CO2 Exchange in soil algal crusts occurring in the trachypogon savannas of the Orinoco Llanos,Venezuela

CO₂ exchange between the atmosphere and soil algal crusts of the Trachypogon savannas of the Orinoco Llanos has been analyzed using an open gas exchange system. These savannas encompass a wide range of physiognomic types, from herbaceous communities to savanna woodlands. A maximum CO₂ flux of 0.207 mg m^-2 s^-1 was measured in the crusts of the Guanipa savannas, while in the other examined crusts (0.035–0.105 mg m^-2 s^-1) the flux was similar to values reported for terrestrial algae. The CO₂ flux data were statistically fitted to the photosynthetically active radiation by a logarithmic relationship, and the photosynthetic efficiencies of the crusts were compared. The activation energy calculated for the CO₂ fixation indicates that limitations by diffusion and photochemical processes were excluded in the Guanipa crusts (above 12 kcal mole^-1), whereas they were evident in the other crust studied. An optimum CO₂ incorporation as a function of the crust water potential was established and carbon gain strategies were proposed on the basis of the results and characteristics of the habitats.     

Oscillations in photosynthesis are initiated and supported by imbalances in the supply of ATP and NADPH to the Calvin cycle

Oscillations in the rate of photosynthesis of sunflower (Helianthus annuus L.) leaves were induced by subjecting leaves, whose photosynthetic apparatus had been activated, to a sudden transition from darkness or low light to high-intensity illumination, or by transfering them in the light from air to an atmosphere containing saturating CO₂. It was found that at the first maximum, light-and CO₂-saturated photosynthesis can be much faster than steady-state photosynthesis. Both Q_A in the reaction center of PS II and P₇₀₀ in the reaction center of PS I of the chloroplast electron-transport chain were more oxidized during the maxima of photosynthesis than during the minima. Maxima of P₇₀₀ oxidation slightly preceded maxima in photosynthesis. During a transition from low to high irradiance, the assimilatory force F_A, which was calculated from ratios of dihydroxyacetone phosphate to phosphoglycerate under the assumption that the reactions catalyzed by NADP-dependent glyceraldehydephosphate dehydrogenase, phosphoglycerate kinase and triosephosphate isomerase are close to equilibrium, oscillated in parallel with photosynthesis. However, only one of its components, the calculated phosphorylation potential (ATP)/(ADP)(P_i), paralleled photosynthesis, whereas calculated NADPH/NADP ratios exhibited antiparallel behaviour. When photosynthetic oscillations were initiated by a transition from low to high CO₂, the assimilatory force F_A declined, was very low at the first minimum of photosynthesis and increased as photosynthesis rose to its second maximum. The observations indicate that the minima in photosynthesis are caused by lack of ATP. This leads to overreduction of the electron-transport chain which is indicated by the reduction of P₇₀₀. During photosynthetic oscillations the chloroplast thylakoid system is unable to adjust the supply of ATP and NADPH rapidly to demand at the stoichiometric relationship required by the carbonreduction cycle.Abbreviations PGA 3-phosphoglycerate - DHAP dihydroxyacetone phosphate - P₇₀₀ electron-donor pigment in the reaction enter of PS I - Q_A quinone acceptor in the reaction center of PS II This work received support from the Estonian Academy of Sciences, the Bavarian Ministry of Science and Art and the Sonderforschungsbereich 251 of the University of Würzburg. We are grateful for criticism by D.A. Walker, Robert Hill Institute, University of Sheffield, U.K. and by Mark Stitt, Institute of Botany, University of Heidelberg, FRG.     

Measurement and effects of gel matric potential and expressibility on production of morphogenic callus by cultured sugarbeet leaf discs

