Binding affinity of bicarbonate and formate in herbicide-resistant D1 mutants of Synechococcus sp. PCC 7942

We examined the effects of mutations at amino acid residues S264 and F255 in the D1 protein on the binding affinity of the stimulatory anion bicarbonate and inhibitory anion formate in Photosystem II (PS II) in Synechococcus sp. PCC 7942. Measurements on the rates of oxygen evolution in the wild type and mutant cells in the presence of different concentrations of formate with a fixed bicarbonate concentration and vice versa, analyzed in terms of an equilibrium activator-inhibitor model, led to the conclusion that the equilibrium dissociation constant for bicarbonate is increased in the mutants, while that of the formate remains unchanged (11±0.5 mM). The hierarchy of the equilibrium dissociation constant for bicarbonate (highest to lowest, ±2 M) was: D1-F255L/S264A (46 M)>D1-F255Y/ S264A (31 M)D1-S264A (34 M)D1-F255Y (33 M)>wild type (25 M). The data suggest the importance of D1-S264 and D1-F255 in the bicarbonate binding niche. A possible involvement of bicarbonate and these two residues in the protonation of Q_B ^-, the reduced secondary plastoquinone of PS II, in the D1 protein is discussed.Abbreviations Chl a chlorophyll a - DBMIB 2,5-dibromo-3-methyl-6-isopropyl-p-benzoquinone - DCMU 3-(3,4-dichlorophenyl)-1,1-dimethylurea - DMQ 2,5-dimethyl-p-benzoquinone - HEPES N-2-hydroxyethylpiperazine-N'-2-ethanesulfonic acid - MES 2-[N-morpholino]ethanesulfonic acid - PSI Photosystem I - PS II Photosystem II - Q_A bound plastoquinone, a one-electron acceptor in Photosystem II - Q_B another bound plastoquinone, a two-electron acceptor in Photosystem II This paper is dedicated to the memory of my dear friend Robin Hill-Govindjee.     

Effect of Photosystem II inhibitor K-15 on photochemical reactions of the isolated D1/D2 cytochrome b559 complex

Effect of a highly efficient inhibitor of Photosystem II (PS II), K-15 (4-[methoxy-bis-(trifluoromethyl)methyl)-2,6-dinitrophenyl hydrazone methyl ketone), was investigated using the D1/D2/cytochrome b559 reaction centre (RC) complex. A novel approach for photoaccumulating reduced pheophytin (Pheo^–) in the absence of the strong reducing agent, sodium dithionite, was demonstrated which involved illumination in the presence of TMPD (from 5 to 100 M) under anaerobic conditions. The addition of K-15 at concentrations of 0.5 M and 2 M resulted in approx. 50% and near 100%, respectively, inhibition of this photoreaction, while subsequent additions of dithionite eliminated the inhibitory effect of K-15. Methyl viologen induced similar inhibition at much higher concentrations (>1 mM). Moreover, K-15 efficiently quenched the variable part of chlorophyll fluorescence (which is the recombination luminescence of the pair P₆₈₀ ⁺ Pheo^–). A 50% inhibition was induced by 5 M K-15 and the effect was maximal in the range 20 to 200 M. Photooxidation of P₆₈₀ in the presence of 0.1 mM silicomolybdate was also efficiently inhibited by K-15 (50% inhibition at 15 M). The data are consistent with the idea put forward earlier (Klimov et al. 1992) that the inhibitory effect of K-15 is based on facilitating a rapid recombination between Pheo^– and P₆₈₀ ⁺ (or Z⁺) via its redox properties. The inhibitor can be useful for suppressing PS II reactions in isolated RCs of PS II which are resistant to all traditional inhibitors, like diuron, and probably functions by substituting for Q_A missing in the preparation.At a concentration of 0.5–50 M K-15 considerably increased both the rate and extent of cytochrome b559 photoreduction in the presence, as well as in the absence, of 5 mM MnCl₂. Consequently it is suggested that K-15 also serves as a mediator for electron transfer from Pheo^– to cytochrome b559.Abbreviations K-15 4-[methoxy-bis-(trifluoromethyl)methyl]-2,6-dinitrophenyl hydrazone methyl ketone - P₆₈₀ the primary electron donor of PS II - Pheo pheophytin - PS II Photosystem II - Q_A and Q_B the primary and the secondary electron acceptor of PS II - RC reaction centre - SiMo silicomolybdate - TMPD N,N,N,,N,-tetramethyl-p-phenylenediamine - Z secondary electron donor of PS II     

