Showing papers on "Photoinhibition published in 1993"

Photosynthesis, photoinhibition and low temperature acclimation in cold tolerant plants.

Abstract: Cold acclimation requires adjustment to a combination of light and low temperature, conditions which are potentially photoinhibitory. The photosynthetic response of plants to low temperature is dependent upon time of exposure and the developmental history of the leaves. Exposure of fully expanded leaves of winter cereals to short-term, low temperature shiftsinhibits whereas low temperature growthstimulates electron transport capacity and carbon assimilation. However, the photosynthetic response to low temperature is clearly species and cultivar dependent. Winter annuals and algae which actively grow and develop at low temperature and moderate irradiance acquire a resistance to irradiance 5- to 6-fold higher than their growth irradiance. Resistance to short-term photoinhibition (hours) in winter cereals is a reflection of the increased capacity to keep QA oxidized under high light conditions and low temperature. This is due to an increased capacity for photosynthesis. These characteristics reflect photosynthetic acclimation to low growth temperature and can be used to predict the freezing tolerance of cereals. It is proposed that the enhanced photosynthetic capacity reflects an increased flux of fixed carbon through to sucrose in source tissue as a consequence of the combined effects of increased storage of carbohydrate as fructans in the vacuole of leaf mesophyll cells and an enhanced export to the crown due to its increased sink activity. Long-term exposure (months) of cereals to low temperature photoinhibition indicates that this reduction of photochemical efficiency of PS II represents a stable, long-term down regulation of PS II to match the energy requirements for CO2 fixation. Thus, photoinhibition in vivo should be viewed as the capacity of plants to adjust photosynthetically to the prevailing environmental conditions rather than a process which necessarily results in damage or injury to plants. Not all cold tolerant, herbaceous annuals use the same mechanism to acquire resistance to photoinhibition. In contrast to annuals and algae, overwintering evergreens become dormant during the cold hardening period and generally remain susceptible to photoinhibition. It is concluded that the photosynthetic response to low temperatures and susceptibility to photoinhibition are consequences of the overwintering strategy of the plant species.

Photoinhibition and D1 Protein Degradation in Peas Acclimated to Different Growth Irradiances

Abstract: The relationship between the susceptibility of photosystem II (PSII) to photoinhibition in vivo and the rate of degradation of the D1 protein of the PSII reaction center heterodimer was investigated in leaves from pea plants (Pisum sativum L. cv Greenfeast) grown under widely contrasting irradiances. There was an inverse linear relationship between the extent of photoinhibition and chlorophyll (Chl) a/b ratios, with low-light leaves being more susceptible to high light. In the presence of the chloroplast-encoded protein synthesis inhibitor lincomycin, the differential sensitivity of the various light-acclimated pea leaves to photoinhibition was largely removed, demonstrating the importance of D1 protein turnover as the most crucial mechanism to protect against photoinhibition. In the differently light-acclimated pea leaves, the rate of D1 protein degradation (measured from [35S]methionine pulse-chase experiments) increased with increasing incident light intensities only if the light was not high enough to cause photoinhibition in vivo. Under moderate illumination, the rate constant for D1 protein degradation corresponded to the rate constant for photoinhibition in the presence of lincomycin, demonstrating a balance between photodamage to D1 protein and subsequent recovery, via D1 protein degradation, de novo synthesis of precursor D1 protein, and reassembly of functional PSII. In marked contrast, in light sufficiently high to cause photoinhibition in vivo, the rate of D1 protein degradation no longer increased concomitantly with increasing photoinhibition, suggesting that the rate of D1 protein degradation is playing a regulatory role. The extent of thylakoid stacking, indicated by the Chl a/b ratios of the differently light-acclimated pea leaves, was linearly related to the half-life of the D1 protein in strong light. We conclude that photoinhibition in vivo occurs under conditions in which the rate of D1 protein degradation can no longer be enhanced to rapidly remove irreversibly damaged D1 protein. We suggest that low-light pea leaves, with more stacked membranes and less stroma-exposed thylakoids, are more susceptible to photoinhibition in vivo mainly due to their slower rate of D1 protein degradation under sustained high light and their slower repair cycle of the photodamaged PSII centers.

