A novel approach to the in situ measurement of oxygen concentrations in the sapwood of woody plants

A novel technique for the physico‐chemical analysis of xylem sap by underwater access to the sapwood of trees is described. In situ measurements of dissolved oxygen in the sapwood are performed by combining this technique with a novel optical method for oxygen detection. In early spring, the oxygen concentration of the sapwood of Betula pendula was in the range of 80–230 µmol O₂ L^?1, corresponding to an oxygen deficit of 40–75% of air saturation. Oxygen concentration maxima and minima occurred early in the morning and in the afternoon, respectively, whereas xylem sap temperatures showed the reverse pattern. In the sapwood, hypoxia increased from the beginning of bud break until frondescence, when a deficit of 86% of air saturation marked the upper limit of oxygen depletion. There seemed to be no relationship between daily variations of oxygen concentration and xylem sap pressure. In summer, sap flow was a major determinant for the diurnal variation of dissolved oxygen concentration. Oxygen supply to the sapwood was determined by both radial influx into the trunk through intercellular gas spaces and transport of dissolved oxygen via xylem sap flow. Radial influx seemed to be favoured during night‐time, when the trunk was warmer than ambient air. During daytime, the hypoxia of the sapwood rose and increased sharply in the evening, when sap flow velocity approximated zero. High temperature in the sapwood enhanced the respiratory oxygen consumption of the wood parenchyma while the supply of dissolved oxygen via the transpiration stream became ineffective.     

Xylem sap flow as a major pathway for oxygen supply to the sapwood of birch (Betula pubescens Ehr.)

The role of xylem sap flow as an aqueous pathway for oxygen supply to the wood parenchyma of Betula pubescens saplings was investigated. Using micro‐optode sensors the oxygen status of the sapwood was quantified in relation to mass flow of xylem sap. Sap flow was gradually reduced by an increasing oxygen depletion in the root space. The effect of sap flow on radial O₂ transport between stem and atmosphere was assessed by a stoichiometrical approach between respiratory CO₂ production and O₂ consumption. Restriction of sap flow set in 36.5 h after the onset of O₂ depletion, and was complete after 71 h. Interruption of sap flow drastically increased the O₂ deficit in the sapwood to 70%. Sap flow contributed about 60% to the total oxygen supply to the sapwood. Diurnal O₂ flow rates varied between 3 and 6.3 nmol O₂ m^?2 leaf area (LA) s^?1 during night‐ and daytime, respectively. Maximum O₂ flow rates of 20 nmol O₂ m^?2 LA s^?1 were reached at highest sap flow rates of 5.7 mmol H₂O m^?2 LA s^?1. Sap flow not only affected the oxygen status of the sapwood but also had an effect on radial O₂ transport between stem and atmosphere.     

A new method to determine the oxygen concentration inside the sapwood of trees

Research into the short-term fluctuations of oxygen concentrations in tree stems has been hampered by the difficulty of measuring oxygen inside tissues. A new method, which is based on fluorescence quenching of a ruthenium complex in the presence of oxygen, has been applied to measure changes of oxygen concentration in the sapwood of trees. During a field day-course oxygen increased with the radiation load and fell during the night (in Fagus orientalis from 20.3% in the afternoon to 17.5% in the morning next day). In a greenhouse experiment the sapwood oxygen concentration of Laurus nobilis could be influenced by flooding the root system. The very fast response, high resolution (better than 0.1%), easy calibration, and dependence only on oxygen and temperature make the technique well suited for measurements of oxygen concentrations in the sapwood.     

<Emphasis Type="Italic">Brettanomyces bruxellensis</Emphasis>: effect of oxygen on growth and acetic acid production

