Showing papers on "Photosynthesis published in 1995"

The Molecular Biology of Cyanobacteria

Abstract: Preface. Color Plates. 1. Molecular Evolution and Taxonomy of the Cyanobacteria A. Wilmotte. 2. The Oceanic Cyanobacterial Picoplankton N.G. Carr, N.H. Mann. 3. Prochlorophytes: the 'Other' Cyanobacteria? H.C.P. Matthijs, et al. 4. Molecular Biology of Cyanelles W. Loffelhardt, H.J. Bohnert. 5. Chloroplast Origins and Evolution S.E. Douglas. 6. Supramolecular Membrane Organization E. Gantt. 7. Phycobilisome and Phycobiliprotein Structures W.A. Sidler. 8. The Use of Cyanobacteria in the Study of the Structure and Function of Photosystem II B.A. Barry, et al. 9. The Cytochrome b6f Complex T. Kallas. 10. Photosystem I in Cyanobacteria J.H. Golbeck. 11. The F-type ATPase in Cyanobacteria: Pivotal Point in the Evolution of the Universal Enzyme W.D. Frasch. 12. Soluble Electron Transfer Catalysts of Cyanobacteria L.Z. Morand, et al. 13. Cyanobacterial Respiration G. Schmetterer. 14. The Biochemistry and Molecular Regulation of Carbon Dioxide Metabolism in Cyanobacteria F.R. Tabita. 15. Physiological and Molecular Studies on the Response of Cyanobacteria to Changes in the Ambient Inorganic Carbon Concentration A. Kaplan, et al. 16. Assimilatory Nitrogen Metabolism and its Regulation E. Flores, A. Herrero. 17. Biosynthesis of Cyanobacterial Tetrapyrrole Pigments: Hemes, Chlorophylls, and Phycobilins S.I. Beale. 18. Carotenoids in Cyanobacteria J. Hirschberg, D. Chamovitz. 18. Genetic Analysis of Cyanobacteria T. Thiel. 20. The Transcription Apparatus and the Regulation of Transcription Initiation S.E. Curtis, J.A. Martin. 21. The Responses of Cyanobacteria to Environmental Conditions: Light and Nutrients A.R. Grossman, et al. 22. Short-Term and Long-Term Adaptation of the Photosynthetic Apparatus: Homeostatic Properties of Thylakoids Y. Fujita, et al. 23. Light-Responsive Gene Expression and the Biochemistry of the Photosystem II Reaction Center S.S. Golden. 24. Thioredoxins in Cyanobacteria: Structure and Redox Regulation of Enzyme Activity F.K. Gleason. 25. Iron Deprivation: Physiology and Gene Regulation N.A. Straus. 26. The Cyanobacterial Heat-Shock Response and the Molecular Chaperones R. Webb, L.A. Sherman. 27. Heterocyst Metabolism and Development C.P. Wolk, et al. 28. Differentiation of Hormogonia and Relationships with Other Biological Processes N. Tandeau de Marsac. Organism Index. Gene and Gene Product Index. Subject Index.

Dissection of Oxidative Stress Tolerance Using Transgenic Plants.

Abstract: Environmental stress is the major limiting factor in plant productivity. Much of the injury to plants caused by stress exposure is associated with oxidative damage at the cellular level. Widespread losses of forests and crops due to ozone pollution provide a highly visible example of oxidative stress (see Tingey et al., 1993, for a review), but less obvious losses caused by oxidative damage associated with periods of cold or drought also take their toll in the accumulation of incremental setbacks during a growing season. The role of ROIs in plant stress damage is indicated by the increased production of ROIs and the increased oxidative damage in tissues during stress. In plants, the highly energetic reactions of photosynthesis and an abundant oxygen supply make the chloroplast a particularly rich source of ROIs. High light intensity can lead to excess reduction of PSI so that CO2 fixation cannot keep pace and NADP+ pools are reduced. Under these conditions, 02 can compete for electrons from PSI, leading to the generation of ROIs through the Mehler reaction. When CO2 fixation is limited by environmental conditions such as cold temperatures or low CO2 availability (closed stomata), excess PSI reduction and increased ROI production can occur even at moderate light intensities. Efficient removal of ROIs from chloroplasts is critical, since H202 concentrations as low as 10 ptM can inhibit photosynthesis by 50% (Kaiser, 1979). Although the toxicity of *?2and H202 themselves is relatively low, their metal-dependent conversion to the highly toxic -OH via the Haber-Weiss reaction is thought to be responsible for the majority of the biological damage associated with these molecules. Antioxidant systems of plant chloroplasts include enzymes such as SOD and APX, and nonenzymatic components such as ascorbic acid and glutathione. The proposed ROI scavenging pathway of chloroplasts is shown in Figure 1 (Asada, 1994). Superoxide radicals are produced by the reduction of molecular oxygen at PSI via the Mehler reaction. This ?2- is rapidly dismuted to H202 by SOD that is associated with the thylakoid. The H202 produced is

Perspectives on photoinhibition and photorespiration in the field: quintessential inefficiencies of the light and dark reactions of photosynthesis?

