Comparative effects of selenite and selenate on nitrate assimilation in barley seedlings

Abstract. The effect of SeO₃ and SeO₄ on NO₃ assimilation in 8-d-old barley (Hordeum vulgare L.) seedlings was studied over a 24-h period. Selenite at 0.1 mol. m^? in the uptake solutions severely inhibited the induction of NO₃ uptake and active nitrate reductases. Selenate, at 1.0 mol m^?3 in the nutrient solution, had little effect on induction of activities of these systems until after 12 h; however, when the seedlings were pretreated with 1.0 mol m^?3 SeO₄ for 24 h, subsequent NO₃ uptake from SeO₄-free solutions was inhibited about 60%. Sulphate partially alleviated the inhibitory effect of SeO₃ when supplied together in the ambient solutions, but had no effect in seedlings pretreated with SeO₃. By contrast, SO₄ partially alleviated the inhibitory effect of SeO₄ even in seedlings pretreated with SeO₄. Since uptake of NO₃ by intact seedlings was also inhibited by SeO₃, the percentage of the absorbed NO₃ that was reduced was not affected. By contrast, SeO₄, which affected NO₃ uptake much less, inhibited the percentage reduced of that absorbed. However, when supplied to detached leaves, both SeO₃ and SeO₄ inhibited the in vivo reduction of NO₃ as well as the induction of nitrate reductase and nitrite reductase activities. Selenite was more inhibitory than SeO₄; approximately a five to 10 times higher concentration of SeO₄ than SeO₃ was required to achieve similar inhibition. In detached leaves, the inhibitory effect of both SeO₃ and SeO₄ on in vivo NO₃ reduction as well as on the induction of nitrate reductase activity was partially alleviated by SO₄. The inhibitory effects of Se salts on the induction of nitrite reductase were, however, completely alleviated by SO₄. The results show that in barley seedlings SeO₃ is more toxic than SeO₄. The reduction of SeO₄ to SeO₃ may be a rate limiting step in causing Se toxicity.     

Assessing the phytotoxicity of mononuclear hydroxy-aluminum

Abstract Al³⁺ is an important rhizotoxic ion in acid soils around the world. Al³⁺ is in equilibrium with mononuclear hydroxy-Al species, such as AlOH²⁺ and AL(OH)₂⁺, but the toxicity of these species has not been determined. Polynuclear Al may also coexist with Al³⁺, and one of these species, AlO₄Al₁₂(OH)₂₄(H₂O)₁₂⁷⁴, is very toxic. In order to determine the toxicity of mononuclear hydroxy-Al we have reanalysed the results of previously published, solution-culture experiments and have performed new experiments. Several problems are inherent in these studies. At pH values less than 5.0, the activities of the mononuclear hydroxy-Al species are low relative to Al³⁺, but attempts to change the ratio by raising the pH generally initiate the formation of polynuclear Al. (We discovered that mononuclear solutions are stable for many days when {Al³⁺}/{H⁺}³≤ 10^8.8, where braces denote activities.) We avoided, or accounted for, polynuclear Al in our studies. In addition, we used up-to-date equilibrium constants to compute Al species, very simple culture media (generally containing CaCl₂, AlCl₃ and HCl as the only inputs), short-term (2d) growth, and an Al-sensitive wheat variety (Triticum aestivum L. cv. Tyler) that permitted low Al levels and, consequently, higher pH values without Al polymerization. Experiments were designed in which the solutions were constant in {Al³⁺} and variable in mononuclear hydroxy-Al or were orthonal (factorial) in {Al³⁺} and {AlOH²⁺}. Linear and nonlinear, simple and multiple, regression analyses of these and previous experiments failed to reveal any toxicity for mononuclear hydroxy-Al to Tyler wheat.     

