Showing papers on "Photosynthesis published in 2006"

Can improvement in photosynthesis increase crop yields

Abstract: The yield potential ( Y p ) of a grain crop is the seed mass per unit ground area obtained under optimum growing conditions without weeds, pests and diseases. It is determined by the product of the available light energy and by the genetically determined properties: efficiency of light capture ( e i ), the efficiency of conversion of the intercepted light into biomass ( e c ) and the proportion of biomass partitioned into grain ( h ). Plant breeding brings h and e i close to their theoretical maxima, leaving e c , primarily determined by photosynthesis, as the only remaining major prospect for improving Y p . Leaf photosynthetic rate, however, is poorly correlated with yield when different genotypes of a crop species are compared. This led to the viewpoint that improvement of leaf photosynthesis has little value for improving Y p . By contrast, the many recent experiments that compare the growth of a genotype in current and future projected elevated [CO 2 ] environments show that increase in leaf photosynthesis is closely associated with similar increases in yield. Are there opportunities to achieve similar increases by genetic manipulation? Six potential routes of increasing e c by improving photosynthetic efficiency were explored, ranging from altered canopy architecture to improved regeneration of the acceptor molecule for CO 2 . Collectively, these changes could improve e c and, therefore, Y p by c . 50%. Because some changes could be achieved by transgenic technology, the time of the development of commercial cultivars could be considerably less than by conventional breeding and potentially, within 10‐15 years.

Structure and function of photosystems i and ii

Abstract: Oxygenic photosynthesis, the principal converter of sunlight into chemical energy on earth, is catalyzed by four multi-subunit membrane-protein complexes: photosystem I (PSI), photosystem II (PSII), the cytochrome b6f complex, and F-ATPase PSI generates the most negative redox potential in nature and largely determines the global amount of enthalpy in living systems PSII generates an oxidant whose redox potential is high enough to enable it to oxidize H2O, a substrate so abundant that it assures a practically unlimited electron source for life on earth During the last century, the sophisticated techniques of spectroscopy, molecular genetics, and biochemistry were used to reveal the structure and function of the two photosystems The new structures of PSI and PSII from cyanobacteria, algae, and plants has shed light not only on the architecture and mechanism of action of these intricate membrane complexes, but also on the evolutionary forces that shaped oxygenic photosynthesis

Keeping a positive carbon balance under adverse conditions: responses of photosynthesis and respiration to water stress

Abstract: Drought and salinity (i.e. soil water stress) are the main environmental factors limiting photosynthesis and respiration and, consequently, plant growth. This review summarizes the current status of knowledge on photosynthesis and respiration under water stress. It is shown that diffusion limitations to photosynthesis under most water stress conditions are predominant, involving decreased mesophyll conductance to CO 2 , an important but often neglected process. A general failure of photochemistry and biochemistry, by contrast, can occur only when daily maximum stomatal conductance (g s ) drops below 0.05-0.10 mol H 2 O m -2 s -1 . Because these changes are preceded by increased leaf antioxidant activities (g s below 0.15-0.20 mol H 2 O m -2 s -1 ), it is suggested that metabolic responses to severe drought occur indirectly as a consequence of oxidative stress, rather than as a direct response to water shortage. As for respiration, it is remarkable that the electron partitioning towards the alternative respiration pathway sharply increases at the same g s threshold, although total respiration rates are less affected. Despite the considerable improvement in the understanding of plant responses to drought, several gaps of knowledge are highlighted which should become research priorities for the near future. These include how respiration and photosynthesis interact at severe stress, what are the boundaries and mechanisms of photosynthetic acclimation to water stress and what are the factors leading to different rates of recovery after a stress period.

Algae: Anatomy, Biochemistry, and Biotechnology

Abstract: General Overview Definition Classification Occurrence and Distribution Structure of Thallus-Cytomorphological Types Nutrition Reproduction Summaries of the 11 Algal Phyla Endosymbiosis and Origin of Eukaryotic Photosynthesis Anatomy Cytomorphology and Ultrastructure Outside the Cell Flagella and Associated Structures The photoreceptor Apparata Chloroplasts The Nucleus, Nuclear Division, and Cytokinesis Ejectile Organelles and Feeding Apparatus Photosynthesis Light Photosynthesis Light-dependent Reactions Light-Independent Reactions The Energy Relationships in Photosynthesis: The Balance Sheet Working with Light How Light Behaves Field instruments, Use and Application Radiometry Photometry Units conversion PAR Detectors The Photosynthesis-Irradiance Response Curve (P vs E Curve) Photoacclimation Biogeochemical Role of Algae The Role of Algae in Biogeochemistry Limiting Nutrients Algae and the Phosphorus Cycle Algae and the Nitrogen Cycle Algae and the Silicon Cycle Algae and the Sulfur Cycle Algae and the Oxygen-Carbon Cycles Algal Culturing Collection, Storage, and Preservation Culture Types Culture Parameters Culture Vessels Media Choice and Preparation Sterilization of Culture Materials Culture Methods Outdoor Ponds Photobioreactors Quantitative Determinations of Algal Density and Growth Algae Utilization Introduction Sources and Uses of Algae Human food Animal feed Extracts Fertilizers Cosmetics Functional Foods and Nutraceuticals Toxins Oddities and Curiosities in the Algal World In the Realm of Darkness Algae-Animal Interaction: Riding a Sloth, Swinging on a Spider Web, Swimming in a Jelly Someone Like it Cold Someone Like it Hot Someone Like it Dry

