Effects of phosphogypsum or calcium sulfate on aluminon reactive aluminum in solutions at varying pH

Abstract

Growing evidence of positive crop responses to gypsum or phosphogypsum (PG) application in acid soils strongly support the use of these amendments as an ameliorant of subsoil acidity. Although gypsum improves Ca availability in subsoils, its role in alleviation of Al toxicity needs careful attention. In the current study, either PG, CaSO₄.2H₂O or CaCl₂.2H₂O was added (to supply 12 mM Ca) to solutions containing 40 μM Al at pH 4.1 + 0.1. Solution pH was gradually raised to 4.5, 4.8 and then to 5.3 at various time intervals during 25 d aging of the solutions at 25 + 1^OC.

Concentration of Al measured by aluminon method without preacidification and preheating, referred to as “reactive Al”; in this paper, was 16 μM in 2 g L^‐1PG solution without added Al. This accounted 38% of total soluble Al in PG solution. Addition of 2 g L^‐1PG to solution containing 40 μM Al, resulted in only 42% of total Al in solution present in forms reactive with aluminon. According to MINTEQ speciation model, Al in solution was present as an entirely complexed form with F^‐. An increase in solution pH up to 5.3 had no effect on measured concentration of reactive Al or predicted distribution of Al species.

Addition of CaSO₄.2H₂O to 40 μMAl solutions had no effect on the concentration of reactive Al within pH 4.1 ‐4.8, however, up to 62% of total Al was in a form complexed with SO₄ ^2‐, as predicted by MINTEQ model. The concentration of reactive Al decreased by 60% at pH 5.3. Addition of CaCl₂.2H₂O also had no effect on the concentration of reactive Al within pH 4.1 ‐ 4.8. Nearly 73 ‐ 94% of total Al was present in Al³⁺form. An increase in pH to 5.3, decreased the concentration of reactive Al by 27%. The results suggest that ion‐pairing of Al with F^‐would appear to be a possible mechanism for alleviation of Al toxicity by PG at pH range 4.1 ‐ 5.3. With regard to CaSO₄.2H₂O, at pH 4.1 ‐ 4.8 ion‐pairing with SO.₄ ^2‐appears to be possible mechanism for the alleviation of Al toxicity. In addition, at pH 5.3 a considerable decrease in reactive Al was evident which would further alleviate Al toxicity.     

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.     

Predicting Initial Tall Fescue Root Growth Response to Calcium/Aluminum Solution Concentrations

A study was conducted to quantify effects of soluble aluminum (Al) and gypsum (CaSO₄) on initial root growth of three varieties of tall fescue (Festuca arundinacea). Experiments were performed in a growth chamber using hydroponic solutions containing Al from 0 to74 µM in combination with CaSO₄ at 0 to10 mM. Seedlings were grown for 7 d, harvested, air dried, scanned, and weighed for treatment comparisons. Significant differences in root length existed between varieties in Al‐only solutions at low Al concentrations. All varieties showed reduced root growth at concentrations greater than 37 µM Al. Increased calcium (Ca²⁺) and sulfate (SO₄ ^2?) at given concentrations of Al resulted in greater root growth. Relative root growth increased approximately 30% to >80% at 37 µM Al as CaSO₄ increased from 2.5 to 10 mM. A simple logistic model adequately described the effects of Al and CaSO₄ on root growth (r² = 0.86, 0.95, and 0.96 for the three varieties).     

Hydrogen and aluminum inhibition of soybean root extension from limed soil into acid subsurface solutions

