The dynamics of protons,aluminium, and calcium in the rhizosphere of maize cultivated in tropical acid soils: experimental study and modelling

The goals of this work were to understand the dynamics of H⁺, Al and Ca in the rhizosphere of maize cultivated in tropical acid soils, and to evaluate the contribution of the dissolution kinetics of the Al-hydroxides to Al dynamics. The study of the dissolution kinetics was based on a comparison between experimental and simulated data, using a model of the chemical processes in the rhizosphere. Two Oxisols, pH 5.1 and 4.6, and one Ultisol, pH 5.2, were studied. An Al-tolerant maize variety (Zea mays L.) was grown for 14 days on a 3-mm thick soil layer. The composition of the soil and the soil solution, together with the concentration of Al in the roots, were determined throughout the experiment. The results showed that root growth (i) decreased the soil solution pH, up to one pH unit, (ii) increased Al concentration in the soil solution, (iii) increased exchangeable Al, and (iv) decreased exchangeable Ca. Soil solution pH, exchangeable Al, and exchangeable Ca were closely linked. Exchangeable Al increased 1.5 – 3.0 times, due to the dissolution of easily mobilised Al components. In addition, Al accumulation in roots depended mainly on Al in the soil solution. Modelling the interactions between H⁺, Al, and Ca proved that the main factor determining Al in the soil solution was the kinetic reactivity of the easily mobilised Al components. These components, probably poorly crystallised Al-hydroxides, are key players in the functioning of the rhizosphere in tropical acid soils.     

Organic acid exudation as an aluminum-tolerance mechanism in maize (Zea mays L.)

In this study, the role of root organic acid synthesis and exudation in the mechanism of aluminum tolerance was examined in Al-tolerant (South American 3) and Al-sensitive (Tuxpeño and South American 5) maize genotypes. In a growth solution containing 6 M Al³⁺, Tuxpeño and South American 5 were found to be two- and threefold more sensitive to Al than South American 3. Root organic acid content and organic acid exudation from the entire root system into the bulk solution were investigated via high-performance liquid chromatographic analysis while exudates collected separately from the root apex or a mature root region (using a dividedroot-chamber technique) were analyzed with a more-sensitive ion chromatography system. In both the Al-tolerant and Al-sensitive lines, Al treatment significantly increased the total root content of organic acids, which was likely the result of Al stress and not the cause of the observed differential Al tolerance. In the absence of Al, small amounts of citrate were exuded into the solution bathing the roots. Aluminum exposure triggered a stimulation of citrate release in the Al-tolerant but not in the Al-sensitive genotypes; this response was localized to the root apex of the Al-tolerant genotype. Additionally, Al exposure triggered the release of phosphate from the root apex of the Al-tolerant genotype. The same solution Al³⁺ activity that elicited the maximum difference in Al sensitivity between Al-tolerant and Al-sensitive genotypes also triggered maximal citrate release from the root apex of the Al-tolerant line. The significance of citrate as a potential detoxifier for aluminum is discussed. It is concluded that organic acid release by the root apex could be an important aspect of Al tolerance in maize.Abbreviations SA3 South American 3, an Al-tolerant maize cultivar - SA5 South American 5, an Al-sensitive maize cultivar The authors would like to express their appreciation to Drs. John Thompson, Ross Welch and Mr. Stephen Schaefer for their training and guidance in the use of the chromatography systems. This work was supported by a Swiss National Science Foundation Fellowship to Didier Pellet, and U.S. Department of Agriculture/National Research Initiative Competitive Grant 93-37100-8874 to Leon Kochian. We would also like to thank Drs. S. Pandey and E. Ceballos from the CIMMYT Regional office at CIAT Cali, Colombia for providing seed for the maize varieties and inbred line.     

Growth characteristics and seasonal allocation patterns of biomass and nutrients in Carex species growing in floating fens

