoids. With these mutants, their function in protecting photosynthetic electron transport under high light stress as well as chlorophyll and carotenoid degradation after initiation of singlet oxygen or radical formation was analyzed. Wild type Synechocystis is very well protected against high light-mediated photooxidation. Absence of echinenone affects photosynthetic electron transport to only a small extent. However, complete depletion of zeaxanthin together with a modification of myxoxanthophyll resulted in strong photoinhibition of overall photosynthetic electron transport as well as the photosystem II reaction. In the double mutant lacking both carotenoids the effects were additive. The light saturation curves of photosynthetic electron transport of the high light-treated mutants exhibited not only a lower saturation value but also smaller slopes. Using methylviologen or methylene blue as a radical or singlet oxygen generators, respectively, massive degradation of chlorophyll and carotenoids, indicative of photooxidative destruction of the photosynthetic apparatus, was observed, especially in the mutants devoid of zeaxanthin.

Leaf development and photosynthetic properties of three tropical tree species with delayed greening

Abstract: Leaf developmental patterns were characterized for three tropical tree species with delayed greening. Changes in the pigment contents, photosynthetic capacity, stomata development, photosystem 2 efficiency, rate of energy dissipation, and the activity of partial protective enzymes were followed in developing leaves in an attempt to elucidate the relative importance of various photoprotective mechanisms during leaf ontogeny. Big leaves of Anthocephalus chinensis, a fast-growing light demanding species, expanded following an exponential pattern, while relatively small leaves of two shade-tolerant species Litsea pierrei and Litsea dilleniifolia followed a sigmoidal pattern. The juvenile leaves of A. chinensis and L. pierrei contained anthocyanin located below the upper epidermis, while L. dilleniifolia did not contain anthocyanin. Leaves of A. chinensis required about 12 d for full leaf expansion (FLE) and photosynthetic development was delayed 4 d, while L. pierrei and L. dilleniifolia required 18 or 25 d for FLE and photosynthetic development was delayed 10 or 15 d, respectively. During the leaf development the increase in maximum net photosynthetic rate was significantly related to changes in stomatal conductance and the leaf maturation period was positively related to the steady-state leaf dry mass per area for the three studied species. Dark respiration rate of leaves at developing stages was greater, and pre-dawn initial photochemical efficiency was lower than that of mature leaves. Young leaves displayed greater energy dissipation than mature leaves, but nevertheless, the diurnal photoinhibition of young L. dilleniifolia leaves was higher than that of mature leaves. The young red leaves of A. chinensis and L. pierrei with high anthocyanin contents and similar diurnal photoinhibition contained more protective enzymes (superoxide dismutase, ascorbate peroxidase) than mature leaves. Consequently, red leaves may have higher antioxidant ability.

Acclimation of emiliania huxleyi (prymnesiophyceae) to photon flux density1

Abstract: Growth rate, pigment composition, and noninvasive chl a fluorescence parameters were assessed for a noncalcifying strain of the prymmesiophyte Emiliania huxleyi Lohman grown at 50, 100, 200, and 800 μmol photons.m - 2 .s - 1 . Emiliania huxleyi grown at high photon flux density (PFD) was characterized by increased specific growth rates, 0.82 d - 1 for high PFD grown cells compared with 0.38 d - 1 for low PFD grown cells, and higher in vivo chl a specific attenuation coefficients that were most likely due to a decreased pigment package, consistent with the observed decrease in cellular photosynthetic pigment content. High PFD growth conditions also induced a 2.5-fold increase in the pool of the xanthophyll cycle pigments diadinoxanthin and diatoxanthin responsible for dissipation of excess energy. Dark-adapted maximal photochemical efficiency (F v /F m ) remained constant at around 0.58 for cells grown over the range of PFDs, and therefore the observed decline, from 0.57 to 0.33, in the PSII maximum efficiency in the light-adapted state, (F v '/F m '), with increasing growth PFD was due to increased dissipation of excess energy, most likely via the xanthophyll cycle and not due to photoinhibition. The PSII operating efficiency (F q '/F m ') decreased from 0.48 to 0.21 with increasing growth PFD due to both saturation of photochemistry and an increase in nonphotochemical quenching. The changes in the physiological parameters with growth PFD enable E. huxleyi to maximize rates of photosynthesis under subsaturating conditions and protect the photosynthetic apparatus from excess energy while supporting higher saturating rates of photosynthesis under saturating PFDs.

