Active ion uptake and maintenance of cation-anion balance: A critical examination of their role in regulating rhizosphere pH

The processes responsible for maintenance of cation-anion balance in plants and their relation to active ion accumulation and changes in rhizosphere pH are outlined and discussed. The major processes involved are: (1) accumulation and degradation of organic acids which occur in the plant mainly as organic acid anions (and their transfer within the plant) and (2) extrusion of H⁺ or OH^– into the rhizosphere. The relative importance of the two processes is determined by the size of the excess anion or cation uptake. Indeed, plants typically absorb unequal quantities of nutritive cations (NH₄ ⁺+Ca²⁺+ Mg²⁺+K⁺+Na⁺) and anions (NO₃ ^–+Cl^–+SO₄ ^2–+H₂PO₄ ^–) and charge balance is maintained by excretion of an amount of H⁺ or OH^– which is stoichiometrically equal to the respective excess cation or anion uptake. The mechanisms and processes by which H⁺ and in particular OH^– ions are excreted in response to unequal cation-anion uptake are, however, poorly understood.The contemporary view is that primary active extrusion of H⁺, catalyzed by a membrane-located ATPase, is the major driving force for secondary transport of cations and anions across the plasma membrane. However, the fact that net OH^– extrusion often occurs (since excess anion absorption commonly takes place) implies there is a yet-to-be characterized OH^– ion efflux mechanism at the plasma membrane that is associated with anion uptake. There is, therefore, a need for future studies of the uptake mechanisms and stoichiometry of anion uptake; particularly that of NO₃ ^– which is often the predominant anion absorbed. Another related phenonenon which requires detailed study in terms of cation-anion balance is localized rhizosphere acidification which can occur in response to deficiencies of Fe and P.     

Flux of SO(2) into Leaf Cells and Cellular Acidification by SO(2)

A comparison of fluxes of SO₂ from the atmosphere into leaves with fluxes across biomembranes revealed that, apart from the cuticle, the main barrier to SO₂ entry into leaves are the stomates. SO₂ fluxes into leaves can be calculated with an accuracy sufficient for many purposes on the assumption that the intracellular SO₂ concentration is zero. SO₂ entering green leaf cells is trapped in the cytoplasm. In the light, the products formed in its reaction with water are processed particularly in the chloroplasts. Flux of SO₂ to the acidic central vacuole of leaf cells is insignificant. Intracellular acidification of barley mesophyll protoplasts by SO₂ was measured by the uptake of ¹⁴C-labeled 5,5-dimethyl-oxazolidine-2,4-dione. The measured acidification was similar to the acidification calculated from known buffer capacities and the rate of SO₂ influx when the H⁺/SO₂ ratio was assumed to be 2. A comparison of photosynthesis inhibition by SO₂ with calculated acidification revealed different mechanisms of inhibition at low and at high concentrations of SO₂. At very low concentrations, inhibition by SO₂ was even smaller than expected from calculated acidification. The data suggest that, if acidification cannot be compensated by pH-stabilizing cellular mechanisms, it is a main factor of SO₂ toxicity at low SO₂ levels. At high levels of SO₂, anion toxicity and/or radical formation during oxidation of SO₂ to sulfate may play a large role in inhibition.     

Influence of anions on the potassium status and productivity of kiwifruit (Actinidia deliciosa) vines