Effects of bicarbonate and oxygen concentration on photoinhibition of thylakoid membranes

The effects of bicarbonate and oxygen on photoinhibition of thylakoid membranes were investigated by varying their concentrations independently of each other. A pretreatment of the thylakoid suspension which lowered the bicarbonate concentration of the medium without affecting its oxygen content, increased the degree of photoinhibition upon illumination. This showed that the normal bicarbonate content of a thylakoid suspension, originating from dissolved carbondioxide from the air, protects against photoinhibition. The resistance against photoinhibition was further increased by addition of extra NaHCO₃ up to about 5 mM. The normal oxygen content can be decreased profoundly without affecting the degree of photoinhibition; in contrast, even small changes from the normal bicarbonate content affected photoinhibition.At oxygen concentrations approximately below 25 M, added NaHCO₃ not only did not protect, but caused a more severe PS 2 inactivation. This was due to a blockage by added NaHCO₃ of the recovery from a reversible photoinhibited state.Furthermore, it is shown that if the bicarbonate ions bound to high-affinity sites in PS 2 were replaced by formate ions, the thylakoid membranes became less susceptible to photoinhibition under normal oxygen tension.Abbreviations chl chlorophyll - HEPES (N-[2-Hydroxyethyl]piperazine-N-[2-ethanesulfonic acid]) - PpBQ phenyl-p-benzoquinone - PS 2 Photosystem 2 - Q_A and Q_B primary and secondary quinone acceptors of Photosystem 2     

NADP+ reduction by a methanogen using HCOOH or H2 as electron donor

Summary The resting cells of methanogen strain HU could be used as biocatalyser for converting exoge nous NADP⁺ into NADPH, using either formate or hydrogen as electron donor. To enhance the conversion efficiency of NADPH from NADP⁺, several inhibitors of methylcoenzyme M reductase were used in order to avoid further oxidation of NADPH to CH₄. When methyl viologen (7.5 mol ml^-1) was added to the reaction mixture (17 mg of dry cells, 2 mg Triton X-100, 294 mol of Na-formate and 12 mol of NADP⁺ per ml reaction mixture), 9.6 mol ml^-1 NADPH (80% yield) could be produced in a 2-h reaction, compared with 7.2 mol ml^-1 NADPH (60% yield) in a 6-h reaction in the absence of methyl viologen. Molecular hydrogen istead of formate also served as electron donor to convert NADP⁺ into NADPH. A gas mixture of H₂/N₂ (75/25) yielded 9.8 mol ml^-1 NADPH (82% yield) in a 3-h reaction in the absence of formate, suggesting that H₂ might be a promising, inexpensive electron donor for this reaction system.     

Two types of PS II centres as manifested by light saturation of delayed fluorescence from DCMU-treated chloroplasts

Intensity of 2 s delayed fluorescence (DF) as a function of steady-state actinic light intensity was investigated in pea chloroplasts in the presence of 10 M DCMU. The light saturation curve of DF was approximated by a sum of two hyperbolic components which differ by an order of magnitude in the half-saturating incident light intensity. The relative contribution of the amplitudes of the components was practically independent of cation (Na⁺ and Mg²⁺) concentration and a short-term heating of the chloroplasts at 45°C. The component saturating at low incident light intensity was selectively suppressed by 100 M DCMU or by 1 mol g^-1 Chl oleic acid. DF intensity following excitation by a single saturating 15 s flash was equal to the intensity of the component saturating at a low incident light intensity. Upon flash excitation, the maximum steady-state DF level was found to be attained only after a series of saturating flashes. It is concluded that the two components of the DF light saturation curves are related to PS II centres heterogeneity in quantum yield of stabilization of the reduced primary quinone acceptor.Abbreviations DF Delayed fluorescence - L₁- and L₂-components DF components saturating at low and high incident light intensity, respectively - I incident light intensity - L DF intensity - P₆₈₀ reaction centre chlorophyll of PS II - Q_A and Q_B primary and secondary quinone acceptors of PS II, respectively     