Beneficial Effects of Nickel on Pseudomonas saccharophila under Nitrogen-Limited Chemolithotrophic Conditions

Addition of nickel stimulated growth and nitrogenase activity of Pseudomonas saccharophila under nitrogen-limited chemolithotrophic conditions, apparently because of a significant increase in expression of uptake hydrogenase activity. Inhibition of hydrogenase expression by 50 μM EDTA was relieved by nickel over a wide concentration range (1 to 200 μM). Co²⁺, Zn²⁺, Mn²⁺, and Cu²⁺ stimulated expression of hydrogenase activity, but to a much lesser degree than nickel, and Fe²⁺, Mg²⁺, SeO₄²⁻, and SeO₃²⁻ did not increase expression. Nickel in individual combination with Mg²⁺, Fe²⁺, SeO₃²⁻, and SeO₄²⁻ resulted in activities that were essentially the same as that with nickel alone. Hydrogenase synthesis required the presence of nickel, and repression by O₂ was alleviated by increasing the concentration of added nickel. Cells placed under hydrogenase derepression conditions showed progressive incorporation of radioactive nickel to a much greater extent than did cells which were not derepressed.     

Uptake of aluminium into Arabidopsis root cells measured by fluorescent lifetime imaging

Background and Aims

Measuring the Al³⁺ uptake rate across the plasma membrane of intact root cells is crucial for understanding the mechanisms and time-course of Al toxicity in plants. However, a reliable method with the sufficient spatial and temporal resolution to estimate Al³⁺ uptake in intact root cells does not exist.

Methods

In the current study, fluorescent lifetime imaging (FLIM) analysis was used to quantify Al³⁺ uptake in the root-cell cytoplasm in vivo. This was performed via the estimation of the fluorescence lifetime of Al–lumogallion {5-chloro-3[(2,4-dihydroxyphenyl)azo]-2-hydroxybenzenesulfonic acid} complexes and measurements of intracellular pH while exposing arabidopsis seedlings to acidic and Al³⁺ stresses.

Key Results

The lifetime of Al–lumogallion complexes fluorescence is pH-dependent. The primary sites for Al³⁺ entry are the meristem and distal elongation zones, while Al³⁺ uptake via the cortex and epidermis of the mature root zone is limited. The maximum rates of Al uptake into the cytoplasm (2–3 µmol m⁻³ min⁻¹ for the meristematic root zone and 3–7 µmol m⁻³ min⁻¹ for the mature zone) were observed after a 30-min exposure to 100 µm AlCl₃ (pH 4·2). Intracellular Al concentration increased to 0·4 µm Al within the first 3 h of exposure to 100 µm AlCl₃.

Conclusions

FLIM analysis of the fluorescence of Al–lumogallion complexes can be used to reliably quantify Al uptake in the cytoplasm of intact root cells at the initial stages of Al³⁺ stress.Key words: Acid stress, Al³⁺, aluminium toxicity, Arabidopsis thaliana, low pH, fluorescent lifetime imaging (FLIM), lumogallion     

Effects of Ni(2+), Co(2+), and Mn(2+) on desensitized butyrylcholinesterase prepared from human serum

Human serum butyrylcholinesterase (BChE) has been converted into a stable but less active desensitized form when heated at 45°C for 24 h. The desensitized BChE follows Michaelis-Menten kinetics, whereas native enzyme exhibits slightly negative cooperativity with respect to butyrylthiocholine binding. In this study, we investigated the effects of Ni²⁺, Co²⁺, and Mn²⁺ on the desensitized BChE. It is found that all three ions were noncompetitive inhibitors of the desensitized BChE, and K _i values have been determined as 7.816±1.060 mM, 48.722±4.635 mM, and 84.795±5.249 mM for Ni²⁺, Co²⁺, and Mn²⁺, respectively. In our previous study, these ions were linear mixed-type inhibitors of the native BChE. This finding confirms that desensitized BChE changes to a different conformation than native BChE. From the comparison of K _i values of the trace elements, it can be said that Ni²⁺ is a more effective inhibitor of the desensitized BChE than Co²⁺ and Mn²⁺.     