Photosynthesis, Productivity, and Yield of Maize Are Not Affected by Open-Air Elevation of CO2 Concentration in the Absence of Drought

Abstract: While increasing temperatures and altered soil moisture arising from climate change in the next 50 years are projected to decrease yield of food crops, elevated CO2 concentration ([CO2]) is predicted to enhance yield and offset these detrimental factors. However, C4 photosynthesis is usually saturated at current [CO2] and theoretically should not be stimulated under elevated [CO2]. Nevertheless, some controlled environment studies have reported direct stimulation of C4 photosynthesis and productivity, as well as physiological acclimation, under elevated [CO2]. To test if these effects occur in the open air and within the Corn Belt, maize (Zea mays) was grown in ambient [CO2] (376 μmol mol−1) and elevated [CO2] (550 μmol mol−1) using Free-Air Concentration Enrichment technology. The 2004 season had ideal growing conditions in which the crop did not experience water stress. In the absence of water stress, growth at elevated [CO2] did not stimulate photosynthesis, biomass, or yield. Nor was there any CO2 effect on the activity of key photosynthetic enzymes, or metabolic markers of carbon and nitrogen status. Stomatal conductance was lower (−34%) and soil moisture was higher (up to 31%), consistent with reduced crop water use. The results provide unique field evidence that photosynthesis and production of maize may be unaffected by rising [CO2] in the absence of drought. This suggests that rising [CO2] may not provide the full dividend to North American maize production anticipated in projections of future global food supply.

Photostasis and cold acclimation: sensing low temperature through photosynthesis

Abstract: Photosynthesis is a highly integrated and regulated process which is highly sensitive to any change in environmental conditions, because it needs to balance the light energy absorbed by the photosystems with the energy consumed by metabolic sinks of the plant. Low temperatures exacerbate an imbalance between the source of energy and the metabolic sink, thus requiring adjustments of photosynthesis to maintain the balance of energy flow. Photosynthesis itself functions as a sensor of this imbalance through the redox state of photosynthetic electron-transport components and regulates photophysical, photochemical and metabolic processes in the chloroplast. Recent progress has been made in understanding how plants sense the low temperature signal. It is clear that photosynthesis interacts with other processes during cold acclimation involving crosstalk between photosynthetic redox, cold acclimation and sugar-signalling pathways to regulate plant acclimation to low temperatures.

Irradiance and phenotype: comparative eco-development of sun and shade leaves in relation to photosynthetic CO2 diffusion

Abstract: The subject of this paper, sun leaves are thicker and show higher photosynthetic rates than the shade leaves, is approached in two ways. The first seeks to answer the question: why are sun leaves thicker than shade leaves? To do this, CO2 diffusion within a leaf is examined first. Because affinity of Rubisco for CO2 is low, the carboxylation of ribulose 1,5-bisphosphate is competitively inhibited by O2, and the oxygenation of ribulose 1,5-bisphosphate leads to energy-consuming photorespiration, it is essential for C3 plants to maintain the CO2 concentration in the chloroplast as high as possible. Since the internal conductance for CO2 diffusion from the intercellular space to the chloroplast stroma is finite and relatively small, C3 leaves should have sufficient mesophyll surfaces occupied by chloroplasts to secure the area for CO2 dissolution and transport. This explains why sun leaves are thicker. The second approach is mechanistic or ‘how-oriented’. Mechanisms are discussed as to how sun leaves become thicker than shade leaves, in particular, the long-distance signal transduction from mature leaves to leaf primordia inducing the periclinal division of the palisade tissue cells. To increase the mesophyll surface area, the leaf can either be thicker or have smaller cells. Issues of cell size are discussed to understand plasticity in leaf thickness.

Copper-containing plastocyanin used for electron transport by an oceanic diatom.