The effects of K fertiliser (160 kg ha^-1) applied with Cl^- or SO₄ ^2- as the accompanying anion on the K nutrition of kiwifruit (Actinidia deliciosa var. deliciosa) were assessed in a field experiment, using vines with varying degrees of K deficiency. Leaf K concentrations in spring were significantly higher for vines receiving KCl, compared to those receiving K₂SO₄. This effect did not interact significantly with the degree of K deficiency, and persisted for about 6 weeks. Subsequently there was no significant difference between the leaf K concentrations for the vines receiving KCl or K₂SO₄. Applying K as KCl increased the leaf Cl concentration, especially in spring, while applying K as K₂SO₄ had no significant effect on the leaf S concentration at that time. These results implied a greater requirement for organic acid anions for K⁺ uptake from K₂SO₄ than from KCl, and the importance of organic acid anions for K⁺ uptake from different sources of K fertiliser is discussed. This transient effect of the accompanying anion on leaf K status was associated with large effects on flowering, and fruit yields were about 28% higher for plants receiving KCl rather than K₂SO₄.The effects on growth and tissue nutrient composition of varying the concentrations of Cl^-, NO₃ ^-, SO₄ ^2- and H₂PO₄ ^- around the roots of kiwifruit vines were examined in a solution culture experiment. For H₂PO₄ ^-, plant growth was very similar over a wide range of rates of addition. For the other anions, the range between deficiency and toxicity was clearly delineated. For Cl^- and NO₃ ^-, toxicity was associated with high tissue concentrations of Cl and N, respectively, and was consistent with competition for uptake between Cl^- and NO₃ ^-. However, for SO₄ ^2-, toxicity was associated with only a small increase in the tissue S concentration relative to that associated with maximum growth, and appeared to result more from effects on uptake of other anions and cations rather than from direct effects of high tissue S concentrations.It is concluded that the sensitivity of kiwifruit to the anion accompanying K⁺ in fertiliser may be related to the unusually high requirement for Cl previously reported for this species.     

Influence of Nitrate and Ammonium Nutrition on the Uptake, Assimilation, and Distribution of Nutrients in Ricinus communis

Ricinus communis L. plants were grown in nutrient solutions in which N was supplied as NO₃⁻ or NH₄⁺, the solutions being maintained at pH 5.5. In NO₃⁻-fed plants excess nutrient anion over cation uptake was equivalent to net OH⁻ efflux, and the total charge from NO₃⁻ and SO₄²⁻ reduction equated to the sum of organic anion accumulation plus net OH⁻ efflux. In NH₄⁺-fed plants a large H⁺ efflux was recorded in close agreement with excess cation over anion uptake. This H⁺ efflux equated to the sum of net cation (NH₄⁺ minus SO₄²⁻) assimilation plus organic anion accumulation. In vivo nitrate reductase assays revealed that the roots may have the capacity to reduce just under half of the total NO₃⁻ that is taken up and reduced in NO₃⁻-fed plants. Organic anion concentration in these plants was much higher in the shoots than in the roots. In NH₄⁺-fed plants absorbed NH₄⁺ was almost exclusively assimilated in the roots. These plants were considerably lower in organic anions than NO₃⁻-fed plants, but had equal concentrations in shoots and roots. Xylem and phloem saps were collected from plants exposed to both N sources and analyzed for all major contributing ionic and nitrogenous compounds. The results obtained were used to assist in interpreting the ion uptake, assimilation, and accumulation data in terms of shoot/root pH regulation and cycling of nutrients.     

Role of organic acids in the formation of the ionic composition in developing glycophyte leaves

The ionic composition in the leaves of some glycophyte plants (Phaseolus vulgaris L., Lycopersicon esculentum L., and Amaranthus cruentus L.) was studied during leaf development. Plants were grown in a stationary hydroponic culture; a growth medium contained equimolar concentrations of inorganic ions (NO ₃ ^? , Cl^?, SO ₄ ^2? , H₂PO ₄ ^? , K⁺, Ca²⁺, Mg²⁺, and Na⁺) equal to 5 mg-equiv./l for each ion. In the juvenile leaf, the main ions were K⁺ and water-soluble anions of organic acids represented mainly by di-and tricarboxylic acids in kidney bean and tomato and oxalic acid in amaranth. An increase in the total amount of organic anions, coinciding with the accumulation of bivalent cations, was registered in leaves of glycophytes during their development. Mature and senescing leaves of tomato and kidney bean accumulated mainly di-and tricarboxylic acid salts with the prevalence of Ca²⁺ ions. In amaranth leaves, the formation of water-insoluble (acid-soluble) oxalate pool containing Ca²⁺ ions (mature leaves) or Ca²⁺ and Mg²⁺ ions (senescing leaves) was registered. The priority role of the metabolism of organic acids in the formation of the ionic composition of glycophyte leaves during their development is discussed. It is supposed that the species-specific ionic composition of glycophyte leaves at different developmental stages is due mainly to the pattern of carbon metabolism causing the accumulation either of di-and tricarboxylic acids or oxalic acid.     

