Photodynamically induced chemoresponses of the colorless flagellate,Astasia longa,in the presence of riboflavin

In the presence of the photosensitizer riboflavin, Astasia accumulates in illuminated fields at high fluence rates. The quenchers of riboflavin excited states, NaN₃ and KI, abolish the photodynamic effect of riboflavin. Crocetin, a ¹O₂ quencher, does not influence the photodynamic action of riboflavin while 1,4-benzoquinone very strongly depresses its effect. This indicates a type I pathway forming H₂O₂ as a photoproduct. The photodynamic effect is abolished by the addition of 10^-5 M catalase which breaks down H₂O₂. Astasia shows chemoaccumulations around the opening of a capillary filled either with riboflavin (under high intensity irradiation) or H₂O₂ which proves the hypothesis that the photobehavioral response in the presence of riboflavin is based on a chemoresponse toward H₂O₂, produced when the dye is irradiated. The accumulation around the capillary opening is not due to a direct chemotactic movement of cells but rather to a chemophobic response which prevents the cells from swimming away from the chemoattractant.     

Photobehavior of the Ciliated Protozoon Loxodes: Taxic,Transient, and Kinetic Responses in the Presence and Absence of Oxygen1

ABSTRACT. Behavioral responses to light at different oxygen tensions were studied in the ciliate Loxodes striatus. In the absence of O₂ it does not react to light. In the presence of O₂ it reacts to light as if the pO₂ had been further increased, with the induction of positive geotaxis, a transient phobic response, and finally with a permanent kinetic response (increased swimming velocity and a decreased rate of tumbling). Cells treated with cyanide behave as cells in an anoxic environment and do not react to light. It is concluded that the light response is due to the photochemical production of oxygen radicals and that the sensory receptors for O₂ and for light are identical. The three types of behavioral response (geotaxis, transient, and kinetic responses) are discussed in terms of their adaptive significance for the orientation of Loxodes in the natural environment.     

Kinetic study on the photostability of riboflavin in the presence of barbituric acid

Abstract

The photochemical fate of riboflavin (vitamin B₂) in the presence of barbituric acid was examined employing polarographic detection of dissolved oxygen and steady-state and time-resolved spectroscopy. Under visible light, riboflavin reacts with barbituric acid – the latter being transparent to this type of photo-irradiation – via radicals and reactive oxygen species, such as singlet molecular oxygen [O₂(¹Δ_g)] and superoxide radical anion, which are generated from the excited triplet state of the vitamin. As a result, both the vitamin and barbituric acid are photodegraded. Kinetic and mechanistic studies on the photoreactions of riboflavin in the presence of barbituric acid indicate the excellent quenching ability of the latter towards O₂(¹Δ_g).     

Inhibition of stomatal opening in sunflower leaves by carbon monoxide,and reversal of inhibition by light

When leaves of Helianthus annuus, whose stomates had been opened in the dark in the absence of CO₂, were exposed to 25% carbon monoxide (CO), stomatal conductivity for water vapor decreased from about 0.4 to 0.2 cm·s^-1. The CO effect on stomatal aperture required a CO/O₂ ratio of about 25. As this ratio was decreased the stomata opened, indicating that inhibitio of cytochrome-c oxidase by CO is competitive in respect to O₂. Photosynthetically active red light was unable to reverse CO-induced stomatal closure even at high irradiances, when CO₂ was absent. When it was present, stomatal opening was occasionally, but not consistently observed. Carbon monoxide did not inhibit photosynthetic carbon reduction in leaves of Helianthus.In contrast to red light, very weak blue light (405 nm) increased the stomatal aperture in the presence of CO. It also increased leaf ATP/ADP ratios which had been decreased in the presence of CO. The blue-light effect was not related to photosynthesis. Neither could it be explained by photodissociation of the cytochrome a ₃-CO complex which has an absorption maximum at 430 nm. The data indicate that ATP derived from mitochondrial oxidative phosphorylation provides energy for stomatal opening in sunflower leaves in the dark as well as in the light. Indirect transfer of ATP from chloroplasts to the cytosol via the triose phosphate/phosphoglycerate exchange which is mediated by the phosphate translocator of the chloroplast envelope can support stomatal opening only if metabolite concentrations are high enough for efficient shuttle transfer of ATP. Blue light causes stomatal opening in the presence of CO by stimulating ATP synthesis.     

