Arabidopsis mutants with increased sensitivity to aluminum.

Al-sensitive (als) mutants of Arabidopsis were isolated and characterized with the aim of defining mechanisms of Al toxicity and resistance. Most als mutants selected on the basis of root growth sensitivity to Al were recessive, and together the mutants constituted eight complementation groups. Also, in most als mutants, Al sensitivity appeared to be specific for Al relative to La (another trivalent cation), except als2, which was more sensitive to La than wild type. The tendency of roots on mutant seedlings to accumulate Al was examined by staining with morin and hematoxylin, dyes used to indicate the presence of Al. A significant increase in morin staining was observed in als5, consistent with its increased sensitivity to Al. Unexpectedly, als7 and als4 showed less morin staining, suggesting that the roots on these mutants accumulate less Al than wild type seedlings after exposure to Al-containing solutions. Roots of wild-type seedlings produce callose in response to AlCl3 concentrations that inhibit root growth. Only als5 accumulated more callose than wild type in response to low levels (25 mu M) of AICI3 However, als4 and als7 did not accumulate callose at this AlCl3 concentration even though root growth was significantly inhibited. The lack of callose accumulation in als4 and als7 suggests that there is not an obligatory relationship between callose deposition and Al-induced inhibition of root growth.     

Aluminium alleviates manganese toxicity to rice by decreasing root symplastic Mn uptake and reducing availability to shoots of Mn stored in roots

Background and Aims Manganese (Mn) and aluminium (Al) phytotoxicities occur mainly in acid soils. In some plant species, Al alleviates Mn toxicity, but the mechanisms underlying this effect are obscure.Methods Rice (Oryza sativa) seedlings (11 d old) were grown in nutrient solution containing different concentrations of Mn²⁺ and Al³⁺ in short-term (24 h) and long-term (3 weeks) treatments. Measurements were taken of root symplastic sap, root Mn plaques, cell membrane electrical surface potential and Mn activity, root morphology and plant growth.Key Results In the 3-week treatment, addition of Al resulted in increased root and shoot dry weight for plants under toxic levels of Mn. This was associated with decreased Mn concentration in the shoots and increased Mn concentration in the roots. In the 24-h treatment, addition of Al resulted in decreased Mn accumulation in the root symplasts and in the shoots. This was attributed to higher cell membrane surface electrical potential and lower Mn²⁺ activity at the cell membrane surface. The increased Mn accumulation in roots from the 3-week treatment was attributed to the formation of Mn plaques, which were probably related to the Al-induced increase in root aerenchyma.Conclusions The results show that Al alleviated Mn toxicity in rice, and this could be attributed to decreased shoot Mn accumulation resulting from an Al-induced decrease in root symplastic Mn uptake. The decrease in root symplastic Mn uptake resulted from an Al-induced change in cell membrane potential. In addition, Al increased Mn plaques in the roots and changed the binding properties of the cell wall, resulting in accumulation of non-available Mn in roots.     

Nitric oxide protects sour pummelo (Citrus grandis) seedlings against aluminum-induced inhibition of growth and photosynthesis

Limited data are available on the amelioration of nitric oxide (NO) on aluminum (Al)-toxicity. Sour pummelo (Citrus grandis) seedlings were irrigated for 18 weeks with nutrient solution containing 0 and 1.2 mM AlCl₃·6H₂O × 0 and 10 μM sodium nitroprusside (SNP, an NO donor). Under Al stress, SNP increased root phosphorus (P) and Al, but decreased shoot Al. Al decreased photosynthesis, maximum quantum yield of primary photochemistry (F_v/F_m) and total performance index (PI_tot,sbs), but increased inactivation of oxygen-evolving complex (OEC), K-band and relative variable fluorescence at I-steps (V_I). SNP alleviated Al-induced changes for all these parameters. SNP stimulated Al-induced secretion of malate and citrate by excised roots from Al-treated seedlings, while Al did not increase their contents in roots. Antioxidant system in leaves and roots was up- and down-regulated by Al, respectively. SNP prevented Al-induced accumulation of malondialdehyde (MDA) in roots and leaves. In conclusion, SNP alleviates Al-induced inhibition of growth and impairment of the whole photosynthetic electron transport chain. This occurs through increasing Al-immobilization and P level in roots and Al-induced secretion of malate and citrate from roots, and decreasing Al accumulation in shoots. Thus, the decrease of photosynthesis is prevented. Increased P level and Al-immobilization in roots through SNP may be effected through enhanced secretion of malate and citrate.     