Abstract: Diatoms are responsible for 40% of ocean production and are strongly limited by the lack of iron salts in the sea The shortage of this important nutrient may explain a surprising discovery: the oceanic diatom Thalassiosira oceanica uses a copper-containing plastocyanin for electron transport All other chlorophyll c-containing taxa, including coastal diatoms, use the iron-containing cytochrome c6 instead The use of these metalloproteins matches the availabilities of copper and iron in the ocean, and this new discovery suggests that copper is a potentially important nutrient in the open sea The supply of some essential metals to pelagic ecosystems is less than the demand, so many phytoplankton have slow rates of photosynthetic production and restricted growth1 The types and amounts of metals required by phytoplankton depends on their evolutionary history2 and on their adaptations to metal availability3,4, which varies widely among ocean habitats Diatoms, for example, need considerably less iron (Fe) to grow than chlorophyll-b-containing taxa2, and the oceanic species demand roughly one-tenth the amount of coastal strains5,6,7 Like Fe, copper (Cu) is scarce in the open sea, but notably higher concentrations of it are required for the growth of oceanic than of coastal isolates8 Here we report that the greater Cu requirement in an oceanic diatom, Thalassiosira oceanica, is entirely due to a single Cu-containing protein, plastocyanin, which—until now—was only known to exist in organisms with chlorophyll b and cyanobacteria Algae containing chlorophyll c, including the closely related coastal species T weissflogii, are thought to lack plastocyanin and contain a functionally equivalent Fe-containing homologue, cytochrome c6 (ref 9) Copper deficiency in T oceanica inhibits electron transport regardless of Fe status, implying a constitutive role for plastocyanin in the light reactions of photosynthesis in this species The results suggest that selection pressure imposed by Fe limitation has resulted in the use of a Cu protein for photosynthesis in an oceanic diatom This biochemical switch reduces the need for Fe and increases the requirement for Cu, which is relatively more abundant in the open sea

Metabolic consequences of susceptibility and resistance (race‐specific and broad‐spectrum) in barley leaves challenged with powdery mildew

Abstract: In a compatible interaction biotrophic fungi often lower the yield of their hosts by reducing photosynthesis and altering the fluxes of carbon within the infected leaf. In contrast, comparatively little is known about the metabolic consequences of activating resistance responses. In this study we investigated the hypothesis that the activation of both race-specific (Mla12) and broad-spectrum (mlo) resistance pathways in barley leaves infected with Blumeria graminis represents a cost to the plant in terms of carbon production and utilization. We have shown, using quantitative imaging of chlorophyll fluorescence, that during a susceptible interaction, photosynthesis was progressively reduced both in cells directly below fungal colonies and in adjacent cells when compared with uninoculated leaves. The lower rate of photosynthesis was associated with an increase in invertase activity, an accumulation of hexoses and a down-regulation of photosynthetic gene expression. During both Mla12- and mlo-mediated resistance, photosynthesis was also reduced, most severely inhibited in cells directly associated with attempted penetration of the fungus but also in surrounding cells. These cells displayed intense autofluorescence under ultraviolet illumination indicative of the accumulation of phenolic compounds and/or callose deposition. The depression in photosynthesis was not due only to cell death but also to an alteration in source-sink relations and carbon utilization. Apoplastic (cell wall-bound) invertase activity increased more rapidly and to a much greater extent than in infected susceptible leaves and was accompanied by an accumulation of hexoses that was localized to areas of the leaf actively exhibiting resistance responses. The accumulation of hexoses was accompanied by a down-regulation in the expression of Rubisco (rbcS) and chlorophyll a/b binding protein (cab) genes (although to a lesser extent than in a compatible interaction) and with an up-regulation in the expression of the pathogenesis-related protein 1 (PR-1). These results are consistent with a role for invertase in the generation of hexoses, which may supply energy for defence reactions and/or act as signals inducing defence gene expression.

The cyanobacterial genome core and the origin of photosynthesis

Abstract: Comparative analysis of 15 complete cyanobacterial genome sequences, including "near minimal" genomes of five strains of Prochlorococcus spp., revealed 1,054 protein families [core cyanobacterial clusters of orthologous groups of proteins (core CyOGs)] encoded in at least 14 of them. The majority of the core CyOGs are involved in central cellular functions that are shared with other bacteria; 50 core CyOGs are specific for cyanobacteria, whereas 84 are exclusively shared by cyanobacteria and plants and/or other plastid-carrying eukaryotes, such as diatoms or apicomplexans. The latter group includes 35 families of uncharacterized proteins, which could also be involved in photosynthesis. Only a few components of cyanobacterial photosynthetic machinery are represented in the genomes of the anoxygenic phototrophic bacteria Chlorobium tepidum, Rhodopseudomonas palustris, Chloroflexus aurantiacus, or Heliobacillus mobilis. These observations, coupled with recent geological data on the properties of the ancient phototrophs, suggest that photosynthesis originated in the cyanobacterial lineage under the selective pressures of UV light and depletion of electron donors. We propose that the first phototrophs were anaerobic ancestors of cyanobacteria ("procyanobacteria") that conducted anoxygenic photosynthesis using a photosystem I-like reaction center, somewhat similar to the heterocysts of modern filamentous cyanobacteria. From procyanobacteria, photosynthesis spread to other phyla by way of lateral gene transfer.

Mechanism of nano-anatase TiO2 on promoting photosynthetic carbon reaction of spinach: inducing complex of rubisco-rubisco activase.