Influence of certain environmental factors on photosynthesis and photorespiration in Simmondsia chinensis

The response of net photosynthesis and apparent light respiration to changes in [O₂], light intensity, and drought stress was determined by analysis of net photosynthetic CO₂ response curves. Low [O₂] treatment resulted in a large reduction in the rate of photorespiratory CO₂ evolution. Lightintensity levels influenced the maximum net photosynthetic rate at saturating [CO₂]. These results indicate that [CO₂], [O₂] and light intensity affect the levels of substrates involved in the enzymatic reactions of photosynthesis and photorespiration. Intracellular resistance to CO₂ uptake decreased in low [O₂] and increased at low leaf water potentials. This response reflects changes in the efficiency with which photosynthetic and photorespiratory substrates are formed and utilized. Water stress had no effect on the CO₂ compensation point or the [CO₂] at which net photosynthesis began to saturate at high light intensity. The relationship between these data and recently published in-vitro kinetic measurements with ribulose-diphosphate carboxylase is discussed.Abbreviations C _w intracellular CO₂ concentration - F _gross gross photosynthesis - F _net net photosynthesis - I light intensity - R _L light respiration rate - r _c carboxylation resistance - r ₈ leaf gas-phase resistance - r _i intracellular resistance; to CO₂ uptake - r _t resistance to CO₂ flux between the intercellular spaces and the carboxylation sites - T _L leaf temperature - _t leaf water potential - CO₂ compensation point     

Reversible inactivation of nitrate reductase in Chlorella vulgaris in vivo

Summary The NADH-nitrate oxidoreductase of Chlorella vulgaris has an inactive form which has previously been shown to be a cyanide complex of the reduced enzyme. This inactive enzyme can be reactivated by treatment with ferricyanide in vitro. In the present study, the activation state of the enzyme was determined after different prior in vivo programs involving environmental variations. Oxygen, nitrate, light and CO₂ all affect the in vivo inactivation of the enzyme in an interdependent manner. In general, the inactivation is stimulated by O₂ and inhibited by nitrate and CO₂. Light may stimulate or inhibit, depending on conditions. Thus, the effects of CO₂ and nitrate (inhibition of reversible inactivation) are clearly manifested only in the light. In contrast, light stimulates the inactivation in the presence of oxygen and the absence of CO₂ and nitrate. Since the inactivation of the enzyme requires HCN and NADH, and it is improbable that O₂ stimulates NADH formation, it is reasonable to conclude that HCN is formed as the result of an oxidation reaction (which is stimulated by light). The formation of HCN is probably stimulated by Mn²⁺, since the formation of reversibly-inactivated enzyme is impaired in Mn²⁺-deficient cells. The prevention of enzyme inactivation by nitrate in vivo is in keeping with previous in vitro results showing that nitrate prevents inactivation by maintaining the enzyme in the oxidized form. A stimulation of nitrate uptake by CO₂ and light could account for the effect of CO₂ (prevention of inactivation) which is seen mainly in the presence of nitrate and light. Ammonia added in the presence of nitrate has the same effect on the enzyme as removing nitrate (promotion of reversible inactivation). Ammonia added in the absence of nitrate has little extra effect. It is therefore likely that ammonia acts by preventing nitrate uptake. The uncoupler, carbonylcyanide-m-chloro-phenylhydrazone, causes enzyme inactivation because it acts as a good HCN precursor, particularly in the light. Nitrite, arsenate and dinitrophenol cause an enzyme inactivation which can not be reversed by ferricyanide in crude extracts. This suggests that there are at least two different ways in which the enzyme can be inactivated rather rapidly in vivo.     