The influence of the oxygen supply on the growth, acetic acid and ethanol production by Brettanomyces bruxellensis in a glucose medium was investigated with different air flow rates in the range 0-300 l h(-1 ) x (0-0.5 vvm). This study shows that growth of this yeast is stimulated by moderate aeration. The optimal oxygen supply for cellular synthesis was an oxygen transfer rate (OTR) of 43 mg O(2) l(-1) x h(-1). In this case, there was an air flow rate of 60 l h(-1) (0.1 vvm). Above this value, the maximum biomass concentration decreased. Ethanol and acetic acid production was also dependent on the level of aeration: the higher the oxygen supply, the greater the acetic acid production and the lower the ethanol production. At the highest aeration rates, we observed a strong inhibition of the ethanol yield. Over 180 l h(-1) x (0.3 vvm, OTR =105 mg O(2) l(-1) x h(-1)), glucose consumption was inhibited and a high concentration of acetic acid (6.0 g x l(-1)) was produced. The ratio of "ethanol + acetic acid" produced per mole of consumed glucose using carbon balance calculations was analyzed. It was shown that this ratio remained constant in all cases. This makes it possible to establish a stoichiometric equation between oxygen supply and metabolite production.     

Glucose kinetics and substrate oxidation during exercise in the follicular and luteal phases.

The purpose of this investigation was to determine whether plasma glucose kinetics and substrate oxidation during exercise are dependent on the phase of the menstrual cycle. Once during the follicular (F) and luteal (L) phases, moderately trained subjects [peak O(2) uptake (V(O(2))) = 48.2 +/- 1.1 ml. min(-1). kg(-1); n = 6] cycled for 25 min at approximately 70% of the V(O(2)) at their respective lactate threshold (70%LT), followed immediately by 25 min at 90%LT. Rates of plasma glucose appearance (R(a)) and disappearance (R(d)) were determined with a primed constant infusion of [6,6-(2)H]glucose, and total carbohydrate (CHO) and fat oxidation were determined with indirect calorimetry. At rest and during exercise at 70%LT, there were no differences in glucose R(a) or R(d) between phases. CHO and fat oxidation were not different between phases at 70%LT. At 90%LT, glucose R(a) (28.8 +/- 4.8 vs. 33.7 +/- 4.5 micromol. min(-1). kg(-1); P < 0.05) and R(d) (28.4 +/- 4.8 vs. 34.0 +/- 4.1 micromol. min(-1). kg(-1); P < 0.05) were lower during the L phase. In addition, at 90%LT, CHO oxidation was lower during the L compared with the F phase (82.0 +/- 12.3 vs. 93.8 +/- 9.7 micromol. min(-1) .kg(-1); P < 0.05). Conversely, total fat oxidation was greater during the L phase at 90%LT (7.46 +/- 1.01 vs. 6.05 +/- 0.89 micromol. min(-1). kg(-1); P < 0.05). Plasma lactate concentration was also lower during the L phase at 90%LT concentrations (2.48 +/- 0.41 vs. 3.08 +/- 0.39 mmol/l; P < 0.05). The lower CHO utilization during the L phase was associated with an elevated resting estradiol (P < 0.05). These results indicate that plasma glucose kinetics and CHO oxidation during moderate-intensity exercise are lower during the L compared with the F phase in women. These differences may have been due to differences in circulating estradiol.     

Oxygen diffusion in biological and artificial membranes determined by the fluorochrome pyrene

Quenching of pyrene fluorescence by oxygen was used to determine oxygen diffusion coefficients in phospholipid dispersions and erythrocyte plasma membranes. The fluorescence intensity and lifetime of pyrene in both artificial and natural membranes decreases about 80% in the presence of 1 atm O2, while the fluorescence excitation and emission spectra and the absorption spectrum are unaltered. Assuming the oxygen partition coefficient between membrane and aqueous phase to be 4.4, the diffusion coefficients for oxygen at 37 degrees C are 1.51 X 10(-5) cm2/s in dimyristoyl lecithin vesicles, 9.32 X 10(-6) cm2/s in dipalmitoyl lecithin vesicles, and 7.27 X 10(-6) cm2/s in erythrocyte plasma membranes. The heats of activation for oxygen diffusion are low (less than 3 kcal/degree-mol). A dramatic increase in the diffusion constant occurs at the phase transition of dimyristoyl and dipalmitoyl lecithin, which may result from an increase in either the oxygen diffusion coefficient, partition coefficient, or both. The significance of the change in oxygen diffusion below and above the phase transition for biological membranes is discussed.     