Microscale heterogeneity of acidity related stress-parameters in the soil solution of a forested cambic podzol

Acid related stress in soils might be caused by high concentrations of H⁺ and Al³⁺ in soil solution. Sampling of soil solution so far integrates over a relatively large soil volume, in the range of dm³. In order to study the microscale heterogeneity of acidity related stress-parameters the soil profile of a podzolic cambisol was covered by a 10×6 matrix of micro suction cups with a grid distance of 2 cm. The soil solution collected at 10 sampling events was analyzed for free cations and anions by capillary electrophoresis and for total metal content by a micro injection technique on ICP-OES. pH and UV absorption were also measured.There was a general trend of increasing pH and decreasing UV absorption with increasing soil depth, however without a clear correlation of concentration isolines to soil horizon borders. The latter was also true for total Al (Al_tot) and Al³⁺, with the exception of the soil horizon border A_he/B_h,which was very well reflected by Al³⁺ and also by the fraction of bound Al. In the A_he horizon less than 30%, in deeper mineral soil less than 50% of Al_tot were present as free Al³⁺. This fact is critical when calculating Ca/Al ratios as a stress parameter, because total metal content measured by ICP clearly overestimates the risk of root damage, even in deeper horizons of acid forest soils, where organic complexation of Al is of minor importance. The heterogeneity of soil solution chemistry and toxicity parameters on the cm-scale was found to be significant, for example with gradients of more than 0,5 pH-units within 2 cm. Because plant roots also experience soil on a microscale, high resolution investigations of soil solution chemistry offer a new approach for looking at the chemical environment relevant for root growth and plant nutrition.     

Buffered,phosphate-containing media suitable for aluminum toxicity studies

Studies of Al rhizotoxicity sometimes require the use of well-defined rooting media. For that reason, buffers and phosphate are often omitted from Al solutions for which species composition must be determined precisely. Homopipes and succinate appear to be suitable buffers for short-term studies with seedlings of an Al-sensitive wheat (Triticum aestivum L. cv. Scout 66) and white clover (Trifoliau repens L. cv. Huia). In the case of homopipes (homopiperazine-N,N-bis-2-[ethane-sulfonic acid]), a slight inhibition of root elongation must be taken into account, but no binding of Al³⁺ was observed. In the case of succinate, no inhibition of root elongation was observed, but Al³⁺ binding must be considered. Phosphate-containing media remain free of solid-phase or polynuclear species whenever {Al³⁺}²{HPO₄ ^2-}{OH^–}³ < 10^–47.0 (or {Al³⁺}{HPO₄ ^2-}{OH^–}< 10^–22.7) and when {Al³⁺}³ / {H⁺}³ < 10^8.8. These ion activity products, that define stable Al solutions in the laboratory, appear to apply in soils also, according to an analysis of published data. The published equilibrium values {AlH₂PO₄ ²⁺} / ({Al³⁺}{H₂PO₄ ^–}) = 10^3.0, {AlHPO₄ ⁺} / ({Al³⁺}{HPO₄ ^2-}) = 10^7.0, and {Alsuccinate⁺} / ({Al³⁺}{succinate ^2-}) = 10^4.62 appear to be suitable, because solution toxicity could be accounted for entirely on the basis of computed Al³⁺ even in solutions containing high levels of Alsuccinate⁺ and AlHPO₄ ⁺ (in every case {AlHPO₄ ⁺}>> {AlH₂PO₄ ²⁺}). Thus, AlHPO₄ ⁺ and Alsuccinate⁺ were not toxic at achieved concentrations.     