Photosynthetic light-response curves

Abstract: Gradients in photosynthetic capacity through the leaf affect the shape of the irradiance-response curve. These gradients in photosynthetic capacity were manipulated by restraining leaves in different orientations. The shape or curvature of the light-response curve can be defined by Θ, where Θ=0 is a rectangular hyperbola and Θ=1 is a Blackman curve. Horizontal leaves had the highest Θ values when their adaxial (top) surface was illuminated and lowest Θ value when their abaxial (bottom) surface was illuminated. Vertical leaves had intermediate Θ values that were similar for illumination from either direction, indicating that both surfaces had similar photosynthetic capacities. The photosynthetic capacity near each surface was probed by measuring the resistance to photoinhibition by 2000 μmol quanta · m −2·s −1 for 2 h followed by 15 min dark relaxation. Resistance to photoinhibition was consistent with the amount of direct sunlight exposure during growth. By measuring three light-response curves for a given leaf, illuminating the leaf from either the adaxial or abaxial surface or with the adaxial and abaxial surfaces illiminated equally, it was possible to infer gradients in the light absorption and photosynthetic capacity of the leaf using a ten-layer model. The gradient in light absorption was not as steep as expected and the photosynthetic capacity declined from the adaxial surface but increased again approaching the abaxial surface, the increase being more pronounced in vertical leaves. The modelled gradients were qualitatively similar for dorsiventral and isolateral leaves. The gradients in light absorption and photosynthetic capacity were not identical and this results (1) in curvilinear relationships between the quantum efficiency of PSII determined by chlorophyll fluorescence and the quantum efficiency of leaf photosynthesis and (2) in light-response curves that slowly reach saturation rather than being abruptly truncated. The Θ value for the photosynthetic light-response curve will remain a parameter that has to be derived empirically, in contrast to the maximum quantum yield and photosynthetic capacity. The curvature factor, Θ, depends on CO2 partial pressure and the interplay between the gradients in light absorption and photosynthetic capacity through the leaf which can change depending on the light environment during growth.

Daily course of photosynthesis and photoinhibition in marine macroalgae investigated in the laboratory and field

Abstract: Photoinhibition of photosynthesis and its recovery was investigated in the laboratory and in the field with fluorescence and oxygen measuring devices. Photosynthetic efficiency measured at non-saturating fluence rates and the ratio of variable fluorescence to maximal fluorescence (FJF,) showed an approximately inverse course compared to the fluence rate of daylight measured continuously during the day. In the morning photosynthetic efficiency was high, but decreased with increasing fluence rate. Maximal photoinhibition of photosynthesis occurred around noon or in the early afternoon. During the afternoon photosynthetic efficiency increased again and full recovery was reached in the evening. These lunetics of recovery differ from those obtained in the laboratory under artificial conditions, where the red algae required up to 48 h to recover from a strong photoinhibition. Different species showed Mferent sensitivity to photoinhibition and different capability for recovery. The red alga Porphyra spp., living in the upper eulittoral, was able to cope with the high fluence rates at the water surface. The red alga Delesseria sanguinea, living in the subtidal zone, shows the highest sensitivity to photoinhibition. Thus, a relation between photoinhibitory sensitivity and the zonation of the algae in the littoral exists.

Dynamics of Photosystem Stoichiometry Adjustment by Light Quality in Chloroplasts

Abstract: Long-term imbalance in light absorption and electron transport by photosystem I (PSI) and photosystem II (PSII) in chloroplasts brings about changes in the composition, structure, and function of thylakoid membranes. The response entails adjustment in the photosystem ratio, which is optimized to help the plant retain a high quantum efficiency of photosynthesis (W.S. Chow, A. Melis, J.M. Anderson [1990] Proc Nat Acad Sci USA 87: 7502–7506). The dynamics of photosystem ratio adjustment were investigated upon the transfer of pea {Pisum sativum} plants from a predominantly PSI-light to a predominantly PSII-light environment and vice versa. The concentration of functional components (primary electron accepting plastoquinone of PSII [QA], P700) and that of constituent proteins were monitored during acclimation by A difference spectrophotometry and immunoblot analysis, respectively. Fully reversible changes in photosystem ratio occurred with a half-time of about 20 h. They involved closely coordinated changes in the concentration of the QA, reaction center protein D1, D2, and the 9-kD apoprotein of the cytochrome b559 for PSII. Similarly, closely coordinated changes in the relative concentration of P700 and reaction center proteins of PSI were observed. The level of chlorophyll b and that of the light-harvesting complex II changed in accordance with the concentration of PSII in the acclimating thylakoids. Overall, adjustments in the photosystem ratio in response to PSI- or PSII-light conditions appeared to be a well-coordinated reaction in the chloroplast. The response was absent in the chlorophyll b-less chlorina f2 mutant of barley (Hordeum vulgare) and in a phycobilisomeless mutant of Agmenellum quadruplicatum, suggesting that photosystem accessory pigments act as the light-quality perception molecules and that PSI and PSII themselves play a role in the signal transduction pathway.