Abstract: Having a photocatalyzed characteristic, our previous research had proved that nano-anatase TiO2 is closely related to the photosynthesis of spinach. It could not only improve the light absorbance and the transformation from light energy to electron energy and to active chemical energy but also promote carbon dioxide (CO2) assimilation of spinach. However, the mechanism of carbon reaction promoted by nano-anatase TiO2 remains largely unclear. By electrophoresis and Western blot methods, the results of the experiments proved that Rubisco from the nano-anatase TiO2-treated spinach during the extraction procedure of Rubisco was found to consist of Rubisco and a heavier molecular-mass protein (about 1200 kDa) comprising both Rubisco and Rubisco activase. The Rubisco carboxylase activity was 2.67 times that of Rubisco from the control and it could hydrolyze ATP in the same manner as Rubisco activase. The total sulfhydryl groups and available sulfhydryl groups of the Rubisco were 32-SH and 21-SH per mole of enzyme more than those of the Rubisco purified from the control, respectively. The circular dichroism spectra showed that the secondary structure of Rubisco from the nano-anatase TiO2-treated spinach was very different from Rubisco of the control. It suggested that the mechanism of nano-anatase TiO2 activating Rubisco of spinach was that the complex of Rubsico and Rubisco activase was induced in spinach, which promoted Rubsico carboxylation and increased the rate of photosynthetic carbon reaction.

Combined effects of water stress and high temperature on photosynthesis, nitrogen metabolism and lipid peroxidation of a perennial grass Leymus chinensis

Abstract: Drought and high-temperature stresses have been extensively studied; however, little is known about their combined impact on plants. In the present study, we determined the photosynthetic gas exchange, chlorophyll fluorescence, nitrogen level, and lipid peroxidation of the leaves of a perennial grass (Leymus chinensis (Trin.) Tzvel.) subjected to three constant temperatures (23, 29 and 32°C), and five soil-moisture levels (75–80%, 60–65%, 50–55%, 35–40% and 25–30% of field capacity, respectively). High temperature significantly decreased plant biomass, leaf green area, leaf water potential, photosynthetic rate (A), maximal efficiency of PSII photochemistry (F v/F m), actual PSII efficiency (ΦPSII), the activities of nitrate reductase (NR; EC 1.6.6.1) and glutamine synthetase (GS; EC 6.3.1.2), but markedly increased the ratio of leaf area to leaf weight (SLA), endopeptidase (EP; EC 3.4.24.11) activity, and malondialdehyde (MDA) content, especially under severe water stress conditions. The A and F v/F m were significantly and positively correlated with leaf-soluble protein content, and the activities of NR and GS. However, both photosynthesis parameters were significantly and negatively correlated with EP activity and MDA content (P < 0.05). It is suggested that high temperature, combined with severe soil drought, might reduce the function of PSII, weaken nitrogen anabolism, strengthen protein catabolism, and provoke lipid peroxidation. The results also indicate that severe water stress might exacerbate the adverse effects of high temperature, and their combination might reduce the plant productivity and distribution range of L. chinensis in the future.

Bistability of atmospheric oxygen and the Great Oxidation

Abstract: The first significant increase in atmospheric oxygen levels on Earth (the 'Great Oxidation') is thought to have occurred at least 300 million years after the evolution of oxygenic photosynthesis, but the reason for this time lag is not clear. Using a new conceptual model of the global redox system, Goldblatt et al. show that atmospheric oxygen levels could have remained either at a low or a high steady state after oxygenic photosynthesis evolved. The Great Oxidation may have been a switch between these states triggered by a relatively small environmental change. The model suggests that oxygenic photosynthesis alone is insufficient to cause an oxygen-rich atmosphere. So in the absence of additional factors, Earth might have an atmosphere containing only a few parts per million of oxygen, not the 21% which we enjoy. Equally, oxygenic photosynthesis could conceivably have evolved on planets that only have low levels of atmospheric oxygen. The history of the Earth has been characterized by a series of major transitions separated by long periods of relative stability1. The largest chemical transition was the ‘Great Oxidation’, approximately 2.4 billion years ago, when atmospheric oxygen concentrations rose from less than 10-5 of the present atmospheric level (PAL) to more than 0.01 PAL, and possibly2 to more than 0.1 PAL. This transition took place long after oxygenic photosynthesis is thought to have evolved3,4,5, but the causes of this delay and of the Great Oxidation itself remain uncertain6,7,8,9,10,11. Here we show that the origin of oxygenic photosynthesis gave rise to two simultaneously stable steady states for atmospheric oxygen. The existence of a low-oxygen (less than 10-5 PAL) steady state explains how a reducing atmosphere persisted for at least 300 million years after the onset of oxygenic photosynthesis. The Great Oxidation can be understood as a switch to the high-oxygen (more than 5 × 10-3 PAL) steady state. The bistability arises because ultraviolet shielding of the troposphere by ozone becomes effective once oxygen levels exceed 10-5 PAL, causing a nonlinear increase in the lifetime of atmospheric oxygen. Our results indicate that the existence of oxygenic photosynthesis is not a sufficient condition for either an oxygen-rich atmosphere or the presence of an ozone layer, which has implications for detecting life on other planets using atmospheric analysis12,13 and for the evolution of multicellular life.