Soybean [Glycine max (L.) Merr. cv. ‘Ransom'] root elongation under varying concentrations of solution hydrogen (H) and aluminum (Al) was investigated in a vertical split‐root system. Roots extending from a limed and fertilized soil compartment grew for 12 days into a subsurface compartment with solutions adjusted to either different pH values from 3.7 to 5.5 or a factorial combination of pH (4.0,4.6, and 5.2) and Al (0,7.5, 15, and 30 μM) levels. Ionic forms of Al were estimated with GEOCHEM and solution Al was determined with ferron. Boron (B) (18.5 μM) and zinc (Zn) (0.5 μM) were supplied to all solution treatments, in addition to 2000 μM Ca, after preliminary studies at pH 5.2 without Al indicated that their omission inhibited length of tap roots and their laterals in the subsurface compartment. Both H⁺ and Al inhibited the length of lateral roots more than tap roots. Lateral roots failed to develop on tap roots at pH<4.3 or in treatments with 30 μM Al. Relative tap root length (RRL) among treatments receiving Al correlated with Al as measured by reaction with ferron for 30s. Ferron‐reactive Al was correlated to GEOCHEM‐predicted Al³⁺ activity (r=0.99). A 50% reduction in RRL occurred with either 2.1 μM Al³⁺ activity or 4.9 uM ferron‐reactive Al. The absence of shoot and soil‐root biomass differences among solution treatments in the split‐root system indicated that differences in root growth in the subsurface compartment were not directly confounded with differences in top growth.     

Influence of phosphate and hydroxyl ions on aluminum toxicity in soybeans and wheat

Aluminum (Al) toxicity to plants in complete nutrient solutions is difficult to relate to Al activity in solution because of precipitation and complexation. Aluminum toxicity was studied for two seedling crops, sorghum (Sorghum bicolor L. Moench) and wheat (Triticum aestivum L. em Thell), at low levels (≤10 μM) in two incomplete nutrient solutions to study plant response to Al alone, Al+PO₄ ^3‐, Al+OH^‐, and Al+PO₄ ³‐+OH^‐. Relative root length was the bioassay for Al toxicity. ‘Monomeric’ Al was measured using Aluminon and both root length and measured Al were compared to the theoretical Al in solution predicted by the MINTEQA2 equilibrium model.

Low levels of Al were toxic to plant roots with sorghum showing a decrease in relative root length from 1 to 10 μM Al, and wheat showing a decrease from 4 to 10 μM. A mono‐salt background solution (400 μM CaCl₂) and a more complex base solution (CaCl₂, KNO₃, and MgCl₂) gave similar root lengths and measured Al values. Phosphate and hydroxyl ameliorated Al toxicity and lowered measured Al in solution, but not to the extent predicted by the model. Adding phosphate (PO₄ ^3‐) or hydroxyl (OH^‐) raised the pH, but again not as high as the model predicted. The difference in toxicity and measured Al were most likely the result of polymers (Al⁺³) which are toxic, but not measured by the procedure used, or included in the model which showed the Al as being removed from solution by precipitation.     

Characterization of phytotoxic aluminum in soil solutions from phosphogypsum amended soils

Phosphogypsum (PG), an industrial by-product from phosphoric acid plants, is being used as an ameliorant for acid soil infertility. Phosphogypsum is primarily CaSO₄ and contains F among several impurities. An increase in SO₄ ^2– and F^– ligands in soil solutions following amendment with PG is important for the alleviation of Al toxicity. In soil solutions containing abundant SO₄ ^2– and F^–, a measure of Al that is not complexed with either of the ligands represents phytotoxic Al. Alleviation of Al toxicity by SO₄ ^2– in some instances is due to SO₄ ^2–-induced precipitation and/or sorption of Al. Some studies have demonstrated a decrease in Al toxicity by SO₄ ^2– even though precipitation of Al was not evident.In such cases, formation of the Al-SO₄ ⁺ ion-pair (predicted by speciation models) has been attributed as the reason for alleviation of Al toxicity. Recent evidence has indicated that the proportion of Al complexed with SO₄ ^2– was much greater than that predicted by using speciation models, which suggests that alleviation is not fully attributable to the formation of AlSO₄ ⁺ species. The existing colorimetric Al assay techniques have failed to discriminate Al complexed with SO₄ ^2– and have therefore, proved to be unsuitable for determination of phytotoxic Al in solutions containing SO₄ ^2–. Fractionation of Al complexed with SO₄ ^2– by size exclusion chromatography (SEC) has permitted a more precise characterization of Al complexed with SO₄ ^2– which has been demonstrated as less phytotoxic than the uncomplexed Al. Therefore, size exclusion chromatography is a promising technique for characterization of phytotoxic Al in solutions in the presence of SO₄ ^2–. In solutions containing F^–, alleviation of Al toxicity is due to formation of Al-F complexes which are less phytotoxic. The 8-hydroxyquinoline method with a 15 second reaction time excludes Al-F complexes and, therefore is a measure of phytotoxic Al in solutions containing F^–.     