A functional model for the role of cytochrome b559 in the protection against donor and acceptor side photoinhibition

Abstract: A quinone-independent photoreduction of the low potential form of cytochrome b559 has been studied using isolated reaction centers of photosystem II. Under anaerobic conditions, the cytochrome can be fully reduced by exposure to strong illumination without the addition of any redox mediators. Under high light conditions, the extent and rate of the reduction is unaffected by addition of the exogenous electron donor Mn2+ and, during this process, no irreversible damage occurs to the reaction center. However, prolonged illumination in strong light brings about irreversible bleaching of chlorophyll, indicative of photoinhibitory damage. When the cytochrome is fully reduced and excess Mn2+ is present, the effect of moderate light is to facilitate the photoaccumulation of reduced pheophytin. The dark reoxidation of the reduced cytochrome is very slow under anaerobic conditions but significantly speeded up on addition of oxidized 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone. From these results it is suggested that the low potential form of cytochrome b559 can accept electrons directly from reduced pheophytin and in so doing help to protect the reaction center against acceptor side photoinhibition as suggested by Nedbal et al. [Nedbal, J., Samson, G. & Whitmarsh, J. (1992) Proc. Natl. Acad. Sci. USA 89, 7929-7933]. This conclusion has been incorporated into a model that further suggests that in its high potential form the cytochrome primarily acts to protect against donor side photoinhibition due to increased lifetime of highly oxidized species as previously proposed by Thompson and Brudvig [Thompson, L. & Brudvig, G. W. (1988) Biochemistry 27, 6653-6658]. The particular feature of our scheme is that it incorporates reversible interconversion between the two redox forms so as to protect against either type of photoinhibition.

Cold-hardening-induced resistance to photoinhibition of photosynthesis in winter rye is dependent upon an increased capacity for photosynthesis

Abstract: Analyses of chlorophyll fluorescence and photosynthetic oxygen evolution were conducted to understand why cold-hardened winter rye (Secale cereale L.) is more resistant to photoinhibition of photosynthesis than is non-hardened winter rye. Under similar light and temperature conditions, leaves of cold-hardened rye were able to keep a larger fraction of the PS II reaction centres in an open configuration, i.e. a higher ratio of oxidized to reduced QA (the primary, stable quinone acceptor of PSII), than leaves of non-hardened rye. Three fold-higher photon fluence rates were required for cold-hardened leaves than for non-hardened leaves in order to establish the same proportion of oxidized to reduced QA. This ability of cold-hardened rye fully accounted for its higher resistance to photoinhibition; under similar redox states of qa cold-hardened and non-hardened leaves of winter rye exhibited similar sensitivities to photoinhibition. Under given light and temperature conditions, it was the higher capacity for light-saturated photosynthesis in cold-hardened than in non-hardened leaves, which was responsible for maintaining a higher proportion of oxidized to reduced QA. This higher capacity for photosynthesis of cold-hardened leaves also explained the increased resistance of photosynthesis to photoinhibition upon cold-hardening.

Fluorescence quenching during photosynthesis and photoinhibition of Ulva rotundata blid.

Abstract: The relationships between photoinhibition and photoprotection in high and low-light-grown Ulva were examined by a combination of chlorophyll-fluorescence-monitoring techniques. Tissues were exposed to a computer-controlled sequence of 5-min exposures to red light, followed by 5-min darkness, with stepwise increases in photon flux. Coefficients of chlorophyll fluorescence quenching (1−qP and NPQ) were calculated following a saturating pulse of white light near the end of each 5-min light treatment. Dark-adapted chlorophyll fluorescence parameters (F0 and FV/FM) were calculated from a saturating pulse at the end of each 5-min dark period. Low-light-grown Ulva showed consistently higher 1−qP, i.e. higher reduction status of Q (high primary acceptor of photosystem II), and lower capacity for nonphotochemical quenching (NPQ) at saturating light than did high-light-grown plants. Consequently, low-light plants rapidly displayed photoinhibitory damage (increased F0) at light saturation in seawater. Removal of dissolved inorganic carbon from seawater also led to photoinhibitory damage of high-light-grown Ulva at light saturation, and addition of saturating amounts of dissolved inorganic carbon protected low-light-grown plants against photoinhibitory damage. A large part of NPQ was abolished by treatment with 3 mM dithiothreitol and the processes so inhibited were evidently photoprotective, because dithiothreitol treatment accelerated photoinhibitory damage in both low- and high-light-grown Ulva. The extent of photoinhibitory damage in Ulva was exacerbated by treatment with chloramphenicol (1 mM) without much effect on chlorophyll-quenching parameters, evidently because this inhibitor of chloroplast protein synthesis reduced the rate of repair processes.