Dithiothreitol,an inhibitor of violaxanthin de-epoxidation,increases the susceptibility of leaves ofNerium oleander L. to photoinhibition of photosynthesis

Leaves ofNerium oleander L. plants, which had been previously kept in a shaded glasshouse for at least two months, were fed 1 mM dithiothreitol (DTT) through their petioles, either for 12h in darkness (overnight) or for 2h in low light (28 μmol photons·m⁻²·s⁻¹), in each case followed by a 3-h exposure to high light (1260 μmol photons·m⁻²·s⁻¹). During exposure to high light, violaxanthin became converted to zeaxanthin in control leaves, to which water had been fed, whereas zeaxanthin did not accumulate in leaves treated with DTT. Total carbon gain was not reduced by DTT during the photoinhibitory treatment. Exposure to high light led to a decrease in the photochemical efficiency of photosystem II, measured as the ratio of variable over maximum fluorescence emission,F _v/F _M, at both 298 K and 77K. The decrease was much more pronounced in the presence of DTT, mainly owing to a sustained increase in the instantaneous fluorescence,F _o. By contrast, in the control leaves,F _o determined immediately after the high-light treatment showed a transient decrease below theF _o value obtained before the onset of the photoinhibitory treatment (i.e. after 12 h dark adaptation), followed by a rapid return (within seconds) to this original level ofF _o during the following recovery period in darkness. Incubation of leaves with DTT led to large, sustained decreases in the photon-use efficiency of photosynthetic O₂ evolution by bright light, whilst the capacity of photosynthetic O₂ evolution at light and CO₂ saturation was less affected. In the control leaves, only small reductions in the photon yield and in the photosynthetic capacity were observed. These findings are consistent with previous suggestions that zeaxanthin, formed in the xanthophyll cycle by de-epoxidation of violaxanthin, is involved in protecting the photosynthetic apparatus against the adverse effects of excessive light.     

Photosynthetic electron flow affects H2O2 signaling by inactivation of catalase in Chlamydomonas reinhardtii

A specific signaling role for H₂O₂ in Chlamydomonas reinhardtii was demonstrated by the definition of a promoter that specifically responded to this ROS. Expression of a nuclear-encoded reporter gene driven by this promoter was shown to depend not only on the level of exogenously added H₂O₂ but also on light. In the dark, the induction of the reporter gene by H₂O₂ was much lower than in the light. This lower induction was correlated with an accelerated disappearance of H₂O₂ from the culture medium in the dark. Due to a light-induced reduction in catalase activity, H₂O₂ levels in the light remained higher. Photosynthetic electron transport mediated the light-controlled down-regulation of the catalase activity since it was prevented by 3-(3′4′-dichlorophenyl)-1,1-dimethylurea (DCMU), an inhibitor of photosystem II. In the presence of light and DCMU, expression of the reporter gene was low while the addition of aminotriazole, a catalase inhibitor, led to a higher induction of the reporter gene by H₂O₂ in the dark. The role of photosynthetic electron transport and thioredoxin in this regulation was investigated by using mutants deficient in photosynthetic electron flow and by studying the correlation between NADP-malate dehydrogenase and catalase activities. It is proposed that, contrary to expectations, a controlled down-regulation of catalase activity occurs upon a shift of cells from dark to light. This down-regulation apparently is necessary to maintain a certain level of H₂O₂ required to activate H₂O₂-dependent signaling pathways.     

Phytochemical, antimicrobial, antioxidant and antigenotoxic potentials of Cyperus rotundus extracts

The aqueous, ethyl acetate, methanolic and Total Oligomer Flavonoids (TOF) enriched extracts, obtained from the aerial parts of Cyperus rotundus, were investigated for their contents in phenolic compounds. Antioxidative activity using the NBT/riboflavin assay system, antimicrobial activity against Gram positive and Gram negative bacterial reference strains as well as antigenotoxic activity tested with the SOS chromotest assay were also studied. Significant antibacterial activity against reference strains; Staphylococcus aureus, Enterococcus faecalis, Salmonella enteritidis and Salmonella typhimurium, was detected in the presence of ethyl acetate and TOF enriched extracts. In addition to their antimicrobial activity, the same extracts showed a significant ability to inhibit nitroblue tetrazolium reduction by the superoxide radical in a non enzymatic O₂^.− generating system, and were also able to reduce significantly the genotoxicity induced by nifuroxazide and Aflatoxin B1. The antioxidant, antimicrobial and antigenotoxic activities exhibited by C. rotundus depend on the chemical composition of the tested extracts.     