Root exudation,organic acids,and element distribution in roots of Norway spruce seedlings treated with aluminum in hydroponics

Seedlings of Norway spruce (Picea abies [L.] Karst.), which had been grown under sterile conditions for three months, were treated for one week in a hydroculture system with either 500 μM AlCl₃ or 750 μM CaCl₂ solutions at pH 4. Organic acids were determined in hot‐water extracts of ground root tissue. Oxalate (3.3—6.6 μmol (g root dry weight)^—1) was most abundant. Malate, citrate, formate, acetate, and lactate concentrations ranged between 1—2 μmol (g root dry weight)^—1. Organic substances and phosphate found in the treatment solutions at the end of the experimental period were considered to be root exudates. Total root exudation within a 2‐day period ranged from 20—40 μmol C (g root weight)^—1. In root exudates, organic acids, and total carbohydrates, total amino acids, and total phenolic substances were quantified. Citrate and malate, although present in hot‐water extracts of root tissue, were not detected in root exudates. Phosphate was released from Ca‐treated plants. In Al treatments, there was indication of Al phosphate precipitation at the root surface. Oxalate and phenolics present in the exudates of Norway spruce seedlings are ligands that can form stable complexes with Al. However, concentrations of these substances in the treatment solutions were at micromolar levels. Their importance for the protection of the sensitive root apex under natural conditions is discussed.     

Response of cotton cultivars to aluminum in solutions with varying silicon concentrations

Abstract

Cotton (Gossypium hirsutum L.) is extremely sensitive to Al toxicity. Increasing Si concentration in solution has been reported to alleviate AI toxicity. In this investigation the effects of varying Si concentrations (700, 1400, and 2800 μM Si) on reactive Al (defined as Al reactive with aluminon during 10‐s reaction time, without acidification and heating) was studied in solutions containing either 50, 100 or 200 μM Al during 50 d of aging. An increase in Si concentration had negligible effects on the reactive Al in solutions with 50 or 100 μM Al. However, in solutions with 200 μM Al the reactive Al decreased by 6 to 15% with an increase in Si concentration from 0 to 2800 μM.

The effects of either 700, 1400 or 2800 μM Si on root growth of Coker 208, Coker 315, DPL 90, McNair 235, Stoneville 506 and Tifcot 56 cotton cultivars were investigated in solutions containing either 0, 10, 20 or 40 μM Al with 500 μM Ca at pH 4.5. In solutions containing no Al, addition of 700 μM Si improved root growth by 69–87% in Coker 315, DPL 90 and McNair 235 cultivars but not in the other cultivars. In solutions containing 10 μM Al, an increase in Si concentration from 0 to 2800 μM improved the root growth by 15–17% in DPL 90 and McNair 235 cultivars only. An increase in Si additions failed to improve root growth of any of the cultivars in solutions with 20 or 40 μM Al.     

Effects of Alum-treated Waste Water Sludge on Barley Growth

Alum sludge derived from a municipal wastewater plant was used as a soil amendment in a greenhouse study with barley (Hordeum vulgare) as the test crop. Treatment variables included the soil pH (4.5, 5.1 and 6.5), the amount of Al in the sludge (control = 30 mg Al_T/g; alum sludges = 38 and 52 mg Al_T/g), and the sludge application rate (100 and 270 kg N_T/ha). Soil amendment with the two alum sludges reduced soil pH, increased Al³⁺ activity in the soil solution, and reduced barley growth over the 6-week experiment. Barley growth decreased as the Al³⁺ activity in the sludged soil solution increased, but for a given Al³⁺ the phytotoxicity of Al was markedly pH dependent. For example, at a pH of 5.0 ± 0.1 an Al³⁺ activity of 0.5 μM was sufficient to inhibit plant growth by about 50% this IC₅₀ value increased five-fold to about 2.5 μM when the soil pH was 4.5 ± 0.1. This decrease in the toxicity of Al50 with acidification was explained in terms of a competitive interaction between the H⁺-ion and Al³⁺ at the root surface. Stepwise multiple regression allowed the prediction of aerial leaf biomass from soil pH and sludge application rate.     