Formation of Hydroxysulphate Green Rust 2 as a Single Iron(II-III) Mineral in Microbial Culture

Although GR2(SO₄ ^2-) can be easily formed by abiotic synthesis, the biotic formation of hydroxysulphate as a single iron(II-III) mineral in microbial culture and its characterization was not achieved. This study was carried out to investigate the sole formation of GR2(SO₄ ^2-) during the reduction of γ-FeOOH by a dissimilatory iron-respiring bacterium, Shewanella putrefaciens CIP 8040^T. Reduction experiments were performed in a non-buffered medium devoid of organic compounds, with 25 mM of sulphate and with a range of lepidocrocite concentrations with H₂ as the electron donor under nongrowth conditions. The resulting biogenic solids, after iron-respiring activity, were characterized by X-ray diffraction (XRD), transmission Mössbauer spectroscopy (TMS) and electron microscopy (SEM and TEM). The sulphate has been identified as the intercalated anion by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). In addition, the structure of this sulphate anion was discussed. Our experimental study demonstrated that, under H₂ atmosphere, the biogenic solid was a GR2(SO₄ ^2-), as the sole iron(II-III) bearing mineral, whatever the initial lepidocrocite concentration. The crystals of the biotically formed GR2(SO₄ ^2-) are significantly larger than those observed for GR2(SO₄ ^2-) obtained through abiotic preparation, < 15 μ m diameter as against 0.5–4 μm, respectively.     

Acid-Base Regulation by Azolla spp. with N2 as Sole N Source and with Supplementation by NH+4 or NO−3

Diazotrophic cultures of three species of Azolla (Az. caroliniana, Az. microphylla, Az. pinnata) symbiotic with Anabaena azollae accumulated some 0.10–0.24 mol organic anion per mol N assimilated (0.010–0.018 mol organic anion per mol C assimilated), with a corresponding efflux of 0.05–0.11 mol H⁺ per mol N assimilated (0.006–0.009 mol H⁺ per mol C assimilated). These values are lower than those found for terrestrial diazotrophic vascular plants; this may be related to the decreased possibility of increasing Fe and P availability by rhizosphere acidification in a free-floating plant. Modification of the organic anion content, and of the quantity (and direction) of H⁺ exchange with the medium, with 5 mol m^?3 NH⁺₄ or NO^?₃ added to diazotrophic cultures, are consistent with substantial N acquisition from combined N as well as N₂ assimilation. This conclusion is consistent with previously published work with ¹⁵N.     