Modulation of the Purine Pathway for Riboflavin Production in Flavinogenic Recombinant Strain of the Yeast Candida famata

Riboflavin (vitamin B₂) is an indispensable nutrient for humans and animals, since it is the precursor of the essential coenzymes flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD), involved in variety of metabolic reactions. Riboflavin is produced on commercial scale and is used for feed and food fortification purposes, and in medicine. Until recently, the mutant strains of the flavinogenic yeast Candida famata were used in industry for riboflavin production. Guanosine triphosphate is the immediate precursor of riboflavin synthesis. Therefore, the activation of metabolic flux toward purine nucleotide biosynthesis is a promising approach to improve riboflavin production. The phosphoribosyl pyrophosphate synthetase and phosphoribosyl pyrophosphate amidotransferase are the rate limiting enzymes in purine biosynthesis. Corresponding genes PRS3 and ADE4 from yeast Debaryomyces hansenii are modified to avoid feedback inhibition and cooverexpressed on the background of a previously constructed riboflavin overproducing strain of C. famata. Constructed strain accumulates twofold more riboflavin when compared to the parental strain.     

Characterisation of a H2O2-oxidisable cytochrome b-559 in intact chloroplasts with a new type of LED Array Spectrophotometer

Cytochrome (cyt) b-559 absorbance changes in intact chloroplasts were deconvoluted using a previously described LED-Array-Spectrophotometer (Klughammer et al. (1990), Photosynth Res 25: 317–327). When intact chloroplasts were isolated in the presence of ascorbate, approx. 15% of the total cyt b-559 could be transiently oxidised by 200 M H₂O₂ in the dark. This fraction displays low-potential properties, as it can be also oxidised by menadione in the presence of 5 mM ascorbate. Heat pretreatment increased the size of this fraction by a factor of 3–4. Low concentrations of cyanide (in the M range) prolonged the oxidation time while high concentrations suppressed the oxidation (I₅₀=1.5 mM KCN). The former KCN-effect relates to inhibition of ascorbate dependent H₂O₂-reduction which is catalysed by ascorbate peroxidase, whereas the latter effect reflects competition between H₂O₂ and CN^– for the same binding site at the cytochrome heme. In the light, much lower concentrations of H₂O₂ were required to obtain oxidation, the amplitude depending on light intensity and on the concentration of the added H₂O₂, but never exceeding approx. 15% of the total cyt b-559. In the light, but not in the dark, H₂O₂ also induced the transient oxidation of a cyt f fraction similar in size to the H₂O₂-oxidisable cyt b-559 fraction. In this case, H₂O₂ serves as an acceptor of Photosystem I in conjunction with the ascorbate peroxidase detoxification system. Light can also induce oxidation of a 15% cyt b-559 fraction without H₂O₂-addition, if nitrite is present as electron acceptor and the chloroplasts are depleted of ascorbate. It is concluded that light-induced cyt b-559 oxidation in vivo is likely to be restricted to the H₂O₂-oxidisable cyt b-559 LP fraction and is normally counteracted by ascorbate.Abbreviations APX ascorbate peroxidase - chl chlorophyll - cyt cytochrome - HP high potential - LP low potential - MDA monodehydroascorbate - PQ plastoquinone - PS I and PS II Photosystems I and II     

Photoinhibition and its wavelength dependence in the cyanobacterium Anabaena variabilis

In the cyanobacterium Anabaena variabilis the dependence of photoinhibition on fluence rate, duration and wavelength of irradiation were studied by measurements of oxygen production and fluorescence emission spectra. The analysis of the photosynthetic activity revealed that photoinhibition affects exclusively photosystem II (PS II), whereas photosystem I (PS I) remained largely unimpaired. Furthermore, PS II fluorescence emission decreased much faster in bleached than in unbleached controls.Studying the wavelength dependence of photoinhibition it was found that only radiation between 520 and 680 nm causes photoinhibition. This is about the same range of wavelengths which causes photobleaching. Fluorescence emission spectra of samples exposed to high fluence rates of 582 and 662 nm, respectively, essentially agree with those samples exposed to high fluence rates of white light, whereas the fluorescence emission spectra of samples exposed to blue light resemble those exposed to dim white light.NaN₃, a substance which prevents photobleaching, inhibits the photosynthetic O₂ production of Anabaena and, hence, enhances the photoinhibitory effect.     

