Journal of Experimental Botany, Vol. 51, No. 347, pp. 1057-1066,
June 2000
© 2000 Oxford University Press
Effects of aluminium on the growth and mineral composition of Betula pendula Roth
Department of Biological Sciences, University of Stirling, Stirling FK9 4LA, Scotland, UK
Received 15 September 1999; Accepted 21 January 2000
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Aluminium (Al) is rhizotoxic and is often present in acidic soils at activities high enough to inhibit root elongation. The objectives of the present study were to assess the level of Al tolerance in different races of Betula pendula Roth (Silver Birch) and to investigate how growth and nutrient acquisition were affected by Al. A solution culture technique was employed which simulated natural soil solutions. Aluminium at low concentrations (2 and 5 mg l-1), enhanced the growth of two races of B. pendula originating from soils poor in Al (FM and KP). In contrast, Al, at all concentrations tested, inhibited growth in an Al-sensitive race (KR) whose provenance was a calcareous soil. At concentrations
10 mg l-1, Al reduced growth in FM and KP races, while growth increased with increasing Al (up to 25 mg l-1) in the Al-tolerant, SMM, race. Aluminium altered both root and leaf architecture. Low Al concentrations (<5 mg l-1) significantly increased leaf expansion, and high concentrations (>25 mg l-1) reduced leaf expansion. In the Al-sensitive race, KR, there was a loss of apical dominance, and both lateral and primary roots were stunted and swollen, with increasing Al concentrations. These results demonstrated pronounced racial differences in tolerance to Al by B. pendula that could be predicted from the soil environment of each race. Key words: Aluminium toxicity, aluminium tolerance, Betula pendula, hydroponics, plant morphology.
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Introduction |
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Aluminium (Al) toxicity is widely considered to be the most important growth-limiting factor for plants in most strongly acid soils (pH <5.0) (Foy, 1984
; Horst, 1995; Ryan et al., 1992; Susaki et al., 1995). The inorganic monomeric octahedral hexahydrate, Al
or Al3+ is generally considered the principal rhizotoxic form (Kinraide, 1993). It dominates in solutions more acidic than pH 5.0 (Martin, 1986). The monohydroxy-Al species (Al
, Al(OH)2+, Al
), which are present in solutions between pH 5.0–6.2, are probably non-toxic (Kinraide, 1997). Organo-Al complexes have frequently been shown not to be toxic (Foy et al., 1990; Harper et al., 1995; Kerven et al., 1991; Kidd, 1998). In acid soils (pH <5.0) where there are minerals present Al buffers the soil pH at around 4 and is thus available to plants in the toxic form (Al3+). Plant populations present in these soils normally evolve some degree of tolerance to Al in the soil solution. In acid organic soils, Al concentrations are low, the Al buffering is lost and the pH falls below 3 (Proctor, 1999; Tan, 1998). Any aluminium present in these soils is likely to be as organo-Al complexes and non-toxic, thus H+ ions are probably the main principal toxic element to plants. In calcareous soils, at the other end of the pH spectrum, concentrations of aluminium are low and present as the non-toxic aluminate (Al). Consequently, the tolerance of plant races to Al toxicity should be predictable from their native soil characteristics.
Aluminium has been suggested to bind to the polar regions of phospholipids or proteins on the plasma membrane (Barceló et al., 1996). The resultant structural and functional alterations affect membrane permeability and transport processes, which in the long-term limit nutrient acquisition (Ryan and Kochian, 1993). Symptoms of prolonged Al stress resemble those of Ca deficiency, implying that Al is especially disruptive to the uptake of Ca. Numerous reports have shown Al-induced inhibition of Ca uptake and species adapted to soil acidity frequently show high Ca efficiency (Barceló et al., 1996).
