Role Of Photorespiration Research Articles

Study of vacuole glycerate transporter NPF8.4 reveals a new role of photorespiration in C/N balance.

The photorespiratory intermediate glycerate is known to be shuttled between the peroxisome and chloroplast. Here, localization of NPF8.4 in the tonoplast, together with the reduced vacuolar glycerate content displayed by an npf8.4 mutant and the glycerate efflux activity detected in an oocyte expression system, identifies NPF8.4 as a tonoplast glycerate influx transporter. Our study shows that expression of NPF8.4 and most photorespiration-associated genes, as well as the photorespiration rate, is upregulated in response to short-term nitrogen (N) depletion. We report growth retardation and early senescence phenotypes for npf8.4 mutants specifically upon N depletion, suggesting that the NPF8.4-mediated regulatory pathway for sequestering the photorespiratory carbon intermediate glycerate in vacuoles is important to alleviate the impact of an increased C/N ratio under N deficiency. Thus, our study of NPF8.4 reveals a novel role for photorespiration in N flux to cope with short-term N depletion.

Photorespiration plays an important role in H2 production by marine Chlorella pyrenoidosa

Marine Chlorella pyrenoidosa was previously reported to produce H2 under nitrogen-deprived condition. In this study, the role of photorespiration in H2 production by C. pyrenoidosa was investigated. It was found that the expression level of genes encoding three enzymes involved in photorespiration decreased, but the expression level of gene encoding glutamate synthase increased in nitrogen-deprived culture compared with control, indicating a complex adjustment of photorespiration activity during H2 production by C. pyrenoidosa. The use of a photorespiration inhibitor led not only to a decrease in H2 production by 15 % under nitrogen deprivation condition, but also to a 16 % decrease in O2 evolution in the complete medium. Moreover, it was demonstrated that the total photosynthetic oxygen evolution and Fv/Fm declined when photorespiration was blocked. This suggested that, the photorespiration pathway does not directly participate in the production of photosynthetic electrons and the transfer of hydrogen producing electrons. However, it played an important role in photoprotection of PSII during H2 production by C. pyrenoidosa, which enabled PSII to provide more photosynthetic electrons to hydrogenase.

The role of photorespiration in plant immunity.

To defend themselves in the face of biotic stresses, plants employ a sophisticated immune system that requires the coordination of other biological and metabolic pathways. Photorespiration, a byproduct pathway of oxygenic photosynthesis that spans multiple cellular compartments and links primary metabolisms, plays important roles in defense responses. Hydrogen peroxide, whose homeostasis is strongly impacted by photorespiration, is a crucial signaling molecule in plant immunity. Photorespiratory metabolites, interaction between photorespiration and defense hormone biosynthesis, and other mechanisms, are also implicated. An improved understanding of the relationship between plant immunity and photorespiration may provide a much-needed knowledge basis for crop engineering to maximize photosynthesis without negative tradeoffs in plant immunity, especially because the photorespiratory pathway has become a major target for genetic engineering with the goal to increase photosynthetic efficiency.

Responses of Physiology, Photosynthesis, and Related Genes to Saline Stress in Cornus hongkongensis subsp. tonkinensis (W. P. Fang) Q. Y. Xiang.