<Emphasis Type="Italic">O</Emphasis>-acetylation of sialic acids is required for the survival of lymphoblasts in childhood acute lymphoblastic leukemia (ALL)

Exploiting the selective affinity of Achatinin-H towards 9-O-acetylneuraminic acid(α2-6)GalNAc, we have demonstrated the presence of 9-O-acetylated sialoglycoproteins (Neu5,9Ac₂-GPs) on hematopoietic cells of children suffering from acute lymphoblastic leukemia (ALL), indicative of defective sialylation associated with this disease. The carbohydrate epitope of Neu5,9Ac₂-GPs_ALL was confirmed by using several synthetic sialic acid analogues. They are functionally active signaling molecules as demonstrated by their role in mediating lymphoproliferative responses and consequential increased production of IFN-γ due to specific stimulation of Neu5,9Ac₂-GPs on PBMC_ALL with Achatinin-H. Cells devoid of 9-O-acetylations (9-O-AcSA⁻) revealed decreased nitric oxide production as compared to 9-O-AcSA⁺ cells on exposure to IFN-γ. Under this condition, a decrease in viability of 9-O-AcSA⁻ cells as compared to 9-O-AcSA⁺ cells was also observed which was reflected from increased caspase 3 activity and apoptosis suggesting the protective role of this glycotope. These Neu5,9Ac₂-GPs are also capable of inducing disease-specific anti-Neu5,9Ac₂-GPs antibodies in ALL children. Additionally, we have observed that disease-specific anti-Neu5,9Ac₂-GPs have altered glycosylation profile, and they are incapable of exerting a few Fc-glycosylation-sensitive effector functions. These observations hint toward a disbalanced homeostasis, thereby enabling the cancer cells to escape host defense. Taken together, it may be hypothesized that Neu5,9Ac₂-GPs and their antibodies play a prominent role in promoting the survival of lymphoblasts in ALL.     

Lipid and lipoprotein oxidation: basic mechanisms and unresolved questions in vivo

Abstract

The kinetics and mechanistic aspects of the riboflavin-photosensitised oxidation of the topically administrable ophthalmic drugs Timolol (Tim) and Pindolol (Pin) were investigated in water–MeOH (9:1, v/v) solution employing light of wavelength > 400 nm. riboflavin, belonging to the vitamin B₂ complex, is a known human endogenous photosensitiser. The irradiation of riboflavin in the presence of ophthalmic drugs triggers a complex picture of competitive reactions which produces the photodegradation of both the drugs and the pigment itself. The mechanism was elucidated employing stationary photolysis, polarographic detection of dissolved oxygen, stationary and time-resolved fluorescence spectroscopy, and laser flash photolysis. Ophthalmic drugs quench riboflavin-excited singlet and triplet states. From the quenching of excited triplet riboflavin, the semireduced form of the pigment is generated, through an electron transfer process from the drug, with the subsequent production of superoxide anion radical (O₂^?–) by reaction with dissolved molecular oxygen. Through the interaction of dissolved oxygen with excited triplet riboflavin, the species singlet oxygen (O₂(¹Δ_g)) is also generated to a lesser extent. Both O₂^?– and O₂(¹Δ_g) induce photodegradation of ophthalmic drugs, Tim being ~3-fold more easily photooxidisable than Pin, as estimated by oxygen consumption experiments.     