Abstract: Taking the long-held view that photoinhibition embraces several processes leading to a reduction in the efficiency of light utilization in photosynthesis, and that photorespiration embraces several processes associated with 02 uptake in the light, photoinhibition and photorespiration now can be considered as inevit able, but essential inefficiencies of photosynthesis which help preserve photosynthetic competence in bright light. Photorespiratory 02 uptake via Rubisco, and 02 uptake via the Mehler reaction, both promote non-assimilatory electron transport, and stimulate photon utilization during C02-Iimited photosynthesis in bright light. Although fluorescence studies show that the proportion of total photon use via oxygenase photorespiration in air may decline to only about 10% in full sunlight, mass spectrometer studies show that 02 uptake in Mehler reaction photorespiration in C3 and CAM plants can still account for up to 50% of electron flow in saturating C02 and light. The Mehler ascorbate peroxidase reaction has an additional role in sustaining membrane energization which promotes dynamic photoinhibition and photon protection (rapidly reversible decrease in PSII efficiency involving dissipa tion of the energy of excess photons in the antennae). Net C02 and 02 exchange studies evidently underesti mate the extent of total electron transport, and hence overestimate the extent of photon excess in bright light, leading to overestimates of the role of energy dependent photon dissipation through dynamic photo inhibition. Nevertheless, in C3 plants in air all of these processes help to mitigate chronic photoinhibition and photon damage (slowly reversible decrease in PSII effi ciency involving loss of reaction centre function). The possibility remains that residual electron transport to 02 from intermediates in the vicinity of PSII may also lead to reactive 02 species that potentiate this photon

In situ estimation of net CO2 assimilation, photosynthetic electron flow and photorespiration in Turkey oak (Q. cerris L.) leaves: diurnal cycles under different levels of water supply

Abstract: Diurnal time courses of net CO2 assimilation rates, stomatal conductance and light-driven electron fluxes were measured in situ on attached leaves of 30-year-old Turkey oak trees (Quercus cerris L.) under natural summer conditions in central Italy. Combined measurements of gas exchange and chlorophyll a fluorescence under low O2 concentrations allowed the demonstration of a linear relationship between the photochemical efficiency of PSII (fluorescence measurements) and the apparent quantum yield of gross photosynthesis (gas exchange). This relationship was used under normal O2 to compute total light-driven electron fluxes, and to partition them into fractions used for RuBP carboxylation or RuBP oxygenation. This procedure also yielded an indirect estimate of the rate of photorespiration in vivo. The time courses of light-driven electron flow, net CO2 assimilation and photorespiration paralleled that of photosynthetic photon flux density, with important afternoon deviations as soon as a severe drought stress occurred, whereas photochemical efficiency and maximal fluorescence underwent large but reversible diurnal decreases. The latter observation indicated the occurrence of a large non-photochemical energy dissipation at PSII. We estimated that less than 60% of the total photosynthetic electron flow was used for carbon assimilation at midday, while about 40% was devoted to photorespiration. The rate of carbon loss by photorespiration (R1) reached mean levels of 56% of net assimilation rates. The potential application of this technique to analysis of the relative contributions of thermal de-excitation at PSII and photorespiratory carbon recycling in the protection of photosynthesis against stress effects is discussed.

The unusually strong stabilizing effects of glycine betaine on the structure and function of the oxygen-evolving Photosystem II complex.

Abstract: Natural osmoregulatory substances (osmolytes) allow a wide variety of organisms to adjust to environments with high salt and/or low water content. In addition to their role in osmoregulation, some osmolytes protect proteins from denaturation and deactivation by, for example, elevated temperature and chaotropic compounds. A ubiquitous protein-stabilizing osmolyte is glycine betaine (N-trimethyl glycine). Its presence has been reported in bacteria, in particular cyanobacteria, in animals and in plants from higher plants to algae. In the present review we describe the experimental evidence related to the ability of glycine betaine to enhance and stabilize the oxygen-evolving activity of the Photosystem II protein complexes of higher plants and cyanobacteria. The osmolyte protects the Photosystem II complex against dissociation of the regulatory extrinsic proteins (the 18 kD, 23 kD and 33 kD proteins of higher plants and the 9 kD protein of cyanobacteria) from the intrinsic components of the Photosystem II complex, and it also stabilizes the coordination of the Mn cluster to the protein cleft. By contrast, glycine betaine has no stabilizing effect on partial photosynthetic processes that do not involve the oxygen-evolving site of the Photosystem II complex. It is suggested that glycine betaine might act, in part, as a solute that is excluded from charged surface domains of proteins and also as a contact solute at hydrophobic surface domains.

A model of the acclimation of photosynthesis in the leaves of C3 plants to sun and shade with respect to nitrogen use

