Alleviate Al Toxicity Research Articles

Overexpression of an Arabidopsis magnesium transport gene, AtMGT1, in Nicotiana benthamiana confers Al tolerance

Aluminium (Al) toxicity is the most important limiting factor for crop production in acid soil environments worldwide. In some plant species, application of magnesium (Mg(2+)) can alleviate Al toxicity. However, it remains unknown whether overexpression of magnesium transport proteins can improve Al tolerance. Here, the role of AtMGT1, a member of the Arabidopsis magnesium transport family involved in Mg(2+) transport, played in Al tolerance in higher plants was investigated. Expression of 35S::AtMGT1 led to various phenotypic alterations in Nicotiana benthamiana plants. Transgenic plants harbouring 35S::AtMGT1 exhibited tolerance to Mg(2+) deficiency. Element assay showed that the contents of Mg, Mn, and Fe in 35S::AtMGT1 plants increased compared with wild-type plants. Root growth experiment revealed that 100 microM AlCl(3) caused a reduction in root elongation by 47% in transgenic lines, whereas root growth in wild-type plants was inhibited completely. Upon Al treatment, representative transgenic lines also showed a much lower callose deposition, an indicator of increased Al tolerance, than wild-type plants. Taken together, the results have demonstrated that overexpression of ATMGT1 encoding a magnesium transport protein can improve tolerance to Al in higher plants.

Alleviation of Al Toxicity in Barley by Addition of Calcium

The potential mechanism by which Ca alleviates Al toxicity was investigated in barley seedlings. It was found that 100 μM Al-alone treatment inhibited barley plant growth and thereby reduced shoot height and root length, and dry weights of root, shoot and leaf; promoted Al accumulation but inhibited Ca absorption in plant tissues; and induced an increase in the activities of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) and in the level of lipid peroxidation (MDA content) in leaves. Except for the increase in Ca concentration in plant tissues, treatment with 0.5 mM Ca in the absence of Al had less effect on the above-mentioned parameters, compared with the control. Addition of Ca efficiently reduced Al toxicity, which is reflected by the promotion of plant growth, reduction in Al concentration and MDA content, increase in Ca concentration and in SOD, POD, and CAT activities compared with the Al-alone-treatment; with increase in Ca level (3.0 mM), the ameliorative effect became more dominant. This indicated that the alleviation of aluminum toxicity in barley seedlings with Ca supplementation could be associated with less absorption of Al and the enhancement of the protective ability of the cell because of increased activity of the antioxidative enzyme.

AGRONOMIC IMPLICATIONS OF THE SUPPLY OF LIME AND GYPSUM BY-PRODUCTS TO PALEXERULTS FROM WESTERN SPAIN

A laboratory experiment with leaching columns reproducing the topmost portion (Ap and AB horizons) of a Palexerult from western Spain was conducted to compare the efficiency of limestone (L), gypsum (G), and various lime and gypsum by-products (sugar foam waste, phosphogypsum [PG], and red gypsum [RG]) to alleviate Al toxicity and improve soil productivity. The effect of the previous five amendments on composition changes in the exchange complex and the soil saturation extract was examined after one or two leaching cycles (equivalent to as many years of field precipitation). The productivity of the Ap horizon after the first leaching cycle was assessed by growing wheat (Triticum aestivum var. Jabato) in a greenhouse. The lime by-products increased the pH of the Ap horizon and the Ca content of the exchange complex to a greater extent than the gypsum amendments; in addition, the former minimized the Al content of the exchange complex and the saturation extract. Changes in the AB horizon were much less marked, and the sugar foam waste was a better liming material than limestone. The changes in the exchange complex and saturation extract from gypsum amendments followed a more complex pattern and affected both horizons (Ap and AB). The pH of the soil was initially decreased relative to the controls through a "salt effect" and then increased after the second leaching cycle, when most of the gypsum was leached from the soil. In both horizons, the Ca content was increased, and the Al content of the exchange complex decreased-albeit to a lesser extent than with the lime amendments in Ap; by contrast, exchangeable Mg and Na were lost in substantial amounts in the first leaching cycle; K losses were comparatively smaller. After the first leaching cycle, aluminum-fluoride ion pairs were the most active Al species in the extracts from Ap and AB in PG amended columns; on the other hand, the aluminum sulfate ion pairs were the most active in the extracts from the G- and RG-amended columns. After the second leaching cycle, the activity of Al3+-the most phytotoxic ionic species of Al-in the Ap extracts from all calcium carbonate-amended columns decreased, whereas that in the gypsum-amended soil only decreased in the PG-treated columns; in the AB horizon, however, the extracts from all gypsic amendments decreased the Al3+ activity, with respect to control and lime amendments. The calcium aluminum balance (Noble et al., 1988) was increased by all treatments. All lime and gypsum amendments-particularly sugar foam waste-increased productivity relative to the controls. Of the three gypsum amendments, RG had the greatest impact on productivity; this can be ascribed to its Zn content, which was very low in the natural soil.