Flavin-mediated reduction of ferric leghemoglobin from soybean nodules

The reduction of ferric leghemoglobin (Lb³⁺) from soybean (Glycine max (L.) Merr.) nodules by riboflavin, FMN and FAD in the presence of NAD(P)H was studied in vitro. The system NAD(P)H + flavin reduced Lb³⁺ to oxyferrous (Lb²⁺ · O₂) or deoxyferrous (Lb²⁺) leghemoglobin in aerobic or anaerobic conditions, respectively. In the absence of O₂ the reaction was faster and more effective (i.e. less NAD(P)H oxidized per mole Lb³⁺ reduced) than in the presence of O₂; this phenomenon was probably because O₂ competes with Lb³⁺ for reductant, thus generating activated O₂ species. The flavin-mediated reduction of Lb³⁺ did not entail production of superoxide or peroxide, indicating that NAD(P)H-reduced flavins were able to reduce Lb³⁺ directly. The NAD(P)H + flavin system also reduced the complexes Lb³⁺ · nicotinate and Lb³⁺ · acetate to Lb²⁺ · O₂, Lb²⁺ or Lb²⁺ · nicotinate, depending on the concentrations of ligands and of O₂. In the presence of 200 M nitrite most Lb remained as Lb³⁺ in aerobic conditions but the nitrosyl complex (Lb²⁺ · NO) was generated in anaerobic conditions. The above-mentioned characteristics of the NAD(P)H + flavin system, coupled with its effectiveness in reducing Lb³⁺ at physiological levels of NAD(P)H and flavins in soybean nodules, indicate that this mechanism may be especially important for reducing Lb³⁺ in vivo.Abbreviations and Terminology FLbR ferric leghemoglobin reductase - Hb²⁺ /Hb³⁺ hemoglobin containing Fe²⁺ /Fe²⁺ - Lb²⁺ /Lb³⁺ leghemoglobin containing Fe²⁺ /Fe³⁺ - Lb³⁺ · nicotinate/acetate Lb in which nicotinate or acetate are complexed to Lb³⁺ - Lb²⁺ · O₂/CO/NO/nicotinate Lb in which O₂, CO, NO or nicotinate are complexed to Lb²⁺ - Rfl riboflavin - SOD superoxide dismutase (EC 1.15.1.1) Published as Paper No. 9237, Journal Series, Nebraska Agricultural Research DivisionWe thank M.B. Crusellas for his skillful drawings. M. Becana thanks the Spanish Ministry of Education and Science/Fulbright Commission for financial support.     

CO-reacting haemoproteins of neutrophils: Evidence for cytochrome b-245 and myeloperoxidase as potential oxidases during the respiratory burst

Room temperature, CO-difference spectra of intact rat polymorphonuclear leucocytes (neutrophils) revealed the presence of a number of CO-binding haemoproteins. Absorption maxima at 413, 540 and 570 nm were attributed to the CO-complex of cytochromeb-245 whereas an absorption maximum at 595 nm was assigned to the contribution from a myeloperoxidase complex, since an identical absorption maximum was observed in CO-difference spectra of purified myeloperoxidase in the presence of H₂O₂. Photochemical action spectra for the relief of CO-inhibited O₂ uptake revealed contributions from both cytochromeb-245 and myeloperoxidase. The potential of these two O₂- and CO-binding haemoproteins to function as oxidases during the respiratory burst is discussed.     

16O2/18O2 analysis of oxygen exchange in Dunaliella tertiolecta. Evidence for the inhibition of mitochondrial respiration in the light

A mass spectrometric ¹⁶O₂/¹⁸O₂-isotope technique was used to analyse the rates of gross O₂ evolution, net O₂ evolution and gross O₂ uptake in relation to photon fluence rate by Dunaliella tertiolecta adapted to 0.5, 1.0, 1.5, 2.0 and 2.5 M NaCl at 25°C and pH 7.0.At concentrations of dissolved inorganic carbon saturating for photosynthesis (200 M) gross O₂ evolution and net O₂ evolution increased with increasing salinity as well as with photon fluence rate. Light compensation was also enhanced with increased salinities. Light saturation of net O₂ evolution was reached at about 1000 mol m^-2s^-1 for all salt concentrations tested. Gross O₂ uptake in the light was increased in relation to the NaCl concentration but it was decreased with increasing photon fluence rate for almost all salinities, although an enhanced flow of light generated electrons was simultaneously observed. In addition, a comparison between gross O₂ uptake at 1000 mol photons m^-2s^-1, dark respiration before illumination and immediately after darkening of each experiment showed that gross O₂ uptake in the light paralleled but was lower than mitochondrial O₂ consumption in the dark.From these results it is suggested that O₂ uptake by Dunaliella tertiolecta in the light is mainly influenced by mitochondrial O₂ uptake. Therefore, it appears that the light dependent inhibition of gross O₂ uptake is caused by a reduction in mitochondrial O₂ consumption by light.Abbreviations DCMU 3-(3, 4-dichlorophenyl)-1, 1-dimethylurea - DHAP dihydroxy-acetonephosphate - DIC dissolved inorganic carbon - DR_a rate of dark respiration immediately after illumination - DR_b rate of dark respiration before illumination - E₀ rate of gross oxygen evolution in the light - NET rate of net oxygen evolution in the light - PFR photon fluence rate - RubP rubulose-1,5-bisphosphate - SHAM salicyl hydroxamic acid - U₀ rate of gross oxygen uptake in the light     