Analysis of oxygen evolution during photosynthetic induction and in multiple-turnover flashes in sunflower leaves

Exchange of CO₂ and O₂ and chlorophyll fluorescence were measured in the presence of 360 1 · 1^–1 CO₂ in nitrogen in Helianthus annuss L. leaves which had been preconditioned in the dark or at a photon flux density (PFD) of 24 mol · m^–2 · s^–1 either in 21 or 0% O₂. An initial light-dependent O₂ outburst of 6 mol · m^–2 was measured after aerobic dark incubation. It was attributed to the reduction of electron carriers, predominantly plastoquinone. The maximum initial rate of O₂ evolution at PFD 8000 mol · m^–2 · s^–1 was 170 mol · m^–2 · s^–2 or about four times the steady CO₂-and light-saturated rate of photosynthesis. Fluorescence measurements showed that the rate was still acceptor-limited. Fast O₂ evolution ceased after electron carriers were reduced in the dark-adapted leaf, but continued for a short time at the lower rate of 62 mol · m^–2 · s^–1 in the light-adapted leaf. The data are interpreted to show that enzymes involved in 3-phosphoglycerate reduction are dark-inhibited, but were fully active in low light. In a dark-adapted leaf, respiratory CO₂ evolution continued under nitrogen; it was partially inhibited by illumination. Prolonged exposure of a leaf to anaerobic conditions caused reducing equivalents to accumulate. This was shown by a slowly increasing chlorophyll fluorescence yield which indicated the reduction of the PSII acceptor Q_A in the dark. When the leaf was illuminated, no O₂ evolution was detected from short light pulses, although transient O₂ production was appreciable during longer light pulses. This indicates that an electron donor (pool size about 2–3 e/PSII reaction center) became reduced in the dark and the first photons were used to oxidise this donor instead of water.Abbreviations Chl chlorophyll - CRC carbon reduction cycle - GAPDH NADP-glyceraldehyde-phosphate dehydrogenase - PFD photon flux density - PGA 3-phosphoglycerate - RuBP ribulose bisphosphate - TCA tricarboxylic acid cycle To whom correspondence should be addressedThis work received support by the Estonian Academy of Sciences, the Gottfried-Wilhelm-Leibniz Program of the Deutsche For-schungsgemeinschaft and the Sonderforschungsbereich 251 of the University of Würzburg.     

P680+ reduction in oxygen-evolving Photosystem II core complexes

The kinetics of P680⁺ reduction in oxygen-evolving spinach Photosystem II (PS II) core particles were studied using both repetitive and single-flash 830 nm transient absorption. From measurements on samples in which PS II turnover is blocked, we estimate radical-pair lifetimes of 2 ns and 19 ns. Nanosecond single-flash measurements indicate decay times of 7 ns, 40 ns and 95 ns. Both the longer 40 ns and 95 ns components relate to the normal S-state controlled Y_z P680⁺ electron transfer dynamics. Our analysis indicates the existence of a 7 ns component which provides evidence for an additional process associated with modified interactions involving the water-splitting catalytic site. Corresponding microsecond measurements show decay times of 4 s and 90 s with the possibility of a small component with a decay time of 20–40 s. The precise origin of the 4 s component remains uncertain but appears to be associated with the water-splitting center or its binding site while the 90 s component is assigned to P680⁺-Q_A ^– recombination. An amplitude and kinetic analysis of the flash dependence data gives results that are consistent with the current model of the oxygen-evolving complex.Abbreviations PS II- Photosystem II- - P680- primary donor (Chl-a_II dimer) of PS II - Y_z- Tyr 161 donor to P680 - Q_A- quinone secondary acceptor to P680 - LHC- light-harvesting chlorophyll protein of PS II - BBY- Berthold, Babcock and Yocum PS II membrane fragment preparation - PPBQ- phenyl-p-benzoquinone     