Abstract: A model of leaf photosynthesis of C3, plants has been developed to describe their nitrogen economy. In this model, photosynthetic proteins are categorized into five groups depending on their functions. The effects of investment of nitrogen in each of these groups on the maximal rate of photosynthesis and/or the initial slope of the light-response curve are described as simple equations. Using this model, the optimal pattern of nitrogen partitioning which maximizes the daily rate of CO2 exchange is estimated for various light environments and leaf nitrogen contents. When the leaf nitrogen content is fixed, the amount of nitrogen allocated to Calvin cycle enzymes and electron carriers increases with increasing irradiance, while that allocated to chlorophyll-protein complexes increases with decreasing irradiance. For chlorophyll-proteins of photosystem II, the amount of light-harvesting complex II relative to that of the core complex increases with decreasing irradiance. At any irradiance, partitioning into ribulose bisphosphate carboxylase increases with increasing leaf nitrogen content Taking the total leaf nitrogen content and the daily CO2 exchange rate as ‘cost’ and ‘benefit’, respectively, the optimal amount and partitioning of nitrogen are examined for various conditions of light environment and nitrogen availability. The leaf nitrogen content that maximizes the rate of daily carbon fixation increases with increasing growth irradiance. It is also predicted that, at low nitrogen availabilities, low leaf nitrogen contents are advantageous in terms, of nitrogen use efficiency. These trends predicted by the present model are largely consistent with those reported for actual plants. The differences in the total amount of leaf nitrogen and in the organization of photosynthetic components that have been reported for plants from different environments would therefore be of adaptive significance, because such differences can contribute to realization of efficient photosynthesis. These results are further discussed in an ecological context.

Physiological ecology of cyanobacteria in microbial mats and other communities

Abstract: In this review some aspects of the physiological ecology of cyanobacteria are discussed by taking a microbial mat as an example. The majority of microbial mats are built and dominated by cyarsobacteria which are primary producers at the basis of the microbial foodweb in microbial mats. These micro-scale ecosystems are characterized by steep and fluctuating physico-chemical gradients of which those of light, oxygen and sulphide are the most conspicuous. Light is strongly attenuated in the sediment, and owing to constant sedimentation, the mat-forming cyanobacteria have to move upwards towards the light. However, at the sediment surface, light intensity, particularly in the u.v. part of the spectrum, is often deleterious. The gliding movement of the cyanobacteria, with photo- and chemotaxis, allows the organism to position itself in a thin layer at optimal conditions. The organic matter produced by cyanobacterial photosynthesis is decomposed by the ruicrobial community. Sulphate-reducing bacteria are important in the end-oxidation of the organic matter. These organisms are obligate anaerobes and produce sulphide. Gradients of sulphide and oxygen move up and down in the sediment as a response to diurnal variations of light intensity. Cyanobacteria, therefore, are sometimes exposed to large concentrations of the extremely toxic sulphide. Some species are capable of sulphide-dependent anoxygenic photosynthesis. Other cyanobacteria show increased rates of oxygenic photosynthesis in the presence of sulphide and have mechanisms to oxidize sulphide while avoiding sulphide toxicity. Iron might play an important role in this process. Under anoxic conditions in the dark, mat-forming cyanobacteria switch to fermentative metabolism. Many species are also capable of fermentative reduction of elemental sulphur to sulphide. The gradients of sulphide and oxygen are of particular importance for nitrogen fixation. Very few microbial mats are formed by heterocystous cyanobacteria, which are best adapted to diazntrophic growth. However, these organisms probably cannot tolerate greater concentrations of sulphide or anoxic conditions or both. Under such conditions non-heterocystous cyanobacteria become dominant as diazotrophs. These organisms avoid conditions of oxygen supersaturation. In the ecosystem, nitrogen fixation and photosynthesis might be separated temporally as well as spatially. In addition, non-heterocystous diazotrophic cyanobacteria have mechanisms at the subcellular level to protect the oxygen-sensitive nitrogenase from inaction. CONTENTS Summary 1 I. Introduction 2 II. Microbial mats 3 III. Cyanobacteria in light gradients 7 IV. Dark metabolism 10 V. Interactions with sulphide 13 VI. Nitrogen fixation 16 VII. References 28.

8 – Functions of Mineral Nutrients: Macronutrients

Abstract: Publisher Summary The chapter discusses the more common classification as well as functions of macro- and micronutrients, with typical examples of the various functions of macronutrients. The importance of the reduction and assimilation of nitrate for plant life is similar to that of the reduction and assimilation of carbon dioxide (CO2) in photosynthesis. Nitrate reductase is an enzyme that is regulated by several different modes exerted at different levels—namely, enzyme synthesis, degradation, and reversible inactivation, as well as regulation of effectors and the concentration of substrate. In addition to its function in inducing synthesis of nitrate reductase, nitrate, together with light, might act as a “signal” altering the partitioning of photosynthetic carbon flow in leaves. With an increasing supply of nitrate, the capacity for nitrate reduction in the roots becomes a limiting factor, and an increasing proportion of the total nitrogen is translocated to the shoots in the form of nitrate. The carbon skeletons for these different amino acids are derived mainly from intermediates of photosynthesis, glycolysis, and the tricarboxylic acid cycle. The highest growth rates and plant yields are obtained by a combined supply of both ammonium and nitrate. Depending on the plant species, their development stage, and organ, the nitrogen content required for optimal growth varies between 2% and 5% of the plant dry weight. When the potassium supply is abundant “luxury consumption” of potassium often occurs, which deserves attention for its possible interference with the uptake and physiological availability of magnesium and calcium.