Modelling anion amelioration of aluminium phytotoxicity

There is good evidence for the ameliorating effect of SO₄ ^2- and F^- on the expression of Al phytotoxicity in acidic solutions. The role of OH^-, in both shifting Al speciation towards hydroxy-Al species and decreasing activities of H⁺ with increasing pH, is still controversially discussed. Grauer and Horst (1992) proposed a model based on the assumption that Al phytotoxicity is a function of the Al saturation (AlS) of exchange sites in the root apoplast and analyzed the predictions of the model in the case of cation amelioration, with special emphasis on H⁺. In this study the model is further developed by considering, in addition to Al³⁺, the complexation of Al with the anions OH^-, F^-, SO₄ ^2-, and Cl^- to form potentially toxic Al species. Association constants of these Al complexes with a ligand (L ^-) which is assumed to simulate the cation exchange sites in the root apoplast, were estimated. Affinity factors for binding to L ^- compared to Al³⁺ were derived from these estimated association constants, and values were, in a first approach, 0.79 for AlOH²⁺, 0.02 for Al(OH)₂ ⁺, and 0.13 for Al(OH)₃ ⁰ (or 0.03 choosing another hydrolysis constant). High toxicity of Al₁₃ (AlO₄Al₁₂(OH)₂₄(H₂O)₁₂ ⁷⁺) could be explained by diminished H⁺ amelioration and a high association constant to L ^-. From estimated association constants for Al-Cl complexes, low affinity factors for L ^- for these complexes were derived. Since the formation of these Al-Cl complexes is not favoured, Cl^- is predicted to have very little ameliorating effect. In the case of SO₄ ^2– and F^- complexes with Al, the derived affinity factors never exceeded 0.05 and, since formation of these complexes is favoured by high association constants, are thus in agreement with experimental results on ameliorating effects. The ranking of the anions for ameliorative effectiveness was estimated to be in the order of OH^->F^->SO₄ ^2–>Cl^-. Hydroxy amelioration in this context is restricted to the speciation effect, which is only significant above pH 4.     

Nutrient budgets in a dry heathland watershed in northeastern Spain

Dissolved nutrient inputs in bulk precipitation and outputs in streamwater were measured during 3 years of contrasting hydrological conditions in a 6.3-ha, grazed heathland watershed on schists in the Montseny mountains (NE Spain), drained by an intermittent stream. On average, 39% of the precipitation became streamflow. Bulk precipitation delivered positive net alkalinity (mean 0.22 keq/ha/yr), sulphate input was moderate (9.0 kg SO₄-S/ha/yr), and the mean input of inorganic N was not exceptionally high (6.6 kg/ha/yr). Ion concentrations were relatively low in streamwater; SO₄ ^2- was the dominant anion. Most concentrations in streamwater varied seasonally, with maxima in late summer or early autumn and minima in spring. This pattern probably resulted from increased availability of ions for leaching due to decomposition of organic matter and chemical weathering during the warm period. Nitrate concentrations were relatively high in winter and dropped sharply in early spring, probably because of biological uptake. Annual element outputs in streamwater varied between years and seemed to be controlled by both the amount of annual streamflow and its seasonal distribution. Annual inputs exceeded outputs for dissolved inorganic N. The watershed accumulated H⁺ and Ca²⁺, had net losses of Na⁺ and Mg²⁺, and was close to steady state for K⁺, SO₄ ^2-, Cl^- and alkalinity. The chloride budgets gave no evidence of substantial dry deposition in this system. The cationic denudation rate was negative (-0.14 keq/ha/yr) because Ca²⁺ retention was higher than net exports of Na⁺ and Mg²⁺ from silicate weathering. Low nutrient export and little production of alkalinity suggest that this watershed has a low buffering capacity.     