Short-term alleviation of aluminum phytotoxicity by urea application in acid soils from south China

A laboratory experiment was conducted to study effects of urea fertilizer on the chemical composition of soil solutions over time, and to determine Al toxicity as a function of rates of urea application. The experiment revealed that addition of urea fertilizer to soils caused drastic changes in soil pH during the hydrolysis and nitrification stages of urea transformation in the experiment. These pH changes, depending on the N rate of urea application and time courses, had variable effects on soil exchangeable Al, extracted with artificial solutions containing 1 mol l −1 KCl. The Al mobilization rate could be resolved into two phases: A declining phase for Al was attributed to the urea-induced hydrolysis while a second rising phase was dependent with the nitrification of added N fertilizer. The decreases in exchangeable Al reached the greatest in 4–7 days after fertilization, consistent with soil pH increase. Decreased Al availability had been observed as a consequence of increasing urea addition and soil pH when using Root elongation of maize seedlings as the estimators. Results from the present study demonstrate that urea fertilizer to the surface of soils may lead to a temporary immobilization of Al and, therefore, alleviated Al toxicity to plant seedlings.

A role for cyclic hydroxamates in aluminium resistance in maize?

Hydroxamate siderophores have been found to alleviate Al toxicity in bacteria. In Poaceae plants cyclic hydroxamates, like DIMBOA (2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one) and its derivatives have mostly been studied in relation to either defence against insects or allelopathy. In this study the influence of Al on concentrations of these benzoxazinoids (Bx) in root tips, whole roots and root xylem exudates of Zea mays L. varieties differing in Al resistance was analyzed by HPLC-MS. Aluminium resistant maize variety Sikuani maintained considerably higher Bx levels in root tips than the Al sensitive variety Bakero. In vitro binding of Al to DIMBOA was shown by fluorescence quenching. Addition of DIMBOA to Al-containing nutrient solution protected the sensitive maize against Al toxicity as shown by bioassays using callose and haematoxylin staining of root tips as stress indicators. This is the first study showing that Bx can detoxify Al in solution. Tissue analysis data provide first, circumstantial, support for a role of Bx in defence against Al toxicity also in planta.

Laboratory and Greenhouse Evaluation of Quicklime Sources for Suitability as Agricultural Liming Materials