A soil incubation and short-term root growth experiment was conducted to investigate the effects of organic matter application on Al toxicity alleviation in a highly weathered acid soil. Ground leaves of a tree legume (Calliandra calothyrsus Meissn.), ground barley (Hordeum vulgare L.) straw, or CaCO₃ were mixed at various rates with A-horizon soil of a red podzolic soil (Epiaquic Haplustult) and incubated at 90% of field capacity for 4 or 10 weeks. After the incubation, a short term (48 h) root growth test was conducted using mung bean (Vigna radiata (L.) Wilczek), followed by the analysis of the solution and solid phases of the post-harvest soil. Adding either CaCO₃ or organic matter increased root length in mung bean largely by decreasing the activity of monomeric Al in the soil solution. With organic matter, the major mechanisms of this decrease were presumed to be precipitation of soluble Al and the formation of Al-organic matter complexes. The former effect was predicted from the pH increase accompanying the organic matter addition, the increase being larger with legume leaves which had the higher exchangeable and soluble Ca and Mg contents. The concentration of Al complexed with soluble organic matter also was shown to increase with increasing rate of organic matter addition, the effect again being larger with legume leaves. The sum of monomeric Al species activity and Al³⁺ activity was negatively correlated with relative root length for the organic matter and CaCO₃ treatments. However, indices which took into account the possible alleviation effects of basic cations in soil solution on Al toxicity provided an improvement in correlation with relative root length. The efficiency of the two organic amendments relative to CaCO₃ in decreasing Al toxicity was assessed by comparing the rates required to reduce Al³⁺ activity below 10 μ M, the value found to be associated with 90% relative root length for mung bean. The rates of CaCO₃, legume leaf and barley straw required to reach this critical value were 0.75, 14, and 42 t ha⁻¹ respectively.     

Assessment of soil aluminium pools along three mountainous elevation gradients

Anthropogenic soil acidification in mountain forests and consequent Al release still present a significant problem in many regions. The effect of deposition may differ according to stand conditions, including altitude. This contribution is focused on three elevation transects, two in the Jizera Mountains strongly influenced by acid deposition, one in the less affected Novohradske Mountains. Quantification of pools of different Al forms and related soil characteristics (organic carbon, exchangeable hydrogen cations, sorption characteristics, etc.) is evaluated. In the Novohradske Mountains, the pool of both organically bound and water-soluble Al increases with increasing altitudes. In the Jizera Mountains, the distribution is more complicated; it is strongly affected by different forest type (beech vs. spruce), deforestation, and other local differences. Higher amounts of Al are bound in the mineral horizons compared to the surface organic horizons, even in the case of organically bound Al pools. Further differences between different altitudes and between soil horizons in Al distribution were revealed by detailed Al speciation using HPLC/IC method.     

The effect of phosphate rock dissolution on soil chemical properties and wheat seedling root elongation

Soils of the Appalachian region of the United States are acidic and deficient in P. North Carolina phosphate rock (PR), a highly substituted fluoroapatite, should be quite reactive in these soils, allowing it to serve both as a source of P and a potential ameliorant of soil acidity. An experiment was conducted to evaluate the influence of PR dissolution on soil chemical properties and wheat (Triticum aestivum cv. Hart) seedling root elongation. Ten treatments including nine rates of PR (0, 12.5, 25, 50, 100, 200, 400, 800, and 1600 mg P kg^-1) and a CaCO₃ (1000 mg kg^-1) control were mixed with two acidic soils, moistened to a level corresponding to 33 kPa moisture tension and incubated for 30 days. Pregerminated wheat seedlings were grown for three days in the PR treated soils and the CaCO₃ control. Root length was significantly (P<0.05) increased both by PR treatments and CaCO₃, indicating that PR dissolution was ameliorating soil acidity. The PR treatments increased soil pH, exchangeable Ca, and soil solution Ca while lowering exchangeable Al and 0.01 M CaCl₂ extractable soil Al. Root growth in PR treatments was best described by an exponential equation (P<0.01) containing 0.01 M CaCl₂ extractable Al. The PR dissolution did not reduce total soil solution Al, but did release Al complexing anions into soil solution, which along with increased pH, shifted Al speciation from toxic to nontoxic forms. These results suggest that North Carolina PR should contribute to amelioration of soil acidity in acidic, low CEC soils of the Appalachian region.     