Detoxification of SO2 in conifers differing in SO2-tolerance

Contents of organic sulfur, sulfate and the inorganic cations K⁺, Ca²⁺, Mg²⁺, Mn²⁺ and Na⁺ were compared in needles of three conifer species differing in tolerance to chronic SO₂ immissions. Sulfate and organic sulfur compounds were also measured in bark and wood. Field material was collected from Norway Spruce (Picea abies (L.) Karst.), Colorado Spruce (Picea pungens Engelm.) and Scots Pine (Pinus sylvestris L.) at sites where the SO₂ concentration in air was high, and at another site where it was low. In general, sulfate contents were higher but cation contents lower at the sites where SO₂ concentrations were high than where they were low. Up to 114mmol · (kg DW)^–1 sulfate was measured in fouryear-old needles of Norway Spruce from the Erzgebirge (annual mean of SO₂ in air 32 nl · 1^–1). Sulfate accumulation in this SO₂-sensitive conifer increased with SO₂ concentration in ambient air and with needle age, indicating that the main part of the sulfate resulted from the oxidative detoxification of SO₂. Loss of inorganic cations from ageing needles was reduced, or cation levels even increased, with increasing needle age, while sulfate accumulated. Apparently, cations served as counter-ions for sulfate, which is sequestered in the vacuoles. Individual trees differed in regard to the nature of cations which accumulated with sulfate. Calcium, potassium and magnesium were the dominating cations. Sodium levels were very low. Needles of the SO₂-tolerant conifers Colorado Spruce and Scots Pine growing next to Norway Spruce in the Erzgebirge did not accumulate, or accumulated less, sulfate with increasing needle age as compared to needles of Norway Spruce. However, somewhat more sulfate was found in the bark of the SO₂-tolerant species than in the bark of Norway Spruce. Scots Pine contained distinctly more sulfate in the wood than the other conifers. Since accumulation of organic sulfur compounds could not be observed with increasing needle age, or in bark and wood, reduction does not appear to play a major role in the detoxification of SO₂ by the investigated species. Physiological mechanisms permitting Colorado Spruce and Scots Pine to avoid the sulfate accumulation in the needles and the accompanying sequestration of cations that are observed in neighbouring Norway Spruce are discussed on the basis of the obtained data.Abbreviations S_org organic sulfur compounds Died June 10, 1991, aged 29, in a traffic accident. He initiated this work.This work was supported by the Sonderforschungsbereich 251 of the University of Würzburg and by the Projektgruppe Bayern zur Erforschung der Wirkung von Umweltschadstoffen (PBWU). The authors with to thank Prof. Dr. W Kaiser and Prof. Dr. W. Urbach (both Julius-von-Sachs-Institut, University of Würzburg, Germany) for HPLC-analysis and ICP-analysis.     

The Uptake of Free CO2 and HCO3- during Photosynthesis of Plankton Algae with Special Reference to the Coccolithophorid Coccolithus huxleyi

From a re-evaluation of experiments with the coccolithophorid Coccolithus hurleyi made by Paasche (1964), curves are presented showing the rate of photosynthesis as a function of the concentration of both free CO₂ and bicarbonate CO₂. It is shown that photosynthesis in a naked clone is due only to the uptake of free CO₂. The problem concerning the high concentration of free CO₂ necessary for photosynthesis in Coccolithus huxleyi is discussed. It is shown that it is not due to lack of the enzyme carbonic anhydrase. Hence it is probable that the formation of coccoliths is the mechanism by which, in Coccolithus, the utilization of HCO₃^- ions in photosynthesis becomes possible. The OH^- ions produced during photosynthesis (₊Ca²⁺ taken up together with the HCO₃^- ions) are thus neutralized. This situation is different from other aquatics utilizing HCO₃^- in photosynthesis. Here the OH^- ions are neutralized via an ion exchange of Ca²⁺ with H⁺ in the surrounding medium.     

Influence of the level of nitrate nutrition on ion uptake and assimilation, organic Acid accumulation, and cation-anion balance in whole tomato plants

Tomato plants (Lycopersicon esculentum L. var. Ailsa Craig) were grown in water culture in nutrient solution in a series of 10 increasing levels of nitrate nutrition. Using whole plant data derived from analytical and yield data of individual plant parts, the fate of anion charge arising from increased NO₃ assimilation was followed in its distribution between organic anion accumulation in the plant and OH⁻ efflux into the nutrient solution as calculated by excess anion over cation uptake. With increasing NO₃ nutrition the bulk of the anion charge appeared as organic anion accumulation in the plants. OH⁻ efflux at a maximum accounted for only 20% of the anion charge shift. The major organic anion accumulated in response to nitrate assimilation was malate. The increase in organic anion accumulation was paralleled by an increase in cation concentration (K⁺, Ca²⁺, Mg²⁺, Na⁺). Total inorganic anion levels (NO₃⁻, SO₄²⁻, H₂PO₄⁻, Cl⁻) were relatively constant. The effect of increasing NO₃ nutrition in stimulating organic anion accumulation was much more pronounced in the tops than in the roots.     