Cation amelioration of aluminum toxicity in wheat

Aluminum is a major constituent of most soils and limits crop productivity in many regions. Amelioration is of theoretical as well as practical interest because understanding amelioration may contribute to an understanding of the mechanisms of toxicity. In the experiments reported here 2-day-old wheat (Triticum aestivum L. cv Tyler) seedlings with 15-millimeter roots were transferred to solutions containing 0.4 millimolar CaCl₂ at pH 4.3 variously supplemented with AlCl₃ and additional amounts of a chloride salt. Root lengths, measured after 2 days in the test solutions, were a function of both Al activity and the cation activity of the added salt. Percent inhibition = 100 {Al³⁺}/({Al³⁺} + K_m + α{C}^β) where {Al³⁺} is the activity of Al³⁺ expressed in micromolar, {C} is the activity of the added cation expressed in millimolar, and K_m (= 1.2 micromolar) is the {Al³⁺} required for 50% inhibition in the absence of added salt. For Ca²⁺, Mg²⁺, and Na⁺ the values of α were 2.4, 1.6, and 0.011, respectively, and the values for β were 1.5, 1.5, and 1.8, respectively. With regard to relative ameliorative effectiveness, Ca²⁺ > Mg²⁺ ≈ Sr²⁺ K⁺ ≈ Na⁺. Other cations were tested, but La³⁺, Sc³⁺, Li⁺, Rb⁺, and Cs⁺ were toxic at potentially ameliorative levels. The salt amelioration is not solely attributable to reductions in {Al³⁺} caused by increases in ionic strength. Competition between the cation and Al for external binding sites may account for most of the amelioration.     

Effects of litter incorporation and nitrogen fertilization on the contents of extractable aluminium in the rhizosphere soil of tea plant (Camallia sinensis (L.) O. Kuntze)

The effects of litter incorporation and nitrogen application on the properties of rhizosphere and bulk soils of tea plants (Camellia sinensis (L.) O. Kuntze) were examined in a pot experiment. Total of 8 treatments included four levels of tea litter additions at 0, 4.9, 9.8, and 24.5 g kg^–1 in combination with two N levels (154.6 mg kg^–1 and without). After 18 months of growth the rhizosphere soil was collected by removing the soil adhering to plant roots and other soil was referred to as bulk soil. The dry matter productions of tea plants were significantly increased by N fertilization and litter incorporation. The effect of litter was time-depending and significantly decreased the content of exchangeable Al (Al_ex, by 1 mol L^–1 KCl) and Al saturation at 9 months after litter incorporation whereas soil pH was not affected, although the litter contained high Al content. After 18 months, the contents of extractable Al by dilute CaCl₂, CuCl₂ + KCl, NH₄OAC, ammonium oxalate and sodium citrate (Al_CaCl2, Al_Cu/KCl, Al_NH4OAC, Al_Oxal, and Al_Cit respectively) and Al_ex, were not affected by litter application, except that of Al_CaCl2 in the rhizosphere soil which was decreased following litter additions. Nitrogen fertilization with NH₄ ⁺ (urea and (NH₄)₂SO₄) significantly reduced soil pH, the contents of exchangeable Ca, K, Mg and base saturation while raised extractable Al levels (Al_CaCl2, Al_Cu/KCl, Al_NH4OAC, and Al_ex). In the rhizosphere soils exchangeable K accumulated in all treatments while exchangeable Ca and Mg depleted in treatments without litter application. The depletions of Ca and Mg were no longer observed following litter incorporation. This change of distribution gradients in rhizosphere was possibly due to the increase of nutrient supplies from litter decomposition and/or preferable root growth in soil microsites rich in organic matter. Lower pH and higher extractable Al (Al_CaCl2, Al_ex, and Al_NH4OAC) in the rhizosphere soils, regardless of N and litter treatments, were distinct and consistent in all treatments. Such enrichments of extractable Al in the rhizosphere soil might be of importance for tea plants capable of taking up large amounts of Al.     

Mitigation of Acute Aluminum Toxicity by Sodium Selenite and N-Acetylcysteine in Adult Male Rats