Photosynthesis in the Archean era.

Abstract: The earliest reductant for photosynthesis may have been H2. The carbon isotope composition measured in graphite from the 3.8-Ga Isua Supercrustal Belt in Greenland is attributed to H2-driven photosynthesis, rather than to oxygenic photosynthesis as there would have been no evolutionary pressure for oxygenic photosynthesis in the presence of H2. Anoxygenic photosynthesis may also be responsible for the filamentous mats found in the 3.4-Ga Buck Reef Chert in South Africa. Another early reductant was probably H2S. Eventually the supply of H2 in the atmosphere was likely to have been attenuated by the production of CH4 by methanogens, and the supply of H2S was likely to have been restricted to special environments near volcanos. Evaporites, possible stromatolites, and possible microfossils found in the 3.5-Ga Warrawoona Megasequence in Australia are attributed to sulfur-driven photosynthesis. Proteobacteria and protocyanobacteria are assumed to have evolved to use ferrous iron as reductant sometime around 3.0 Ga or earlier. This type of photosynthesis could have produced banded iron formations similar to those produced by oxygenic photosynthesis. Microfossils, stromatolites, and chemical biomarkers in Australia and South Africa show that cyanobacteria containing chlorophyll a and carrying out oxygenic photosynthesis appeared by 2.8 Ga, but the oxygen level in the atmosphere did not begin to increase until about 2.3 Ga.

Photoprotection, photoinhibition, gene regulation, and environment

Abstract: Editorial, Preface, A Random Walk To and Through the Xanthophyll Cycle.- Photoinhibition: Then and Now.- The D1 Protein: Past and Future Perspectives.- Characteristics and Species-Dependent Employment of Flexible Versus Sustained Thermal Dissipation and Photoinhibition.- Energy Dissipation and Photoinhibition: A Continuum of Photoprotection.- Photoinhibition and Photoprotection under Nutrient Deficiencies, Drought, and Salinity.- Photoinhibition and UV Response in the Aquatic Environment.- Phosphorylation of Thylakoid Proteins.- Molecular Analysis of Photoprotection of Photosynthesis.- A Protein Family Saga: From Photoprotection to Light-harvesting (and Back?).- Photoprotection of Photosystem II: Reaction Center Quenching Versus Antenna Quenching.- Photoinhibition and Recovery in Oxygenic Photosynthesis: Mechanism of a Photosystem-II Damage and Repair Cycle.- Regulation by Environmental Conditions of the Repair of Photosystem II in Cyanobacteria.- Photosystem I and Photoprotection: Cyclic Electron Flow and Water-Water Cycle.- Integration of Signaling in Antioxidant Defenses.- Signaling and Integration of Defense Functions of Tocopherol, Ascorbate, and Glutathione.- Redox Regulation of Chloroplast Gene Expression.- Intracellular Signaling and Chlorophyll Synthesis.- The Role of Peroxiredoxins in Oxygenic Photosynthesis of Cyanobacteria and Higher Plants: Peroxide Detoxification or Redox Sensing?- Lipoxygenases, Apoptosis, and the Role of Antioxidants.- Regulation of Photosynthetic Gene Expression by the Environment: From Seedling De-etiolation to Leaf Senescence.

Water relations and photosynthesis in Cucumis sativus L. leaves under salt stress

Abstract: Hydroponically grown cucumber plants were exposed to 14-d period of salinity (0, 50, 100 mM NaCl). NaCl caused reduction in the relative water content in the leaves. The Na+ content increased and the K+ content decreased. The net photosynthetic rate, stomatal conductance and transpiration rate were markedly decreased by all of the salt treatments. Salinity decreased also the maximum quantum efficiency of photosystem 2 (PS 2) determined as the variable to maximum fluorescence ratio, the photochemical quantum yield of PS 2 and the photochemical fluorescence quenching, while the non-photochemical quenching increased. Above results indicate that NaCl affects photosynthesis through both stomata closure and non-stomatal factors.

Balancing the energy flow from captured light to biomass under fluctuating light conditions.

Abstract: Summary • The balance of energy flow from light absorption into biomass was investigated under simulated natural light conditions in the diatom Phaeodactylum tricornutum and the green alga Chlorella vulgaris. • The energy balance was quantified by comparative analysis of carbon accumulation in the new biomass with photosynthetic electron transport rates per absorbed quantum, measured both by fluorescence quenching and oxygen production. The difference between fluorescence- and oxygen-based electron flow is defined as ‘alternative electron cycling’. • The photosynthetic efficiency of biomass production was found to be identical for both algae under nonfluctuating light conditions. In a fluctuating light regime, a much higher conversion efficiency of photosynthetic energy into biomass was observed in the diatom compared with the green alga. • The data clearly show that the diatom utilizes a different strategy in the dissipation of excessively absorbed energy compared with the green alga. Consequently, in a fluctuating light climate, the differences between green algae and diatoms in the efficiency of biomass production per photon absorbed are caused by the different amount of alternative electron cycling.