ALUMINUM SPECIATION OF AMENDED ACID TROPICAL SOIL AND ITS EFFECTS ON PLANT ROOT GROWTH

Exchangeable and soluble soil aluminum (Al) is limiting plant growth in many soils worldwide. This study evaluated the effects of increasing rates of dolomite and magnesium carbonate (MgCO₃) on Al³⁺, pH, dissolved organic carbon, cations, anions, and Al speciation on oil palm Deli dura × AVROS pisifera root growth. Dolomite and MgCO₃ additions significantly raised linearly soil solution pH, magnesium (Mg²⁺), nitrate (NO₃ ^?) and chlorine (Cl^?) concentrations; exponentially decreased the activity of phytotoxic Al species [aluminum (Al³⁺), aluminum sulfate (Al₂SO₄), and aluminum fluoride (AlF₃)]; and reduced manganese (Mn) concentration and activity. High activity of those species exponentially reduced root dry weight. Optimum oil palm growth was achieved at: <50 μM monomeric Al, < 30 μM Mn, and <0.20 unit of the ratio Al+Mn to calcium (Ca)+Mg. High activity of Al species and Mn in acidic soil solution cause significant reduction of the root growth. Soil acidity alleviation either with dolomite or MgCO₃ mitigates the toxic effect of Al and Mn.     

Supernatant solutions containing various levels of aluminum and similar concentrations of phosphorus for aluminum phytotoxicity studies

Abstract

Precipitation of Al(OH)₃ and aluminum phosphate may occur in nutrient solution if a large amount of Al and P have been added to a relatively high pH. The objective of this study was to develop and test a supernatant‐solution method for Al phytotoxicity studies with large and/or old plant seedlings. Effects of pH and additions of Al and P on ionic strength and concentrations of Al and P in supernatant nutrient solutions were investigated. Two sets of supernatant nutrient solutions at two pH levels were prepared. The pH 4.0 set and 4.5 set contained seven levels of Al (maximum Al concentration of 6355 and 378 μM) and similar P concentration about 32 and 6 μM P, respectively. The Al concentrations in supernatant solutions were dependent on preparation procedure. The pH 4.0 set was tested in the greenhouse study with 6‐month‐old citrus seedlings and found to be successful as culture solutions for Al phytotoxicity studies. These two sets are suitable for growth of large (about 0.3 m) and/or old (about 6 mon.) seedlings. This supernatant‐solution method makes it possible to study Al phytotoxicity of large and/or old seedlings, to avoid the confounding effects of P on Al with respect to plant growth, and to report the actual concentrations of Al and P in growth solutions.     

Aluminum diffusion in Oxisols as influenced by soil water matric potential,ph, lime,gypsum, potassium chloride,and calcium phosphate

Abstract

Plant root exposure to soil aluminum (Al) depends on the soil solution Al concentration and transport to the root by diffusion. Changes in Al diffusive flux for two Oxisols was measured under laboratory conditions as a function of pH, water matric potential, and applications of gypsum, potassium chloride, and calcium phosphate. Double‐faced cation exchange resin sheets served as sinks for Al transported during 10‐day incubations through chambers containing 314 cm³ of soil. Across a range of soil pH values from 4.5 to 5.5, maximum diffusive flux of Al occurred at pH values of 4.7–4.8 in both soils and corresponded to increases of 2.2–3.0% relative to the unlimed treatment. Between pH values of 4.7–4.8 and 5.4, diffusive flux of Al decreased by 38 and 46% in the two Oxisols. Diffusive flux of Al decreased by 16–20% for the two Oxisols as soil water potentials decreased from ‐10 to ‐200 kPa. Magnitude of the reductions in diffusive flux of Al with decreasing soil water potential were less than those reported for diffusive flux of phosphorus (P) in prior investigations. Diffusive flux of Al increased by as much as 4‐fold with additions of CaSO₄ and KCl, which increased the soil solution Al concentration. Additions of 400 mg P dm^‐3 of soil had no effect on Al diffusion in either Oxisol.     