The objective of this study is to investigate the toxic effects of aluminum and the potential alleviation of selenite and N-acetylcysteine (NAC) on this toxicity. Acute aluminum toxicity was induced by intraperitoneal (i.p.) injection of AlCl₃ (30 mg Al³⁺/kg) for four consecutive days. Al³⁺ damaged the synthetic capability and regeneration power of liver cells and induced inflammation. It also damaged the kidney and disturbed the lipid profile enhancing the total cholesterol level and LDL-cholesterol level increasing the risks of atherosclerosis. Al³⁺ reduced the cellular antioxidant milieu typified by the decrease in reduced glutathione, vitamin E, and four antioxidant enzymes and induced lipid peroxidation (LPO). Selenite at 1 mg Se/kg and NAC at 150 mg/kg injected either simultaneously with or after Al³⁺ mitigated most of these damaging effects probably by the virtue of scavenging the free radicals, binding aluminum and stimulating its excretion and reducing its bioavailability, bolstering the endogenous antioxidant defense systems, stabilizing the cell membrane, and preventing LPO. The beneficial effects of selenite and NAC against aluminum toxicity were also confirmed by the light and electron histopathology study. There were no significant differences between the two regimens used (protection and therapeutic) in the current study probably due to the short time of exposure, and the abrogation of Al³⁺ toxicity offered by selenite was better than that provided by NAC on the histopathology level.     

Interactive effects of Al, h, and other cations on root elongation considered in terms of cell-surface electrical potential

The rhizotoxicities of Al³⁺ and of La³⁺ to wheat (Triticum aestivum L.) were similarly ameliorated by cations in the following order of effectiveness: H⁺ ≈ C³⁺ > C²⁺ > C¹⁺. Among tested cations of a given charge, ameliorative effectiveness was similar except that Ca²⁺ was slightly more effective than other divalent cations and H⁺ was much more effective than other monovalent cations. H⁺ rhizotoxicity was also ameliorated by cations in the order C³⁺ > C²⁺ > C¹⁺. These results suggest a role for cell-surface electrical potential in the rhizotoxicity of Al³⁺, La³⁺, H⁺, and other toxic cations: negatively charged cell surfaces of the root accumulate the toxic cations, and amelioration is effected by treatments that reduce the negativity of the cell-surface electrical potential by charge screening or cation binding. Membrane-surface activities of free Al³⁺ or La³⁺ computed according to a Gouy-Chapman-Stern model correlated well with growth inhibition, which correlated only poorly with Al³⁺ or La³⁺ activities in the external medium. The similar responses of Al-intoxicated and La-intoxicated roots to ameliorative treatments provide evidence that Al³⁺, rather than AlOH²⁺ or Al(OH)₂⁺, is the principal toxic species of mononuclear Al. Comparisons of the responses of Al-sensitive and Al-tolerant wheats to Al³⁺ and to La³⁺ did not support the hypothesis that varietal sensitivity to Al³⁺ is based upon differences in cell-surface electrical potential.     

Non-phytotoxicity of the aluminum sulfate ion, AlSO+4

Aluminum is a phytotoxic element in many soils and occurs in a variety of chemical species. In order to determine whether AlSO⁺₄ is toxic, seedlings of wheat (Triticum aestivum L. cv. Tyler) and red clover (Trifolium pratense L. cv. Kenland) were transferred to solutions containing controlled activities of Al³⁺, AlSO⁺₄, Na⁺ and Ca²⁺. Root elongation was inhibited by Al³⁺ (or mononuclear hydroxy-Al species that are in equilibrium with Al³⁺), but not by AlSO⁺₄. We assumed a formation constant (K_AlSO⁺₄= {AlSO⁺₄}/[{Al³⁺} {SO^2-₄}]; braces indicate activities} of 10^3.2 for AlSO⁺₄ in the computation of ionic activities, but use of K_AlSO⁺₄ values ranging from 10^2.8 to 10^3.6 had very little effect on the computed toxicities of Al³⁺ and AlSO⁺₄. Sulfate did not promote the formation of polynuclear Al complexes in our experiments. A practice in studies of Al phytotoxicity has been to attribute toxicity to mononuclear Al, but now it would seem advisable to exclude AlSO⁺₄. That AlSO⁺₄ is non-toxic, or is at least 10-fold less toxic than Al³⁺, has implications for the physiology of Al toxicity and for the use of sulfate salts in experimental work and in agriculture.     