The physiology and biophysics of an aluminum tolerance mechanism based on root citrate exudation in maize

Al-induced release of Al-chelating ligands (primarily organic acids) into the rhizosphere from the root apex has been identified as a major Al tolerance mechanism in a number of plant species. In the present study, we conducted physiological investigations to study the spatial and temporal characteristics of Al-activated root organic acid exudation, as well as changes in root organic acid content and Al accumulation, in an Al-tolerant maize (Zea mays) single cross (SLP 181/71 x Cateto Colombia 96/71). These investigations were integrated with biophysical studies using the patch-clamp technique to examine Al-activated anion channel activity in protoplasts isolated from different regions of the maize root. Exposure to Al nearly instantaneously activated a concentration-dependent citrate release, which saturated at rates close to 0.5 nmol citrate h(-1) root(-1), with the half-maximal rates of citrate release occurring at about 20 microM Al(3+) activity. Comparison of citrate exudation rates between decapped and capped roots indicated the root cap does not play a major role in perceiving the Al signal or in the exudation process. Spatial analysis indicated that the predominant citrate exudation is not confined to the root apex, but could be found as far as 5 cm beyond the root cap, involving cortex and stelar cells. Patch clamp recordings obtained in whole-cell and outside-out patches confirmed the presence of an Al-inducible plasma membrane anion channel in protoplasts isolated from stelar or cortical tissues. The unitary conductance of this channel was 23 to 55 pS. Our results suggest that this transporter mediates the Al-induced citrate release observed in the intact tissue. In addition to the rapid Al activation of citrate release, a slower, Al-inducible increase in root citrate content was also observed. These findings led us to speculate that in addition to the Al exclusion mechanism based on root citrate exudation, a second internal Al tolerance mechanism may be operating based on Al-inducible changes in organic acid synthesis and compartmentation. We discuss our findings in terms of recent genetic studies of Al tolerance in maize, which suggest that Al tolerance in maize is a complex trait.     

Aluminum resistance in common bean (Phaseolus vulgaris) involves induction and maintenance of citrate exudation from root apices

Two common bean (Phaseolus vulgaris L.) genotypes differing in aluminum (Al) resistance, Quimbaya (Al‐resistant) and VAX‐1 (Al‐sensitive) were grown in hydroponics for up to 25 h with or without Al, and several parameters related to the exudation of organic acids anions from the root apex were investigated. Al treatment enhanced the exudation of citrate from the root tips of both genotypes. However, its dynamic offers the most consistent relationship between Al‐induced inhibition of root elongation and Al accumulation in and exclusion from the root apices. Initially, in both genotypes the short‐term (4 h) Al‐injury period was characterized by the absence of citrate efflux independent of the citrate content of the root apices, and reduction of cytosolic turnover of citrate conferred by a reduced Nicotinamide adenine dinucleotide phosphate–isocitrate dehydrogenase (EC 1.1.1.42) activity. Transient recovery from initial Al stress (4–12 h) was found to be dependent mainly on the capacity to utilize internal citrate pools (Al‐resistant genotype Quimbaya) or enhanced citrate synthesis [increased activities of NAD‐malate dehydrogenase (EC 1.1.1.37) and ATP‐phosphofructokinase (EC 2.7.1.11) in Al‐sensitive VAX‐1]. Sustained recovery from Al stress through citrate exudation in genotype Quimbaya after 24 h Al treatment relied on restoring the internal citrate pool and the constitutive high activity of citrate synthase (CS) (EC 4.1.3.7) fuelled by high phosphoenolpyruvate carboxylase (EC 4.1.1.31) activity. In the Al‐sensitive genotype VAX‐1 the citrate exudation and thus Al exclusion and root elongation could not be maintained coinciding with an exhaustion of the internal citrate pool and decreased CS activity.     

Salicylic acid-induced aluminum tolerance by modulation of citrate efflux from roots of<Emphasis Type="Italic"> Cassia tora</Emphasis> L.

Aluminum-induced exudation of organic acids from roots has been proposed as a mechanism for Al tolerance in plants. To better understand the regulatory process leading to efflux of organic acids, the possible involvement of salicylic acid (SA) in regulating Al-induced citrate release in Cassia tora L. was identified. The response of citrate efflux to exogenous SA was concentration-dependent. Application of SA at 5 microM in solution containing 20 microM Al increased citrate efflux to levels 1.76-fold higher than in controls (20 microM Al alone). However, inhibition of citrate release was observed when SA concentrations increased to more than 20 microM. Increased citrate efflux due to the SA treatment was associated with decreased inhibition of root growth and Al content in root tips, suggesting that exogenous SA could confer Al tolerance by increasing citrate efflux. We also examined citrate synthase activities (EC 4.1.3.7) and citrate concentrations in root tips exposed to Al and/or SA. However, both citrate synthase activities and citrate accumulation remained unaffected. These results indicate that SA-promotion of Al-induced citrate efflux is not correlated with increase in citrate production. Total endogenous SA concentrations were measured in root tips and the SA concentrations were significantly enhanced by Al at levels of 10-50 microM.     