Quicklime or burnt lime (CaO), is a highly soluble liming material recognized as an acceptable means for neutralization of acid soils in the United States. Blending CaO and MgO with agricultural lime (CaCO3) may prove an alternative to ground agricultural limestone for raising soil pH and providing fertilizer Ca and Mg for crop growth. The objectives of this study were to determine the chemical properties of several CaO, MgO, and CaCO3 liming blends compared to carbonate limestones, and analyze their respective effects on soil solution chemistry and shoot yield of a sorghum-sudangrass hybrid grown under greenhouse conditions. Three formulations of oxide liming materials (CaO, CaO + CaCO3, and CaO + MgO + CaCO3) were evaluated in the laboratory to determine their respective effectiveness as soil amendments compared to two dolomitic limestones (ground and pelletized) and reagent-grade calcium hydroxide [Ca(OH)2]. A 150-day incubation study was conducted to determine the effects of liming materials on soil pH and extractable nutrients, and a greenhouse study was completed to observe the effects of liming materials and rates on the herbage yield of a sorghum-sudangrass hybrid. Oxide liming materials averaged higher calcium carbonate equivalence (CCE) than ground and pelletized dolomitic limestones. An incubation study found CaO, CaO + CaCO3, and CaO + MgO + CaCO3 resulted in rapid pH adjustments (from pH 4.1 and 4.4 to 6.0 +) at one day after test initiation for two soil types and gave higher mean pH values than carbonate limes when applied at equivalent rates based upon CCE. Oxide liming materials also resulted in higher concentrations of soil extractable calcium (Ca) and magnesium (Mg), and the use of all liming materials, regardless of chemical form, alleviated Al toxicity. In a nine week greenhouse bioassay, all oxide liming materials and reagent-grade Ca(OH)2 were applied at 1/2X, 1X, and 2X the recommended application rate of ground dolomitic limestone based on CCE, resulting in similar or higher shoot yields comparatively at conclusion of this study. Because oxide liming materials have higher CCE values and provide more water-soluble sources of fertilizer Ca and Mg compared to ground and pelletized limestone, producers have potential for obtaining a more complete neutralization of acid soil conditions and shoot yields similar to or higher than those obtained with dolomitic limestone, using less material in the process. This study did not determine if changes in current liming practices would result in reductions of total fertilizer costs per year using oxide limes in place of carbonate forms.

Interactive effects of Al, Ca and other cations on root elongation of rice cultivars under low pH.

As with other crop species, Al tolerance in rice (Oryza sativa) is widely different among cultivars, and the mechanism for tolerance is unknown. The Ca2+-displacement hypothesis, that is, Al displaces Ca2+ from critical sites in the root apoplast, was predicted to be the essential mechanism for causing Al toxicity in rice cultivars. If displacement of Ca is an essential cause of Al toxicity in rice, Al toxicity may show the same trend as toxicities of elements such as Sr and Ba that are effective in displacing Ca. The interactive effects of Al, Ca, Sr and Ba on root elongation of rice cultivars with different Al tolerances were evaluated in hydroponic culture. Al and Ca accumulation in root tips was also investigated. Not only Al but also Sr and Ba applications inhibited root growth of rice cultivars under low Ca conditions. As expected, rice cultivars more tolerant of Sr and Ba were also tolerant of Al (japonica > indica). Although Mg application did not affect Sr or Ba toxicity, Mg alleviated Al toxicity to the same level as Ca application. In addition, Ca application decreased the Al content in root tips without displacement. These results suggest that Ca does not have a specific, irreplaceable role in Al toxicity, unlike Sr and Ba toxicities. Alleviation of Al toxicity with increasing concentrations of Ca in rice cultivars is due to increased ionic strength, not due to decreased Al activity. The difference in Al tolerance between indica and japonica cultivars disappears under high ionic strength conditions, suggesting that different electrochemical characteristics of root-tip cells are related to the significant difference in Al tolerance under low ionic strength conditions.