Process development and optimisation of lactic acid purification using electrodialysis

Cell free sodium lactate solutions were subjected to purification based on mono- and bi-polar electrodialysis. Lactate concentration in the product stream increased to a maximum of 15% during mono-polar electrodialysis. Stack energy consumption averaged 0.6 kW h kg(-1) lactate transported at current efficiencies in the 90% range. Under optimum feed concentration (125 g l(-1)) and process conditions (auto-current mode with conductivity setpoints of minimum 5 and maximum 40 mS cm(-1)), lactate flux reached 300 g m(-2) h(-1) and water flux were low for mono-polar electrodialysis averaging 0.3 kg H(2)O per M lactate transported. Glucose in the concentrate stream solutions was reduced to < 2 g l(-1). Acetate impurities enriched from about 0.5 g l(-1) in the feed stream to 1.5 g l(-1) in the concentrate stream solutions. After mono-polar electrodialysis, the concentrated sodium lactate solutions were further purified using bi-polar electrodialysis. Water transport during bi-polar electrodialysis reached figures of 0.070 - 0.222 kg H(2)O per M lactate. Free lactic acid concentration reached 16% with lactate flux of up to 300 g m(-2) h(-1). Stack energy consumption ranged from 0.6 to 1 kW h per kg lactate. Under optimised process conditions current efficiency during bi-polar electrodialysis was consistently around 90%. Glucose was further reduced from 2 to <1 g l(-1) in the free lactic acid solution. Acetic acid impurity remained at around 1 g l(-1). Significant reduction in colour and minerals in the product streams was observed during electrodialysis purification.     

Sublethal effects of copper on the freshwater crab Potamonautes warreni

The mass specific rates of oxygen consumption (M (O(2)) M(b)(-1)), ammonia excretion (M (NH(4)-N) M(b)(-1)) and carbon dioxide production (M (CO(2)) M(b)(-1)) were measured after 7, 14 and 21 days exposure of adult Potamonautes warreni to a sublethal concentration of 1.0 mg Cu l(-1) (15.75 micromol l(-1)). Under control (non-copper-exposed) conditions M (O(2)) M(b)(-1) was 35.7+/-8.5 micromol kg(-1)min(-1) (mean+/-S.D.), M (NH(4)-N) M(b)(-1) 2.92+/-0.26 micromol kg(-1)min(-1) and M (CO(2)) M(b)(-1) 25.6+/-9.0 micromol kg(-1)min(-1). The oxygen:nitrogen (O:N) ratio and respiratory quotient (RQ) were 24.5+/-3.0 and 0.80+/-0.06, respectively. M (O(2)) M(b)(-1) of copper-exposed crabs showed a significant increase after 7 and 14 days, but decreased significantly by 40% after 21 days. From the increased O:N ratio and RQ below 0.7, it is clear that crabs exposed to 1 mg Cul(-1) metabolize lipids during the entire 21-day exposure period. Free fatty acids in the midgut gland were determined by GC-MS, and showed increases of up to 600% in some C14 to C18 fatty acids. It is proposed that the excess lipids inhibit the pyruvate dehydrogenase complex, leading to the acceleration of the gluco- and glyco-neogenic pathways. Increased glyconeogenesis results in elevated glycogen concentrations in all tissues after 21 days. Experiments on acutely exposed P. warreni show increased incorporation of 14C-labelled lactate into glycogen.     

Phase and amplitude relations between transpiration, water potential and stem shrinkage

Abstract An electrical analogue describing the phase and amplitude relations between transpiration, water potential and stem shrinkage for trees was developed. Observations of shrinking and swelling at various heights up a Pinus radiata tree were obtained over several weeks in summer and autumn. The relative amplitude in shrinkage increased by a factor of two up the stem, but phase lags were small. The data obtained were used in conjunction with the electrical analogue of the How pathway to obtain an estimate of the relative magnitude of the capacitance of the living bark and the sapwood, and to predict phase lags between transpiration and leaf water potential, and lags in transpirational flux up the stem. The results suggest that if water potentials recover by dawn, phase lags in water potential down tree stems arc small and that the exchange between water stored in the sapwood and the transpiration stream is irreversible over the diurnal time scale.     

Anti-oxidant and pro-oxidant behaviour of bucillamine.