Effects of limestone and gypsum application to a Malaysian ultisol on soil solution composition and yields of maize and groundnut

A field experiment was conducted on an Ultisol in Malaysia to assess changes in soil solution composition and their effects on maize and groundnut yields, resulting from limestone and gypsum application. The results showed that soil solution Ca in the lime treatment remained mainly in the zone of incorporation, but in the gypsum treatment some Ca moved into 15–30 cm zone. Al³⁺ and AlSO₄ ⁺ were dominant Al species in the soil solution of nil treatment. Liming decreased Al³⁺ and AlSO₄ ⁺, but increased hydroxy-Al monomer activities. However, gypsum application resulted in an increase of AlSO₄ ⁺ activity and in a decrease of Al³⁺ activity. Relative maize and groundnut yields were negatively correlated with Al³⁺, Al(OH)²⁺ and Al_sum activities. Likewise, relative yields were negatively correlated with Al concentration and the Al concentration ratio and positively correlated with soil solution Mg concentration and Ca/Al ratio.     

Identity of the rhizotoxic aluminium species

The aluminium (III) released from soil minerals to the soil solution under acid conditions may appear as hexaaquaaluminium (Al(H₂O)₆ ³⁺, or Al³⁺ for convenience) or may react with available ligands to form additional chemical species. That one or more of these species is rhizotoxic (inhibitory to root elongation) has been known for many decades, but the identity of the toxic species remains problematical for the following reasons. 1. Several Al species coexist in solution so individual species cannot be investigated in isolation, even in artificial culture media. 2. The activities of individual species must be calculated from equilibrium data that may be uncertain. 3. The unexpected or undetected appearance of the extremely toxic triskaidekaaluminium (AlO₄Al₁₂(OH)₂₄(H₂O)₁₂ ⁷⁺ or Al₁₃) may cause misatribution of toxicity to other species, especially to mononuclear hydroxy-Al. 4. If H⁺ ameliorates Al³⁺ toxicity, or vice versa, then mononuclear hydroxy-Al may appear to be toxic when it is not. 5. The identity and activities of the Al species contacting the cell surfaces are uncertain because of the H⁺ currents through the root surface and because of surface charges. This article considers the implications of these problems for good experimental designs and critically evaluates current information regarding the relative toxicities of selected Al species. It is concluded that polycationic Al (charge >2) is rhizotoxic as are other polyvalent cations.     

Chemical composition of soil solutions extracted from New Zealand beech forests and West German beech and spruce forests

Concentrations of ions were measured in soil solutions from beech (Nothofagus) forests in remote areas of New Zealand and in solutions from beech (Fagus sylvatica) and Norway spruce (Picea abies) forests in North-East Bavaria, West Germany, to compare the chemistry of soil solutions which are unaffected by acid deposition (New Zealand) with those that are affected (West Germany). In New Zealand, soil solution SO₄ ^2– concentrations ranged between <2 and 58 mol L^–1, and NO₃ ^– concentrations ranged between <1 and 3 mol L^–1. In West Germany, SO₄ ^2– concentrations ranged between 80 and 700 mol L^–1, and NO₃ ^– concentrations at three of six sites ranged between 39 and 3750 mol L^–1, but was not detected at the remaining three sites. At all sites in New Zealand, and at sites where the soil base status was moderately high in West Germany, pH levels increased, and total Al (Al_t) and inorganic monomeric Al (Al_i) levels decreased rapidly with increasing soil depth. In contrast, at sites on soils of low base status in West Germany, pH levels increased only slightly, and Al levels did not decline with increasing soil depth.Under a high-elevation Norway spruce stand showing severe Mg deficiency and dieback symptoms in West Germany, soil solution Mg²⁺ levels ranged between 20 and 60 mol L^–, and were only half those under a healthy stand. Al_t and Al_i levels were substantially higher the healthy stand than under the unhealthy stand, indicating that Al toxicity was not the main cause of spruce decline.     