Comparative effects of salt and alkali stresses on growth, osmotic adjustment and ionic balance of an alkali-resistant halophyte Suaeda glauca (Bge.)

Effects of salt and alkali stresses on growth, osmotic adjustment and ionic balance of Suaeda glauca (Bge.), an alkali-resistant succulent halophyte, were compared. The results showed that alkali stress clearly inhibited the growth of S. glauca. Moreover, the concentrations of Na⁺ and K⁺ both increased with increasing salinity under both stresses, suggesting no competitive inhibition between absorptions of Na⁺ and K⁺. The mechanism underlying osmotic adjustment during salt stress was similar to alkali stress in shoots. The shared essential features were that organic acids, betaine and inorganic ions (dominated by Na⁺) mostly accumulated. On the other hand, the mechanisms governing ionic balance under both stresses were different. Under salt stress, S. glauca accumulated organic acids and inorganic anions to maintain the intracellular ionic equilibrium, but the anion contribution of inorganic ions was greater than that of organic acids. However, the concentrations of inorganic anions under alkali stress were significantly lower than those under salt stress of the same intensity, suggesting that alkali stress might inhibit uptake of anions, such as NO₃ ^– and H₂PO₄ ^–. Under alkali stress, organic acids were the dominant factor in maintaining ionic equilibrium. The contribution of organic acids to anions was 74.1%, while that of inorganic anions was only 25.9%. S. glauca enhanced the synthesis of organic acids, dominated by oxalic acid, to compensate for the shortage of inorganic anions.     

H+/OH- Excretion and Nutrient Uptake in Upper and Lower Parts of Lupin (Lupinus angustifolius L.) Root Systems

The cultivation of narrow-leafed lupins (Lupinus angustifoliusL.) increase rates of subsoil acidification, and this is thoughtto be partly related to their pattern of nutrient uptake andH⁺/OH^- excretion. The main hypothesis of this study was thatH⁺ and OH^- excretion is not distributed evenly over the entirelength of the root system but is limited to zones where excesscation or anion uptake occur. Seedlings of nodulated lupinswere grown in solution culture using vertically split pots thatallowed the upper and lower zones of the root system to be suppliedwith varying concentrations of K⁺ and NO^-₃. Net H⁺/OH^- excretionwas equated to the addition of NaOH/HCl required to maintaina constant pH in the nutrient solution during a 4-d treatmentperiod and nutrient uptake was measured by depletion from solutionin each zone of the split pots. The excess of cation over anion uptake was positively correlatedwith H⁺ excretion in each rooting zone. In zones where K⁺ wassupplied at 1200 µM, cation uptake was dominated by K⁺and up to twice as much H⁺ was excreted than in zones whereK⁺ was absent. In zones where NO^-₃ was supplied at 750 µM,the anion/cation uptake was balanced, however H⁺ excretion continuedto occur in the zone. When NO^-₃ was supplied at 5000 µM,anion uptake exceeded cation uptake but there was no OH^- excretion.Organic acid anions may be excreted by lupins to maintain theirinternal electroneutrality when anion uptake exceeds cationuptake. Rhizosphere pH would not increase unless the pK_a ofthe excreted organic anions was greater than the external pH.Copyright1993, 1999 Academic Press Lupinus angustifolius L., H⁺/OH^- excretion, nutrient uptake, cation-anion balance, vertical split root     

An estimation of the proton conductance of the inner membrane of turnip (Brassica napus L.) mitochondria

The permeability of the inner membrane of turnip mitochondria to H⁺ and OH^- ions has been investigated using an acid pulse technique. The rate of decay of a H⁺ pulse across the inner membrane is exponential having first-order kinetics and gives t 1/2 values of approx 54 s at neutral pH and at 25° C. Valinomycin or 1799 alone have little effect on t 1/2 values, whereas in combination, values of <15 s are observed. Nigericin produces a similar effect. The effective proton conductance of the inner membrane near pH 7 at 25° C is 0.27 nmol H⁺ min^-1 mg protein^-1 mV^-1. The results suggest that at neutral pH, the inner membrane of plant mitochondria is relatively impermeable to H⁺ and OH^- ions.     