Effects of Boron on Aluminum Toxicity on Seedlings of Two Soybean Cultivars

Boron (B) is an essential micronutrient for higher plants. It has been suggested that addition of B at an optimal concentration might alleviate Al toxicity. Little information is available about B effects on soybean seedling growth under Al stress and differential alleviation of B on Al toxicity. In the present study, the seedlings of two soybean cultivars (Williams and Nanrong 73-935) were grown in a solution with factorial combinations of Al (0, 2, and 5 mM) and B (0, 5, and 50 μM) in a randomized complete block design experiment for 18 days. The results showed that high B was found to ameliorate Al toxicity by significantly increasing the growth characters including root length under 2 mM Al stress, and epicotyl length and fresh weight under 5 mM Al stress of the two cultivars. However, high B concentration did not significantly increase chlorophyll content under Al stress. Williams was more sensitive than Nanrong 73-935 on the growth characters and chlorophyll content under double stresses (B deficiency combined with Al toxicity), although they had similar sensitivity to B deficiency stress alone for growth characters. In addition, high B concentration was found to cause toxicity symptoms on root length and older leaves of both cultivars under no Al stress.

Combined effects of Mg and Ca supply on alleviation of Al toxicity in wheat plants

A Bangladeshi wheat (Triticum aestivum) cultivar, Kalyansona, was exposed to 100 J.µ Al with varying concentrations of Ca (250, 500, 2,500 J.µM) and Mg (103, 1,029, 4,115 J.µ) in the nutrient solution for 14 d. Exposure to Al along with low concentrations of both Ca and Mg caused severe damage to the root apex. Under AI-stressed conditions, an increased supply of Mg from 103 to 1,029 J.µ enhanced the root length by 44, 22, and 3% at 250, 500, and 2,500 J.µ Ca, respectively. Root growth at 2,500 J.µ Ca was not affected by additional Mg concentration. Again, an increased supply of Mg from 103 to 4,115 J.µ enhanced the root length by 56% at 500 J.µ Ca. Under the Al-stressed conditions, the increase in the root length was correlated with the decrease in the Al content in roots. Our results suggest that Mg alleviates Al toxicity efficiently in the presence of Ca at a low level in the culture solution by decreasing Al accumulation in roots.

Testing the ability of organic ligands and plant materials to reduce the toxic effects of aluminium in soils

An acid soil from the Sedgwick region of central Victoria was modified to provide a range of aluminium (Al) concentrations in order to test whether incorporation of organic ligands, or plant material, could reduce plant-available (or 'reactive') Al in soils. Al concentrations in the soil were increased by addition of varying amounts of a solution of AlK(SO4)2, chosen after measurement of the adsorption of Al onto the soil. A similar study of citrate adsorption allowed estimation of the amount of citrate required to achieve a 1 : 1 Al : citrate ratio in the soil in order to test the effectiveness of organic ligands in alleviating Al toxicity. Citrate was found to decrease the level of reactive Al in the soil. Pot trials also showed that addition of citrate to Al amended soil caused some improvement in root length and dry weight of soybean plants (Glycine max) compared with the Al amended soil. Addition of oxalate also reduced the level of reactive Al but did not improve root growth. Incorporation of dried leaves from lucerne (Medicago sativa), rhubarb (Rheum rhubarbarum), oxalis (Oxalis pes-caprae), and soybean at application rates equivalent to 5 and 10 t/ha to a soil containing added Al tended to increase root length. The most surprising result was the relatively small effect of high Al concentrations on plant growth, with 2000 μm reactive Al reducing root length by only 50% after 15 days of growth. This is a much smaller reduction in root length than has previously been obtained in hydroponic systems with 500 μm reactive Al after 15 days growth (Ginting et al. 1998) and points to a major difference between plant growth trials in solution culture and soil systems. One possible explanation for this difference is that the roots are in contact with only a small volume of soil solution, and this can be modified by root exudates. Further research is required to test this hypothesis, which will require the development of a method of analysis for Al in soil systems that more accurately reflects reactive Al levels in the root-zone.

Role of exudation of organic acids and phosphate in aluminum tolerance of four tropical woody species.