Evidence for involvement of photosynthetic processes in the stomatal response to CO2.

Abstract: Stomatal conductance (gs) typically declines in response to increasing intercellular CO2 concentration (ci). However, the mechanisms underlying this response are not fully understood. Recent work suggests that stomatal responses to ci and red light (RL) are linked to photosynthetic electron transport. We investigated the role of photosynthetic electron transport in the stomatal response to ci in intact leaves of cocklebur (Xanthium strumarium) plants by examining the responses of gs and net CO2 assimilation rate to ci in light and darkness, in the presence and absence of the photosystem II inhibitor 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), and at 2% and 21% ambient oxygen. Our results indicate that (1) gs and assimilation rate decline concurrently and with similar spatial patterns in response to DCMU; (2) the response of gs to ci changes slope in concert with the transition from Rubisco- to electron transport-limited photosynthesis at various irradiances and oxygen concentrations; (3) the response of gs to ci is similar in darkness and in DCMU-treated leaves, whereas the response in light in non-DCMU-treated leaves is much larger and has a different shape; (4) the response of gs to ci is insensitive to oxygen in DCMU-treated leaves or in darkness; and (5) stomata respond normally to RL when ci is held constant, indicating the RL response does not require a reduction in ci by mesophyll photosynthesis. Together, these results suggest that part of the stomatal response to ci involves the balance between photosynthetic electron transport and carbon reduction either in the mesophyll or in guard cell chloroplasts.

Dehydroascorbate Reductase Affects Leaf Growth, Development, and Function

Abstract: Ascorbic acid (Asc) is a major antioxidant in plants that detoxifies reactive oxygen species (ROS) and maintains photosynthetic function. Expression of dehydroascorbate reductase (DHAR), responsible for regenerating Asc from an oxidized state, regulates the cellular Asc redox state, which in turn affects cell responsiveness and tolerance to environmental ROS. Because of its role in Asc recycling, we examined whether DHAR is important for plant growth. Suppression of DHAR expression resulted in a preferential loss of chlorophyll a, a lower steady state of Rubisco as measured by the amount of the large subunit of Rubisco (RbcL), and a lower rate of CO 2 assimilation. As a consequence, a slower rate of leaf expansion and reduced foliar dry weight were observed. In addition, an accelerated rate of loss of chlorophyll, RbcL, light-harvesting complex II, and photosynthetic functioning was observed in mature leaves, resulting in premature leaf aging. Reduced growth rate as measured by plant height and leaf number was consistent with the DHAR-mediated reduction of photosynthetic function. Increasing DHAR expression maintained higher levels of chlorophyll, RbcL, light-harvesting complex II, and photosynthetic functioning, resulting in delayed leaf aging. The effect of DHAR expression on leaf aging inversely correlated with the level of lipid peroxidation, indicating that DHAR functions to protect against ROS-mediated damage. These observations support the conclusion that through its Asc recycling function, DHAR affects the level of foliar ROS and photosynthetic activity during leaf development and as a consequence, influences the rate of plant growth and leaf aging.

Phytoplankton photoacclimation and photoadaptation in response to environmental gradients in a shelf sea

Abstract: Variability in the photosynthetic performance of natural phytoplankton communities, due to both taxonomic composition and the physiological acclimation of these taxa to environmental conditions, was assessed at contrasting sites within a temperate shelf sea region. Physiological parameters relating to the structure of the photosystem II (PSII) antenna and processes downstream from PSII were evaluated using a combination of fast repetition rate fluorescence, oxygen flash yields, spectral fluorescence, and 14 C photosynthesis versus irradiance measurements. Parameters relating to PSII antenna structure, specifically the functional absorption cross-section (sPSII) and the chlorophyll to PSII reaction center ratio, varied principally as a result of spatial (horizontal) taxonomic differences. Phenotypic plasticity in the size of the PSII light-harvesting antenna appeared to be limited. In contrast, parameters related to electron transport rates (ETRs) downstream of PSII, including the maximum ETR (1/tPSII), the chlorophyll-specific maximum rate of carbon fixation (P ), and the light-saturation intensity ( Ek), all decreased from the surface to the subsurface chlorophyll * maximum (SCM) in stratified waters. The primary photoacclimation response to the vertical light gradient thus resulted in decreasing light-saturated carbon fixation per reaction center with increasing depth. Increases in the ratio of PSII reaction centers to carbon fixation capacity thus dominated the phenotypic response to decreased irradiance within the SCM. Perhaps counterintuitively, phytoplankton populations within fully mixed water columns, characterized by low mean irradiance, were acclimated or adapted to relatively high irradiance. Photoacclimation describes the phenotypic response of algae to changes in irradiance at the organism level (Falkowski and LaRoche 1991) and can be assessed by measuring dif