Assessing the Rhizotoxicity of the Aluminate Ion, Al(OH)(4)

Dissolved aluminum (III) in acidic soils or culture media is often rhizotoxic (inhibitory to root elongation). Alkaline solutions of Al are also sometimes rhizotoxic, and for that reason toxicity has been attributed to the aluminate ion, Al(OH)₄⁻. In the present study, seedlings of wheat (Triticum aestivum L. cv Tyler) and red clover (Trifolium pratense L. cv Kenland) were cultured in aerated aluminate solutions at pH 8.0 to 8.9. The bulk phases of these solutions were free of reactive polynuclear hydroxy-Al (including the extremely toxic species AlO₄Al₁₂[OH]₂₄[H₂O]⁷⁺₁₂ [Al₁₃]) according to the ferron (8-hydroxy-7-iodo-5-quinolinesulfonic acid) assay. At an aluminate concentration of 25 micromolar (23 micromolar activity) and a pH of 8, root elongation was less than 40% of Al-free controls, but at pH 8.9 elongation was 100% of controls. The hypothesis is offered that aluminate is nontoxic and that the inhibition at lower pH values is attributable to Al₁₃ postulated to have formed in the acidic free space of the roots where the ratio /{Al³⁺/}//{H⁺/}³ may rise above 10¹⁰. At this value hydroxy-Al in over-saturated, alkaline solutions begins to undergo rapid conversion to polynuclear species.     

Complexation of Al(III) with reduced glutathione in acidic aqueous solutions

The complexation of reduced glutathione (GSH) in its free and Al(III)-bound species in acidic aqueous solutions was characterized by means of multi-analytical techniques: pH-potentiometry, multinuclear (¹H, ¹³C and ²⁷Al) and two-dimensional nuclear Overhauser enhancement NMR spectroscopy (¹H, ¹H-NOESY), electrospray mass spectroscopy (ESI-MS), and ab initio electronic structure calculations. The following results were found. In the 25 °C 0.1 M KCl and 37 °C 0.15 M NaCl ionic medium systems, Al³⁺ coordinates with the important biomolecule GSH through carboxylate groups to form various mononuclear 1:1 (AlHL, AlH₂L and AlH₋₁L), 1:2 (AlL₂) complexes, and dinuclear (Al₂H₅L₂) species, where H₄L⁺ denotes totally protonated GSH. Besides the monodentate complexes through carboxylate groups, the amino groups and the peptide bond imino and carbonyl groups may also be involved in binding with Al³⁺ in the bidentate and tridentate complexes. The present data reinforce that the glycine carboxylate group of GSH has a higher microscopic complex formation constant than γ-glutamyl carboxylate. Compared with simple amino acids, the tripeptide GSH displays a greater affinity for the Al³⁺ ion and thus may interfere with aluminum’s biological role more significantly.     

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.     

Conostegia xalapensis (Melastomataceae): an aluminum accumulator plant

In acidic soils, an excess of Al³⁺ is toxic to most plants. The Melastomataceae family includes Al‐accumulator genera that tolerate high Al³⁺ by accumulating it in their tissues. Conostegia xalapensis is a common shrub in Mexico and Central America colonizing mainly disturbed areas. Here, we determined whether C. xalapensis is an Al accumulator, and whether it has internal tolerance mechanisms to Al. Soil samples collected from two pastures in the state of Veracruz, Mexico, had low pH and high Al³⁺ concentrations along with low Ca²⁺ levels. Leaves of C. xalapensis from pastures showed up to 19 000 mg Al kg^?1 DW (dry weight). In laboratory experiments, 8‐month‐old seedlings treated with 0.5 and 1.0 mM AlCl₃ for 24 days showed higher number of lateral roots and biomass. Pyrocatechol violet and hematoxylin staining evidenced that Al localized in epidermis and mesophyll cells in leaves and in epidermis and vascular pith in roots. Scanning electron microscope‐energy dispersive X‐ray microanalysis of Al‐treated leaves corroborated that Al is in abaxial and adaxial epidermis and in mesophyll cells (31.2%) in 1.0 mM Al‐treatment. Roots of Al‐treated plants had glutathione reductase (EC 1.6.4.2) and superoxide dismutase (EC 1.15.1.1) activity higher, and low levels of O and H ₂O₂. C. xalapensis is an Al‐accumulator plant that can grow in acidic soils with higher Al³⁺ concentrations, and can be considered as an indicator species for soils with potential Al toxicity.