Magnesium ameliorates aluminum rhizotoxicity in soybean by increasing citric acid production and exudation by roots

Superior effectiveness of Mg over Ca in alleviating Al rhizotoxicity cannot be accounted for by predicted changes in plasma membrane Al3+ activity. The influence of Ca and Mg on the production and secretion of citrate and malate, and on Al accumulation by roots was investigated with soybean genotypes Young and PI 416937 which differ in Al tolerance. In the presence of a solution Al3+ activity of 4.6 microM, citrate and malate concentrations of tap root tips of both genotypes increased with additions of either Ca up to 3 mM or Mg up to 50 microM. Citrate efflux rate from roots exposed to Al was only enhanced with Mg additions and exceeded malate efflux rates by as much as 50-fold. Maximum citrate release occurred within 12 h after adding Mg to solution treatments. Adding 50 microM Mg to 0.8 mM CaSO4 solutions containing Al3+ activities up to 4.6 microM increased citrate concentration of tap root tips by 3- to 5-fold and root exudation of citrate by 6- to 9-fold. Plants treated with either 50 microM Mg or 3 mM Ca had similar reductions in Al accumulation at tap root tips, which coincided with the respective ability of these ions to relieve Al rhizotoxicity. Amelioration of Al inhibition of soybean root elongation by low concentrations of Mg in solution involved Mg-stimulated production and efflux of citrate by roots.     

Phosphorus and aluminum interactions in soybean in relation to aluminum tolerance. Exudation of specific organic acids from different regions of the intact root system

Aluminum (Al) toxicity and phosphorus (P) deficiency often coexist in acid soils that severely limit crop growth and production, including soybean (Glycine max). Understanding the physiological mechanisms relating to plant Al and P interactions should help facilitate the development of more Al-tolerant and/or P-efficient crops. In this study, both homogeneous and heterogeneous nutrient solution experiments were conducted to study the effects of Al and P interactions on soybean root growth and root organic acid exudation. In the homogenous solution experiments with a uniform Al and P distribution in the bulk solution, P addition significantly increased Al tolerance in four soybean genotypes differing in P efficiency. The two P-efficient genotypes appeared to be more Al tolerant than the two P-inefficient genotypes under these high-P conditions. Analysis of root exudates indicated Al toxicity induced citrate exudation, P deficiency triggered oxalate exudation, and malate release was induced by both treatments. To more closely mimic low-P acid soils where P deficiency and Al toxicity are often much greater in the lower soil horizons, a divided root chamber/nutrient solution approach was employed to impose elevated P conditions in the simulated upper soil horizon, and Al toxicity/P deficiency in the lower horizon. Under these conditions, we found that the two P-efficient genotypes were more Al tolerant during the early stages of the experiment than the P-inefficient lines. Although the same three organic acids were exuded by roots in the divided chamber experiments, their exudation patterns were different from those in the homogeneous solution system. The two P-efficient genotypes secreted more malate from the taproot tip, suggesting that improved P nutrition may enhance exudation of organic acids in the root regions dealing with the greatest Al toxicity, thus enhancing Al tolerance. These findings demonstrate that P efficiency may play a role in Al tolerance in soybean. Phosphorus-efficient genotypes may be able to enhance Al tolerance not only through direct Al-P interactions but also through indirect interactions associated with stimulated exudation of different Al-chelating organic acids in specific roots and root regions.     

模拟酸雨对主要酸性土壤中铝的溶出及形态的影响

本文研究了模拟酸雨对主要酸性土壤中铝的溶出及形态变化的影响。结果表明,模拟酸雨对土壤酸化的影响较小,但对土壤铝的溶出却影响明显,尤其在pH<4.0时;模拟酸雨对不同类型土壤的影响是不同的,其中以高度风化的酸性土壤较为敏感。模拟酸雨对土壤游离铝形态的影响是重要的,酸处理后,交换性铝略有增加,无定形活性铝增加较多,而有机络合态铝有减少的趋势。这表明在酸雨的长期作用下,铝终将转化为交换性铝和水溶性铝而进入环境并危害生态系统。     

Al-induced efflux of organic acid anions is poorly associated with internal organic acid metabolism in triticale roots