Identification of dominant mutations that confer increased aluminium tolerance through mutagenesis of the Al-sensitive Arabidopsis mutant, als3-1

Aluminium (Al) toxicity is a global agricultural problem that occurs in acid soil environments and severely limits root growth and crop productivity. The isolation and characterization of a gene, ALS3, which is absolutely required by Arabidopsis seedlings for growth in an Al-toxic environment was reported previously. Since the als3-1 loss-of-function mutant has extreme root growth inhibition even in the presence of very low levels of Al, it was an excellent candidate for using a mutagenesis approach to identify suppressor mutations that would increase either Al resistance or tolerance in Arabidopsis roots. EMS-mutagenized als3-1 seedlings were screened for mutants that could sustain root growth in an Al-containing environment that is highly toxic to als3-1 but not Col-0 wt. This approach resulted in identification of 12 strong suppressor mutants that reversed the als3-1 phenotype and grew as well or better than Col-0 wt in the presence of high levels of Al. Subsequent analysis of three representative suppressor mutants revealed that the phenotype of each probably arises from dominant gain-of-function mutations at the same locus. Detailed analysis of one of these, alt1-1 (Al tolerant), suggests that this mutation positively impacts Al resistance in a manner dependent on pH adjustment rather than enhanced Al exclusion. Identification of these suppressor mutations, should not only further elucidate the biochemical and molecular mechanisms underlying Al toxicity and tolerance but also will develop a collection of mutations that may be useful for engineering crop plants that can grow and thrive in Al-toxic environments.     

<Emphasis Type="Italic">In vitro</Emphasis> organogenesis and antioxidant enzymes activity in <Emphasis Type="Italic">Acanthophyllum sordidum</Emphasis>

The effect of various hormonal combinations on callus formation and regeneration of shoot and root from leaf derived callus of Acanthophyllum sordidum Bunge ex Boiss. has been studied. Proteins and activity of antioxidant enzymes were also evaluated during shoot and root organogenesis from callus. Calli were induced from leaf explants excised from 30-d-old seedlings grown on Murashige and Skoog medium containing 4.52 μM 2,4-dichlorophenoxyacetic acid + 4.65 μM kinetin. Maximum growth of calli and the most efficient regeneration of shoots and roots occurred with 2.69 μM 1-naphthalene acetic acid (NAA), 2.69 μM NAA + 4.54 μM thidiazuron and 2.46 μM indole-3-butyric acid. Protein content decreased in calli and increased significantly during regeneration of shoots from callus. Superoxide dismutase activity decreased in calli comparing to that of seedlings, then increased in regenerated shoots and roots. High catalase activity was detected in seedlings and regenerated shoots, whereas high peroxidase activity was observed in calli and regenerated roots.     

Elevated oxalate oxidase activity is correlated with Al-induced plasma membrane injury and root growth inhibition in young barley roots

In order to characterise the possible mechanisms involved in Al toxicity some functional characteristics were analysed in young barley (Hordeum vulgare L.) seedlings cultivated between moistened filter paper. Transfer of germinated barley seeds into hydroponic culture system caused significant stress, which was manifested by root-growth inhibition and elevated Evans blue uptake of root tips. Hydroponics caused stress unabled the analysis of Al-induced stress in the young barley roots during the first day of cultivation. Several (3–4) days are required for adaptation of barley seedlings to hydroponics in spite of strong aeration of the medium. Using filter paper compared to cultivation in solution application of much higher Al concentrations were required to inhibit root growth. Al-induced root growth inhibition, Al uptake, damage of plasma-membrane (PM) permeability of root cells, as well as elevated oxalate oxidase - OxO (EC 1.2.3.4) activity were significantly correlated. While 1 mM Al concentration had no effect on barley roots growing on filter paper, 5 to 100 mM Al concentration inhibited root growth, enhanced cell death and induced oxalate oxidase activity with increasing intensity. The time course analysis of OxO gene expression and OxO activity showed that 10 mM Al increased OxO activity as soon as 3 h after exposure of roots to Al reaching its maximum at about 18 h after Al application. These results indicate that expression of OxO is activated very early after exposure of barley to Al, suggesting its role in oxidative stress and subsequent cell death caused by Al toxicity in plants.     