Although regarded as a toxic element, Al frequently stimulates growth at low concentrations (Foy, 1984; Kinraide, 1993; Taylor, 1988). Early explanations for this enhancement in growth include increased Fe solubility and availability, prevention of internal Fe deficiency through displacement of Fe from inactive sites in calcicolous plants, prevention of P toxicity or promotion of P uptake, reduction in growth rate and prevention of Ca depletion, alteration of growth regulators, protection against Cu/Mn toxicity, and protection against fungal pathogens (Clark, 1977; Foy et al., 1978; Grime and Hodgson, 1969; Ko and Hora, 1972; Konishi et al., 1985; Mullette, 1975). However, hypotheses such as those listed above have only been shown to apply in certain cases (Kinraide, 1993). In addition, there is now substantial evidence that the nature of these beneficial effects occur through the alleviation of H+ toxicity by Al3+. This hypothesis was proposed by Kinraide (Kinraide, 1993) using Triticum aestivum L. Growth enhancement by Al3+ only occurred under acidic conditions that reduced root elongation. Furthermore, alleviation of H+ toxicity was a general phenomenon achieved by cations (not solely Al3+), and the effectiveness was dependent upon the charge (C3+ C2+ C1+). Aluminium ions (Al3+) increased cell membrane electrical polarity and stimulated H+ extrusion (essential for continued root growth at low pH; Yan et al., 1992). Kinraide suggested that this reduction in electrical polarity by the ameliorative cation could reduce the cell-surface activity of the toxic cation (Kinraide, 1993).
Early ecological work (Rorison, 1958, 1960a, b, 1969), attempted to explain the primary factors limiting growth of plants on acid and calcareous soils. Rorison selected typical calcifuge (Deschampsia flexuosa (L.) Trin) and calcicole (Briza media L.) species and compared their growth responses to Al. More recent studies involving Al toxicity and tolerance in higher plants have largely focused on crop species. The objectives of this study were to investigate the Al-induced changes in root growth, architecture, leaf expansion, and mineral composition of four races of the naturally occurring species, B. pendula. In addition, this study investigated whether or not the growth response of these different populations was in accordance with their ecological distribution and native soil characteristics. The origin of the races covered a range of soil environments from acidic (organic and mineral) to calcareous. The races were grown in hydroponic solutions designed to simulate native soil solutions. The beneficial effects of low Al concentrations on birch growth and the changes in the leaf expansion with Al, relative to the race provenance, have not to date been quantified.
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Materials and methods |
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To establish whether or not Al-tolerance of different populations was in accordance with their ecological distribution, soils and seeds were collected from four Scottish sites which covered a range of soils from acid (organic and mineral) to calcareous: East Flanders Moss (FM, NS 639973), Sheriffmuir (SMM, NN 831029), Kippenrait Glen (KP, NS 794994), and Kinloch Rannoch (KR, NN 717574). The acid organic soils of FM and the acid mineral soils of SMM have an average soil pH (1 : 2.5 fresh soil: 0.01 M CaCl2) of 3.2±0.03 and 4.3±0.11, and a monomeric Al concentration ([Al]mono) of 3.7±0.1 and 21.1±1.4 mg l-1, respectively. In contrast, the Brown Forest soils of KP and the calcareous soils of KR have an average soil pH of 4.8±0.05 and 6.1±0.16, and [Al]mono of 5.3±1.9 and 2.0±0.3 mg l-1, respectively. The soils were described in full by Kidd (Kidd, 1998
).
Seedling growth
Seeds of birch (B. pendula Roth) were collected from the four sites in August/September 1995 and were germinated in Petri dishes containing 1% agar in December 1995. The Petri dishes were kept under a photoperiod of 16/8 h light/dark with a PAR of 200 µmol m-2 s-1. Temperature was maintained at 20 °C during the day and 15 °C during the night. Seeds germinated after 5 d and were kept in agar for a further 7–14 d. At the first leaf stage they were removed from the agar and carefully threaded through thin glass tubes suspended from the lids of 600 ml beakers containing culture solution (composition given below) with no added Al at pH 5.6. After 28 d in this solution, seedlings of similar size were selected and placed into beakers, each holding one seedling. At this stage seedlings ranged from 5.9–8.6 cm in height, with 3–5 roots with a combined length of 40.4–73.0 cm, and 10–17 leaves of 26.1–40.9 cm2 total area. There were five replicate seedlings (and five beakers) per treatment.