Comparative ecophysiology of leaf and canopy photosynthesis

Abstract: Leaves and herbaceous leaf canopies photosynthesize efficiently although the distribution of light, the ultimate resource of photosynthesis, is very biased in these systems. As has been suggested in theoretical studies, if a photosynthetic system is organized such that every photosynthetic apparatus photosynthesizes in concert, the system as a whole has the sharpest light response curve and is most adaptive. This condition can be approached by (i) homogenization of the light environment and (ii) acclimation of the photosynthetic properties of leaves or chloroplasts to their local light environments. This review examines these two factors in the herbaceous leaf canopy and in the leaf. Changes in the inclination of leaves in the canopy and differentiation of mesophyll into palisade and spongy tissue contribute to the moderation of the light gradient. Leaf and chloroplast movements in the upper parts of these systems under high irradiances also moderate light gradients. Moreover, acclimation of leaves and chloroplasts to the local light environment is substantial. These factors increase the efficiency of photosynthesis considerably. However, the systems appear to be less efficient than the theoretical optimum. When the systems are optically dense, the light gradients may be too great for leaves or chloroplasts to acclimate. The loss of photosynthetic production attributed to the imperfect adjustment of photosynthetic apparatus to the local light environment is most apparent when the photosynthesis of the system is in the transition between the light-limited and light-saturated phases. Although acclimation of the photosynthetic apparatus and moderation of light gradients are imperfect, these markedly raise the efficiency of photosynthesis. Thus more mechanistic studies on these adaptive attributes are needed. The causes and consequences of imperfect adjustment should also be investigated.

An evaluation of direct and indirect mechanisms for the "sink-regulation" of photosynthesis in spinach: Changes in gas exchange, carbohydrates, metabolites, enzyme activities and steady-state transcript levels after cold-girdling source leaves

Abstract: Mature source leaves of spinach (Spinacia oleracea L.) plants growing hydroponically in a 9 h light (350 μmol photons·m−2 · s−1)/15 h dark cycle at 20° C in a climate chamber were fitted with a cold girdle around the petiole, 2 h into the light period. Samples were taken 1, 3 and 7 h later, and at the end of the photoperiod for the following 4 d. Control samples were taken from ungirdled leaves. In the first 7 h after fitting the cold girdle there was (compared to the control leaves) a two to five-fold accumulation of sucrose, glucose, fructose and starch, a 40–50% increase of hexose-phosphates and ribulose-1,5-bisphosphate, a decrease of glycerate-3-phosphate, a small decrease in sucrose-phosphate synthase activation, an increase of fructose-2,6-bisphosphate, increased activation of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), but no significant change in photosynthetic rate or stomatal conductance. Steady-state transcript levels for rbcS (small subunit of Rubisco) and atp-D (D-subunit of the thylakoid ATP synthase) decreased 30%, cab (chlorophyll-a-binding protein) decreased by 15% and agp-S (S-isoenzyme of ADP-glucose pyrophosphorylase) and nra (nitrate reductase) rose twofold. On the following days, levels of carbohydrates continued to rise and the changes of metabolites were maintained. Transcripts for rbcS, cab and atpD declined to 20, 70 and 25% of the control values. From day 3 onward the maximum activity of Rubisco declined. This was accompanied by a further increase of Rubisco activation to over 90% and, from day 4 onwards, an inhibition of photosynthesis which was associated with high internal CO2 concentration (ci), high ribulose-1,5-bisphosphate, and low glycerate-3-phosphate. When the cold-girdle was removed on day 5 there was a gradual recovery of photosynthesis and decline of ci over the next 2 d. Hexose-phosphates levels and transcripts for rbcS, cab and atp-D completely recovered within 2 d, even though the levels of carbohydrates had not fully recovered. Activity of Rubisco only reverted partly after 2 d, and Rubisco activation state and the ribulose-1,5-bisphosphate/glycerate-3-phosphate ratio were still higher than in control leaves. Transcripts for nra and agp-S were also still higher than in control leaves. It is concluded (i) that a reversible modulation of gene expression in response to the export rate plays a central role in the mid-term feedback “sink” regulation of photosynthesis, and (ii) that feedback regulation of CO2 fixation by changes of Pi are of little importance in spinach under these conditions. Further (iii) the rapid and reciprocal changes in nra and agpS transcripts, compared to rbcS, provide evidence that gene expression could also contribute to the modulation of nitrate assimilation and carbohydrate storage in conditions of decreased sink demand.

Seasonal changes in photosystem II organisation and pigment composition in Pinus sylvestris

Abstract: Conifers of the boreal zone encounter considerable combined stress of low temperature and high light during winter, when photosynthetic consumption of excitation energy is blocked. In the evergreen Pinus sylvestris L. these stresses coincided with major seasonal changes in photosystem II (PSII) organisation and pigment composition. The earliest changes occurred in September, before any freezing stress, with initial losses of chlorophyll, the D1-protein of the PSII reaction centre and of PSII light-harvesting-complex (LHC II) proteins. In October there was a transient increase in F0, resulting from detachment of the light-harvesting antennae as reaction centres lost D1. The D1-protein content eventually decreased to 90%, reaching a minimum by December, but PSII photochemical efficiency [variable fluorescence (Fv)/maximum fluorescence (Fm)] did not reach the winter minimum until mid-February. The carotenoid composition varied seasonally with a twofold increase in lutein and the carotenoids of the xanthophyll cycle during winter, while the epoxidation state of the xanthophylls decreased from 0.9 to 0.1 from October to January. The loss of chlorophyll was complete by October and during winter much of the remaining chlorophyll was reorganised in aggregates of specific polypeptide composition, which apparently efficiently quench excitation energy through non-radiative dissipation. The timing of the autumn and winter changes indicated that xanthophyll de-epoxidation correlates with winter quenching of chlorophyll fluorescence while the drop in photochemical efficiency relates more to loss of D1-protein. In April and May recovery of the photochemistry of PSII, protein synthesis, pigment rearrangements and zeaxanthin epoxidation occurred concomitantly. Indoor recovery of photosynthesis in winter-stressed branches under favourable conditions was completed within 3 d, with rapid increases in F0, the epoxidation state of the xanthophylls and in light-harvesting polypeptides, followed by recovery of D1-protein content and Fv/Fm, all without net increase in chlorophyll. The fall and winter reorganisation allow Pinus sylvestris to maintain a large stock of chlorophyll in a quenched, photoprotected state, allowing rapid recovery of photosynthesis in spring.