Effects of varying phosphorus additions on aluminum in solutions

Abstract

Effects of varying additions of phosphorus (0, 0.8, 8 and 80 μM P) on the concentrations of total and monomeric aluminum (Al) and on calculated sum of activities of monomeric Al species (Sa_{Al mono}) were investigated during 21 d of aging in deionised water containing 40 μM Al with either 0 or 1500 μM calcium (Ca). These P and Ca treatments were also maintained in deionised water without the addition of Al. The concentrations of total and monomeric Al decreased with an increase in P additions at the 8 and 80 μM P although the effect was negligible at the 0.8 μM P. The effects of increase in P additions on the monomeric Al were almost instant as evident from 12 to 49 percent decrease in its concentration after only an hour of aging. However, marked effects of P on the total Al were observed after 3 d of aging. On 21 d of aging in solutions with 8 and 80 pH P, the concentrations of total and monomeric Al decreased by 17 to 34 percent and 20 to 60 percent, respectively. The presence of Ca had negligible effects on the concentrations of either total or monomeric Al at the varying P additions. However, the calculated Sa_{Al mono} for a given P concentration treatment over Al mono 21 d of aging were considerably lower in solutions with 1500 μM Ca than with 0 Ca. At each P concentration treatment, Sa_{Al mono} were considerably lower than the concentration of monomeric Al in solutions with 1500 μM Ca, while in solutions with 0 Ca the above difference was negligible. During the 21 d of aging, in solutions containing 40 μM Al, the measured P concentrations in the 0.8, 8 and 80 μM nominal P concentration treatments decreased by 44, 37, and 19 percent respectively, at the 0 Ca treatments and 50, 56 and 24 percent respectively, at the 1500 μM Ca treatments.     

Sand culture of mycorrhizal plants

In this work, a sand culture system for ecto- or arbuscular mycorrhizal plants was developed. Nutrients were added into the sand at a constant rate using solutions with similar concentrations of nutrients as those found in forest soil solutions. Plants grew well in the system and inoculated plants developed abundant ecto- or arbuscular mycorrhizas. To test the suitability of the culture system for studies on metal toxicity, aluminium was added to the nutrient solutions of non-mycorrhizal and mycorrhizal spruce seedlings. Measurments of labile Al in the solution draining from the sand revealed that Al was mainly present as phytotoxic monomeric Al. In addition, concentrations of Ca²⁺, Mg²⁺ and SO₄^2— ions which have the potential to alleviate Al toxicity, were similar in the draining as in the nutrient solutions. After 10 weeks of exposure to 400 μM Al, taproot growth as well as Ca and Mg uptake of the seedlings were impaired by Al. Mycorrhizal colonization had no effect on the degree of inhibition of these processes. We conclude that this culture system is suitable for investigations on effects of Al and other toxic metals on mycorrhizal tree seedlings.     

Short‐term response of the Ca cycle of a montane forest in Ecuador to low experimental CaCl2 additions

The tropical montane forests of the E Andean cordillera in Ecuador receive episodic Sahara‐dust inputs particularly increasing Ca deposition. We added CaCl₂ to isolate the effect of Ca deposition by Sahara dust to tropical montane forest from the simultaneously occurring pH effect. We examined components of the Ca cycle at four control plots and four plots with added Ca (2 × 5 kg ha^–1 Ca annually as CaCl₂) in a random arrangement. Between August 2007 and December 2009 (four applications of Ca), we determined Ca concentrations and fluxes in litter leachate, mineral soil solution (0.15 and 0.30 m depths), throughfall, and fine litterfall and Al concentrations and speciation in soil solutions. After 1 y of Ca addition, we assessed fine‐root biomass, leaf area, and tree growth. Only < 3% of the applied Ca leached below the acid organic layer (pH 3.5–4.8). The added CaCl₂ did not change electrical conductivity in the root zone after 2 y. In the second year of fertilization, Ca retention in the canopy of the Ca treatment tended to decrease relative to the control. After 2 y, 21% of the applied Ca was recycled to soil with throughfall and litterfall. One year after the first Ca addition, fine‐root biomass had decreased significantly. Decreasing fine‐root biomass might be attributed to a direct or an indirect beneficial effect of Ca on the soil decomposer community. Because of almost complete association of Al with dissolved organic matter and high free Ca²⁺ : Al³⁺ activity ratios in solution of all plots, Al toxicity was unlikely. We conclude that the added Ca was retained in the system and had beneficial effects on some plants.     