Calculation of leaf photosynthetic parameters from light-response curves for ecophysiological applications

Measurement of the light response of photosynthetic CO₂ uptake is often used as an implement in ecophysiological studies. A method is described to calculate photosynthetic parameters, such as the maximum rate of whole electron transport and dissimilative respiration in the light, from the light response of CO₂ uptake. Examples of the light-response curves of flag leaves and ears of wheat (Triticum aestivum cv. ARKAS) are shown.Abbreviations and symbols A net photosynthesis rate - D ¹ rate of dissimilative respiration occurring in the light - f loss factor - I incident PPFD - I effective absorbed PPFD - J rate of whole electron transport - J _m maximum rate of whole electron transport - p ^c intercellular CO₂ partial pressure - PPFD photosynthetic photon flux density - q effectivity factor for the use of light (electrons/quanta) - absorption coefficient - I ^* CO₂ compensation point in the absence of dissimilative respiration (bar) - II conversion factor for calculation of CO₂ uptake from the rate of whole electron transport - convexity factor Gas-exchange rates relate to the projective area and are given in mol·m^-2·s^-1. Electron-transport rates are given in mol electrons·m^-2·s^-1; PPFD is given in mol quanta·m^-2·s^-1.     

Tissue culture ofKarwinskia humboldtiana—a plant producing toxins with antitumoural effects

Isolated embryos ofKarwinskia humboldtiana were cultured in vitro. The growth of embryos and development to plantlets on woody plant medium supplemented with indole-3-acetic acid 6.10^-2 mol l^–1, gibberellic acid (GA₃) 3.10^-2 mol l^–1, and 6-benzylaminopurine (BA) 2 mol l^–1 was obtained. Multiplication of shoots and rooting of excised shoots has been achieved. Callus formation on modified Murashige-Skoog medium supplemented with 1-naphthaleneacetic acid 10 mol l^–1, GA₃ 14 mol l^–1, and kinetin 5 mol l^–1 on hypocotyls, or on root cultures on medium supplemented with 2.4-dichlorophenoxyacetic acid 10 mol l^–1 and BA 10 mol l^–1 was induced.Abbreviations BA 6-benzylaminopurine - 2,4-d 2,4-dichlorophenoxyacetic acid - GA₃ gibberellic acid - IAA indole-3-acetic acid - NAA 1-naphthaleneacetic acid - TEM transmission electron microscopy     

Transient state characteristics of adaptation to changes in light conditions for the cyanobacterium Oscillatoria agardhii

Transitions in growth irradiance level from 92 to 7 Em^-2 s^-1 and vice versa caused changes in the pigment contents and photosynthesis of Oscillatoria agardhii. The changes in chlorophyll a and C-phycocyanin contents during the transition from high to low irradiance (HL) were reflected in photosynthetic parameters. In the LH transition light utilization efficiencies of the cells changed faster than pigment contents. This appeared to be related to the lowering of light utilization efficiencies of photosynthesis. As a possible explanation it was hypothesized that excess photosynthate production led to feed back inhibition of photosynthesis. Time-scales of changes in the maximal rate of O₂ evolution were discussed as changes in the number of reaction centers of photosystem II in relation to photosynthetic electron transport. Parameters that were subject to change during irradiance transitions obeyed first order kinetics, but hysteresis occurred when comparing HL with LH transients. Interpretation of first order kinetic analysis was discussed in terms of adaptive response vs changes in growth rate.Non-standard abbreviations Chla chlorophyll a - CPC C-phycocyanin - PS II photosystem II - PS I photosystem I - RC II reaction center of photosystem II - P photosynthetic O₂-evolution - I irradiance, Em^-2 s^-1 - light utilization efficiency of cells, mmol O₂·mg dry wt^-1·h^-1/Em^-2 s^-1 - light utilization efficiency of photosynthetic apparatus, mol O₂·mol Chla ^-1·h^-1/Em^-2 s^-1 - P_max maximal rate of O₂ evolution by cells, mol O₂·mg dry wt^-1·h^-1 - P_max maximal rate of O₂ evolution by photosynthetic apparatus, mol O₂·mol·Chla ^-1·h^-1 - LL low light, E m^-2 s^-1 - HL high light, E m^-2 s^-1 - LH low to high light transition - HL high to low light transition - k specific rate of adaptation, h^-1 - specific growth rate, h^-1 - Q pool size of cell constituent, mol·mg dry wt^-1 - q net synthesis rate of cell constituent, mol·mg dry wt^-1·h^-1     