Effect of monovalent ion adsorption on the electric charge of phosphatidylcholine - decylamine liposomal membranes

We examined the effect of adsorbed monovalent ions on the surface charge of phosphatidylcholine (PC) – decylamine (DA) liposomal membranes. Surface charge density values were determined from electrophoretic mobility measurements of lipid vesicles performed at various pH levels. The interaction between solution ions and the PC-DA liposomal surface was described by a six component equilibrium model. The previously determined association constants of the -PO^(-) and –N⁽⁺⁾(CH₃)₃ groups of PC with H⁺, OH^-, Na⁺ and Cl^- ions (K _A1H, K _B1OH, K _A1Na, K _B1C1) were used to calculate K _B2OH, and K _B2C1, the association constants of the –N⁽⁺⁾H₃ group of DA with OH^- and Cl^- ions, providing an experimental verification for the proposed model.     

H2O2-Induced Oxidative Stress Affects SO4= Transport in Human Erythrocytes

The aim of the present investigation was to verify the effect of H₂O₂-induced oxidative stress on SO₄⁼ uptake through Band 3 protein, responsible for Cl^-/HCO₃^- as well as for cell membrane deformability, due to its cross link with cytoskeletal proteins. The role of cytoplasmic proteins binding to Band 3 protein has been also considered by assaying H₂O₂ effects on hemoglobin-free resealed ghosts of erythrocytes. Oxidative conditions were induced by 30 min exposure of human erythrocytes to different H₂O₂ concentrations (10 to 300 μM), with or without GSH (glutathione, 2 mM) or curcumin (10 μM), compounds with proved antioxidant properties. Since SO₄⁼ influx through Band 3 protein is slower and better controllable than Cl^- or HCO₃^- exchange, the rate constant for SO₄⁼ uptake was measured to prove anion transport efficiency, while MDA (malondialdehyde) levels and –SH groups were estimated to quantify the effect of oxidative stress. H₂O₂ induced a significant decrease in rate constant for SO₄⁼ uptake at both 100 and 300 μM H₂O₂. This reduction, observed in erythrocytes but not in resealed ghosts and associated to increase in neither MDA levels nor in –SH groups, was impaired by both curcumin and GSH, whereas only curcumin effectively restored H₂O₂-induced changes in erythrocytes shape. Our results show that: i) 30 min exposure to 300 μM H₂O₂ reduced SO₄⁼ uptake in human erythrocytes; ii) oxidative damage was revealed by the reduction in rate constant for SO₄⁼ uptake, but not by MDA or –SH groups levels; iii) the damage was produced via cytoplasmic components which cross link with Band 3 protein; iv) the natural antioxidant curcumin may be useful in protecting erythrocytes from oxidative injury; v) SO₄⁼ uptake through Band 3 protein may be reasonably suggested as a tool to monitor erythrocytes function under oxidative conditions possibly deriving from alcohol consumption, use of drugs, radiographic contrast media administration, hyperglicemia or neurodegenerative diseases.     

Leaching of solutes by the action of acidic rain: a comparison of efflux from twigs and single needles of Picea abies (L. Karst.)