Screening wheat and sorghum cultivars for aluminum sensitivity at low aluminum levels

Screening cultivars for aluminum (Al) tolerance is often conducted in acid soils or in complete nutrient solutions. The former method lacks precise measurements of Al, and the second requires high Al concentrations because of precipitation and chelation of the Al and is less representative of the actual environmental stresses to which plants must adapt. These experiments were designed to determine Al tolerance of wheat (Triticum aestivum L. em Thell) and sorghum (Sorghum bicolor L. Moench) using incomplete solutions with very low Al concentrations. Six wheat and five sorghum cultivars were screened for Al tolerance in solution culture with 0 to 10 μM Al and only Ca, K, Mg, NO₃, and Cl in the solutions. Plants were subjected to the solutions for 4 d, and the change in relative root length was measured. Solution Al levels and pH were measured after the termination of the experiments. ‘Atlas’ 66 and ‘Stacy’ were the most tolerant wheat cultivars ('Atlas 66’ = ‘Stacy’ ≥ ‘Monon’ ≥ ‘Scout 66’ ≥ ‘Arthur 71’ = ‘Oasis'). The wheat cultivars were effectively separated on a genetic response basis at 2 μM Al. Sorghum cultivars were uniform in their Al tolerance, but did show some separation at 1 μM Al (SC56 > Tx430 > ‘Funk GS22DR’ > SC283 = SC599). The pH and Al variations did not account for any of the differences observed, indicating that root length differences were caused by genetic control of response to high Al.     

Proton toxicity interferes with the screening of common bean (Phaseolus vulgaris L.) genotypes for aluminium resistance in nutrient solution

Common bean (Phaseolus vulgaris L.) proved to be very sensitive of low pH (4.3), with large genotypic differences in proton sensitivity. Therefore, proton toxicity did not allow the screening of common bean genotypes for aluminium (Al) resistance using the established protocol for maize (0.5 mM CaCl₂, 8 μM H₃BO₃, pH 4.3). Increasing the pH to 4.5, the Ca²⁺ concentration to 5 mM, and addition of 0.5 mM KCl fully prevented proton toxicity in 28 tested genotypes and allowed to identify differences in Al resistance using the inhibition of root elongation by 20 μM Al supply for 36 h as parameter of Al injury. As in maize, Al treatment induced callose formation in root apices of common bean. Aluminium‐induced callose formation well reflected the effect of Ca supply on Al sensitivity as revealed by root‐growth inhibition. Aluminum‐induced callose formation in root apices of 28 bean genotypes differing in Al resistance after 36 h Al treatment was positively correlated to Al‐induced inhibition of root elongation and Al contents in the root apices. However, the relationship was less close than previously reported for maize. Also, after 12 h Al treatment, callose formation and Al contents in root apices did not reflect differences in Al resistance between two contrasting genotypes, indicating a different mode of the expression of Al toxicity and regulation of Al resistance in common bean than in maize.     