Tolerance of endolithic algae to elevated temperature and light in the coral Montipora monasteriata from the southern Great Barrier Reef.

Abstract: Photosynthetic endolithic algae and cyanobacteria live within the skeletons of many scleractinians. Under normal conditions, less than 5% of the photosynthetically active radiation (PAR) reaches the green endolithic algae because of the absorbance of light by the endosymbiotic dinoflagellates and the carbonate skeleton. When corals bleach (loose dinoflagellate symbionts), however, the tissue of the corals become highly transparent and photosynthetic microendoliths may be exposed to high levels of both thermal and solar stress. This study explores the consequence of these combined stresses on the phototrophic endoliths inhabiting the skeleton of Montipora monasteriata, growing at Heron Island, on the southern Great Barrier Reef. Endoliths that were exposed to sun after tissue removal were by far more susceptible to thermal photoinhibition and photo-damage than endoliths under coral tissue that contained high concentrations of brown dinoflagellate symbionts. While temperature or light alone did not result in decreased photosynthetic efficiency of the endoliths, combined thermal and solar stress caused a major decrease and delayed recovery. Endoliths protected under intact tissue recovered rapidly and photoacclimated soon after exposure to elevated sea temperatures. Endoliths under naturally occurring bleached tissue of M. monasteriata colonies (bleaching event in March 2004 at Heron Island) acclimated to increased irradiance as the brown symbionts disappeared. We suggest that two major factors determine the outcome of thermal bleaching to the endolith community. The first is the microhabitat and light levels under which a coral grows, and the second is the susceptibility of the coral-dinoflagellates symbiosis to thermal stress. More resistant corals may take longer to bleach allowing endoliths time to acclimate to a new light environment. This in turn may have implications for coral survival.

PsbU Provides a Stable Architecture for the Oxygen-Evolving System in Cyanobacterial Photosystem II†

Abstract: PsbU is a lumenal peripheral protein in the photosystem II (PS II) complex of cyanobacteria and red algae. It is thought that PsbU is replaced functionally by PsbP or PsbQ in plant chloroplasts. After the discovery of PsbP and PsbQ homologues in cyanobacterial PS II [Thornton et al. (2004) Plant Cell 16, 2164-2175], we investigated the function of PsbU using a psbU deletion mutant (DeltaPsbU) of Synechocystis 6803. In contrast to the wild type, DeltaPsbU did not grow when both Ca2+ and Cl- were eliminated from the growth medium. When only Ca2+ was eliminated, DeltaPsbU grew well, whereas when Cl- was eliminated, the growth rate was highly suppressed. Although DeltaPsbU grew normally in the presence of both ions under moderate light, PS II-related disorders were observed as follows. (1) The mutant cells were highly susceptible to photoinhibition. (2) Both the efficiency of light utilization under low irradiance and the chlorophyll-specific maximum rate of oxygen evolution in DeltaPsbU cells were 60% lower than those of the wild type. (3) The decay of the S2 state in DeltaPsbU cells was decelerated. (4) In isolated PS II complexes from DeltaPsbU cells, the amounts of the other three lumenal extrinsic proteins and the electron donation rate were drastically decreased, indicating that the water oxidation system became significantly labile without PsbU. Furthermore, oxygen-evolving activity in DeltaPsbU thylakoid membranes was highly suppressed in the absence of Cl-, and 60% of the activity was restored by NO3- but not by SO4(2-), indicating that PsbU had functions other than stabilizing Cl-. On the basis of these results, we conclude that PsbU is crucial for the stable architecture of the water-splitting system to optimize the efficiency of the oxygen evolution process.