Influence of aluminium on biomass, nutrients, soluble carbohydrates and phenols in beech (Fagus sylvatica)

The effects of aluminium on biomass, nutrients and soluble carbohydrates and phenols were studied in beech ( Fagus sylvatica L.) seedlings. After germination, seedlings with cotyledons and the buds of the first leaf-pair developed, were preconditioned for two weeks and then grown for 31 days in nutrient solutions containing 0.1, 0.5, 1.0 or 2.0 m M A1C1₃. Aluminium did not affect the dry weights of roots but at Al concentrations ≥ 1.0 m M the development of the terminal shoot above the first leaf pair, was reduced by 80% or more. The concentrations of most nutrients (P, Ca, Mg, Zn, Cu) in the plant tissues decreased strongly even at the lowest Al levels, but K increased in the shoots. The tissue concentration of N was not affected of Al. but the distribution between the organs was changed to a higher content of N in the roots. At ≥1.0 m M Al the concentrations of starch in both the shoots and the roots were significantly increased, and at ≥ 0.5 m M the roots contained more of total phenols than untreated seedlings. The elevated concentrations and contents of starch and phenols in the seedlings may partly be related to the reduced shoot growth. The observed effects of Al were marked already at Al levels found in soil waters from beech forests in southern Sweden.     

Root-shoot signalling in sunflower plants with confined root systems

Sunflower plants were grown hydroponically under controlled conditions with the root systems confined in small containers. Root confinement inhibited the growth of sunflower plants as indicated by reduction in both leaf and cotyledon area and root and shoot fresh weight. This effect was more pronounced in shoots. Root confinement favored the accumulation of potassium in the roots and shoots, and the exudation of potassium and water in excised roots. Xylem sap from root confined plants inhibited cotyledon expansion as revealed by bioassay with decapited sunflower seedlings. In addition decapited control plants incubated in ABA solution also showed cotyledon growth reduction. Xylem sap ABA analysis indicated a 7-times higher concentration in root confined than control plants. Our results suggest the synthesis of a chemical signal in the roots of plants subjected to mechanical stress which can be responsible for the inhibition of plant growth.     

Influence of aluminum and phosphorus on growth and xylem sap composition in Melastoma malabathricum L.

Melastoma (Melastoma malabathricum L.) is an aluminum-accumulating woody plant that accumulates more than 10 000 mg kg^–1 of aluminum (Al) in mature leaves. The influence of Al and phosphorus (P) applications on plant growth and xylem sap was examined in the present study in order to elucidate the interaction between Al-induced growth enhancement and P nutrition, and to determine the form of Al for translocation from roots to shoots. Although the Al application significantly increased the growth of Melastomaseedlings with the high P pre-treatment, and P concentrations in the leaves and Pi concentrations in the xylem sap regardless of the P pre-treatment, we could not come to the conclusion that a primary cause of the Al-induced growth enhancement in Melastoma is the stimulation of P uptake. The degree of Al-induced growth enhancement corresponded not with the P concentrations but with the Al concentrations in the plant tissue, suggesting that the Al-induced growth enhancement in Melastoma is primarily caused by Al itself in the plant tissue rather than by the stimulation of P uptake. Through the analysis of organic acids and Al in the xylem sap and plant tissue, the form of Al for translocation from roots to shoots was shown to be an Al-citrate complex that was transformed into Al-oxalate complex for Al storage in the leaves. In addition, the xylem sap of Melastoma seedlings grown in the absence of Al contained higher concentrations of malate. In the presence of Al, however, higher concentrations of citrate were found, indicating that Melastoma changes its organic acid metabolism in the presence or absence of Al; more specifically, it increases the synthesis of citrate.     

Hormonal control of root development on epiphyllous plantlets of Bryophyllum (Kalanchoe) marnierianum: role of auxin and ethylene

Epiphyllous plantlets develop on leaves of Bryophyllum marnierianum when they are excised from the plant. Shortly after leaf excision, plantlet shoots develop from primordia located near the leaf margin. After the shoots have enlarged for several days, roots appear at their base. In this investigation, factors regulating plantlet root development were studied. The auxin transport inhibitor 2,3,5-triiodobenzoic acid (TIBA) abolished root formation without markedly affecting shoot growth. This suggested that auxin transport from the plantlet shoot induces root development. Excision of plantlet apical buds inhibits root development. Application of indole-3-acetic acid (IAA) in lanolin at the site of the apical buds restores root outgrowth. Naphthalene acetic acid (NAA), a synthetic auxin, reverses TIBA inhibition of plantlet root emergence on leaf explants. Both of these observations support the hypothesis that auxin, produced by the plantlet, induces root development. Exogenous ethylene causes precocious root development several days before that of a control without hormone. Ethylene treatment cannot bypass the TIBA block of root formation. Therefore, ethylene does not act downstream of auxin in root induction. However, ethylene amplifies the effects of low concentrations of NAA, which in the absence of ethylene do not induce roots. Ag(2)S(2)O(3), an ethylene blocker, and CoCl(2), an ethylene synthesis inhibitor, do not abolish plantlet root development. It is therefore unlikely that ethylene is essential for root formation. Taken together, the experiments suggest that roots develop when auxin transport from the shoot reaches a certain threshold. Ethylene may augment this effect by lowering the threshold and may come into play when the parent leaf senesces.     