High thermal acclimation potential of both photosynthesis and respiration in two lowland Plantago species in contrast to an alpine congeneric

Abstract: Thermal acclimation of photosynthesis and respiration can enable plants to maintain near constant rates of net CO2 exchange, despite experiencing sustained changes in daily average temperature. In this study, we investigated whether the degree of acclimation of photosynthesis and respiration of mature leaves differs among three congeneric Plantago species from contrasting habitats [two fast-growing lowland species (Plantago major and P. lanceolata), and one slow-growing alpine species (P. euryphylla)]. In addition to investigating some mechanisms underpinning variability in photosynthetic acclimation, we also determined whether leaf respiration in the light acclimates to the same extent as leaf respiration in darkness, and whether acclimation reestablishes the balance between leaf respiration and photosynthesis. Three growth temperatures were provided: constant 13, 20, or 27°C. Measurements were made at five temperatures (6–34°C). Little acclimation of photosynthesis and leaf respiration to growth temperature was exhibited by P. euryphylla. Moreover, leaf masses per area (LMA) were similar in 13°C-grown and 20°C-grown plants of the alpine species. In contrast, growth at 13°C increased LMA in the two lowland species; this was associated with increased photosynthetic capacity and rates of leaf respiration (both in darkness and in the light). Alleviation of triose phosphate limitation and increased capacity of electron transport capacity relative to carboxylation were also observed. Such changes demonstrate that the lowland species cold-acclimated. Light reduced the short-term temperature dependence (i.e. Q10) of leaf respiration in all three species, irrespective of growth temperature. Collectively, our results highlight the tight coupling that exists between thermal acclimation of photosynthetic and leaf respiratory metabolism (both in darkness and in the light) in Plantago. If widespread among contrasting species, such coupling may enable modellers to assume levels of acclimation in one parameter (e.g. leaf respiration) where details are only known for the other (e.g. photosynthesis).

Sink strength regulates photosynthesis in sugarcane.

Abstract: The relationship in sugarcane (Saccharum spp.) between photosynthetic source tissue and sink material was examined through manipulation of the sink:source ratio of field-grown Saccharum spp. hybrid cv. N19 (N19). To enhance sink strength, all leaves, except for the third fully expanded leaf, were enclosed in 90% shade cloth for varying periods of time. Variations in sucrose, glucose and fructose concentrations were measured and the effects of shading on the leaf gas exchange and fluorescence characteristics recorded. Changes in carbon partitioning caused by shading were examined based on the uptake and translocation of fixed 14CO2. Following a decline in sucrose concentrations in young internodal tissue and shaded leaves, significant increases in the CO2-saturated photosynthetic rate (Jmax), carboxylation efficiency (CE) and electron transport rate were observed in unshaded leaves after 8 d of shading treatment. It was concluded that up-regulation of source-leaf photosynthetic capacity is correlated with a decrease in assimilate availability to acropetal culm sink tissue. Furthermore, a significant relationship was revealed between source hexose concentration and photosynthetic activity.

Light‐driven Hydrogen Production by a Hybrid Complex of a [NiFe]‐Hydrogenase and the Cyanobacterial Photosystem I

Abstract: In order to generate renewable and clean fuels, increasing efforts are focused on the exploitation of photosynthetic microorganisms for the production of molecular hydrogen from water and light. In this study we engineered a 'hard-wired' protein complex consisting of a hydrogenase and photosystem I (hydrogenase-PSI complex) as a direct light-to-hydrogen conversion system. The key component was an artificial fusion protein composed of the membrane-bound [NiFe] hydrogenase from the beta-proteobacterium Ralstonia eutropha H16 and the peripheral PSI subunit PsaE of the cyanobacterium Thermosynechococcus elongatus. The resulting hydrogenase-PsaE fusion protein associated with PsaE-free PSI spontaneously, thereby forming a hydrogenase-PSI complex as confirmed by sucrose-gradient ultracentrifuge and immunoblot analysis. The hydrogenase-PSI complex displayed light-driven hydrogen production at a rate of 0.58 mumol H(2).mg chlorophyll(-1).h(-1). The complex maintained its accessibility to the native electron acceptor ferredoxin. This study provides the first example of a light-driven enzymatic reaction by an artificial complex between a redox enzyme and photosystem I and represents an important step on the way to design a photosynthetic organism that efficiently converts solar energy and water into hydrogen.

Contribution of fructose-1,6-bisphosphatase and sedoheptulose-1,7-bisphosphatase to the photosynthetic rate and carbon flow in the Calvin cycle in transgenic plants.