Responses of Melaleuca leucadendra (L.) L., Melaleuca cajuputi Powell, Acacia auriculiformis A. Cunn. ex Benth. and Eucalyptus camaldulensis Dehnh. to aluminum (Al) toxicity at low pH are poorly understood. We investigated effects of low pH and exudation of ligands by roots on Al tolerance of these species. Seedlings were grown hydroponically in nutrient solutions at pH 4.2 or 3.5 containing AlCl3 at concentrations ranging from 0 to 4 mM Al. The presence of 4 mM Al at pH 3.5 depressed growth in all species. Growth depression was greatest in E. camaldulensis, least in A. auriculiformis. In the low Al treatment (0.5 mM Al), roots of M. cajuputi tended to have the highest Al concentration among species, whereas in the 4 mM Al treatment, the highest Al concentration was found in roots of E. camaldulensis. Aluminum application enhanced root exudation of citrate in all species, with the enhancement in M. cajuputi, M. leucadendra and A. auriculiformis being similar and much greater than in E. camaldulensis. Exudation of oxalate and phenolic compounds was greater in E. camaldulensis than in the other species. The presence of Al enhanced phosphate exudation in all species, particularly in A. auriculiformis. Acacia auriculiformis was tolerant to low pH, probably because the presence of an unknown substance increased the pH. Application of 0.38 mM Al alleviated the toxicity of the pH 3.5 treatment in E. camaldulensis and M. cajuputi, whereas low pH alleviated Al toxicity in A. auriculiformis. We conclude that exudation of ligands such as citrate and phosphate only partly accounts for interspecific differences in Al tolerance among the tropical woody plants studied, whereas the reciprocal alleviation of Al toxicity and low pH differed considerably among the species.

Evaluation of industrial by‐products as soil acidity amendments: chemical and mineralogical implications

SummaryLime, mined gypsum and some gypsum‐like by‐products have been frequently applied to soil to counter acidity. We have examined the changes in the chemical and mineralogical properties of three acid soils resulting from the application of three industrial by‐products (phosphogypsum, red gypsum and sugar foam). We did so in laboratory experiments on suspensions of soil in saturated solutions of the by‐products. A sizeable fraction of the Al released by exchange with Ca of the amendments remained on the mineral surfaces as Al polymers, as suggested by changes in the CuCl2‐, oxalic–oxalate‐ and DTPA‐extractable Al contents. Polymerization of Al was promoted by corresponding neutral and basic pH of red gypsum and sugar foam saturated solutions, respectively. Sorption of these Al polymers was particularly favourable in those horizons with more clay fraction and variable‐charge clay minerals. On the other hand, in most cases there was little or no detectable sorption of sulphate, thus excluding precipitation of crystalline or amorphous basic Al sulphates to alleviate Al toxicity. Based on the reduction of the Al saturation of the exchange complex in the soils, as well as on the small contents of heavy metals and natural radionuclides of the three by‐products, these can be considered effective alternatives to mined gypsum and lime for alleviating soil acidity and reducing toxic concentrations of Al in agricultural, acid soils.

Magnesium is more efficient than calcium in alleviating aluminum rhizotoxicity in soybean and its ameliorative effect is not explained by the Gouy-Chapman-Stern model.

The mechanistic basis for cation amelioration of Al rhizotoxicity in soybean was investigated through a series of studies comparing protective effects of Ca and Mg against Al inhibition of root elongation in a background 0.8 mM CaSO4 solution (pH 4.3). A modified Gouy-Chapman-Stern model was used to evaluate the effect of cations on electrical potential and Al3+ activity at root plasma membrane surfaces. Activities of Al3+ up to 4.6 microM in the background solution inhibited soybean tap root elongation by more than 80%. There was little or no response in root elongation when Ca and Mg were added to background solutions in the absence of AL: When added to Al-toxic solutions in the micromolar concentration range, Mg was 100-fold more effective than Ca in alleviating Al toxicity, whereas both cations were equally effective when added in the millimolar concentration range. The protective effect of micromolar additions of Mg on root elongation was specific for Al and it failed to alleviate La rhizotoxicity. In contrast to wheat, Mg amelioration of Al toxicity to soybean root elongation at low Mg concentration could not be explained by changes in potential and Al3+ activity at the root plasma membrane surfaces as predicted by a Gouy-Chapman-Stern model. These results suggest that Mg is not acting as an indifferent cation when present at low concentration and implies the involvement of a mechanism other than pure electrostatic effects at the root surface.