The role of roots and cotyledons as storage organs in early stages of establishment in Quercus crispula: a quantitative analysis of the nonstructural carbohydrate in cotyledons and roots

Quercus seedlings have hypogeal cotyledons and tap roots, both of which act as storage organs. The importance of the storage function in the two organs may change as the seedling develops. Therefore, changes in carbohydrate reserves in cotyledons and roots of Q. crispula grown under 40 % and 3 % of full light from shoot emergence to the completion of the first leaf flush were monitored. In addition, a shoot-clipping treatment was performed to examine the relative contribution of the cotyledons and tap roots to resprouting. Cotyledons maintained large amounts of nonstructural carbohydrates during shoot development, and carbohydrates were still present in the cotyledons during the final phase of leaf flush. In addition, a notable increase in the amount of carbohydrates was observed in tap roots before leaf flush at both light levels. Since root development occurred before leaf flush, even in plants grown under 3 % light, the carbohydrate found in them presumably originated from seed reserves and was translocated to roots as storage reserves. When shoots were clipped at the leaf flushing stage, the amount of carbohydrate decreased only in the cotyledons after resprouting, suggesting that cotyledons act as the main storage organs during shoot development stages. However, it could be advantageous as a 'risk avoidance strategy' for the seedlings to store reserves in both cotyledons and roots, since cotyledons may be removed by predators during shoot development.     

Al-Fe interactions and growth enhancement in Melastoma malabathricum and Miscanthus sinensis dominating acid sulphate soils

Plants growing in acid sulphate soils are subject to high levels of Al availability, which may have effects on the growth and distribution of these species. Although Fe availability is also high in acid sulphate soils, little is known about the effect of Fe on the growth of native plants in these soils. Two species dominating this soil type in Asia, viz. Melastoma malabathricum and Miscanthus sinensis were grown hydroponically in a nutrient solution with different concentrations of Al and Fe. Melastoma malabathricum is found to be sensitive to Fe (40 and 100 microm). Application of 500 microm Al, however, completely ameliorates Fe toxicity and is associated with a decrease of Fe concentration in shoots and roots. The primary reason for the Al-induced growth enhancement of M. malabathricum is considered to be the Al-induced reduction of toxic Fe accumulation in roots and shoots. Therefore, Al is nearly essential for M. malabathricum when growing in acid sulphate soils. In contrast, application of both Fe and Al does not reduce the growth of M. sinensis, and Al application does not result in lower shoot concentrations of Fe, suggesting that this grass species has developed different mechanisms for adaptation to acid sulphate soils.     

Removal of root apices enables study of direct toxic effects of aluminum on rice (Oryza sativa L.) leaf cells

Root growth inhibition is a well-known symptom of aluminum (Al) toxicity in intact plants, mainly because the mechanisms of Al exclusion or resistance that operate outside the root endodermis prevent the ascent of this metal from roots into shoots. This work presents a new method to better understand the direct effects of this metal on rice leaves. For this, Al-sensitive and tolerant rice genotypes, having had their root apices removed, were incubated in AlCl₃ solutions to evaluate unblocked metal ascension toward the leaf cells. To avoid regrowth and closing of roots, apices were removed daily and also verified for a lack of tyloses production and consequent obstruction in tracheal elements. Thus, seedlings of both cultivars, which were root apex-free, accumulated differentially high amounts of Al in the leaves, highlighting the importance of mechanisms of Al exclusion or resistance in roots of intact plants. Also, Al moved freely toward leaf cells, clearly inducing necrosis-like mesophyll alterations in both genotypes. Added ultrastructural analyses revealed significant cytoplasmatic damage, mainly in chloroplasts. These results suggest that differential responses to Al sensitivity/tolerance preserved in roots between the genotypes studied are also expressed in leaves. Therefore, this method allowed for development of a possible biological model suitable for investigating the direct effect of Al on cells and, alternatively, other compounds in plant leaf cell physiology.     