Photoinhibition as a control on photosynthesis and production of Sphagnum mosses.

Abstract: The effect of high light intensity on photosynthesis and growth of Sphagnum moss species from Alaskan arctic tundra was studied under field and laboratory conditions. Field experiments consisted of experimental shading of mosses at sites normally exposed to full ambient irradiance, and removal of the vascular plant canopy from above mosses in tundra water track habitats. Moss growth was then monitored in the experimental plots and in adjacent control areas for 50 days from late June to early August 1988. In shaded plots total moss growth was 2–3 times higher than that measured in control plots, while significant reductions in moss growth were found in canopy removal plots. The possibility that photoinhibition of photosynthesis might occur under high-light conditions and affect growth was studied under controlled laboratory conditions with mosses collected from the arctic study site, as well as from a temperate location in the Sierra Nevada, California. After 2 days of high-light treatment (800 μmol photons m−2 s−1) in a controlled environmental chamber, moss photosynthetic capacity was significantly lowered in both arctic and temperate samples, and did not recover during the 14-day experimental period. The observed decrease in photosynthetic capacity was correlated (r2=0.735, P<0.001) with a decrease in the ratio of variable to maximum chlorophyll fluorescence (Fv/Fm) in arctic and temperate mosses. This relationship indicates photoinhibition of photosynthesis in both arctic and temperate mosses at even moderately high light intensities. It is suggested that susceptibility to photoinhibition and failure to photoacclimate to higher light intensities in Sphagnum spp. may be related to low tissue nitrogen levels in these exclusively ombrotrophic plants. Photoinhibition of photosynthesis leading to lowered annual carbon gain in Sphagnum mosses may be an important factor affecting CO2 flux at the ecosystem level, given the abundance of these plants in Alaskan tussock tundra.

Theoretical assessment of alternative mechanisms for non-photochemical quenching of PS II fluorescence in barley leaves

Abstract: The components of non-photochemical chlorophyll fluorescence quenching (qN) in barley leaves have been quantified by a combination of relaxation kinetics analysis and 77 K fluorescence measurements (Walters RG and Horton P 1991). Analysis of the behaviour of chlorophyll fluorescence parameters and oxygen evolution at low light (when only state transitions — measured as qNt — are present) and at high light (when only photoinhibition — measured as qNi — is increasing) showed that the parameter qNt represents quenching processes located in the antenna and that qNi measures quenching processes located in the reaction centre but which operate significantly only when those centres are closed. The theoretical predictions of a variety of models describing possible mechanisms for high-energy-state quenching, measured as the residual quenching, qNe, were then tested against the experimental data for both fluorescence quenching and quantum yield of oxygen evolution. Only one model was found to agree with these data, one in which antennae exist in two states, efficient in either energy transfer or energy dissipation, and in which those photosynthetic units in a dissipative state are unable to exchange energy with non-dissipative units.

Rapid interchange between two distinct forms of cyanobacterial photosystem II reaction-center protein D1 in response to photoinhibition.

Abstract: We have studied photoinhibition of photosynthesis in the cyanobacterium Synechococcus sp. PCC 7942, which possesses two distinct forms of the photosystem II reaction-center protein D1 (D1:1 and D1:2). We report here that when cells adapted to a growth irradiance of 50 mumol.m-2.s-1 are exposed to an irradiance of 500 mumol.m-2.s-1, the normally predominant D1 form (D1:1) is rapidly replaced with the alternative D1:2. This interchange is not only complete within the first hour of photoinhibition but is also fully reversible once cells are returned to 50 mumol.m-2 x s-1. By using a mutant that synthesizes only D1:1, we show that the failure to replace D1:1 with D1:2 during photoinhibition results in severe loss of photosynthetic activity as well as a diminished capacity to recover after the stress period. We believe that this interchange between D1 forms may constitute an active component in a protection mechanism unique among photosynthetic organisms that enables cyanobacteria to effectively cope with and recover from photoinhibition.

Photosystem II Reaction Center Damage and Repair in Dunaliella salina (Green Alga) (Analysis under Physiological and Irradiance-Stress Conditions).