Stem photosynthesis in a desert ephemeral,Eriogonum inflatum

Summary The gas exchange characteristics of photosynthetic tissues of leaves and stems of Eriogonum inflatum are described. Inflated stems were found to contain extraordinarily high internal CO₂ concentrations (to 14000 bar), but fixation of this internal CO₂ was 6–10 times slower than fixation of atmospheric CO₂ by these stems. Although the pool of CO₂ is a trivial source of CO₂ for stem photosynthesis, it may result in higher water-use efficiency of stem tissues. Leaf and stem photosynthetic activities were compared by means of CO₂ fixation in CO₂ response curves, light and temperature response curves in IRGA systems, and by means of O₂ exchange at CO₂ saturation in a leaf disc O₂ electrode system. On an area basis leaves contain about twice the chlorophyll and nitrogen as stems, and are capable of up to 4-times the absolute CO₂ and O₂ exchange rates. However, the stem shape is such that lighting of the shaded side leads to a substantial increase in overall stem photosynthesis on a projected area basis, to about half the leaf rate in air. Stem conductance is lower than leaf conductance under most conditions and is less sensitive to high temperature or high VPD. Under most conditions, the ratio C _i /C _a is lower in stems than in leaves and stems show greater water-use efficiency (higher ratio assimilation/transpiration) as a function of VPD. This potential advantage of stem photosynthesis in a water limited environment may be offset by the higher VPD conditions in the hotter, drier part of the year when stems are active after leaves have senesced. Stem and leaf photosynthesis were similarly affected by decreasing plant water potential.     

Light-driven reduction of oxygen as a method for studying electron transport in the green photosynthetic bacterium Chlorobium limicola

The use of O₂ uptake as a valid assay for non-cyclic photosynthetic electron flow in membranes from Chlorobium limicola is discussed. It is recommended that methyl viologen, catalase and superoxide dismutase should be added to the experimental medium. The addition of methyl viologen more than doubled the rate of O₂ uptake observed on illumination with 1 mM sulphide as donor. Superoxide dismutation was shown to be efficient under the experimental conditions by means of standard additions of potassium superoxide dissolved in dimethylsulphoxide. The highest rates of light stimulated O₂ uptake were obtained with sulphide as electron donor, and approached 50 mol O₂ · h^-1 · mg bacteriochlorophyll c ^-1 with 0.2 mM sulphide. The presence of 5 mM 2-mercaptoethanol or 3 mM sulphite as electron donor led to lower light stimulated rates of O₂ uptake, while 5 mM thiosulphate had little effect. The rates were insensitive to uncoupler. The light stimulated O₂ uptake with 0.2 mM sulphide as donor was 20–30% inhibited by 10 M antimycin A and 50 M cyanide.Abbreviations APS Adenosine 5-phosphosulphate - FCCP carbonylcyanide-p-trifluoromethoxyphenylhydrazone - HEPES 4-(2-hydroxyethyl)-1-piperazineethanesulphonic acid - MeV methyl viologen - P-840 the photoreactive bacteriochlorophyll     

Mineral nutrient deficiency increases the sensitivity of photosynthesis to sulphur dioxide in needles of a coniferous tree,Abies nordmanniana

Summary Abies nordmanniana (Stev.) Spach was cultivated in rooting media either rich in nutrients (control) or low in magnesium (low Mg) or low in magnesium and nitrogen (low Mg-N), respectively. Intact, attached needles were exposed, in the light (460 mol photons m^-2 s^-1), to an atmosphere containing 1 ppm SO₂ for 5 h. Measurements of light- and CO₂-saturated rates of photosynthetic O₂ evolution, A _max, were performed before and after SO₂ treatments. In needles from well fertilized plants, A _max was high (about 50 mol m^-2 s^-1) and was not affected by SO₂. Needles from low-Mg and low-Mg-N plants had lower photosynthetic rates and showed a marked decline in A _max in response to the SO₂ treatment. Stomatal conductance was similar in the three groups of plants during SO₂ treatments.Abbreviations A _max photosynthetic capacity (CO₂- and light-saturated rate of O₂ evolution) - DW dry weight - F_o yield of dark level fluorescence - F_M maximum yield of fluorescence, induced in a pulse of saturating light - F_v yield of variable fluorescence (= F_M–F_O) - FW fresh weight; g, conductance to water vapor transfer