Aluminum Effects on Calcium (45Ca2+) Translocation in Aluminum-Tolerant and Aluminum-Sensitive Wheat (Triticum aestivum L.) Cultivars (Differential Responses of the Root Apex versus Mature Root Regions)

The influence of Al exposure on long-distance Ca2+ translocation from specific root zones (root apex or mature root) to the shoot was studied in intact seedlings of winter wheat (Triticum aestivum L.) cultivars (Al-tolerant Atlas 66 and Al-sensitive Scout 66). Seedlings were grown in 100 [mu]M CaCl2 solution (pH 4.5) for 3 d. Subsequently, a divided chamber technique using 45Ca2+-labeled solutions (100 [mu]M CaCl2 with or without 5 or 20 [mu]M AlCl3, pH 4.5) was used to study Ca2+ translocation from either the terminal 5 to 10 mm of the root or a 10-mm region of intact root approximately 50 mm behind the root apex. The Al concentrations used, which were toxic to Scout 66, caused a significant inhibition of Ca2+ translocation from the apical region of Scout 66 roots. The same Al exposures had a much smaller effect on root apical Ca2+ translocation in Atlas 66. When a 10-mm region of the mature root was exposed to 45Ca2+, smaller genotypic differences in the Al effects effects on Ca2+ translocation were observed, because the degree of Al-induced inhibition of Ca2+ translocation was less than that at the root apex. Exposure of the root apex to Al inhibited root elongation by 70 to 99% in Scout 66 but had a lesser effect (less than 40% inhibition) in Atlas 66. When a mature root region was exposed to Al, root elongation was not significantly affected in either cultivar. These results demonstrate that genotypic differences in Al-induced inhibition of Ca2+ translocation and root growth are localized primarily in the root apex. The pattern of Ca2+ translocation within the intact root was mainly basipetal, with most of the absorbed Ca2+ translocated toward the shoot. A small amount of acropetal Ca2+ translocation from the mature root regions to the apex was also observed, which accounted for less than 5% of the total Ca2+ translocation within the entire root. Because Ca2+ translocation toward the root apex is limited, most of the Ca2+ needed for normal cellular function in the apex must be absorbed from the external solution. Thus, continuous Al disruption of Ca2+ absorption into cells of the root apex could alter Ca2+ nutrition and homeostasis in these cells and could play a pivotal role in the mechanisms of Al toxicity in Al-sensitive wheat cultivars.     

Aluminium Toxicity in Roots: An Investigation of Spatial Sensitivity and the Role of the Root Cap

The primary symptom of aluminium (Al) toxicity in higher plantsis inhibition of root growth. In this study, we investigatedthe spatial sensitivity of maize (Zea mays L.) roots to Al.A divided-chamber technique indicated that only exposure ofthe terminal 10 to 15 mm of the root to Al resulted in inhibitionof growth. Application of Al to all but this apical region ofthe root had little or no effect on growth for 24 h and causedminimal damage to the root tissue. Small agar blocks infusedwith Al were then applied to discrete areas of the apex of maizeroots to determine which section (root cap, meristem or elongationzone) was more important to Al-induced inhibition of growth.The terminal 20 to 30 mm of root (root cap and meristem) mustbe exposed to Al for inhibition. Application of Al to the 30mm of root proximal to this terminal zone (elongation zone)resulted in damage to the root tissue but no significant inhibitionof growth. Therefore, the visible injuries incurred by rootsduring Al-stress are not associated directly with the inhibitionof root growth. Furthermore, removal of the root cap had noeffect on the Al-induced inhibition of root growth in solutionexperiments and argues against the root cap providing protectionfrom Al stress or serving an essential role in the mechanismof toxicity. We suggest that the meristem is the primary siteof Al-toxicity. Key words: Aluminium, toxicity, root growth, root cap     

Effect of Whole Plant and Local Heating on the ABA Content in Cucumber Seedling Leaves and Roots and on Their Heat Tolerance

The effects of heating at 38°C of whole cucumber (Cucumis sativus L.) seedlings or local heating of their shoots and roots on ABA content and heat tolerance of leaves and roots were investigated. During the initial period of the high-temperature treatment of whole seedlings, the ABA concentration in leaves and roots increased considerably. Local heating of the shoot or root resulted in an increase in the ABA concentration not only in the heated organ, but also in unheated seedling parts. A high-temperature treatment of the whole seedlings and the local treatment of shoots or roots caused an increase in the heat tolerance of leaf cells. The heat tolerance of root cells virtually did not change after heating of the whole seedlings or shoots, but decreased after heating of roots. The possible role of ABA in changing the heat tolerance of leaf and root cells by local heating of the seedling is discussed.     