During studies to optimize production of morphogenic callus from cultured leaf discs of sugarbeet (Beta vulgaris L.) large differences were observed associated with the gelling agent employed. Water availability, as determined mainly by gel matric potential, was found to be the dominant factor. A simple method was devised to measure the relative matric potential of different gels. A precisely moistened filter-paper disc was placed on the gel surface, allowed to equilibrate, removed and weighed. The relative gain or loss of water from the paper disc was a measure of the matric potential of the gel and varied with both gel type and concentration. Leaf disc expansion and production of callus-derived embryos and shoots were shown to be directly proportional to gel matric potential. Water availability may also be affected by the ease with which liquid is expressed from gels in response to localized pressure caused by explant expansion and contortion. This property, called gel expressibility, was easily measured with a weight and capillary pipette and shown also to vary with gel type and concentration. Validity of the technique for measuring relative matric potential was verified physiologically by culturing leaf discs on filter-paper overlays to eliminate expressibility differences among gels. Additionally, comparison of leaf disc growth on uncovered gel surfaces versus filter-paper overlays demonstrated the contribution of liquid expression to overall water availability. Expression of liquid by explants on uncovered gel surfaces greatly enhanced the production of morphogenic callus.     

γ-氨基丁酸、荷包牡丹碱和L-谷氨酸钠对猫和家兔同侧和对侧图形视觉诱发电位的影响

在猫和家兔大脑半球一侧视区17/18交界处施加γ—氨基丁酸(GABA)、荷包牡丹碱和L—谷氨酸钠,以及用氯化钾和冷冻阻遏的方法,记录对侧和同侧皮层相应处图形视觉诱发电位(PVEP)的变化。讨论了GABA、荷包牡丹碱和L—谷氨酸钠对猫和兔的对侧和同侧PVEP的影响。     

Assessment of membrane potential changes using the carbocyanine dye,diS-C3-(5): synchronous excitation spectroscopy studies

The fluorescence of the voltage sensitive dye, diS-C₃-(5), has been analyzed by means of synchronous excitation spectroscopy. Using this rather rare fluorescence technique we have been able to distinguish between the slightly shifted spectra of diS-C₃-(5) fluorescence from cells and from the supernatant. It has been found that diS-C₃-(5) fluorescence in the supernatant can be selectively monitored at _exc = 630 nm and _em= 650 nm, while the cell associated fluorescence can be observed at _exc= 690 nm and _em = 710 nm. A modified theory for the diSC₃-(5) fluorescence response to the membrane potential is presented, according to which a linear relationship exists between the logarithmic increment of the dye fluorescence intensity in the supernatant, In I/I°, and the underlying change in the plasma membrane potential, _p=_p-°_p. The theory has been tested on human myeloid leukemia cells (line ML-1) in which membrane potential changes were induced by valinomycin clamping in various K⁺ gradients. It has been demonstrated that the membrane potential change, _p,can be measured on an absolute scale. Offprint requests to: J. Plasek     

Effects of different rates of cardiac pacing on rat myocardial energy status

The energy status of mammalian cells is a finely regulated phenomenon. This is especially true in cardiac muscle cells in which energy requirements are high and the system must provide rapid turnover of the adenine nucleotides and instant response to changes in energetic demands. We have examined the acute response of the rat myocardium to ventricular pacing up to 2.5 times the resting heart rate. The purpose of this study was to determine at what level of pacing the normal energy status could be maintained and at what point it was compromised. Myocardial energy charge (EC = (ATP + 0.5 ADP)/(ATP + ADP + AMP)) was maintained at 1, 1.5 and 2 times the resting heart rate but declined significantly at 2.5 times. In contrast, phosphorylation potential (PP = ATP/ADP₁ × Pi) was drastically altered in hearts paced at 1.5, 2 and 2.5 times the resting rate. Tissue lactate increased and glycogen decreased in a linear fashion as pacing rate increased, indicating that the metabolic challenge was proportional to the pacing rate. EC seems to reflect the overall status of the cell and its ability to maintain a dynamic equilibrium. PP may reflect the immediate and necessary driving force for mitochondrial respiration in times of increased demand. These data suggest that the myocardium may meet the increased energy demands of acute ventricular pacing by shifting the molar ratio of ATP to ADP times Pi in favour of driving phosphorylation.     