Abstract: Mechanistic aspects of the photosystem II (PSII) damage and repair cycle in chloroplasts were investigated. The D1/32-kD reaction center protein of PSII (known as the psbA chloroplast gene product) undergoes a frequent light-dependent damage and turnover in the thylakoid membrane. In the model organism Dunaliella salina (green alga), growth under a limiting intensity of illumination (100 [mu]mol of photons m-2 s-1; low light) entails damage, degradation, and replacement of D1 every about 7 h. Growth under irradiance-stress conditions (2000 [mu]mol of photons m-2 s-1; high light) entails damage to and replacement of D1 about every 20 min. Thus, the rate of damage and repair of PSII appears to be proportional to the light intensity during plant growth. Low-light-grown cells do not possess the capacity for high rates of repair. Upon transfer of low-light-grown cells to high-light conditions, accelerated damage to reaction center proteins is followed by PSII disassembly and aggregation of neighboring reaction center complexes into an insoluble dimer form. The accumulation of inactive PSII centers that still contain the D1 protein suggests that the rate of D1 degradation is the rate-limiting step in the PSII repair cycle. Under irradiance-stress conditions, chloroplasts gradually acquire a greater capacity for repair. The induction of this phenomenon occurs with a half-time of about 24 h.

Influence of light and temperature on photoinhibition of photosynthesis in Spirulina platensis

Abstract: Photoinhibition of photosynthesis and its recovery in the cyanobacteriumSpirulina platensis was studied to find how photosynthetic rates were influenced by light and temperature. By exposing cell samples from a turbidostat culture to combinations of light and temperature, a connection between light, temperature and photoinhibition was found. The experiments showed that a 10 degree increase from 20 °C to 30 °C considerably reduced the photoinhibition. At 25 °C a photon flux density of 1720 µmol m−2 s−1 reduced the photosynthetic rate by 50 % in 1 h, but a similarly high photon flux density had nearly no negative effect at 35 °C. Reactivation in low light from 50% photoinhibition was fast and complete in 60 min at 30 °C, while at 20 °C only about 1/6 of the full capacity was regained in the same time. Addition of the protein synthesis inhibitor streptomycin to cultures undergoing photoinhibition and regeneration indicated the presence also in this organism of a repair mechanism based on protein synthesis.

Effect of Nitrogen Supply on the Photosynthetic Performance of Leaves from Coffee Plants Exposed to Bright Light

Abstract: Although Coffea arabica L. grows naturally in shaded habitats, it can be cultivated under high light intensity, but not without severe photoinhibition mainly during the period of transfer from the nursery into the field. The present work examines some of the changes in the photosynthetic performance induced by exposure to high light and the possibility of using enhanced nitrogen levels to overcome photoinhibition. For that purpose, young plants of Coffea arabica L. (cv. Catuai) grown in a shaded greenhouse were treated with 0, 1 and 2 mmol of nitrogen and 4 weeks later exposed to full solar irradiation, outside. Visible damage due to exposure to full sunlight appeared within 2 d in all plants, resulting in a reduced photosynthetic leaf area and drastic shedding of leaves in the unfertilized plants. These effects were considerably less in plants with the highest N dose. After 130 d of exposure, there was 100% mortality in plants receiving no extra nitrogen, compared with 30% in the plants treated with 2 mmol nitrogen. Photosynthesis rates, leaf conductance and transpiration presented minimum values after 4 d of light stress. Large changes in the photosynthetic capacity (measured at high C02 concentration and high light intensity), quantum efficiency and fluorescence yield (Fy/Fm) indicate that net photosynthesis rate in the air had been reduced by both stomatal closure and by changes at the photochemical level. All indicators show that N-fertilized plants were less affected by photoinhibition.

End product feedback effects on photosynthetic electron transport.

Abstract: The inhibition of photosynthetic electron transport when starch and sucrose synthesis limit the overall rate of photosynthesis was studied inPhaseolus vulgaris L. andXanthium strumarium L. The starch and sucrose limitation was established by reducing photorespiration by manipulation of the partial pressure of O2 and CO2. Chlorophylla fluorescence quenching, the redox state of Photosystem I (estimated by the redox status of NADP-dependent malate dehydrogenase), and the intermediates of the xanthophyll cycle were investigated. Non-photochemical fluorescence quenching increased, NADP-dependent malate dehydrogenase remained at 100% activity, and the amount of violaxanthin decreased when starch and sucrose synthesis limited photosynthesis. In addition, O2-induced feedback caused a decrease in photochemical quenching. These results are consistent with a downward regulation of photosynthetic electron transport during end product feedback on photosynthesis. When leaves were held in high CO2 for 4 hours, the efficiency of Photosystem II was reduced when subsequently measured under low light. The results indicate that the quantum efficiency of open Photosystem II centers was reduced by the 4 hour treatment. We interpret the results to indicate that feedback from starch and sucrose synthesis on photosynthetic electron transport stimulates mechanisms for dissipating excess light energy but that these mechanisms do not completely protect leaves from long-term inhibition of photosynthetic electron transport capacity.