Tissue culture and plant regeneration of okra (Abelmoshus esculentus)

Explants (hypocotyl, cotyledon, cotyledonary node and leaf segment) were excised from aseptically grown okra (Abelmoschus esculentus) seedlings. The explants were cultured on a Murashige and Skoog basal nutrient medium supplemented with auxins, cytokinins and auxin-cytokinin combinations. Callus formation and root differentiation occurred in a medium containing naphthaleneacetic acid (NAA) or indoleacetic acid. There was a greater proliferation of roots on medium supplemented with NAA. The addition of 2,4-dichlorophenoxyacetic acid (2,4-D) to the growth medium suppressed root formation. No shoot bud or shoot development was observed at any of the auxin levels tested. Both kinetin (KN) and zeatin (Z) also proved ineffective in inducing shoot buds or shoots. Shoots were produced on cotyledon and cotyledonary node explants cultured in a medium supplemented with benzyladenine and NAA. These shoots developed roots on the same medium. The plantlets, on transfer to soil, grew normally.     

Antioxidant Responses of Lentil to Cold and Drought Stress

The effects of cold and drought stress on antioxidant responses and growth parameters in shoots and roots of lentil (Lens culinaris M cv Sultan 1) seedlings were investigated. Ten-day-old hydroponically grown seedlings were subjected to drought and cold (4°C) stress for 5 days. The length and fresh weight of shoots decreased significantly under both stress conditions, contrary to the increase in these growth parameters for roots under the same conditions. The oxidative damage as generation of malondialdehyde and hydrogen peroxide, was markedly higher in shoots under cold. Both stress conditions caused a significant increase in malondialdehyde levels in root tissues. The increase in proline levels was more pronounced under cold stress in shoots and roots. The tested stress conditions had no significant effect on chlorophyll contents. Superoxide dismutase activity was differentially altered in shoot and root tissues under drought and cold stress. The catalase activity was higher in roots under drought stress. On the other hand, ascorbate peroxidase activity increased in root tissues under cold stress. The results indicate that improved tolerance to cold and drought stress in root and shoot tissues of lentil might be correlated to the increased capacity of antioxidative defense system.     

Aluminium accumulation in root cell walls coincides with inhibition of root growth but not with inhibition of magnesium uptake in Norway spruce

High levels of aluminium in the soil solution of forest soils cause stress to forest trees. Within the soil profile, pH and aluminium concentration in the soil solution vary considerably with soil depth. pH strongly influences the speciation of A1 in solution, and is a factor when considering toxicity of A1 to roots. Norway spruce ( Picea abies [L.] Karst.) seedlings were grown for 7 weeks in nutrient solutions at pH 3.2, 4.0 or 5.0 containing 0, 100 or 400 µ M A1. At the end of this period, seedling growth, the cation exchange capacity of the roots and the amount of exchangeable Ca and Mg in roots were determined. A1 concentrations in whole roots, root segments, and in needles were measured. Using X‐ray microanalysis, the concentrations of Al, Ca, Mg and P were determined in cortical cell walls. We wanted to test the hypotheses that (1) the amount of Al bound to cation exchange sites can be used as a marker for Al toxicity and (2) the Mg concentration of needles is controlled by the amount of Mg bound to cation exchange sites. Low pH reduced the inhibition of Al on root growth and shoot length. Both low pH and Al lowered the concentration of Ca and Mg in needles. Al concentrations in the roots decreased as the pH decreased. In the roots, Al displaced Mg and Ca from binding sites at the root cortical cell walls. A comparison of the effects of Al at the different pH values on root growth and Mg concentration in the needles, suggests that, at pH 5.0, an Al fraction in the apoplast inhibits root growth, but does not affect Mg uptake. This fraction of Al is not available for transport to the shoots. In contrast, Mg uptake is strongly affected by Al at pH 3.2, although only very low levels of Al were detected in the roots. Thus, Al accumulation in the apoplast is a positive marker for Al effects on root growth, but not Mg uptake. The Mg concentration of needles is not controlled by the amount of Mg bound to cation exchange sites.