Chilling-enhanced photooxidation: The production, action and study of reactive oxygen species produced during chilling in the light

Abstract: Chilling-enhanced photooxidation is the light- and oxygen-dependent bleaching of photosynthetic pigments that occurs upon the exposure of chilling-sensitive plants to temperatures below approximately 10 °C. The oxidants responsible for the bleaching are the reactive oxygen species (ROS) singlet oxygen (1O2), superoxide anion radical (O 2 ∸ ,hydrogen peroxide (H2O2), the hydroxyl radical (OH·), and the monodehydroascorbate radical (MDA) which are generated by a leakage of absorbed light energy from the photosynthetic electron transport chain. Cold temperatures slow the energy-consuming Calvin-Benson Cycle enzymes more than the energy-transducing light reactions, thus causing leakage of energy to oxygen. ROS and MDA are removed, in part, by the action of antioxidant enzymes of the Halliwell/Foyer/Asada Cycle. Chloroplasts also contain high levels of both lipid- and water-soluble antioxidants that act alone or in concert with the HFA Cycle enzymes to scavenge ROS. The ability of chilling-resistant plants to maintain active HFA Cycle enzymes and adequate levels of antioxidants in the cold and light contributes to their ability to resist chilling-enhanced photooxidation. The absence of this ability in chilling-sensitive species makes them susceptible to chilling-enhanced photooxidation. Chloroplasts may reduce the generation of ROS by dissipating the absorbed energy through a number of quenching mechanisms involving zeaxanthin formation, state changes and the increased usage of reducing equivalents by other anabolic pathways found in the stroma. During chilling in the light, ROS produced in chilling-sensitive plants lower the redox potential of the chloroplast stroma to such a degree that reductively-activated regulatory enzymes of the Calvin Cycle, sedohepulose 1,7 bisphosphatase (EC 3.1.3.37) and fructose 1,6 bisphosphatase (EC 3.1.3.11), are oxidatively inhibited. This inhibition is reversible in vitro with a DTT treatment indicating that the enzymes themselves are not permanently damaged. The inhibition of SBPase and FBPase may fully explain the inhibition in whole leaf gas exchange seen upon the rewarming of chilling-sensitive plants chilled in the light. Methods for the study of ROS in chilling-enhanced photooxidation and challenges for the future are discussed.

Light quality affects photosynthesis and leaf anatomy of birch plantlets in vitro

Abstract: Cultures in vitro of Betula pendula Roth were subjected to light of different spectral qualities. Photosynthetic capacity was highest when the plantlets were exposed to blue light (max recorded photosynthesis, 82 μmol CO2 dm−2 h−1) and lowest when irradiated with light high in red and/or far-red wave lengths (max recorded photosynthesis, 40 μmol CO2 dm−2 h−1). Highest chlorophyll content (2.2 mg dm−2 leaf area) was found in cultures irradiated with blue light, which also enhanced the leaf area. Morphometric analysis of light micrographs showed that the epidermal cell areas were largest in plantlets subjected to blue light and smallest in those subjected to red light. Morphometric analysis of electron micrographs of palisade cells, showed that the functional chloroplast area was largest in chloroplasts of leaves subjected to blue light and smallest in those exposed to red light. We suggest that light quality affects photosynthesis both through effects on the composition of the photosynthetic apparatus and on translocation of carbohydrates from chloroplasts.

Light-Harvesting Complexes in Oxygenic Photosynthesis: Diversity, Control, and Evolution

Abstract: This article focuses on light-harvesting complexes (LHCs) in oxygen evolving photosynthetic organisms. These organisms include cyanobacteria, red algae, plants, green algae, brown algae, diatoms, chrysophytes, and dinoflagellates. We highlight the diversity of pigment-protein complexes that fuel the conversion of radiant energy to chemical bond energy in land plants and the diverse groups of the algae, detail the ways in which environmental parameters (i.e. light quantity and quality, nutrients) modulate the synthesis of these complexes, and discuss the evolutionary relationships among the LHC structural polypeptides.