Abstract: To clarify the contributions of fructose-1,6-bisphosphatase (FBPase) and sedoheptulose-1,7-bisphosphatase (SBPase) separately to the carbon flux in the Calvin cycle, we generated transgenic tobacco plants expressing cyanobacterial FBPase-II in chloroplasts (TpF) or Chlamydomonas SBPase in chloroplasts (TpS). In TpF-11 plants with 2.3-fold higher FBPase activity and in TpS-11 and TpS-10 plants with 1.6- and 4.3-fold higher SBPase activity in chloroplasts compared with the wild-type plants, the amount of final dry matter was approximately 1.3-, 1.5- and 1.5-fold higher, respectively, than that of the wild-type plants. At 1,500 micromol m(-2) s(-1), the photosynthetic activities of TpF-11, TpS-11 and TpS-10 were 1.15-, 1.27- and 1.23-fold higher, respectively, than that of the wild-type plants. The in vivo activation state of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) and the level of ribulose-1,5-bisphosphate (RuBP) in TpF-11, TpS-10 and TpS-11 were significantly higher than those in the wild-type plants. However, the transgenic plant TpF-9 which had a 1.7-fold higher level of FBPase activity showed the same phenotype as the wild-type plant, except for the increase of starch content in the source leaves. TpS-11 and TpS-10 plants with 1.6- and 4.3-fold higher SBPase activity, respectively, showed an increase in the photosynthetic CO(2) fixation, growth rate, RuBP contents and Rubisco activation state, while TpS-2 plants with 1.3-fold higher SBPase showed the same phenotype as the wild-type plants. These data indicated that the enhancement of either a >1.7-fold increase of FBPase or a 1.3-fold increase of SBPase in the chloroplasts had a marked positive effect on photosynthesis, that SBPase is the most important factor for the RuBP regeneration in the Calvin cycle and that FBPase contributes to the partitioning of the fixed carbon for RuBP regeneration or starch synthesis.

Is C4 photosynthesis less phenotypically plastic than C3 photosynthesis

Abstract: C4 photosynthesis is a complex specialization that enhances carbon gain in hot, often arid habitats where photorespiration rates can be high. Certain features unique to C4 photosynthesis may reduce the potential for phenotypic plasticity and photosynthetic acclimation to environmental change relative to what is possible with C3 photosynthesis. During acclimation, the structural and physiological integrity of the mesophyll-bundle sheath (M-BS) complex has to be maintained if C4 photosynthesis is to function efficiently in the new environment. Disruption of the M-BS structure could interfere with metabolic co-ordination between the C3 and C4 cycles, decrease metabolite flow rate between the tissues, increase CO2 leakage from the bundle sheath, and slow enzyme activity. C4 plants have substantial acclimation potential, but in most cases lag behind the acclimation responses in C3 plants. For example, some C4 species are unable to maintain high quantum yields when grown in low-light conditions. Others fail to reduce carboxylase content in shade, leaving substantial over-capacity of Rubisco and PEP carboxylase in place. Shade-tolerant C4 grasses lack the capacity for maintaining a high state of photosynthetic induction following sunflecks, and thus may be poorly suited to exploit subsequent sunflecks compared with C3 species. In total, the evidence indicates that C4 photosynthesis is less phenotypically plastic than C3 photosynthesis, and this may contribute to the more restricted ecological and geographical distribution of C4 plants across the Earth.

Effects of Rubisco kinetics and Rubisco activation state on the temperature dependence of the photosynthetic rate in spinach leaves from contrasting growth temperatures

Abstract: Recently, several studies reported that the optimum temperature for the initial slope [IS(Ci)] of the light-saturated photosynthetic rate (A) versus intercellular CO2 concentration (Ci) curve changed, depending on the growth temperature. However, few studies compare IS(Ci) with ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) properties. Here, we assessed Rubisco activation state and in vitro Rubisco kinetics, the main determinants of IS(Ci), in spinach leaves grown at 30/25 [high temperature (HT)] and 15/10 °C [low temperature (LT)]. We measured Rubisco activation state and A at a CO2 concentration of 360 µL L−1 (A360) at various temperatures. In both HT and LT leaves, the Rubisco activation state decreased with increasing temperatures above the optimum temperatures for A360, while the activation state remained high at lower temperatures. To compare Rubisco characteristics, temperature dependences of the maximum rate of ribulose 1,5-bisphosphate (RuBP) carboxylation (Vcmax), specificity factor (Sc/o) and thermal stability were examined. We also examined Vcmax and thermal stability in the leaves that were transferred from HT to LT conditions and were subsequently kept under LT conditions for 2 weeks (HL). Rubisco purified from HT, LT and HL leaves are called HT, LT and HL Rubisco, respectively. Thermal stabilities of LT and HL Rubisco were similar and lower than that of HT Rubisco. Both Vcmax and Sc/o in LT Rubisco were higher than those of HT Rubisco at low temperatures, while these were lower at high temperatures. Vcmax in HL Rubisco were similar to those of LT Rubisco at low temperatures, and to those of HT Rubisco at high temperatures. The predicted photosynthetic rates, taking account of the Rubisco kinetics and the Rubisco activation state, agreed well with A360 in both HT and LT leaves. This study suggests that photosynthetic performance is largely determined by the Rubisco kinetics at low temperature and by Rubisco Kinetics and the Rubisco activation state at high temperature.