Bucillamine (BUC) is used clinically for the treatment of rheumatoid arthritis. Some of the pharmacological action of BUC has been reported as being dependent on the production of reactive oxygen species (ROS). In this paper the reactivity of BUC with superoxide anion radical (O(2) (*-)) generated from potassium superoxide/18-crown-6 ether dissolved in DMSO, hydroxyl radical (HO(*)) produced in the Cu(2+)-H(2)O(2) reaction, peroxyl radical (ROO(*)) from 2,2'-azobis (2-amidino-propane) dichloride decomposition, and singlet oxygen ((1)O(2)) from a mixture of alkaline aqueous H(2)O(2) and acetonitrile, have been investigated. Chemiluminescence, fluorescence, electron paramagnetic resonance (EPR) spin-trapping techniques and the deoxyribose and oxygen radical absorbance capacity towards ROO(*) (ORAC(ROO)) assays were used to elucidate the anti- and pro-oxidative behaviours of BUC towards ROS. The results indicated that BUC efficiently inhibited chemiluminescence from the O(2) (*-)-generating system at relatively high concentrations (0.5-2 mmol/L); however, at lower concentrations (<0.5 mmol/L) the drug enhanced light emission. The behaviour of BUC was correlated with a capacity to decrease the chemiluminescence signal from the Cu(2+)-H(2)O(2) system; scavenging HO(*) was effective only at high concentrations (1-2 mmol/L) of the drug. Bucillamine also prevented deoxyribose degradation induced by HO(*) in a dose-dependent manner, reaching maximal inhibition (24.5%) at a relative high concentration (1.54 mmol/L). Moreover, BUC reacts with ROO(*); the relative ORAC(ROO) was found to be 0.34 micromol/L Trolox equivalents/micromol sample. The drug showed quenching of (1)O(2)-dependent 2,2,6,6-tetramethylpiperidine-N-oxide radical formation from 2,2,6,6-tetramethyl-piperidine (e.g. 90% inhibition was found at 1 mmol/L concentration). The results showed that BUC may directly scavenge ROS or inhibit reactions generating them. However, the drug may have pro-oxidant activity under some reaction conditions.     

Apamin-sensitive nitric oxide- and ATP-mediated motor effects on the guinea pig small intestine

The involvement of nitric oxide and ATP in both spontaneous and electrically-induced nonadrenergic noncholinergic (NANC) motor activity with special interest in the apamin-sensitive mechanisms was studied in a guinea pig ileum model. Depending on the concentration (0.1 or 1 micromol/l), apamin, a blocker of the calcium-activated potassium channels and antagonist of ATP action, induced either TTX (0.1 micromol/l)-resistant increase in tone or contractions. SNP, a nitric oxide donor, applied cumulatively (0.1-100 micromol/l) evoked a concentration-dependent relaxation, the EC50 value being 0.39 +/- 0.12 micromol/l. At concentrations of 0.1 or 1 micromol/l, apamin decreased the SNP effects and shifted the concentration-response curves for SNP to the right. The EC50 value for SNP in the presence of apamin at a concentration of 0.1 micromol/l increased to 59.34 +/- 36.53 micromol/l. ATP (1 or 50 micromol/l) induced TTX-resistant contractions. The effects of ATP were reduced by apamin (1 micromol/l). The contractile effect of ATP occurred in the presence of SNP. SNP provoked relaxation on the background of ATP. The NANC responses to electrical stimulation (0.8 ms, 40 V, 2 or 20 Hz, 20 s) consisted of an initial relaxation phase followed by a phase of contractions, twitch-like and tonic. L-NNA (0.5 mmol/l), an inhibitor of nitric oxide syntheses, abolished the relaxation phase. L-arginine (0.5 mmol/l) restored it. Apamin (0.1 or 1 micromol/l) completely eliminated the relaxation phase and concentration-dependently inhibited the tonic contraction of the phase of contractions. The present findings indicate that the apamin-sensitive nitric oxide-evoked relaxation could be realized by calcium-activated potassium channels and that the apamin-sensitive ATP-induced contraction is mediated via contraction-producing P2 purinoceptors.     