Cornus hongkongensis subsp. tonkinensis (W. P. Fang) Q. Y. Xiang is a native evergreen species with high ornamental value for abundant variations in leaf, bract, fruit, and tree gesture. To broaden its cultivation in coastal saline soil, salt damage and survival rate, physiological responses, photosynthetic performance, and related genes were evaluated for annual seedlings exposed to 0.3% salt (ST) concentrations for 60 days. Syndromes of salt damage were aggravated, and the survival rate decreased with prolonged stress duration; all stressed seedlings displayed salt damage, and 58.3% survived. Under short-term saline stress (5 d), marked increases in malondialdehyde (MDA), relative electrical conductivity (REC), and decreases in superoxide dismutase (SOD), photosynthetic rate (Pn), stomatal conductance (gs), and internal carbon dioxide concentration (Ci) were recorded. The stable leaf water use efficiency (WUE) and chlorophyll content were positive physiological responses to ensure photosynthetic performance. Meanwhile, the expression levels of genes related to photosystem II (psbA) and photorespiration (SGAT and GGAT) were upregulated, indicating the role of photorespiration in protecting photosynthesis from photoinhibition. After 30 days of stress (≥30 d), there was a significant increase in MDA, REC, soluble sugar (SS), soluble protein (SP), and Ci, whereas descending patterns in Pn, gs, WUE, the maximal photochemical efficiency of photosystem II (Fv/Fm), and potential activities of PSII (Fv/F0) occurred in salt-stressed seedlings, compared with CK. Meanwhile, the expression levels of related genes significantly dropped, such as psbA, LFNR, GGAT, GLYK, and PGK, indicating photoinhibition and worse photosynthetic performance. Our results suggest that the moderate salt tolerance of C. hongkongensis subsp. tonkinensis mostly lies in a better photosynthetic system influenced by active photorespiration. Hence, these results provide a framework for better understanding the photosynthetic responses of C. hongkongensis subsp. tonkinensis to salt stress.

Crystal Structure Of Photorespiratory Alanine:Glyoxylate Aminotransferase 1 (AGT1) From Arabidopsis thaliana.

Photorespiration is an energetically costly metabolic pathway for the recycling of phosphoglycolate produced by the oxygenase activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (RUBISCO) to phosphoglycerate. Arabidopsis alanine:glyoxylate aminotransferase 1 (AGT1) is a peroxisomal aminotransferase with a central role in photorespiration. This enzyme catalyzes various aminotransferase reactions, including serine:glyoxylate, alanine:glyoxylate, and asparagine:glyoxylate transaminations. To better understand structural features that govern the specificity of this enzyme, its crystal structures in the native form (2.2-Å resolution) and in the presence of l-serine (2.1-Å resolution) were solved. The structures confirm that this enzyme is dimeric, in agreement with studies of the active enzyme in solution. In the crystal, another dimer related by noncrystallographic symmetry makes close interactions to form a tetramer mediated in part by an extra carboxyl-terminal helix conserved in plant homologs of AGT1. Pyridoxal 5′-phosphate (PLP) is bound at the active site but is not held in place by covalent interactions. Residues Tyr35′ and Arg36′, entering the active site from the other subunits in the dimer, mediate interactions between AGT and l-serine when used as a substrate. In comparison, AGT1 from humans and AGT1 from Anabaena lack these two residues and instead position a tyrosine ring into the binding site, which accounts for their preference for l-alanine instead of l-serine. The structure also rationalizes the phenotype of the sat mutant, Pro251 to Leu, which likely affects the dimer interface near the catalytic site. This structural model of AGT1 provides valuable new information about this protein that may enable improvements to the efficiency of photorespiration.

Role of Photorespiration and Cyclic Electron Transport in C4 Photosynthesis Evolution in the C3–C4 Intermediate Species Sedobassia sedoides

Plants from two Sedobassia sedoides (Pall.) Aschers populations (Makan and Valitovo) (Chenopodiaceae) with C2 photosynthesis (precursor of C4 photosynthesis in phylogenesis) and photorespiratory CO2-concentrating mechanism were studied. Genetic polymorphism and isotope discrimination (δ13С) levels of the plants were determined under natural conditions, and their morpho-physiological parameters such as fresh and dry biomass of the above ground parts of plants, functioning of photosystem I (PSI) and photosystem II (PSII), intensity of net photosynthesis (A), transpiration (E), photorespiration and water use efficiency (WUE) of plants were calculated under control and salinine conditions (0 and 200 mM NaCl). Results of the population-genetic analysis showed that the Makan population is polymorphic (plastic) and the Valitovo population is monomorphic (narrowly specialized). There were no significant differences between the populations based on δ13С values or growth parameters, PSII, A, E and WUE under control conditions. Under saline conditions, dry biomass accumulation decreased in the Makan population by 15% and by more than 2- fold in the Valitovo population. Population differences were revealed in terms of photorespiration intensity and P700 oxidation kinetics under control and saline conditions. Under control conditions, Makan plants were characterized by a higher photorespiration intensity, which decreased by 2-fold under saline conditions to the photorespiration level of Valitovo plants. Cyclic electron transport activity was minimal in the control Makan plants, and it increased by almost 2-fold under saline conditions to the level of that in Valitovo plants under control and saline conditions. Under control conditions, photosynthesis in Makan plants can be specified as the proto-Kranz type (transitional type from C3 to C2) and that in Valitovo plants can be specified as the C2 type (C4 photosynthesis with photorespiratory CO2-concentrating mechanism), based on their photorespiration level and cyclic electron transport activity. Under saline conditions, Makan plants exhibited features of C2 photosynthesis. Intraspecific functional differences of photosynthesis were revealed in different populations of intermediate C3–C4 plant species S. sedoides which reflect the initial stages of formation of a photorespiratory CO2-concentrating mechanism during C4 photosynthesis evolution, accompanied by decrease in salt tolerance.