Variations in Chlorophyll Fluorescence Yields in Phytoplankton in the World Oceans

Abstract: The ocean is optically thin and lends itself to large-scale measurements of in vivo chlorophyll fluorescence. In the open ocean, however, phytoplankton chlorophyll concentrations average only 0.2 μg L-1, and hence high sensitivity is required for precise measurements of the fluorescence yields. Over the past decade, we have developed two approaches to achieve the required sensitivity; these are the pump- and probe-technique and a fast repetition rate (FRR) method. Both methods have been adapted for in situ studies and are used to rapidly measure the maximum change in the quantum yield (ΔOmax) of photosystem II (PSII), as well as the effective absorption cross-section of PSII (σPSII). Sections of variable fluorescence across the Pacific and Atlantic Oceans reveal the influence of geophysical processes in controlling the quantum yields of phytoplankton photosynthesis. Areas of upwelling, such as off the coast of north-westem Africa, have Fv/Fm values of 0.65, which are close to the maximum achievable values in nutrient-replete cultures. Throughout most of the nutrient-deficient central ocean basins, this quantum efficiency is reduced by more than 50%. In high-nutrient, low- chlorophyll regions of the eastern Equatorial Pacific, the deliberate, large-scale addition of nanomolar iron directly to the ocean leads to a rapid increase in quantum efficiency of the natural phytoplankton community, thereby revealing that in these regions phytoplankton photosynthetic energy conversion efficiency is iron limited. Diel patterns of variation in the upper ocean display midday, intensity- dependent reductions in both upsII and AOmax. We interpret the former as indicative of non- photochemical quenching in the antenna, while the latter is a consequence of both rapidly reversible and slowly reversible damage to reaction centres. From knowledge of the incident spectral irradiance, ΔOmax, σPSII, and photochemical quenching, the absolute photosynthetic electron transport rate can be derived in real-time. Using unattended, moored continuous measurements of in vivo fluorescence parameters, the derived in situ electron transport rates can be related to satellite observations of the global ocean with basin-scale, seasonal estimates of phytoplankton carbon fixation. Thus, unlike any other photosynthetic parameter, chlorophyll fluorescence can be used to bridge the scales of biophysical responses to ecosystem dynamics.

Unsaturation of the membrane lipids of chloroplasts stabilizes the photosynthetic machinery against low-temperature photoinhibition in transgenic tobacco plants

Abstract: Using tobacco plants that had been transformed with the cDNA for glycerol-3-phosphate acyltransferase, we have demonstrated that chilling tolerance is affected by the levels of unsaturated membrane lipids. In the present study, we examined the effects of the transformation of tobacco plants with cDNA for glycerol-3-phosphate acyltransferase from squash on the unsaturation of fatty acids in thylakoid membrane lipids and the response of photosynthesis to various temperatures. Of the four major lipid classes isolated from the thylakoid membranes, phosphatidylglycerol showed the most conspicuous decrease in the level of unsaturation in the transformed plants. The isolated thylakoid membranes from wild-type and transgenic plants did not significantly differ from each other in terms of the sensitivity of photosystem II to high and low temperatures and also to photoinhibition. However, leaves of the transformed plants were more sensitive to photoinhibition than those of wild-type plants. Moreover, the recovery of photosynthesis from photoinhibition in leaves of wild-type plants was faster than that in leaves of the transgenic tobacco plants. These results suggest that unsaturation of fatty acids of phosphatidylglycerol in thylakoid membranes stabilizes the photosynthetic machinery against low-temperature photoinhibition by accelerating the recovery of the photosystem II protein complex.

Effects of Iron Excess on Nicotiana plumbaginifolia Plants (Implications to Oxidative Stress).

Abstract: Fe excess is believed to generate oxidative stress. To contribute to the understanding of Fe metabolism, Fe excess was induced in Nicotiana plumbaginifolia grown in hydroponic culture upon root cutting. Toxicity symptoms leading to brown spots covering the leaf surface became visible after 6 h. Photosynthesis was greatly affected within 12 h; the photosynthetic rate was decreased by 40%. Inhibition of photosynthesis was accompanied by photoinhibition, increased reduction of photosystem II, and higher thylakoid energization. Fe excess seemed to stimulate photorespiration because catalase activity doubled. To cope with cellular damage, respiration rate increased and cytosolic glucose-6-phosphate dehydrogenase activity more than doubled. Simultaneously, the content of free hexoses was reduced. Indicative of generation of oxidative stress was doubling of ascorbate peroxidase activity within 12 h. Contents of the antioxidants ascorbate and glutathione were reduced by 30%, resulting in equivalent increases of dehydroascorbate and oxidized glutathione. Taken together, moderate changes in leaf Fe content have a dramatic effect on plant metabolism. This indicates that cellular Fe concentrations must be finely regulated to avoid cellular damage most probably because of oxidative stress induced by Fe.

Summer survival of leaves in a soft-leaved shrub (Phlomis fruticosa L., Labiatae) under Mediterranean field conditions: avoidance of photoinhibitory damage through decreased chlorophyll contents