Gas diffusion through wood: implications for oxygen supply

Living tissue in tree stems has to be supplied with oxygen, which can be transported upwards with the transpiration stream; but in times of zero sapflow, the only source is the oxygen stored or diffusing radially through bark and xylem. We measured radial and axial diffusion of oxygen against nitrogen gas in wood of coniferous (Picea abies (L.) Karst. and Taxus baccata L.), ring-porous (Quercus robur L. and Fraxinus excelsior L.) and diffuse-porous (Fagus sylvatica L. and Carpinus betulus L.) trees at different water and gas contents in the laboratory. The diffusion coefficient (D) in radial direction was mostly between 10⁻¹¹ and 10⁻⁷ m² s⁻¹ and was strongly related to the gas content. At 40% gas volume, D increased 5–13-fold in Picea, Taxus and Quercus, 36-fold in Fraxinus, and about 1000-fold in Carpinus and Fagus relative to D at 15% gas volume. In the axial direction, diffusion was 1 or 2 orders of magnitude faster. Between-species differences in diffusion velocities can largely be explained by wood structure. In general, D was lowest in conifers, highest in diffuse-porous and intermediate in ring-porous hardwoods, where the large vessels were mostly blocked by tyloses. Model calculations showed that at very high water content, radial diffusion can be too low to ensure the supply of respiring sapwood with sufficient oxygen and an important function of gas in living stems appears to be the supply of oxygen through storage and diffusion.     

Internal oxygen levels decrease during the growing season and with increasing stem height

The oxygen levels in the sapwood of Norway spruce [Picea abies Karst. (L.)] trees were measured by GC-MS (gas chromatography – mass spectrometry) as stem growth progressed during the growing season. The measurements were made on 30-year-old, 13–15 m high, trees growing on three plots which were irrigated, left undisturbed or subjected to drought. The oxygen measurements were done at 1.5, 4 and 8 m above the ground. The internal oxygen levels dropped to about 5% of the level in air in irrigated trees, 1–3% in control trees and to below 1% in trees subject to drought between mid-July and mid-August. In the trees subjected to drought there where no differences in internal oxygen concentrations between the positions on the tree during the growing season except for a faster increase back to ambient concentration occurring in late August in the 1.5 m position. In trees growing on the two other plots the internal oxygen concentration was higher near the ground than at 4 or 8 m above ground during July and August. At all other dates there were no differences between the positions. The results indicate that there is an oxygen gradient with levels decreasing acropetally, giving indirect support to the hypothesis that oxygen is supplied to the growing stem in the transpiration stream. Received: 26 January 1999 / Accepted: 1 June 1999     

Effect of low oxygen concentrations on growth and alpha-amylase production of Aspergillus oryzae in model solid-state fermentation systems

Oxygen transfer in the fungal mat is a major concern in solid-state fermentation (SSF). Oxygen supply into the mycelial layers is hampered by diffusion limitation. For aerobic fungi, like Aspergillus oryzae, this oxygen depletion can be a severely limiting factor for growth and metabolite production. This paper describes the effects of a low oxygen concentration on growth at the levels of individual hyphae, colonies and overcultures, and on alpha-amylase production in overcultures. PDA medium was used to study the effect of a low oxygen concentration on hyphal elongation rate and branching frequency of hyphae, and radial extension rate of colonies of A. oryzae. We found similar saturation constants (K(O2)) of 0.1% (v/v in the gas phase) for oxygen concentration described with Monod kinetics, for branching frequency of hyphae and colony extension rate. When A. oryzae was grown as an over-culture on wheat-flour model substrate at 0.25% (v/v) oxygen concentration, the reduction in growth was more pronounced than as individual hyphae and a colony on PDA medium. Experimental results also showed that the specific alpha-amylase production rate under the condition of 0.25% (v/v) oxygen was reduced. Because the value of K(O2) is relatively low, it is reasonable to simplify the kinetics of growth of A. oryzae to zero-order kinetics in coupled diffusion/reaction models.     