The role of photorespiration during H2 photoproduction in Chlorella protothecoides under nitrogen limitation.

Photorespiration in Chlorella protothecoides plays an important role in photoprotection of photosystem (PS) II in the late phase of H(2) photoproduction, allowing PSII to supply more electrons to hydrogenase.

Photoprotective function of energy dissipation by thermal processes and photorespiratory mechanisms in Jatropha curcas plants during different intensities of drought and after recovery

The role of photorespiration in plants under drought is not well understood. It can act both as an alternative electron sink and a source of reactive oxygen species (ROS). In this study, we evaluated in which stage of water deficit the energy dissipation by thermal processes and photorespiration act as photoprotective mechanisms, alleviating the photodamage in photosystem II (PSII) of Jatropha curcas plants. The experiment lasted 20 days; every 5 days (0, 5, 10, 15, and 20 days), measurements of gas exchange, chlorophyll fluorescence, and biochemical analysis were performed. On the 10th day of treatment, re-hydration in a separate group of drought-stressed plants was applied. The electron flow used for ribulose bisphosphate (RuBP) carboxylation (JC) progressively decreased with time of exposure to increasing drought. On the other hand, the electron flow used for RuBP oxygenation (JO) decreased during 5–10 days of exposure to water deficit and was significantly enhanced from 10 to 20 days. The JC/JO ratio was lower in plants exposed to drought compared to non-stressed plants. In addition, the photorespiration and the PR/PN and JT/PN ratios also were stimulated by drought. The non-photochemical quenching coefficient (NPQ) and relative energy excess level at PSII (EXC) in drought-stressed plants significantly increased between 5 and 10 days of treatment compared to control plants. Leaf H2O2 content was enhanced in plants exposed to water deficit reaching their concentrations more elevated in the period of 15–20 days when compared to control. In addition, the drought stress stimulated ascorbate peroxidase activity. On the other hand, the glycolate oxidase (GO) activity intensely increased, while catalase activity progressively decreased with increasing water deficit. In general, our results reveal that both excessive energy dissipation as heat and photorespiration play important roles in protecting the photosynthetic apparatus from photoinhibition.

The role of photorespiration during the evolution of C4 photosynthesis in the genus Flaveria.

C4 photosynthesis represents a most remarkable case of convergent evolution of a complex trait, which includes the reprogramming of the expression patterns of thousands of genes. Anatomical, physiological, and phylogenetic and analyses as well as computational modeling indicate that the establishment of a photorespiratory carbon pump (termed C2 photosynthesis) is a prerequisite for the evolution of C4. However, a mechanistic model explaining the tight connection between the evolution of C4 and C2 photosynthesis is currently lacking. Here we address this question through comparative transcriptomic and biochemical analyses of closely related C3, C3-C4, and C4 species, combined with Flux Balance Analysis constrained through a mechanistic model of carbon fixation. We show that C2 photosynthesis creates a misbalance in nitrogen metabolism between bundle sheath and mesophyll cells. Rebalancing nitrogen metabolism requires anaplerotic reactions that resemble at least parts of a basic C4 cycle. Our findings thus show how C2 photosynthesis represents a pre-adaptation for the C4 system, where the evolution of the C2 system establishes important C4 components as a side effect.