The secretion of organic acid anions from roots has been identified as a mechanism of resistance to Al. However, the process leading to the secretion of organic acid anions is poorly understood. The effect of Al on organic acid metabolism was investigated in two lines of triticale (xTriticosecale Wittmark) differing in Al-induced secretion of malate and citrate and in Al resistance. The site of Al-induced secretion of citrate and malate from a resistant line was localized to the root apices (terminal 5 mm). The levels of citrate (root apices and mature root segments) and malate (mature segments only) in roots increased during exposure to Al, but similar changes were observed in both triticale genotypes. The in vitro activities of four enzymes involved in malate and citrate metabolism (citrate synthase, phosphoenolpyruvate carboxylase, malate dehydrogenase, and NADP-isocitrate dehydrogenase) were similar for sensitive and resistant lines in both root apices and mature root segments. The response of these enzymes to pH did not differ between tolerant and sensitive lines or in the presence and absence of Al. Moreover, cytoplasmic and vacuolar pH were not affected by exposure to Al in either line. Together, these results indicate that the Al-dependent efflux of organic acid anions from the roots of triticale is not regulated by their internal levels in the roots or by the capacity of the root cells to synthesize malate and citrate.     

Role of root derived organic acids in the mobilization of nutrients from the rhizosphere

The role of organic acids in the mobilization of plant nutrients from the rhizosphere was assessed in seven contrasting soil types. The results indicated that malate was poor at mobilizing micronutrients from all the test soils, whilst citrate was capable of mobilizing significant quantities. Citrate was also capable of mobilizing P from one soil which possessed a large Ca-P fraction. This mobilization of P was due to both the complexing action of the citrate anion and due to the dissolution properties of the protons released from citric acid upon equilibrium with the soil solution. The reaction of citrate with cations was found to be near instantaneous with significant absorption to the solid phase in some soils at low concentrations. Soil decomposition studies indicated that citrate was rapidly broken down in organic soils but was more resistant to degradation in subsoil horizons. It was concluded that organic acids can be expected to be of little consequence in nutrient mobilization from high pH soils, whilst in acid soils they may be involved both in a more general mechanism for micronutrient uptake or as a potential Al detoxification mechanism.Abbreviations C₆H₈O₇ Citric acid and H-citrate indicates - C₆H₅O₇Na₃ whilst Na-citrate indicates     

The effects of acid deposition on the biogeochemical cycles of major nutrients in miniature red spruce ecosystems

As part of an experimental study of air pollution effects on tree growth and health, we combined process studies with an ecosystem approach to evaluate the effects of acidic deposition on soil acidification, nutrient cycling and proton fluxes in miniature red spruce ecosystems. Ninety red spruce saplings were transplanted into 1-m diameter pots containing reconstructed soil profiles and exposed to simulated acid rain treatments of pH 3.1, 4.1 and 5.1 for four consecutive growing seasons. All the principal fluxes of the major elements were measured. During the first year of treatments, the disturbance associated with the transplanting of the experimental trees masked any treatment effects by stimulating N mineralization rates and consequent high N0₃ ⁻ cation, and H⁺ flux through the soil profile. In subsequent years, leaching of base cations and labile Al was accelerated in the most intensive acid treatment and corresponding declines in soil pH and exchangeable pools of Ca and Mg and increases in exchangeable Al concentrations were observed in the organic horizon. Leaching of Ca²⁺ and Mg²⁺ also was significantly higher in the pH 4.1 than in the pH 5.1 treatment. Flux of Ca from foliage and soil was increased in response to strong acid loading and root uptake increased to compensate for foliar Ca losses. In contrast, K cycling was dominated by root uptake and internal cycling and was relatively insensitive to strong acid inputs. Cation leaching induced by acidic deposition was responsible for the majority of H⁺ flux in the pH 3.1 treatment in the organic soil horizon whereas root uptake accounted for most of the H⁺ flux in the pH 4.1 and 5.1 treatments. Although no measurable effects on tree nutrition or health were observed, base cation leaching was significantly accelerated by acidic deposition, even at levels below that observed in the eastern U.S., warranting continued concern about acid deposition effects on the soil base status of forested ecosystems.     