Accumulation of silver by growing and non-growing populations of Citrobacter intermedius B6

Summary Accumulation of silver is reported for growing and non-growing populations of Citrobacter intermedius B6. In non-growing cultures a maximum uptake of 4.35% (w/w) was observed at an initial silver concentration of 2111.2 mol Ag⁺ l^-1. In contrast the maximum uptake of silver by growing bacteria was 2.81% (w/w) at an effective concentration of silver of 217.8 mol·l^-1. Silver accumulation rates in both resting (460 mol Ag⁺ g^-1 per hour) and continuously grown (41 mol Ag⁺ g^-1 per hour) bacteria are higher than previously reported. Cell fractionation and electron microscopy of continuously grown bacteria indicated that accumulation of silver was associated with the cell envelope, in the form of dense deposits of macromolecular proportions. This observation discounts simple surface adsorption as the process of accumulation in growing cultures.     

Seasonal changes in inorganic and organic phosphorus in the soil of a riparian forest

This study addresses the temporal distribution of forms of phosphorus in the soil of a temporarily flooded riparian forest of the valley of the river Garonne (Southwest of France). A sequential extraction for forms of phosphorus of increasing chemical stability was used. During the study period (13 months), the forest was flooded a few days during March and May. In winter, resin-Pi concentration was high (26 g g^–1) in comparison to spring values (<9 g g^–1). NaHCO₃-Po, NaHCO₃-Pi or NaOH-Pi concentrations increased during winter (up to 74, 124 and 78 g g^–1 respectively) and decreased significantly during spring (32, 44 and 32 g g^–1 respectively). This pattern was attributed to simultaneous mineralization and plant uptake during the growing season and to the flood events (erosional processes and P-release). During summer and fall, resin-Pi concentration increased significantly (up to 26 g g^–1 in October). NaHCO₃-Po concentrations remained low during spring and summer (<33 g g^–1), and increased significantly in fall (>45 g g^–1 NaHCO₃-Pi or NaOH-Pi increased in late spring or summer (90 g g^–1 and 68 g g^–1 respectively). Increasing concentrations of the labile forms during late spring or summer were ascribed to the warm temperature and soil dryness that limited plant growth. HCl-Pi increased regularly after the floods (174 g g^–1 before the flood events to 254 g g^–1 after the floods). Residual P presented a similar pattern i.e. 214 g g^–1 and 279 g g^–1 respectively before and after the flood events. This pattern was attributed to a progressive incorporation of flood deposits to the soil.     

Asymmetric diffusion into the postsynaptic neuron: An extremely efficient mechanism for removing excess GABA from synaptic clefts on the Deiters' neurone plasma membrane

Microdissected Deiters' neuron plasma membranes have been used for studying the passage of GABA through the membrane both in the inward and outward direction. Working with 0.2 mM GABA in the compartment simulating the outside of the neurone and with 2.0 mM GABA in the one simulating the inside we found a net transport of GABA towards the inside. This mechanism does not require a Na⁺ ion gradient across the membrane. The nature of the transport process involved was studied by determining the rate of [³H]-GABA inward passage as a function of GABA concentration (1 nM–800 M) on the outward side of the membrane. The results have shown that until 50 M a diffusion process (v=D₁×C, where D₁=3.1×10^–11 1/m²×sec) is the sole mechanism involved. Above 50 M a second diffusion process is activated v=D₂×(C–50×10^–6), where D₂=2.8×10^–11 1/m²×sec. Taking in account both inward and outward directed diffusion, one can calculate 16 M as the equilibrium concentration of GABA on the outward side of the membrane. From a kinetic point of view, these diffusion processes are able to reduce GABA concentration in a synaptic cleft from 3 mM to 20 M within 3 sec. These diffusion systems are discussed as extremely efficient in removing the excess of released GABA in the synaptic cleft.     