Flux Balance Analysis of Cyanobacterial Metabolism: The Metabolic Network of Synechocystis sp. PCC 6803

Cyanobacteria are versatile unicellular phototrophic microorganisms that are highly abundant in many environments. Owing to their capability to utilize solar energy and atmospheric carbon dioxide for growth, cyanobacteria are increasingly recognized as a prolific resource for the synthesis of valuable chemicals and various biofuels. To fully harness the metabolic capabilities of cyanobacteria necessitates an in-depth understanding of the metabolic interconversions taking place during phototrophic growth, as provided by genome-scale reconstructions of microbial organisms. Here we present an extended reconstruction and analysis of the metabolic network of the unicellular cyanobacterium Synechocystis sp. PCC 6803. Building upon several recent reconstructions of cyanobacterial metabolism, unclear reaction steps are experimentally validated and the functional consequences of unknown or dissenting pathway topologies are discussed. The updated model integrates novel results with respect to the cyanobacterial TCA cycle, an alleged glyoxylate shunt, and the role of photorespiration in cellular growth. Going beyond conventional flux-balance analysis, we extend the computational analysis to diurnal light/dark cycles of cyanobacterial metabolism.

Knockdown of GDCH gene reveals reactive oxygen species‐induced leaf senescence in rice

Glycine decarboxylase complex (GDC) is a multi-protein complex, comprising P-, H-, T- and L-protein subunits, which plays a major role in photorespiration in plants. While structural analysis has demonstrated that the H subunit of GDC (GDCH) plays a pivotal role in GDC, research on the role of GDCH in biological processes in plants is seldom reported. Here, the function of GDCH, stresses resulting from GDCH-knockdown and the interactions of these stresses with other cellular processes were studied in rice plants. Under high CO(2), the OsGDCH RNA interference (OsGDCH-RNAi) plants grew normally, but under ambient CO(2), severely suppressed OsGDCH-RNAi plants (SSPs) were non-viable, which displayed a photorespiration-deficient phenotype. Under ambient CO(2), chlorophyll loss, protein degradation, lipid peroxidation and photosynthesis decline occurred in SSPs. Electron microscopy studies showed that chloroplast breakdown and autophagy took place in these plants. Reactive oxygen species (ROS), including O2(-) and H(2)O(2), accumulated and the antioxidant enzyme activities decreased in the leaves of SSPs under ambient CO(2). The expression of transcription factors and senescence-associated genes (SAGs), which was up-regulated in SSPs after transfer to ambient CO(2), was enhanced in wild-type plants treated with H(2)O(2). Evidences demonstrate ROS induce senescence in SSPs, and transcription factors OsWRKY72 may mediate the ROS-induced senescence.

Evidence for the presence of photorespiration in desiccation-sensitive leaves of the C4 'resurrection' plant Sporobolus stapfianus during dehydration stress

The possible role of photorespiration as a general stress protection mechanism, and in C(4) plant metabolism, is controversial. In particular, the potential involvement of photorespiration in the acquisition of desiccation tolerance in 'resurrection' plants is unknown. An investigation was carried out into whether photorespiration is present in leaves of the C(4) resurrection plant Sporobolus stapfianus Gandoger (Poaceae) and whether it functions as a mechanism of stress resistance in the desiccation-tolerant younger leaves (YL) of this plant. It is shown that the enzymes involved in the photorespiratory pathway maintain their activity until 88% relative water content (RWC) in both YL and desiccation-sensitive older leaves (OL). In subsequent stages of dehydration stress, the enzymatic activity declined similarly in both YL and OL. The content of the phorespiratory metabolite, serine, and ethanolamine, a direct product of serine decarboxylation, is higher in the early stages of dehydration (88% RWC) in OL, suggesting a transiently enhanced photorespiratory activity in these leaves. This was confirmed by simultaneous gas exchange and fluorescence measurements, showing suppression of the electron transport rate in OL exposed to non-photorespiratory conditions (2% O(2)) at 85% RWC. It is concluded that a higher photorespiratory electron transport occurs in desiccation-sensitive OL, and it is therefore proposed that the capacity to scavenge excess electrons through photorespiration does not contribute to protect leaves of the desiccation-tolerant YL of S. stapfianus during the stress.