Two functionally distinct forms of the photosystem II reaction-center protein D1 in the cyanobacterium Synechococcus sp. PCC 7942.

Abstract: The cyanobacterium Synechococcus sp. PCC 7942 possesses a small psbA multigene family that codes for two distinct forms of the photosystem II reaction-center protein D1 (D1:1 and D1:2). We showed previously that the normally predominant D1 form (D1:1) was rapidly replaced with the alternative D1:2 when cells adapted to a photon irradiance of 50 mumol.m-2.s-1 are shifted to 500 mumol.m-2.s-1 and that this interchange was readily reversible once cells were allowed to recover under the original growth conditions. By using the psbA inactivation mutants R2S2C3 and R2K1 (which synthesize only D1:1 and D1:2, respectively), we showed that this interchange between D1 forms was essential for limiting the degree of photoinhibition as well as enabling a rapid recovery of photosynthesis. In this report, we have extended these findings by examining whether any intrinsic functional differences exist between the two D1 forms that may afford increased resistance to photoinhibition. Initial studies on the rate of D1 degradation at three photon irradiances (50, 200, and 500 mumol.m-2.s-1) showed that the rates of degradation for both D1 forms increase with increasing photon flux density but that there was no significant difference between D1:1 and D1:2. Analysis of light-response curves for oxygen evolution for the mutants R2S2C3 and R2K1 revealed that cells with photosystem II reaction centers containing D1:2 have a higher apparent quantum yield (approximately 25%) than cells possessing D1:1. Further studies using chlorophyll a fluorescence measurements confirmed that R2K1 has a higher photochemical yield than R2S2C3; that is, a more efficient conversion of excitation energy from photon absorption into photochemistry. We believe that the higher photochemical efficiency of reaction centers containing D1:2 is causally related to the preferential induction of D1:2 at high light and thus may be an integral component of the protection mechanism within Synechococcus sp. PCC 7942 against photoinhibition.

Analysing the responses of photosynthetic CO2 assimilation to long-term elevation of atmospheric CO2 concentration

Abstract: Understanding how photosynthetic capacity acclimatises when plants are grown in an atmosphere of rising CO2 concentrations will be vital to the development of mechanistic models of the response of plant productivity to global environmental change. A limitation to the study of acclimatisation is the small amount of material that may be destructively harvested from long-term studies of the effects of eleva tion of CO2 concentration. Technological developments in the measurement of gas exchange, fluorescence and absorption spectroscopy, coupled with theoretical developments in the interpretation of measured values now allow detailed analyses of limitations to photosynthesis in vivo. The use of leaf chambers with Ulbricht integrating spheres allows separation of change in the maximum efficiency of energy transduction in the assimilation of CO2 from changes in tissue absorptance. Analysis of the response of CO2 assimilation to intercellular CO2 concentration allows quantitative determination of the limitation imposed by stomata, carboxylation efficiency, and the rate of regeneration of ribulose 1:5 bisphosphate. Chlorophyll fluorescence provides a rapid method for detecting photoinhibition in heterogeneously illuminated leaves within canopies in the field. Modulated fluorescence and absorption spectroscopy allow parallel measurements of the efficiency of light utilisation in electron transport through photo systems I and II in situ.

Effects on Photosystem II Function, Photoinhibition, and Plant Performance of the Spontaneous Mutation of Serine-264 in the Photosystem II Reaction Center D1 Protein in Triazine-Resistant Brassica napus L.

Abstract: Wild-type and an atrazine-resistant biotype of Brassica napus, in which a glycine is substituted for the serine-264 of the D1protein, were grown over a wide range of constant irradiances in a growth cabinet. In the absence of serine-264, the function of photosystem II (PSII) was changed as reflected by changes in chlorophyll fluorescence parameters and in photosynthetic oxygen-evolving activity. The photochemical quenching coefficient was lower, showing that a larger proportion of the primary quinone acceptor is reduced at all irradiances. At low actinic irradiances, the nonphotochemical quenching coefficient was higher, showing a greater tendency for heat emission. Decreased rates of light-limited photosynthesis (quantum yield) and lower oxygen yields per single-turnover flash were also observed. These changes were observed even when the plants had been grown under low irradiances, indicating that the changes in PSII function are direct and not consequences of photoinhibition. In spite of the lowered PSII efficiency under light-limiting conditions, the light-saturated photosynthesis rate of the atrazine-resistant mutant was similar to that of the wild type. An enhanced susceptibility to photoinhibition was observed for the atrazine-resistant biotype compared to the wild type when plants were grown under high and intermediate, but not low, irradiance. We conclude that the replacement of serine by glycine in the D1 protein has a direct effect on PSII function, which in turn causes increased photoinhibitory damage and increased rates of turnover of the D1 protein. Both the intrinsic lowering of light-limited photosynthetic efficiency and the increased sensitivity to photoinhibition probably contribute to reduced crop yields in the field, to different extents, depending on growth conditions.