Identification and characterization of high-affinity Ca2+-ATPase associated with axonal plasma membranes of dog mesenteric nerves

The microsomal fraction isolated from dog mesenteric nerve fibres was found to contain ATPase activity stimulated by micromolar concentrations of Ca ions. Such a high-affinity Ca²⁺-ATPase (hereafter referred to as HA Ca-ATPase) followed a Michaelis-Menten kinetics with K_m for Ca ions of 0.4 M and V_max=12.5±2.4 mol P_i.mg^–1h^–1. The examination of the subcellular origin of HA Ca-ATPase revealed that this enzyme is associated with axonal plasma membranes as documented by its co-purification with several plasma membrane marker enzymes and with tetrodotoxin-sensitive³H-saxitoxin binding. The addition of exogenous magnesium ions (Mg) resulted in a non-competitive inhibition of HA Ca-ATPase with K_i=0.5 mM. The reaction velocity of HA Ca-ATPase was also inhibited by other divalent ions with the order of potency Mg>Mn >ZnCo>Ni. In contrast to low affinity (high K_m) Mg- and Ca-ATPase, the HA Ca-ATPase was insensitive to the inhibition by sodium azide (10 mM) and sodium fluoride (10 mM). Similarly, the specific activity of HA Ca-ATPase was unaffected by vanadate (100 M) and N-ethylmaleinimide (100 M). It is concluded that axonal plasma membranes of dog mesenteric nerves contain HA Ca-ATPase which seems to be unrelated to calcium-transporting Mg-dependent, Ca-stimulated ATPase.Abbreviations used BSA bovine serum albumin - HA Ca-ATPase high-affinity Ca²⁺-ATPase - K-pNPPase onabain-sensitive, K⁺-stimulated p-nitrophenyl phosphatase - NEM N-ethylmaleinimide - SIM 250 mM sucrose, 10 mM imidazole-HCl pH 7.4 - TRIS tris (hydroxymethyl) aminomethane     

Adaptation of the thylakoid membranes of pea chloroplasts to light intensities. II. Regulation of electron transport capacities,electron carriers,coupling factor (CF1) activity and rates of photosynthesis

The electron transport rates of photosystems II and I, amounts of electron carriers, coupling factor activity and photosynthetic rates were investigated in thylakoids isolated from pea plants grown under a wide range of light intensities (16 h light-8 h dark). The electron transport rates of PS II and PS I, as partial reactions or in whole chain, and coupling factor activity on a unit chlorophyll basis, all increased as the light intensity available for growth was altered from a very low intensity of 10 E m^-2s^-1 to a high intensity of 840 E m^-2s^-1. Similarly, there were increases in the amounts of atrazine binding sites, plastoquinine, cytochrome f and P700 per unit chlorophyll; significantly, the amounts of reaction centres of PS II and PS I were not equal at any light intensity. The rate of change of all parameters with respect to light intensity could be represented by two straight lines of different slopes which met at a transition point corresponding to approximately 200 E m^-2s^-1 during growth. These photoadaptations were similar to those observed for both the relative distribution of chlorophyll in chlorophyll-protein complexes and the chl a/chl b ratios [Leong and Anderson, 1984, Photosynthesis Research 5:117–128]. Since these thylakoid components and functions were affected in the same direction by light intensity during growth and all show linear relationships with chl a/chl b ratios, it indicates that they are closely regulated and markedly well co-ordinated. Plants compensate for the limited amount of low light intensities by drastically increasing the light-harvesting antenna unit size of photosystem II and to a lesser extent that of photosystem I. Changes in the composition of the thylakoid membranes exert a regulatory effect on the overall photosynthetic rate up to approximately 450 E m^-2s^-1.Abbreviations chl chlorophyll - cyt cytochrome - PQ plastoquinone - PS photosystem     

Localization of sodium absorption and chloride secretion in an intestinal epithelium