Effect of ajmaline on action potential and ionic currents in rat ventricular myocytes

The effect of ajmaline on action potential (AP) and ionic current components has been investigated in right ventricular myocytes of rat at room temperature using the whole cell patch clamp technique. Ajmaline decreased the upstroke velocity ((dV/dt)max) of AP and the AP amplitude, increased the AP duration measured at 50 and 90% repolarization, and reversibly inhibited most components of membrane ionic current in a concentration-dependent manner. The following values of IC50 and of the Hill coefficient (nH) resulted from approximation of the measured data by the Hill formula: for fast sodium current (INa) IC50=27.8+/-1.14 micromol/l and nH=1.27+/-0.25 at holding potential -75 mV, IC50=47.2+/-1.16 micromol/l and nH=1.16+/-0.21 at holding potential -120 mV; for L-type calcium current (ICa-L) IC50=70.8+/-0.09 micromol/l and n(H)=0.99+/-0.09; for transient outward potassium current (Ito) IC50=25.9+/-2.91 micromol/l and nH=1.07+/-0.15; for ATP-sensitive potassium current (IK(ATP)) IC50=13.3+/-1.1 micromol/l and nH=1.16+/-0.15. The current measured at the end of 300 ms depolarizing impulse was composed of an ajmaline-insensitive component and a component inhibited with IC50=61.0+/-1.1 micromol/l and nH=0.91+/-0.08. At hyperpolarizing voltages, ajmaline at high concentration of 300 micromol/l reduced the inward moiety of time-independent potassium current (IK1) by 36%. The results indicate that the inhibition of INa causes both the decreased rate of rise of depolarizing phase and the lowered amplitude of AP. The inhibition of Ito is responsible for the ajmaline-induced AP prolongation.     

Myocardial oxygen consumption regulation in isolated mouse heart: assessment by intracoronary administration of exogenous nitric oxide

In our previous work we have shown that in mouse heart basal level of endothelial produced nitrite, as a marker of nitric oxide (NO) formation, was 9.7 nmol l(-1). Bradykinin (10 microl l(-1)) induced a 5-fold rise in nitrite release, the coronary venous effluent concentration being 58 nmol l(-1), but there was no effect on myocardial oxygen consumption (MVO2). The aim of this study was to assess the levels of authentic nitric oxide solution, exogenously applied, on myocardial oxygen consumption. Isolated mouse hearts (n=36) were paced (500 imp./min) and perfused at constant flow (16.0 +/- 0.3 ml g(-1) min(-1)). When coronary vasculature resistance was carefully controlled by adenosine (1 micromol l(-1)), authentic nitric oxide solution, in a concentration less than 5 micromol l(-1) did not alter myocardial oxygen consumption. Only concentrations of nitric oxide higher than 5 micromol l(-1) induced reduction in myocardial oxygen consumption. Thus in the saline perfused mouse heart, with carefully controlled vasodilatation, modulating myocardial nitric oxide levels using an arterial application of authentic nitric oxide, concentrations higher than 5 micromol l(-1) of nitric oxide were required to induce a decrease in myocardial oxygen consumption.     

Human muscle sarcoplasmic reticulum function during submaximal exercise in normoxia and hypoxia.

In this study, the response of the sarcoplasmic reticulum (SR) to prolonged exercise, performed in normoxia (inspired O(2) fraction = 0.21) and hypoxia (inspired O(2) fraction = 0.14) was studied in homogenates prepared from the vastus lateralis muscle in 10 untrained men (peak O(2) consumption = 3.09 +/- 0.25 l/min). In normoxia, performed at 48 +/- 2.2% peak O(2) consumption, maximal Ca(2+)-dependent ATPase activity was reduced by approximately 25% at 30 min of exercise compared with rest (168 +/- 10 vs. 126 +/- 8 micromol.g protein(-1) x min(-1)), with no further reductions observed at 90 min (129 +/- 6 micromol x g protein(-1) x min(-1)). No changes were observed in the Hill coefficient or in the Ca(2+) concentration at half-maximal activity. The reduction in maximal Ca(2+)-dependent ATPase activity at 30 min of exercise was accompanied by oxalate-dependent reductions (P < 0.05) in Ca(2+) uptake by approximately 20% (370 +/- 22 vs. 298 +/- 25 micromol x g protein(-1) x min(-1)). Ca(2+) release, induced by 4-chloro-m-cresol and assessed into fast and slow phases, was decreased (P < 0.05) by approximately 16 and approximately 32%, respectively, by 90 min of exercise. No differences were found between normoxia and hypoxia for any of the SR properties examined. It is concluded that the disturbances induced in SR Ca(2+) cycling with prolonged moderate-intensity exercise in human muscle during normoxia are not modified when the exercise is performed in hypoxia.     