Nitrate photo‐assimilation in tomato leaves under short‐term exposure to elevated carbon dioxide and low oxygen

ABSTRACTThe role of photorespiration in the foliar assimilation of nitrate (NO3–) and carbon dioxide (CO2) was investigated by measuring net CO2 assimilation, net oxygen (O2) evolution, and chlorophyll fluorescence in tomato leaves (Lycopersicon esculentum). The plants were grown under ambient CO2 with ammonium nitrate (NH4NO3) as the nitrogen source, and then exposed to a CO2 concentration of either 360 or 700 µmol mol−1, an O2 concentration of 21 or 2%, and either NO3– or NH4+ as the sole nitrogen source. The elevated CO2 concentration stimulated net CO2 assimilation under 21% O2 for both nitrogen treatments, but not under 2% O2. Under ambient CO2 and O2 conditions (i.e. 360 µmol mol−1 CO2, 21% O2), plants that received NO3– had 11–13% higher rates of net O2 evolution and electron transport rate (estimated from chlorophyll fluorescence) than plants that received NH4+. Differences in net O2 evolution and electron transport rate due to the nitrogen source were not observed at the elevated CO2 concentration for the 21% O2 treatment or at either CO2 level for the 2% O2 treatment. The assimilatory quotient (AQ) from gas exchange, the ratio of net CO2 assimilation to net O2 evolution, indicated more NO3– assimilation under ambient CO2 and O2 conditions than under the other treatments. When the AQ was derived from gross O2 evolution rates estimated from chlorophyll fluorescence, no differences could be detected between the nitrogen treatments. The results suggest that short‐term exposure to elevated atmospheric CO2 decreases NO3– assimilation in tomato, and that photorespiration may help to support NO3– assimilation.

Effect of Photorespiratory C2Acids on CO2Assimilation, PS II Photochemistry and the Xanthophyll Cycle in Maize

The photorespiration cycle plays an important role in avoiding carbon drainage from the Calvin cycle and in protecting plants from photoinhibition. The role of photorespiration is frequently underestimated in C(4) plants, since these are characterized by low photorespiration rates. The aim of this work was to study the relationship between CO(2) assimilation, PS II photochemistry and the xanthophyll cycle when the photorespiratory cycle is disrupted in Zea mays L. To this end, the photorespiration inhibitor phosphinothricin (PPT) was applied individually or together with the photorespiratory C(2) acids, glycolate and glyoxylate to maize leaves. Application of PPT alone led to the inhibition of CO(2) assimilation. Moreover, feeding with glycolate or glyoxylate enhanced the effect of PPT on CO(2) assimilation. Our results confirm that the avoidance of the accumulation of the photorespiratory metabolites glycolate, glyoxylate or phosphoglycolate, is of vital importance for coordinated functioning between the glycolate pathway and CO(2) assimilation. Relatively early changes in PS II photochemistry also took place when the photorespiratory cycle was interrupted. Thus, fluorescence photochemical quenching (qP) was slightly reduced (10%) due to the application of PPT together with glycolate or glyoxylate. A decrease in the efficiency of excitation-energy capture by open PS II reaction centres (F'v/F'm) and an increase in thermal energy dissipation (non-photochemical quenching, NPQ) were also measured. These observations are consistent with a limitation of activity of the Calvin cycle and a subsequent lower demand for reduction equivalents. The increase in NPQ is discussed on the basis of changes in the xanthophyll cycle in maize, which seem to provide a limited protective role to avoid photoinhibition when the glycolate pathway is blocked. We conclude that C(2) photorespiratory acids can act as physiological regulators between the photorespiratory pathway and the Calvin cycle in maize.

The role of photorespiration in redox and energy balance of photosynthetic plant cells: A study with a barley mutant deficient in glycine decarboxylase.