Summary Hen coprodeum absorbs sodium electrogenically and, when stimulated by theophylline, secretes chloride. In this study the vibrating microprobe technique was used to localize the transport of these ions to intestinal villi/folds and crypts. With the isolated, stretched epithelium, controlled by light microscopy and scanning electron microscopy, in open circuit, currents were inward, 40±7 A/cm², 50 m vertically above villi, and outward, 36±7 A/cm² above crypts. The currents decayed exponentially to near zero at 300 m with the same length constant. A physical model simulating the observed loci of current sources and sinks predicts potential profiles consistent with our data. Extrapolation of the currents gives a surface potential of 45 V, negative on villi and positive above crypts. Short circuiting increased villus current to 86±27 A/cm² at 50 m, and amiloride treatment reduced it to –8 A/cm²; in both cases crypt currents were abolished. The inward currents are compatible with sodium absorption. Induction of chloride secretion after amiloride treatment, resulted in current circuits similar to those induced by sodium absorption, with villus currents of 23±7 A/cm². This is in accord with chloride secretion at the villi. Quantitative estimates of crypt number (860/cm²) and opening diameter (15 m), in conjunction with isotopic measurements of active and electrical potential-driven ion fluxes demonstrate, however, that only 4% of the potential-driven co-ion transport occurs through the crypts. This indicates that nearly all chloride secretion comes from the sodium-absorbing villar area. Were the chloride secretion to occur solely from the crypts, the current should have been in the opposite direction and 10,000-fold larger.     

A lyophilized aqueous extract of<Emphasis Type="Italic"> Maytenus ilicifolia</Emphasis> leaves inhibits histamine-mediated acid secretion in isolated frog gastric mucosa

Lyophilized aqueous extract of Maytenus ilicifolia leaves (LAEMIL) is commonly used in Brazilian folk medicine in the treatment of dyspepsia as well as gastric ulcers. We have investigated the effect and the possible mechanism of action of the LAEMIL on acid secretion in isolated frog gastric mucosa incubated in an Ussing chamber. It was observed that LAEMIL (7–28 mg%) as well as cimetidine (125–4,000 M), a well-known histamine H₂ receptor antagonist, decreased basal acid secretion in a concentration-dependent manner. Similarly to cimetidine (190 M), LAEMIL (21 mg%) also inhibited gastric acid secretion induced by increasing concentrations of histamine (50–800 M). The EC₅₀ values for histamine alone and histamine in the presence of LAEMIL or cimetidine were 94.6 M (71.1–125.9 M), 244.9 M (209.4–286.4 M) and 142.2 M (23.6–855.0 M), respectively. LAEMIL, histamine and cimetidine were effective on acid secretion only when added to the serosal surface of the mucosa. Furthermore, simultaneous addition of LAEMIL and cimetidine at concentrations, per se, ineffective, caused a 16% reduction in the basal acid secretion [from 8.3±0.3 to 6.9±0.2 Eq g^–1 (15 min)^–1, n=4]. Although effects such as inhibition of histamine biosynthesis and/or histamine release can not be ruled out, our data suggest that LAEMIL, like cimetidine, reduces acid secretion in the isolated frog gastric mucosa by antagonising histamine H₂ receptors.Abbreviations LAEMIL Lyophilized aqueous extract of Maytenus ilicifolia leaves - Hist Histamine - Cim Cimetidine     

The effects of ultraviolet irradiation on P680+ reduction in PS II core complexes measured for individual S-states and during repetitive cycling of the oxygen-evolving complex

Flash-induced absorbance measurements at 830 nm on both nanosecond and microsecond timescales have been used to characterise the effect of ultraviolet light on Photosystem II core particles. A combination of UV-A and UV-B, closely simulating the spectrum of sunlight below 350 nm, was found to have a primary effect on the donor side of P680. Repetitive measurements indicated reductions in the nanosecond components of the absorbance decay with a concomitant appearance and increase in the amplitude of a component with a 10 s time constant attributed to slow reduction of P680⁺ by Tyr_z when the function of the oxygen evolving complex is inhibited. Single-flash measurements show that the nanosecond components have amplitudes which vary with S-state. Increasing UV irradiation inhibited the amplitude of these components without changing their S-state dependence. In addition, UV irradiation resulted in a reduction in the total amplitude, with no change in the proportion of the 10 s contribution.Abbreviations BBY- PS II membrane fragments - P680- primary electron donor of PS II - PS II- Photosystem II - Q_A and Q_B– primary and secondary quinone electron acceptors of PS II - S-state- redox state of the oxygenevolving complex - Tyr_z– tyrosine residue in PS II - UV-A- ultraviolet radiation 320–400 nm - UV-B- ultraviolet radiation 280–320 nm     