Aluminium/silicon interactions in conifers

It is widely accepted that acidic precipitation has been responsible for the die-back of trees in Western Europe and North America. The mechanisms responsible for the die-back are, however, not very well understood. It is known that free aluminium (Al) is mobilised in soils that are affected by acidic precipitation, and Al toxicity is probably a major factor in the die-back. Al toxicity in conifers has been attributed to the induction of calcium (Ca) or magnesium deficiency. However, the potential role of silicon (Si) in alleviating Al toxicity has received little attention. Here we review the literature concerning Al and Si in conifers, and present some new data documenting mineral deposition in their needles. Needles of Picea glauca, Pinus strobus, Larix laricina and Abies balsamea were analysed using X-ray microanalysis to determine mineral distribution in the leaf tissues. The distribution of Al, Ca and Si varied with the species, but Al was invariably codeposited with Si. This may be a mechanism for sequestering toxic Al and thus could represent a defence against Al toxicity.

Metal Solubility in Phosphogypsum‐Amended Sediment under Controlled pH and Redox Conditions

AbstractThe storage of phosphogypsum (PG, CaSO4 xH2O) has occupied vast areas of land that remain abandoned and subject to leaching and erosion by wind and water. The major constraints for PG use in the environment are the presence of several heavy metals and its radioactivity. Mississippi River alluvial sediment amended with PG (1%) was equilibrated under controlled redox (−250, −100, 0, 150, 300, and 500 mV) and pH (5.0 and 7.0) conditions to evaluate the feasibility of using PG to reduce aqueous concentrations of potentially toxic heavy metals and as a nutrients source. Phosphogypsum is an effective nutrient source since it increased water soluble Ca, Mg, K, and P concentrations (essential plant nutrients). At the same time, PG significantly reduced soluble Al at pH 7.0 (normal pH value of anaerobic wetland sediments), thereby reducing Al toxicity to plants growing in the sediment. Phosphogypsum addition resulted in a large increase in sulfide levels in reduced sediment suspensions. As a result, at low redox potential values, the solubility of spiked heavy metals (Cd and Ni at rates of 1000 mg kg−1) and natural trace elements was substantially reduced by precipitation as insoluble sulfides. Low sediment pH (pH 5.0) resulted in the highest release of all the studied metals [Ca, Mg, P, K, Fe, Mn, Cd, and Ni] into the sediment solution, likely due to H+, Fe2+, Mn2+, and Al3+ displacing these metals from the exchange complex. This study, therefore, indicates that PG application to Louisiana Mississippi River alluvial sediments is useful for alleviating Al toxicity (neutral pH), increasing nutrient availability under watterlogged conditions, and reducing aqueous concentrations of toxic heavy metals to trace levels under anoxic conditions by precipitating these toxic elements as insoluble sulfides. If PG or PG‐products are placed in neutral to alkaline sediments/soils and/or reducing environments, metals release at toxic levels should be of little concern to the wetland environment.