Photosynthesis : photoreactions to plant productivity

Abstract: Part 1 Chloroplast genome: the chloroplast genome - genetic potential and its expression Part 2 Photosystems: photosystem I reaction centre in oxygenic photosynthetic organisms - current views and the future oxidation of wate to molecular oxygen stoichometry of proton uptake by thylakoids during electron transport in chloroplasts Part 3 Coordination and regulation: regulation of the 32 kD-D1 photosystem II reaction centre protein regulation of electron transport at the acceptor side of photosystem II by herbicides, bicarbonate and formate photosynthesis and Herbicides - effects of pyridazinones on chloroplast function and biogenesis interactions between electron transport and carbon assimilation in leaves - coordination of activities and control leaf senescence-induced alterations in structure and function of higher plant chloroplasts maximizing light interception - the role of chloroplast disposition in cells and thylakoid membrane architecture Part 4 Carbon assimilation and partitioning: structure, function and regulation of ribulose 1,5-Bisphosphate carboxylase in higher plants C4 photosynthesis and C3-C4 intermediacy - adaptive strategies for semiarid tropics interaction between carbon and nitrogen metabolism assimilate partitioning within leaves of small grain cereals source and sink relationship photosynthetic characteristics of fruiting structures cultivated crops Part 5 Stress - CO2 enrichment: effects of water stress on photosynthesis of crops and the biochemical mechanism effect of heavy metals on photoynthesis in higher plants the role of carotenoids in protection against photoinhibition carbon dioxide enrichment effects on photosynthesis and plant growth the influence of atmospheric CO2 enrichment on allocation patterns of carbon and nitrogen in plants from natural vegetations Part 6 Genetic variation - productivity: genetic variation in photosynthetic characteristics in wheat - causes and consequences the significance of light - limiting photosynthesis to crop canopy carbon gain and productivity - a theoretical analysis leaf photosynthesis in rice in relation to grain yield photosynthesis improvement as a way to increase crop yield

Preferential photoinactivation of catalase and photoinhibition of photosystem II are common early symptoms under various osmotic and chemical stress conditions

Abstract: Activity of catalase (EC 1.11.1.6) and variable fluorescence (F) were measured in sections of rye leaves (Secale cereale L. cv. Halo) that were exposed for 24 h to moderately high irradiance under osmotic or chemical stress conditions (paraquat, DCMU, mannitol, NaCl, CdCl2 , CuSO4 , Pb(NO3 )2 , KNO2 , or K2 SO3 ). Changes of the chlorophyll content and of enzyme activities related to peroxide metabolism, such as glycolate oxidase, glutathione reductase, and peroxidase, were assayed for comparison. In the presence of the herbicides paraquat and low DCMU concentrations that exert only partial inhibition of photosynthesis, as well as after most treatments with osmotic or chemical stress factors, catalase markedly declined due to a preferential photoinactivation. At higher DCMU levels catalase did not decline. At low KNO2 concentrations catalase activity was preferentially increased. In general, photoinactivation of catalase was accompanied by a decline of the F/Fm ratio, indicating photoinhibition of photosystem II, while other parameters were much more stable. Inasmuch as both catalase and the D1 reaction center protein of photosystem II have a rapid turnover in light, their steady state levels appear to decline whenever stress effects either excessively enhance deleterious oxidative conditions and degradation (e. g. Paraquat, low DCMU), or inhibit repair synthesis. Photoinactivation of catalase and of photosystem II represent specific and widely occurring early symptoms of incipient photodamage indicating stress conditions where the repair capacity is not sufficient. During prolonged exposures, e. g. to NaCl and CuSO4 , chlorophyll was bleached in light and the rate of its photodegradation increased in proportion as the catalase level had declined. The results suggest that the enhanced susceptibility of leaf tissues to photooxidative damage which is widely observed in stressed plants is related to the early loss of catalase.