Abstract: Photosynthetic pigments and relative water content of young leaves of P. fruticosa decreased considerably with the onset of the summer dry period and stabilized at low values for the last two summer months, while leaf growth was arrested. Corresponding decreases in photochemical efficiency of photosystem II, as judged by chlorophyll fluorescence measurements of predarkened leaves, were, however, negligible. Following the first autumn heavy rains, growth was restored and photosynthetic pigments and relative water content increased to the pre-drought values. The results indicate that the reduction of chlorophylls does not result from severe photoinhibitory damage but, instead, it may be an adaptive response against the adverse conditions of the Mediterranean summer. Some photosynthetic and photoprotective characteristics of P. fruticosa leaves at two stages of their development, i.e. at the severely dehydrated state with arrested growth during late summer and after their revival following the first heavy autumn rains were compared. Apart from the chlorophyll loss, the photon yield of O 2 evolution and the photosynthetic capacity at saturated CO 2 were considerably suppressed during the summer, indicating that the extremely low net photosynthetic rates observed in the field were the combined result of stomatal and mesophyll limitations. Epoxidation state was low at midday during the summer, indicating an active, photodissipative xanthophyll cycle. Although zeaxanthin content did not increase at midday after the rains, the potential of the cycle was maintained in the revived leaves, as judged by the high concentrations of the cycle components. After the rains, the activities of the anti-oxidant enzymes (superoxide dismutase, ascorbate peroxidase) remained relatively unchanged on a chlorophyll basis, but increased when expressed on a leaf surface area or protein basis. It may be concluded that P. fruticosa leaves avoid severe photoinhibitory and oxidative damage during the long, warm, dry and sunny Mediterranean summer by reducing light harvesting and electron flow capacity, whilst maintaining an adequate photoprotective ability. The preservation of a remarkable photodissipative and anti-oxidative potential after the rains may be related to the low predictability of precipitation even during the rainy winter.

8 – Functions of Mineral Nutrients: Macronutrients

Abstract: The chapter summarizes the role of micronutrients followed by their deficiencies and toxicities. The section on iron covers chloroplast development and photosynthesis, localization and binding state of iron, and root responses to iron deficiency and iron toxicity. The section on manganese covers manganese-containing enzymes, manganese-dependent or activated enzymes, photosynthesis and oxygen (O2) evolution, proteins, carbohydrates, lipids, and cell division. Copper covers copper proteins, carbohydrates, lipid and nitrogen metabolism, lignification, pollen formation, and fertilization. The section on zinc covers zinc-containing enzymes, zinc-activated enzymes, protein synthesis, carbohydrate metabolism, tryptophan and indoleacetic acid synthesis, membrane integrity, zinc-binding forms, and bioavailability. Nickel covers nickel-containing enzymes, role of nickel in nitrogen metabolism, and nickel content in plants. The enzymes covered under molybdenum are nitrate reductase, nitrogenase, xanthine oxidase/dehydrogenase, and sulfite reductase. The section also documents gross metabolic changes of molybdenum. Boron covers boron complexes with organic structures, root elongation and nucleic acid metabolism, cell wall synthesis, phenol metabolism, auxin (IAA) and tissue differentiation, membrane function, pollen germination and pollen tube growth, carbohydrate, and protein metabolism. The section on chlorine discusses photosynthetic O2 evolution, tonoplast proton-pumping ATPase, stomatal regulation, chlorine supply and plant growth, and chlorine supply and osmoregulation. The chapter also includes a summary on the mechanism of (heavy) metal tolerance in higher plants and the various mechanisms into which they can be grouped.

Copper and photosystem II: A controversial relationship

Abstract: Copper is an essential micronutrient for higher plants and algae and has a direct impact on photosynthesis. It is a constituent of the primary electron donor in photosystem I, the Cu-protein plastocyanin. Many authors have also described Cu as a constituent of photosystem II (PSII). However, high Cu concentrations inhibit the photosynthetic electron transport, especially in PSII. In addition, both Cu deficiency and Cu toxicity interfere with pigment and lipid biosynthesis and, consequently, with chloroplast ultrastructure thus negatively influencing the photosynthetic efficiency. In this review, the different functions proposed for the metal in PSII are reviewed. With reference to the effect of toxic Cu on PSII, the polemic results concerning its mechanism of action and Cu-binding sites are discussed. Other effects of Cu toxicity and Cu deprivation on the thylakoid membrane are also briefly described.

Regulation of Photosynthesis in C3 and C4 Plants: A Molecular Approach.

Abstract: Most plants use the C3 pathway of photosynthesis, also called the photosynthetic carbon reduction cycle (PCR), shown in Figure 1A. C3 plants have a single chloroplast type that performs all of the reactions that convert light energy into the chemical energy that is used to fix COp and to synthesize the reduced carbon compounds upon which all life depends. Ribulose-i ,5-bisphosphate carboxylaseloxygenase (Rubisco) catalyzes primary carbon fixation, in which a fivecarbon sugar phosphate, ribulose-l,5-bisphosphate (RuBP), and COp are converted to two molecules of the threecarbon compound 3-phosphoglycerate (hence the name C3). Phosphoglycerate is then phosphorylated and reduced by the products of the light reactions of photosynthesis (ATP and NADPH) to produce triose phosphate (TP). TP can be exported from the chloroplast via the chloroplast envelope phosphate (Pi) transporter to the cytosol and used in the synthesis of sucrose, which is then translocated throughout the plant (see Sonnewald et al., 1994), or it can be retained within the chloroplast for starch synthesis or recycling to RuBP. Rubisco also catalyzes the fixation of Op in a process known as photorespiration, which competes directly with fixation of COp. At air levels of Coa, for every three COp molecules fixed by Rubisco to form 3-phosphoglycerate, approximately one O2 molecule is fixed, producing Sphosphoglycerate and 3-phosphoglycolate (Figure 1A). Because 3-phosphoglycolate cannot be used in the PCR cycle, it must be recycled to phosphoglycerate via the photorespiratory pathway, expending ATP and NADPH. This competition between 0 2 and COp and the energy costs associated with recycling phosphoglycolate largely determine the efficiency of C3 photosynthesis in air (Hatch, 1988; Woodrow and Berry, 1988). The C4 pathway is a complexadaptation of the C3 pathway that overcomes the limitation of photorespiration and is found in a diverse collection of species, many of which grow in hot climates with sporadic rainfall. The C4 pathway effectively suppresses photorespiration by elevating the C02 concentration at the site of Rubisco using a biochemical C02 pump. C4 plants have two chloroplast types, each found in a specialized cell type. Leaves of C4 plants show extensive vascularization,

Increased Accumulation of Carbohydrates and Decreased Photosynthetic Gene Transcript Levels in Wheat Grown at an Elevated CO2 Concentration in the Field.