A light-enhanced metabolism of sulfite in cells of Cucumis sativus L. cotyledons

The effects of light and several photosynthetic inhibitors on the rate of sulfite metabolism in cells obtained from Cucumis sativus L. cotyledons was studied. The cells were treated with 200 M Na₂SO₃ and the disappearance of sulfite was monitored using either dithiobisnitrobenzoic acid or fuchsin. The rate of sulfite disappearance in light was double the dark rate. Disalicylidene propanediamine at 1 mM increased this light-enhanced metabolism approx. 50%; neither 1 M 3,4-dichlorophenyl-N,N-dimethylurea nor 0.1 mM cyanazine, which completely inhibited CO₂-dependent oxygen evolution, affected the rate of sulfite metabolism. Addition of 200 M Na₂SO₃ to the cells partially inhibited ¹⁴CO₂ fixation. The rate of sulfite consumption by the cells did not affect this inhibition. We conclude that light-dependent sulfite metabolism is cucumber cells may utilize reduced ferredoxin generated as a result of photosynthetic electron transport. An injurious interaction between CO₂ fixation and sulfite appears to occur independently of the sulfite-metabolism process.Abbreviations DCMU 3,4-dichlorophenyl-N,N-dimethylurea - DSPD disalicylidene propanediamine - DTNB 5,5-dithiobis-(2-nitrobenzoic acid)     

Relationship Between Photosystem 2 Electron Transport and Photosynthetic CO2 Assimilation Responses to Irradiance in Young Apple Tree Leaves

The responses to irradiance of photosynthetic CO₂ assimilation and photosystem 2 (PS2) electron transport were simultaneously studied by gas exchange and chlorophyll (Chl) fluorescence measurement in two-year-old apple tree leaves (Malus pumila Mill. cv. Tengmu No.1/Malus hupehensis Rehd). Net photosynthetic rate (P _N) was saturated at photosynthetic photon flux density (PPFD) 600-1 100 (mol m^-2 s^-1, while the PS2 non-cyclic electron transport (P-rate) showed a maximum at PPFD 800 mol m^-2 s^-1. With PPFD increasing, either leaf potential photosynthetic CO₂ assimilation activity (F_d/F_s) and PS2 maximal photochemical activity (F_v/F_m) decreased or the ratio of the inactive PS2 reaction centres (RC) [(F_i – F_o)/(F_m – F_o)] and the slow relaxing non-photochemical Chl fluorescence quenching (q_s) increased from PPFD 1 200 mol m^-2 s^-1, but cyclic electron transport around photosystem 1 (RFp), irradiance induced PS2 RC closure [(F_s – F_o)/F_m – F_o)], and the fast and medium relaxing non-photochemical Chl fluorescence quenching (q_f and q_m) increased remarkably from PPFD 900 (mol m^-2 s^-1. Hence leaf photosynthesis of young apple leaves saturated at PPFD 800 mol m^-2 s^-1 and photoinhibition occurred above PPFD 900 mol m^-2 s^-1. During the photoinhibition at different irradiances, young apple tree leaves could dissipate excess photons mainly by energy quenching and state transition mechanisms at PPFD 900-1 100 mol m^-2 s^-1, but photosynthetic apparatus damage was unavoidable from PPFD 1 200 mol m^-2 s^-1. We propose that Chl fluorescence parameter P-rate is superior to the gas exchange parameter P _N and the Chl fluorescence parameter F_v/F_m as a definition of saturation irradiance and photoinhibition of plant leaves.