Acceptor side mechanism of photoinduced proteolysis of the D1 protein in photosystem II reaction centers.

Abstract: A 23-kDa breakdown product, containing the N terminus of the D1 protein, has been detected after photoinhibitory treatment of isolated photosystem II (PSII) reaction centers. The ability to induce charge separation in the reaction center and the presence of oxygen seem to be required for the generation of this fragment. It is suggested that, under these conditions, the initial light-induced damage to the complex occurs via singlet oxygen generated by the P680 triplet state and contrasts with the situation when an electron acceptor is present and donor-side photoinhibition gives rise to a 24-kDa C-terminal fragment of the D1 protein. The temperature sensitivity of the appearance of the 23-kDa N-terminal fragment suggests that the cleavage is not by a direct photochemical process but that it is proteolytic in nature, being triggered possibly by a conformational change induced by singlet oxygen-mediated photodestruction of the P680 chlorophylls. The existence of an intrinsic serine-type protease, within the reaction center itself, is supported by inhibition of the appearance of the 23-kDa N-terminal fragment by stoichiometric levels of soybean trypsin inhibitor. It seems likely that the 23-kDa N-terminal fragment which we have detected is the same as that identified in vivo by Greenberg et al. [Greenberg, B. M., Gaba, V., Mattoo, A. K., & Edelman, M. (1987) EMBO J. 6, 2865-2869] and originates from the acceptor-side mechanism advocated by Vass et al. [Vass, I., Styring, S., Hundal, T., Koivuniemi, A., Aro, E.-M., & Andersson, B. (1992) Proc. Natl. Acad. Sci. U.S.A. 89, 1408-1412].

Developmental Variability of Photooxidative Stress Tolerance in Paraquat-Resistant Conyza.

Abstract: Paraquat-resistant hairy fleabane (Conyza bonariensis L. Cronq.) has been extensively studied, with some contention. A single, dominant gene pleiotropically controls levels of oxidant-detoxifying enzymes and tolerance to many photooxidants, to photoinhibition, and possibly to other stresses. The weed forms a rosette on humid short days and flowers in dry long days and, thus, needs plasticity to photooxidant stresses. In a series of four experiments over 20 months, the resistant and susceptible biotypes were cultured in constant 10-h low-light short days at 25[deg]C. Resistance was measured as recovery from paraquat. The concentration required to achieve 50% inhibition of the resistant biotype was about 30 times that of the susceptible one just after germination, increased to >300 times that of the susceptibles at 10 weeks of growth, and then decreased to 20-fold, remaining constant except for a brief increase while bolting. Resistance increased when plants were induced to flower by long days. The levels of plastid superoxide dismutase and of glutathione reductase were generally highest in resistant plants compared to those of the susceptibles at the times of highest paraquat resistance, but they were imperceptibly different from the susceptible type at the times of lower paraquat resistance. Photoinhibition tolerance measured as quantum yield of oxygen evolution at ambient temperatures was highest when the relative amounts of enzymes were highest in the resistant biotype. Resistance to photoinhibition was not detected by chlorophyll a fluorescence. Enzyme levels, photoinhibition tolerance, and paraquat resistance all increased during flowering in both biotypes. Imperceptibly small increases in enzyme levels would be needed for 20-fold resistance, based on the moderate enzyme increases correlated with 300-fold resistance. Thus, it is feasible that either these enzymes play a role in the first line of defense against photooxidants, or another, yet unknown mechanism(s) facilitate(s) the lower level of resistance, or the enzymes and unknown mechanisms act together.

Photosynthetic acclimation to low temperature by western red cedar seedlings

Abstract: Imposition of low, but above freezing, temperatures resulted in a gradual increase in the cold hardiness of western red cedar seedlings. This was associated with a decrease in the maximum rates of photosynthetic CO2 fixation and O2 evolution, and changes in chlorophyll a fluorescence transients which indicated that photoinhibition had occurred. Maximum photosynthetic rates declined approximately 40% during cold hardening. The leaves changed colour from green to red-brown during the hardening process. The colour change was due to the synthesis of large amounts of the carotenoid rhodoxanthin. Lutein levels doubled, while chlorophyll declined slightly. Dehardening resulted in the rapid recovery of photosynthesis to control levels, the rapid disappearance of rhodoxanthin, and the return of lutein levels to control. It is suggested that rhodoxanthin accumulation at low temperature functions to decrease the light intensity reaching the photosynthetic apparatus. The combination of photoinhibition and rhodoxanthin synthesis probably serves to protect the photosynthetic capacity of the seedlings at low temperature.