A comparative study on the aluminium- and copper-induced organic acid exudation from wheat roots

It is well established that aluminium (Al) and some heavy metals can elicit organic acid exudation from a range of species. In the present research we found that copper (Cu) can also induce organic acid exudation from the roots of wheat, rye, triticale, maize and soybean. Using intact wheat plants, we made a comparative study of Al- and Cu- induced organic acid exudation. In 5-day-old wheat seedlings, severe Cu stress (40 µ M CuCl₂) mainly induced the exudation of malate and citrate, and Al-tolerant genotypes could release significantly greater amounts of malate than Al-sensitive genotypes. The time course of the exudation of malate and citrate from the roots of 5-day-old seedlings of wheat (cv. Atlas) in 200 µ M AlCl₃ was similar to that in 40 µ M CuCl₂. In older wheat plants (15-day-old), moderate Cu stress (12 µ M CuCl₂) induced the exudation of large amounts of citrate and addition of Al or La sharply reduced Cu-induced citrate exudation, while Cu or La did not affect Al-induced malate efflux. When half of the root system of Atlas wheat was immersed in Al- or Cu-containing solution and the remaining half in Al- or Cu-free solution, organic acids were only exuded into the solution containing Al or Cu. This suggests that no long distance signal transport is involved in organic acid exudation induced by Al or Cu, and that direct contact of Al or Cu with plant roots is a prerequisite for the induction of organic acid exudation. The anion-channel inhibitor niflumic acid (NIF) significantly stimulated the exudation of both citrate and malate from 5-day-old wheat seedlings under severe Al or Cu stress. Our results suggest that Cu-induced organic acid efflux may be a common response, which may play a role in alleviating Cu toxicity in plants.     

Phosphate solubilization by organic anion excretion from rice (Oryza sativa L.) growing in aerobic soil

A mathematical model of P solubilization by organic anion excretion from roots is described and used to account for P solubilization by rice (Oryza sativa L.) plants growing in aerobic soil. In previous experiments with rice in an aerobic, highly-weathered, P-deficient soil, we found that the plants were able to solubilize P from an alkali-soluble pool and thereby increase their P uptake. The solubilization could not be explained by pH changes nor by the release of phosphatases. In subsequent experiments we found excretion of citrate from rice roots into nutrient solutions, and the synthesis and excretion of citrate tended to increase under P starvation. The model allows for the diffusion of the organic anion away from a root, its decomposition by soil microbes, its reaction with the soil in solubilizing P, and diffusion of the solubilized P back towards the root as well as away from it. We calculated the rate of citrate excretion from rice roots growing in soil based on measured steady-state citrate concentrations in the rhizosphere and calculated rates of decomposition. Calculations using these and other model parameters obtained independently showed that the observed solubilization and increased P uptake by rice growing in soil could be accounted for. A sensitivity analysis of the model is given. This revised version was published online in June 2006 with corrections to the Cover Date.     

Phosphorus saturation and pH differentially regulate the efficiency of organic acid anion-mediated P solubilization mechanisms in soil

Exudation of organic acid anions by plants as well as root-induced changes in rhizosphere pH can potentially improve phosphate (P_i) availability in the rhizosphere and are frequently found to occur simultaneously. In non-calcareous soils, a major proportion of P_i is strongly sorbed to metal oxi(hydr)oxides of mainly iron (Fe) and aluminium (Al) and organic anions are known to compete with P_i for the same sorption sites (ligand exchange) or solubilize P_i via ligand-promoted mineral dissolution. Root-induced co-acidification may also further promote P_i release from soil. The relative efficiency of these different solubilization mechanisms, however, is poorly understood. The aims of this study were to gain a better mechanistic understanding of the solubilizing mechanisms of four carboxylates (citrate, malate, oxalate, malonate) in five soils with high and low P surface site saturation. Results indicate that at a lower P saturation of solid phase sorption sites, ligand-promoted mineral dissolution was the main P_i solubilization mechanism, while ligand exchange became more important at higher soil P concentrations. Co-acidification generally increased P_i solubility in the presence of carboxylates; however the relative solubilizing effect of carboxylates compared to the background electrolyte (KCl) control decreased by 20–50%. In soils with high amounts of exchangeable calcium (Ca), the proton-induced Ca solubilization reduced soluble P_i, presumably due to ionic-strength-driven changes in the electric surface potential favoring a higher P_i retention. Across a wider soil pH range (pH 3–8), P_i solubility increased with increasing alkalinity, as a result of both, more negatively charged sorption sites, as well as DOC-driven changes in Fe and Al solubility, which were further enhanced by the presence of citrate. Overall, the relative efficiency of carboxylates in solubilizing P_i was greatest in soils with medium to high amounts of anionic binding sites (mainly Fe- and Al-oxy(hydr)oxides) and a medium P sorption site coverage, with citrate being most effective in solubilizing P_i.     