Abstract: Repression of photosynthetic genes by increased soluble carbohydrate concentrations may explain acclimation of photosynthesis to elevated CO2 concentration. This hypothesis was examined in a field crop of spring wheat (Triticum aestivum L.) grown at both ambient (approximately 360 [mu]mol mol-1) and elevated (550 [mu]mol mol-1) atmospheric CO2 concentrations using free-air CO2 enrichment at Maricopa, Arizona. The correspondence of steady-state levels of mRNA transcripts (coding for the 83-kD photosystem I apoprotein, sedoheptulose-1,7-bisphosphatase, phosphoribulokinase, phosphoglycerokinase, and the large and small subunits of ribulose-1,5-bisphosphate carboxylase/oxygenase) with leaf carbohydrate concentrations (glucose-6-phosphate, glucose, fructose, sucrose, fructans, and starch) was examined at different stages of crop and leaf development and through the diurnal cycle. Overall only a weak correspondence between increased soluble carbohydrate concentrations and decreased levels for nuclear gene transcripts was found. The difference in soluble carbohydrate concentration between leaves grown at elevated and current ambient CO2 concentrations diminished with crop development, whereas the difference in transcript levels increased. In the flag leaf, soluble carbohydrate concentrations declined markedly with the onset of grain filling; yet transcript levels also declined. The results suggest that, whereas the hypothesis may hold well in model laboratory systems, many other factors modified its significance in this field wheat crop.

High susceptibility to photoinhibition of young leaves of tropical forest trees

Abstract: Photoinhibition of photosynthesis was studied in young (but almost fully expanded) and mature canopy sun leaves of several tropical forest tree species, both under controlled conditions (exposure of detached leaves to about 1.8 mmol photons·m-2·s-1) and in the field. The degree of photoinhibition was determined by means of the ratio of variable to maximum chlorophyll (Chl) fluorescence emission (FV/FM) and also by gas-exchange measurements. For investigations in situ, young and mature leaves with similar exposure to sunlight were compared. The results show a consistently higher degree of photoinhibition in the young leaves. In low light, fast recovery was observed in both types of leaves in situ, as well as in the laboratory. The fluorescence parameter 1 — FS/F′M (where FS = stationary fluorescence and f′M = maximum fluorescence during illumination) was followed in situ during the course of the day in order to test its suitability as a measure of the photosynthetic yield of photosystem II (PSII). Electron-transport rates were calculated from these fluorescence signals and compared with rates of net CO2 assimilation. Measurements of diurnal changes in PSII ‘yield’ confirmed the increased susceptibility of young leaves to photoinhibition. Calculated electron transport qualitatively reflected net CO2 uptake in situ during the course of the day. Photosynthetic pigments were analyzed in darkened and illuminated leaves. Young and mature leaves showed the same Chl a/b ratio, but young leaves contained about 50% less Chl a + b per unit leaf area. The capacity of photosynthetic O2 evolution per unit leaf area was decreased to a similar extent in young leaves. On a Chl basis, young leaves contained more α-carotene, more xanthophyll cycle pigments and, under strong illumination, more zeaxanthin than mature leaves. The high degree of reversible photoinhibition observed in these young sun leaves probably represents a dynamic regulatory process protecting the photosynthetic apparatus from severe damage by excess light.

Interaction of ozone pollution and light effects on photosynthesis in a forest canopy experiment

Abstract: Ozone pollution may reduce net carbon gain in forests, yet data from mature trees are rare and the effects of irradiance on the response of photosynthesis to ozone remain untested. We used an open-air system to expose 10 branches within the upper canopy of an 18-m-tall stand of sugar maple (Acer saccharum Marsh.) to twice-ambient concentrations of ozone (95 nmol mol -1 , 0900 to 1700, 1h mean) relative to 10 paired, untreated controls (45nmol mol -1 ) over 3 months. The branch pairs were selected along a gradient from relatively high irradiance (PPFD 14.5 mol m -2 d -1 ) to deep shade (0.7 mol m -2 d -1 ). Ozone reduced light-saturated rates of net photosynthesis (A sat ) and increased dark respiration by as much as 56 and 40%, respectively. Compared to sun leaves, shade leaves exhibited greater proportional reductions in A sat and had lower chlorophyll concentrations, quantum efficiencies, and leaf absorptances when treated with ozone relative to controls. With increasing ozone dose over time, A sat became uncoupled from stomatal conductance as ratios of internal to external concentrations of carbon dioxide increased, reducing water-use efficiency. Ozone reduced net photosynthesis and impaired stomatal function, with these effects depending on the irradiance environment of the canopy leaves. Increased ozone sensitivity of shade leaves compared to sun leaves has consequences for net carbon gain in canopies.