Abstract. Leaching of inorganic cations (K⁺, Mg²⁺) and in some cases of inorganic anions and sugars from detached twigs and single needles of spruce Picea abies L. Karst.) in the presence of acid rain (H₂SO₄, 1 mol m^?3) or salt solutions (Na₂ SO₄, 1 mol m^?3) was examined under laboratory conditions. Cation leaching (as percentage of the total water soluble ion content of the tissue per hour) was: K⁺: 0.01-0.02%; Mg²⁺: 0.005-0.01%; Ca²⁺: 0.1-0.2%. Leaching rates of anions were even lower than that and concentrations in the leachate were often below the detection limit of anion chromatography. Spraying with H₂SO₄ (pH 2.95, 1 mol m^?3) increased leaching only transiently. Similar effects were found when Na₂SO₄ was used instead of H₂SO₄. The transiently enhanced leaching was apparently due to H⁺/cation or cation/cation exchange at the twig or leaf surfaces. Feeding of K⁺ or Al³⁺ through the stems increased leaching of all cations within a few hours, again demonstrating rapid ion exchange in the apoplast. Leaching of potassium and magnesium from single needles occurred at similar relative rates as from twigs. Loss of Ca²⁺ ions, however, was even smaller from needles than from twigs. Apparently, a large part of the Ca²⁺ lost from twigs originated from the bark and not from the needles. Efflux of ions from longitudinal needle sections was about 1000 times taster than the rates obtained with intact needles, indicating that the cuticle was the main barrier Preventing solute loss. In relation to the total amount of mineral nutrients in trees, leaching is considered to be too small to be the primary cause of damage to trees stressed by acid rain, as has been suggested in the literature.     

Netuptake of sulphate and its transport to the shoot in tobacco plants fumigated with H2S or SO2

In tobacco plants the net uptake of sulphate and its transport to the shoot were determined after cultivation with low, normal, and high sulphate supply. The relative amount of the sulphate taken up that was transported to the shoot was used as a measure of xylem loading. Net uptake of sulphate and its transport to the shoot were low in tobacco plants grown with low sulphate, and high in plants cultivated with high sulphate. Xylem loading, however, was relatively low in tobacco plants grown with high sulphate and relatively high in tobacco plants grown with low sulphate supply. Pre-culture in low sulphate containing nutrient solution also resulted in a high proportion of the absorbed sulphate being transported into the xylem if normal sulphateconcentration was supplied afterwards. Fumigation with H₂S or SO₂ reduced net uptake of sulphate in tobacco plants grown with normal, but not with high sulphate supply. Sulphate transport to the shoots was diminished by H₂S or SO₂ fumigation in tobacco plants grown with normal and high sulphate supply. Also the relative amount of the sulphate taken up that was transported to the shoot was lowered by fumigation with H₂S or SO₂ in tobacco plants grown with normal sulphate supply. Apparently, the diminished sulphate transport to the shoot upon H₂S or SO₂ fumigation can only partially be explained by a smaller sulphate uptake. Sulphur nutrition of tobacco plants also seems to be controlled by xylem loading of sulphate. The possible role of glutathione as a signal regulating sulphur nutrition of tobacco plants upon fumigation with H₂S and SO₂ is discussed.     

Anion sensitivity of motility and step-down photophobic responses of Euglena gracilis

The motility and step-down photophobic responses of Euglena are influenced by inorganic and organic anions. Persistent motility (with Ca²⁺, Mg²⁺ and K⁺ present) is supported with chloride or sulfate but not with acetate, nitrate or propionate as the only added anions. Cells in media containing acetate displayed a cell aggregation (clumping) behavior that was both red light sensitive and, under some conditions, was accompanied by suppression of the step-down photophobic response. Addition of sodium salts (Cl^-, SO ₄ ^2- , acetate or propionate) to cells in Cl^- or SO ₄ ^2- based media had differential effects on the duration of the step-down photophobic responses induced by blue light removal: anions alter the response. In addition, cells in all Cl^- containing media showed constant photophobic response duration following repeated stimulation. Cells in some SO₄ ^2- containing media, however, showed response summation to repeated stimulation. This latter effect was reversible and was overcome by the addition of chloride anions.