Protoplasts and mitochondria were isolated from leaves of homozygous barley (Hordeum vulgare L.) mutant deficient in glycine decarboxylase complex (GDC, EC 2.1.2.10) and wild-type plants. The photosynthetic rates of isolated protoplasts from the mutant and wild-type plants under saturating CO2 were similar, but the respiratory rate of the mutant was two-fold higher. Respiration in the mutant plants was much more strongly inhibited by antimycin A than in wild-type plants and a low level of the alternative oxidase protein was found in mitochondria. The activities of NADP- and NAD-dependent malate dehydrogenases were also increased in mutant plants, suggesting an activation of the malate-oxaloacetate exchange for redox transfer between organelles. Mutant plants had elevated activities of NADH- and NADPH-dependent glyoxylate/hydroxypyruvate reductases, which may be involved in oxidizing excess NAD(P)H and the scavenging of glyoxylate. We estimated distribution of pools of adenylates, NAD(H) and NADP(H) between chloroplasts, cytosol and mitochondria. Under photorespiratory conditions, ATP/ADP and NADPH/NADP ratios in the mutant were higher in chloroplasts as compared to wild-type plants. The cytosolic NADH/NAD ratio was increased, whereas the ratio in mitochondria decreased. It is concluded that photorespiration serves as an effective redox transfer mechanism from the chloroplast. Plants with a lowered GDC content are deficient in this mechanism, which leads to over-reduction and over-energization of the chloroplasts.

Role of photorespiration in dissipation of absorbed photon excess in Spinacia oleracea L. plants exposed to low temperature and high irradiance

Plants subjected to drought stress have a decreased rate of photosynthesis and, therefore, are unable to use absorbed light energy. As a consequence, the plant is at risk of oxidative damage through the formation of reactive oxygen species (Asada, 1999). Regulation of electron transport to minimise the production of these radicals is a mechanism whereby protection may be achieved (Genty and Harbinson, 1996; Ott et al., 1999). Non-photochemical quenching (NPQ) dissipates excitation energy from the reaction centres of PSII and so protects against photoinhibition under conditions of excess energy (Heber and Walker, 1992). Cyclic electron transport around PSI has also been suggested as a protective mechanism (Clarke and Johnson, 2001) and is capable of generating the DpH required to support NPQ. The response of the electron transport chain to moderate and severe drought has been studied in Barley (Hordeum Vulgare c.v Chariot). Measurements of gas exchange, chlorophyll fluorescence and absorbance changes at 830 nm have been made in plants subjected to varying sorbitol concentrations for 24 hours. It was found that PSII electron transport capacity is unaffected by moderate drought but is inhibited by severe drought. By contrast, PSI electron transport rate is increased under both conditions. This increase in PSI electron transport rate indicates increased cyclic electron transport, which may be important in generating the DpH required to support NPQ. NPQ is also increased under drought.

Photorespiration: metabolic pathways and their role in stress protection.

Photorespiration results from the oxygenase reaction catalysed by ribulose-1,5-bisphosphate carboxylase/oxygenase. In this reaction glycollate-2-phosphate is produced and subsequently metabolized in the photorespiratory pathway to form the Calvin cycle intermediate glycerate-3-phosphate. During this metabolic process, CO2 and NH3 are produced and ATP and reducing equivalents are consumed, thus making photorespiration a wasteful process. However, precisely because of this inefficiency, photorespiration could serve as an energy sink preventing the overreduction of the photosynthetic electron transport chain and photoinhibition, especially under stress conditions that lead to reduced rates of photosynthetic CO2 assimilation. Furthermore, photorespiration provides metabolites for other metabolic processes, e.g. glycine for the synthesis of glutathione, which is also involved in stress protection. In this review we describe the use of photorespiratory mutants to study the control and regulation of photorespiratory pathways. In addition, we discuss the possible role of photorespiration under stress conditions, such as drought, high salt concentrations and high light intensities encountered by alpine plants.