Effect of citric acid on aluminum toxicity in the growth of mungbean seedlings

Citric acid was reported to chelate aluminum (Al) strongly and alleviate Al toxicity. Detoxification of Al by adding citric acid was studied in experiments with mungbean (Phaseolus aureus Roxb.) seedlings and cuttings (without roots) grown in a growth chamber. The seedlings and cuttings were grown for sixteen days in a dilute nutrient (pH 4.1) at two concentrations of citric acid (0, 5 mM) and three concentrations of Al (0, 2, and 5 mM) using a complete randomized design method. Results showed that 5 mM citric acid increased the height of mungbean seedlings, length of epicotyl, and fresh weight of seedlings and roots significantly. Although the mitigation by adding citric acid was more effective at higher Al concentrations, it seemed that citric acid was harmful to fresh and dry weights of cuttings under higher Al stress. This was perhaps attributed to the absence of roots. The content of soluble protein and chlorophyll in the seedlings were decreased under high Al stress by adding citric acid, but were unaffected in the cuttings at 2 mM Al. This also implied that alleviation of Al toxicity by adding citric acid was related to the root function.

Calcium alleviation of hydrogen and aluminum inhibition of soybean root extension from limed soil into acid subsurface solutions

Alleviation by calcium (Ca) of inhibition of soybean [Glycine max (L.) Merr. cv. ‘Ransom'] root elongation by hydrogen (H) and aluminum (Al) was evaluated in a vertical split‐root system. Roots extending from a limed and fertilized soil compartment grew for 12 days into a subsurface compartment containing nutrient solution with treatments consisting of factorial combinations of either pH (4.0, 4.6, and 5.5) and Ca (0.2, 2.0, 10, and 20 mM), Al (7.5, 15, and 30 μM) and Ca (2.0,10, and 20 mM) at pH 4.6, or Ca (2, 7, and 12 mM) levels and counter ions (SO4 and Cl) at pH 4.6 and 15 μM Al. Length of tap roots and their laterals increased with solution Ca concentration and pH value, but decreased with increasing Al level. Length of both tap and lateral roots were greater when Ca was supplied as CaSO4 than as CaCl2, but increasing Ca concentration from 2 to 12 mM had a greater effect on alleviating Al toxicity than Ca source. In the absence of Al, relative root length (RRL) of tap and lateral roots among pH and Ca treatments was related to the Ca:H molar activity ratio of solutions (R2≥0.82). Tap and lateral RRL among solutions with variable concentrations of Al and Ca at pH 4.6 were related to both the sum of the predicted activities of monomeric Al (R2≥0.92) and a log‐transformed and valence‐weighted balance between activities of Ca and selected monomeric Al species (R2≥0.95). In solutions with 15 μM Al at pH 4.6, response of tap and lateral RRL to variable concentrations of CaSO4 and CaCl2 were related to predicted molar activity ratios of both Ca:Al3+ (R2≥0.89) and Ca:3 monomeric Al (R2≥0.90), provided that AISO4 and AI(SO4)2 species were excluded from the latter index. In all experiments H and Al inhibited length of lateral roots more than tap roots, and a greater Ca:H or Ca:Al concentration ratio was required in solutions to achieve similar RRL values as tap roots.

Effects of aluminum on nutrient solution pH and nitrate/ammonium uptake by triticale

Aluminum (Al) plant tolerance has been frequently associated with a pH increase in the rhizosphere. The changes in pH are dependent on plant genotypes and ionic composition and strength of nutrient solutions. This work was performed in order to study in triticale (Triticosecale Wittm.) the association of pH change with nitrogen (N) uptake and growth performance in acid conditions. Three‐day‐old seedlings were treated with Al (185 μM) in solutions having different proportion nitrate/ammonium (NO3/NH4), 15/1 and 8/1, but the same total N content. Along the period with Al treatment, several measurements have been made: pH, every day; NO3 and NH4 uptake from the solution as well as shoot and root biomass production every two days (five and seven days of plant age). The maximum growth inhibition (30%) of fresh weight was found in roots of plants in the 15/1 (NO/NH,) nutrient solution. The presence of a higher proportion of NH4 (8/1 solution) had a protective effect on Al damage as shown by less growth inhibition and less reduction in NO3 uptake. Changes in pH apparently were not relevant for the tolerance. The results suggest that NH4 fertilization may be useful for alleviating Al toxicity in triticale.