The effect of level of CaCO3, inoculation and lime pelleting on the nodulation and growth of beans in five acid soils

Summary The effect of level of CaCO₃, inoculation and lime pelleting on the nodulation, dry matter yield and % N content of common bean plants (Phaseolus vulgaris) grown in five acid soils was investigated in a greenhouse study. The soils represented a range in pH from 3.9 to 5.1, in exchangeable Al from 0.0 to 4 meq/100 gm, in exchangeable Mn from 0.35 to 2.32 me/100 gm, and in %C from 0.69 to 5.60.Nodule weight decreased with increasing %C and for the soil with highest %C (5.60) no nodules were observed. In soils with low organic matter and low exchangeable Al and Mn, inoculation increased nodule weight, dry matter yield and %N especially at the lowest pH level. Where the seeds were not inoculated, nodule weight and dry matter yield increased with soil pH. No such increases were observed where the seeds were inoculated. There was no apparent advantage in lime pelleting in such soils.In soils with low organic matter content and with substantial amounts of Al and/or Mn, liming increased nodule weight and dry matter yield, and decreased exchangeable Al and/or Mn. Lime pelleting was superior to mere inoculation in increasing nodule weight particularly at low lime rates.In soils with relatively high organic matter content, nodulation was very low or none at all. Low lime rates had little effect on exchangeable Al and Ca and dry matter yield. Higher lime rates, however, decreased exchangeable Al and dry matter yield but increased exchangeable Ca.     

Aluminum resistance of cowpea as affected by phosphorus-deficiency stress

Plants growing in acid soils suffer both phosphorus (P) deficiency and aluminum (Al) toxicity stresses. Selection of genotypes for adaptation to either P deficiency or Al toxicity has sometimes been unsuccessful because these two soil factors often interact. Two experiments were conducted to evaluate eight cowpea genotypes for Al resistance and to study the combined effect of P deficiency and Al toxicity stress on growth, P uptake, and organic acid anion exudation of two genotypes of contrasting Al resistance selected from the first experiment. Relative root inhibition by 30 μM Al ranged from 14% to 60% and differed significantly among the genotypes. Al significantly induced callose formation, particularly in Al-sensitive genotypes. P accumulation was significantly reduced (28% and 95%) by Al application for both the Al-resistant and the Al-sensitive genotypes. Al supply significantly enhanced malate release of root apices of both genotypes. However, the exudation rate was significantly higher in the Al-resistant genotype. P deprivation induced an enhanced malate exudation in the presence of Al only in the Al-resistant genotype IT89KD-391. Citrate exudation rate of the root apices was lower than malate exudation by a factor of about 10, and was primarily enhanced by P deficiency in both genotypes. Al treatment further enhanced citrate exudation in P-sufficient, but not in P-deficient plants. The level of citrate exudation was consistently higher in the Al-resistant genotype IT89KD-391 particularly in presence of Al.It is concluded that the Al-resistant genotype is better adapted to acid Al-toxic and P-deficient soils than the Al-sensitive genotype since both malate and citrate exudation were more enhanced by combined Al and P-deficiency stresses.     

Differential aluminum tolerance in soybean: An evaluation of the role of organic acids

The role of organic acids in aluminum (Al) tolerance has been the object of intensive research. In the present work, we evaluated the roles of organic acid exudation and concentrations at the root tip on Al tolerance of soybean. Exposing soybean seedlings to Al³⁺ activities up to 4.7 μ M in solution led to different degrees of restriction of primary root elongation. Al tolerance among genotypes was associated with citrate accumulation and excretion into the external media. Citrate and malate efflux increased in all genotypes during the first 6 h of Al exposure, but only citrate efflux in Al-tolerant genotypes was sustained for an extended period. Tolerance to Al was correlated with the concentration of citrate in root tips of 8 genotypes with a range of Al sensitivities (r²=0.75). The fluorescent stain lumogallion indicated that more Al accumulated in root tips of the Al-sensitive genotype Young than the Al-tolerant genotype PI 416937, suggesting that the sustained release of citrate from roots of the tolerant genotype was involved in Al exclusion. The initial stimulation of citrate and malate excretion and accumulation in the tip of all genotypes suggested the involvement of additional tolerance mechanisms. The experiments included an examination of Al effects on lateral root elongation. Extension of lateral roots was more sensitive to Al than that of tap roots, and lateral root tips accumulated more Al and had lower levels of citrate.