The role of photorespiration during drought stress: an analysis utilizing barley mutants with reduced activities of photorespiratory enzymes

C i, intercellular CO2 concentration Fv/Fm, quantum efficiency of excitation capture by open photosystem II centres FBPase, fructose‐1,6‐bisphosphatase GAPDH, glyceraldehyde‐3‐phosphate dehydrogenase GDC, glycine decarboxylase GS‐2, chloroplastic glutamine synthetase HPR, hydroxypyruvate reductase PFD, photon flux density ΦCO2, quantum efficiency of CO2 assimilation ΦPSII, quantum efficiency of photosystem II electron transport ψ, water potential qN, non‐photochemical chlorophyll a fluorescence quenching qP, photochemical chlorophyll a fluorescence quenching RuBP, ribulose‐1,5‐bisphosphate Rubisco, ribulose‐1,5‐bisphosphate carboxylase‐oxygenase SBPase, sedoheptulose‐1,7‐bisphosphatase SGAT, serine : glyoxylate aminotransferase The significance of photorespiration in drought‐stressed plants was studied by withholding water from wild‐type barley (Hordeum vulgare L.) and from heterozygous mutants with reduced activities of chloroplastic glutamine synthetase (GS‐2), glycine decarboxylase (GDC) or serine : glyoxylate aminotransferase (SGAT). Well‐watered plants of all four genotypes had identical rates of photosynthesis. Under moderate drought stress (leaf water potentials between –1 and –2 MPa), photosynthesis was lower in the mutants than in the wild type, indicating that photorespiration was increased under these conditions. Analysis of chlorophyll a fluorescence revealed that, in the GDC and SGAT mutants, the lower rates of photosynthesis coincided with a decreased quantum efficiency of photosystem II and increased non‐photochemical dissipation of excitation energy. Correspondingly, the de‐epoxidation state of xanthophyll‐cycle carotenoids was increased several‐fold in the drought‐stressed GDC and SGAT mutants compared with the wild type. Accumulation of glycine in the GDC mutant was further evidence for increased photorespiration in drought‐stressed barley. The effect of drought on the photorespiratory enzymes was determined by immunological detection of protein abundance. While the contents of GS‐2 and P‐ and H‐protein of the GDC complex remained unchanged as drought stress developed, the content of NADH‐dependent hydroxypyruvate reductase increased. Enzymes of the Benson–Calvin cycle, on the other hand, were either not affected (ribulose‐1,5‐bisphosphate carboxylase‐oxygenase and plastidic fructose‐1,6‐bisphosphatase) or declined (sedoheptulose‐ 1,7‐bisphosphatase and NADP‐dependent glyceraldehyde‐3‐phosphate dehydrogenase). These data demonstrate that photorespiration was enhanced during drought stress in barley and that the control exerted by photorespiratory enzymes on the rate of photosynthetic electron transport and CO2 fixation was increased.

Factors Influencing Photosynthetic Productivity ofChara vulgarisL. in a Moderately Productive Hardwater Lake

ABSTRACT In situ seasonal patterns and ecophysiology of photosynthesis were investigated in the macroalga Chara vulgaris L. growing in Shoe Lake, Michigan, in an effort to understand controlling factors and to determineé the role of photorespiration. C. vulgaris was shown to be a C3 plant with inducible photorespiration. Its photosynthetic productivity in Shoe Lake in early summer was maximal for submersed macrophytes. Subsequent seasonal declines and dialy fluctations in productivity were correlated with presence of photorespiration, but the photorespiration represented a minor percentage of photosynthesis. In the shallow light-saturated water both nitrogen and carbon availability were limiting factors at times during mid- and late summer.

The influence of reduced photorespiration on long‐term growth and development of wheat

The long‐term role of photorespiration was investigated by comparing growth, development, gas exchange characteristics and mineral nutrition of a wheat crop (Triticum aestivum L. cv. Courtot) cultivated in a culture chamber during a life cycle, either in 4% O2 or in normal O2 Low O2 pressure reduced photorespiration, but CO2 was controlled so that net photosynthesis remained the same as in the control crop. The growth and development of the low O2 crop was slowed down. Ear appearance was 16 days late, but the rate of tillering was the same as in the control and was maintained longer so that the final number of tillers was doubled. Pigment, ribulose bisphosphate carboxylase (EC 4.1.1.39) and soluble sugar contents were similar. The response of photosynthesis to CO2 and O2 was not appreciably changed by the low O2 treatment. There was almost no seed formation, and the senescence of the leaves was delayed. It appears that in non‐stress conditions most of the photorespiration can be suppressed without damage to the photosynthetic apparatus. Retardation of development and inhibition of reproduction are likely due to other effects of O2.