Phytoextraction: simulating uptake and translocation of arsenic in a soil-plant system

The uptake, transport, and accumulation of metals by plants are functions central to successful phytoextraction. This study investigates the uptake and translocation of arsenic from a contaminated sandy soil by a mature Chinese brake fern (Pteris vittata L.). An existing mathematical model for the coupled transport of water, heat, and solutes in the soil-plant-atmosphere continuum (CTSPAC) was modified to examine the flow of water as well as the uptake and translocation of total arsenic in the xylem of the fern. This model was calibrated using greenhouse measurements before its application. Simulation results showed that about 20% of the soil arsenic was removed by the fern in 10 d, of which about 90% of the arsenic was stored in the fronds and 10% in the roots. Although arsenic mass in the plant tissues increased consecutively with time, arsenic concentration in the xylem sap of the root tips has a typical diurnal distribution pattern: increasing during the day and decreasing at night, resulting from daily variations of frond surface water transpiration. The largest difference in simulated arsenic concentration in the root tips between the day and night was about 5%. This study also suggests that the use of transpiration stream concentration factor (TSCF), which is defined as the ratio of chemical concentration in the xylem sap to that in the external solution, to evaluate the translocation efficiency of arsenic for the hyperaccumulator Chinese brake fern (Pteris vittata L.) could be limited.     

A comparison of three models for the diffusion of oxygen in electrolyte solutions

Diffusion of oxygen through aqueous solutions is of great importance in biological systems. In this work, three models for the diffusion of oxygen through aqueous salt solutions are compared. One model uses mole fraction as the driving force (Fick's Law) and another uses chemical potential. The third model uses the gradient in oxygen activity as the driving force. This new model was chosen because of the availability of oxygen electrodes which directly measure oxygen activity in aqueous solution. These models have been used to reevaluate the technique of measuring O(2) diffusivities. We show that Pick's Law diffusion coefficients do not vary strongly with salt concentration as was erroneously reported in the literature. In addition, we compare the predicted O(2) fluxes of the three models over a wide range in O(2) concentrations. For oxygen concentrations of biological interest, the three models give identical predictions of the flux.     

Mechanisms of iron-induced oxidative modifications of creatine kinase in rat brain in vitro. possible involvement of HNE

The model of oxidative stress induced by Fe/ascorbate in rat brain in vitro was used to compare the antioxidant capacity of known antioxidants. Creatine kinase (CK) was selected as a marker of protein injury in such studies. Of the antioxidant enzymes (catalase, superoxide dismutase), oxygen radical scavengers (mannitol, glutathione), and the chelator (EDTA) tested in this work and this system, only catalase and glutathione prevented the injury induced by oxidative stress, indicating that H2O2 and the glutathione peroxidase reaction were involved in the preventive effect. Additionally, the preventive effect of glutathione may be caused also by the fact that glutathione easily reacts with 4-hydroxynonenal (HNE), generated in rat brain homogenate, thus protecting CK from inactivation by this aldehyde. To find out whether and if at which concentrations CK may be oxidatively modified by HNE, pure CK was incubated in the presence of 10 and 64 micromol/l HNE for 30 min at 37 degrees C. The activity of CK incubated with HNE decreased significantly. Simultaneously, the protein carbonyls, determined by electrophoresis and immunoblotting increased at 10 micromol/l HNE or disappeared probably due to crosslinking of CK at 64 micromol/l HNE. The concentration of HNE in rat brain homogenates after oxidative stress was determined by HPLC and was in the range of 10-16 nmol/mg prot., corresponding to a concentration of 10-16 micromol/l HNE. This indicates that CK of rat brain homogenates oxidized by Fe/ascorbate may be impaired not only directly by oxygen radicals but also secondarily by HNE.