A re-assessment of bacterial growth efficiency: the heat production and membrane potential of Streptococcus bovis in batch and continuous culture

Glucose-limited, continuous cultures (dilution rate 0.1 h^-1) of Streptococcus bovis JB1 fermented glucose at a rate of 3.9 mol mg protein^-1 h^-1 and produced acctate, formate and ethanol. Based on a maximum ATP yield of 32 cells/mol ATP (Stouthamer 1973) and 3 ATP/glucose, the theoretical glucose consumption for growth would have been 2.1 mol mg protein^-1 h^-1. Because the maintenance energy requirement was 1.7 mol/mg protein/h (Russell and Baldwin 1979), virtually all of the glucose consumption could be explained by growth and maintenance and the Y_ATP was 30. Glucose-limited, continuous cultures produced heat at a rate of 0.29 mW/mg protein, and this value was similar to the enthalpy change of the fermentation (0.32 mW/mg protein). Batch cultures (specific growth rate 2.0 h^-1) fermented glucose at a rate of 81 mol mg protein^-1 h^-1, and produced only lactate. The heat production was in close agreement with the theoretical enthalpy change (1.72 versus 1.70 mW/mg protein), but only 80% of the glucose consumption could be accounted by growth and maintenance. The Y_ATP of the batch cultures was 25. Nitrogen-limited, glucose-excess, non-growing cultures fermented glucose at a rate of 6.9 mol mg protein^-1 h^-1, and virtually all of the enthalpy for this homolactic fermentation could be accounted as heat (0.17 mW/mg protein). The nitrogenlimited cultures had a membrane potential of 150 mV, and nearly all of the heat production could be explained by a futile cycle of protons through the cell membrane (watts = amperes x voltage where H⁺/ATP was 3). The membrane voltage of the nitrogen-limited cells was higher than the glucose-limited continuous cultures (150 versus 80 mV), and this difference in voltage explained why nitrogen-limited cultures consumed glucose faster than the maintenance rate. Batch cultures had a membrane potential of 100 mV, and this voltage could not account for increased glucose consumption (more than growth plus maintenance). It appears that another mechanism causes the increased heat production and lower growth efficiency of batch cultures.     

The membrane potential modulates the ATP-dependent Ca pump of cardiac sarcolemma

The effect of membrane potential on the activity of the ATP-dependent Ca²⁺ pump of isolated canine ventricular sarcolemmal vesicles were investigated. The membrane potential was controlled by the intravesicular and extravesicular concentration of K⁺, and the initial rates of Ca²⁺ uptake both in the presence and the absence of valinomycin were determined. The rate of Ca²⁺ uptake was stimulated by a inside-negative potential induced in the presence of valinomycin. The valinomycin-dependent stimulation was enhanced by the addition of K⁺ channel blocker, tetraethylammonium ion or Ba²⁺. The electrogenicity of cardiac sarcolemmal ATP-dependent Ca²⁺ pump is suggested from the increase of Ca²⁺ uptake by negative potential induced by valinomycin.     

Rhodamine 123 as a probe of transmembrane potential in isolated rat-liver mitochondria: spectral and metabolic properties

The spectral and metabolic properties of Rhodamine 123, a fluorescent cationic dye used to label mitochondria in living cells, were investigated in suspensions of isolated rat-liver mitochondria. A red shift of Rhodamine 123 absorbance and fluorescence occurred following mitochondrial energization. Fluorescence quenching of as much as 75% also occurred. The red shift and quenching varied linearly with the potassium diffusion potential, but did not respond to ΔpH. These energy-linked changes were accompanied by dye uptake into the matrix space. Concentration ratios, in-to-out, approached 4000:1. A large fraction of internalized dye was bound. At concentrations higher than those needed to record these spectral changes, Rhodamine 123 inhibited ADP-stimulated (State 3) respiration of mitochondria (K_i = 12 μM) and ATPase activity of inverted inner membrane vesicles (K_i = 126 μM) and partially purified F₁-ATPase (K_i = 177 μM). The smaller K_i for coupled mitochondria was accounted for by energy-dependent Rhodamine 123 uptake into the matrix. Above about 20 nmol/mg protein (10 μM), Rhodamine 123 caused rapid swelling of energized mitochondria. Effects on electron-transfer reactions and coupling were small or negligible even at the highest Rhodamine 123 concentrations employed. Δψ-dependent Rhodamine 123 uptake together with Rhodamine 123 binding account for the intense fluorescent staining of mitochondria in living cells. Inhibition of mitochondria ATPase likely accounts for the cytotoxicity of Rhodamine 123. At concentrations which do not inhibit mitochondrial function, Rhodamine 123 is a sensitive and specific probe of Δψ in isolated mitochondria.