Aluminium-organic matter-fluoride interactions during soil development in oxidised mine waste

A three year field trial assessing the suitability of mine waste rock as a surrogate subsoil with different depths of topsoil showed that pasture yield was significantly lower in the unmodified waste rock (UWR) treatments compared to waste rock modified with lime and potassic superphosphate (MWR) during the first two years but there was no difference in yield in the third year between the waste rock treatments. The reduced yield in UWR during the first two years was considered to be due to phytotoxic levels of aluminium (Al) in the UWR soil solution. Soil samples from the waste rock interface (A) and waste rock at depth ≥ 300 mm (B) collected at the end of the three year trial were analysed for different forms of soil Al, total soluble fluoride (F), dissolved organic carbon (DOC) and soil pH to determine whether organic matter and F leachate from the topsoil reduced Al toxicity at the waste rock interface (A) during soil development over 3 years. Results showed that after 3 years, 0.02 M CaCl 2 extractable Al (Al Ca) and 1 M KCl extractable exchangeable + soluble Al (Al K) were effectively reduced in the MWR interface (A) by lime and fertiliser applications but they remained at very high phytotoxic levels (Al Ca = 17–21 μg g −1, Al K = 261–339 (μg g −1) in the UWR interface (A) irrespective of topsoil depth treatment. The corresponding organic bound Al (Al OM) ranged from 200 to 214 μg g −1 and DOC was in the range 169–203 μg g −1 in both UWR and MWR interfaces (A). These values were a significant two-fold higher than the values at depth ≥ 300 mm (B) which were considered to be the original levels of Al OM and DOC in UWR. The increase in Al OM and DOC by the third year is probably due to progressive leaching of organic ligands produced from the decomposition of organic matter in the topsoil subject to an annual rainfall of 2500 mm. This evidence for the reduction in Al toxicity by organic matter is further supported by significant dry root density (DRD) in the UWR interface (A). This indicated that organic matter may have complexed toxic Al in UWR and alleviated Al toxicity in the third year of the field trial. Total soluble F levels were similar between interface (A) and waste rock depth (B) of the UWR, suggesting that F levels have not changed during the three years of the trial and therefore there was no evidence of F complexing with phytotoxic Al in the soil solution in the presence of organic ligands.

Effects of organic acid fractions extracted from Eucalyptus camaldulensis leaves on root elongation of maize (Zea mays) in the presence and absence of aluminium

Complexes of aluminium (Al) with organic ligands are believed to represent an important detoxification mechanism in acid soils. However, relatively little is known about the particular ligands produced by decomposing vegetation or about their effects on plant growth in the presence or absence of toxic Al. This paper reports an experiment on the effects of decomposition products of Eucalyptus camaldulensis leaves on the root elongation of maize (Zea mays) cv. DK687 in the presence or absence of Al. The static solution culture experiment used fulvic acid (FA) and humic acid (HA), extracted from E. camaldulensis leaves, at three nominal concentrations, viz. 40, 120 and 360 mg C L-1, replicated 4 times in the presence and absence of 30 µM Al. In the absence of Al, root elongation was increased by 30% by HA at 40 mg C L-1 and by 36% by FA at 120 mg C L-1. In the presence of 30 µM Al, the effects of toxic Al on root elongation were negated by FA and HA at all concentrations. Aluminium was totally complexed in all treatments except FA at 40 mg C L-1 in which treatment only 2.7 µM Al was present in the monomeric form. The E. camaldulensis FA and HA at concentrations of 40 and 120 mg C L-1, either in the presence or absence of Al, stimulated maize root elongation. Aluminium was strongly complexed by the E. camaldulensis FA and HA. The present results, in which FA and HA alleviated Al toxicity limitations on root elongation of maize, are relevant to the protection afforded to plant growth in acid soils amended with organic materials. They highlight the need to focus more on the role of FA and HA.