Aluminum tolerance in maize: Minimizing pH changes in screening solutions 1

Preliminary screening of maize (Zea mays L.) genotypes for aluminum (Al) tolerance in nutrient solutions over a 12‐day growth period showed greater plant‐induced pH changes in solutions without Al than in solutions containing Al. Such pH changes may alter the specific effect of Al on relative root length (length in Al‐containing solution/length in 0 Al solution) commonly used as an index to rank genotypes with respect to Al tolerance. The objective of this study was to examine several screening methods for identifying Al‐tolerant maize genotypes, and to identify those procedures which resulted in minimal pH fluctuations during the course of screening. The following methods of controlling pH in nutrient solutions were compared: (i) 12‐day exposure to 0 or 5 mg Al/L in nutrient solutions (a) with or (b) without daily pH adjustment or (c) with different NO₃ ^‐/NH₄ ⁺ ratios, and (ii) 2‐day exposure to 0, 5, 10, 25 or 40 mg Al/L treatment solutions followed by a 3‐day recovery period in solutions with an initial pH at (a) 4.6 or (b) 4.0. In the 12‐day experiments, daily pH adjustment to 4.6 did not eliminate large pH fluctuations in the control (0 Al) solutions, and it substantially decreased the soluble Al concentration in the Al‐treatment solution. Varying the ratio of NO₃ ^‐ to NH₄ ⁺ did not eliminate large pH fluctuations. Exposing the seedlings for 2 days to Al solutions at pH 4.6 resulted in large pH differences between 0 Al and Al‐containing solutions and in precipitation of large amounts of Al. In contrast, the 2‐day procedure using solutions with an initial pH at 4.0 was more satisfactory in that the pH was maintained between 4.0 and 3.7 in all solutions, and Al precipitation was minimized. When the 2‐day method at pH 4.0 was used to screen the genotypes, PDMR3 had consistently higher relative root lengths in Al‐containing solutions than did Kalimpos, IPB Varl, UPCA Varl and Trinidad Grp1&2.     

Excess sulfate supply and onion‐induced human antiplatelet activity

Awareness is growing of health‐promoting functional foods and the use of various plants as nutraceuticals. Due to a suite of organosulfur compounds, onion, and other vegetable Alliums possess a unique antiplatelet effect that may promote cardiovascular health because aggregating platelets can lead to heart attack and stroke. Investigation of the consequences of S nutrition in onion has primarily focused on levels of S ranging from deficiency to just above sufficiency; however, little work has been conducted to examine the effects of supraoptimal S nutrition on organosulfur‐based traits. Four onion selections were grown in hydroponic solutions with 2, 7, and 12 mM SO₄^2—‐S in greenhouse experiments during 1995—1996 and 1996—1997. Onion plants were separated into root, leaf, and bulb portions and analyzed for mineral content. Onion bulb extracts were assayed for in vitro human antiplatelet activity. Bulb and leaf S were not affected by increasing solution culture S, but root S was increased by 98 % when solution culture S was increased from 2 mM to 12 mM. Similar increases in root Ca and Al were detected, suggesting CaSO₄ and AlSO₄⁺ were accumulated in and on the root. No directional change in antiplatelet activity was detected as S was increased from 2 to 12 mM. This lack of significant change in antiplatelet activity with increasing S levels suggests that modification of this trait by simply increasing S levels may not be feasible in a solution culture environment.     

Changes in net proton release by roots of intact maize plants after local nutrient supply in a split root system

The effect of local nutrient supply to maize roots (Zea mays L. cv. Blizzard) on net proton release was studied using the split root technique (SRNS, SRCa) to compare plants that were cultivated with their roots completely in either nutrient solution (NS) or 0.1 mM CaSO₄ (Ca). Roots in NS released more protons than roots in Ca. This higher net proton release was associated with significantly higher ATP concentrations in the root tissue. Higher net proton release and ATP concentrations were also observed after a 4 h lag phase when 20 μM abscisic acid were exogenously applied to roots in 0.1 mM CaSO₄. It is suggested that higher metabolic activity in roots supplied with nutrients increased ATP concentrations and thus the substrate supply of the plasma membrane H⁺ ATPase. When only half of the root system was supplied with nutrient solution with the other half bathed in 0.1 mM CaSO₄, the roots in the SRNS compartment released significantly higher amounts of protons relative to the NS control plants. Conversely, roots in the SRCa compartment showed net proton uptake in contrast to the roots of control plants in 0.1 mM CaSO₄ which significantly acidified the root medium. These differences in proton release by roots in the split root system and control roots could not be explained in terms of differences in ATP concentrations. It is therefore suggested that an internal signal may lead to a modification of the plasma membrane H⁺ ATPase as shown earlier during plant adaptation to low pH in the root medium.