Growth solutions
The composition of the culture solutions was (µM): 776 NH4OH, 350 Na2SO4, 74 Ca(NO3)2.4H2O, 74 CaCl2.6H2O, 58 Mg(NO3)2.6H2O, 56 NH4H2PO4, 22 KH2PO4, 9.9 KFeEDDHA, 4.6 H3BO3, 0.91 MnSO4.4H2O, 0.076 ZnSO4.7H2O, 0.032 CuSO4.5H2O, and 0.0074 (NH4)6Mo7O24.4H2O. Stock solutions of 100- (macronutrients) and 1000-strength (micronutrients) were made up and diluted appropriately. The composition of macronutrients in the culture solution simulated soil solutions extracted from fresh and rewetted air-dried soil (see below), and micronutrient concentrations were based on those used by Johnston and Proctor (Johnston and Proctor, 1981) which were 1/10 of those used by Hoagland and Arnon (Hoagland and Arnon, 1950). KFeEDDHA was used instead of NaFeEDTA following cautions by Chaney and Bell (Chaney and Bell, 1987) about the possible confounding effects of NaFeEDTA in micronutrient experiments. The software program GEOCHEM-PC (Parker et al., 1995) predicted that 63.8% of Fe3+ in solutions remained bound to EDDHA compared with 11.9% using EDTA. Culture solutions were stirred daily and the pH corrected where necessary to 4.2 using 1 M NaOH or 1 M HCl. Culture solutions were renewed every 3 d.
Aluminium was added to the culture solutions in the form Al(NO3)3.9H2O and at the following concentrations: 0 (control), 2, 5, 10, 15, 25, and 35 mg Al l-1. The following abbreviations are used corresponding to the treatments 0 Al, 2 Al, 5 Al, 10 Al, 15 Al, 25 Al, and 35 Al. Subsamples of 5 ml from each of six beakers, from each of the seven treatments, were withdrawn from fresh culture solution, and from solutions 1-, 2-, and 3-d-old, during the first 14 d of the experiment. Solutions were analysed to monitor element concentrations using the same analytical techniques as for soil solution extraction (described below).
Soil solution extraction and element analyses
Twenty soil samples (to a depth of 10 cm) were collected randomly from each of the four sites in February and July 1995. Ten samples were stored at 5 °C (but extracted within 24 h) and the remaining ten samples were air-dried at room temperature, ground, and sieved through a 2 mm mesh. The air-dried soils were slowly saturated with water over a 2 d period. Subsamples of 25 g were centrifuged for 30 min at 12 000 rpm in a High Speed MSE Centrifuge. Concentrations of K, Ca, Mg, Fe, and Na were measured using a Varian AA-575 S atomic absorption spectrophotometer (AAS). Total Al was measured with a Pye Unicam SP9 AAS fitted with a Unicam GF90 furnace and FS90 furnace autosampler. Unicam 919 series atomic absorption software was used. The anions Cl, SO4 and NO3 were measured using ion chromotography: a Dionex QIC analyser fitted with Dionex AI450 software connected to a Dionex ACI with a Dionex AS40 autosampler. The columns used were both 4 mm versions: Dionex IonPac AG4 guard column and Dionex IonPac AS4A analytical column. Phosphorus and NH4 were measured on a Tecator FIAstar 5010 flow-injection auto-analyser. The concentration of monomeric Al species, [Al]mono, in nutrient solutions and soil solutions were determined by the 60 s Pyrocatechol violet method (PCV), at wavelength 585 nm, as described by Kerven et al. (Kerven et al., 1989).
Growth measurements
The number of roots and their lengths, the number of leaves and their length and maximum breadth, the height of the seedlings, and the number of buds were recorded before seedlings were put into treatments and after treatments at harvesting. To determine the leaf area expansion over the treatment period, a relationship between actual leaf area and measured values of length and breadth was established. One hundred leaves, collected from separate birch seedlings, which were grown alongside experimental seedlings, were scanned and their areas were measured on a Power Macintosh (8500/180) computer using the public domain NIH 5b Image program (developed at the US National Institutes of Health, National Technical Information Service, Springfield, Virginia). A regression equation between leaf area and maximum breadth and length was then determined and used to estimate the leaf area of experimental seedlings before treatments began and after harvesting (Area=(3.44(Length)+1.26(Breadth)-3.88), F=82.31, P<0.001). Both absolute growth (AGR=cm2 d-1) and relative growth (RGR=cm2cm-2 d-1) rates were then determined. Seedlings were harvested after 84 d growth in treatment (14 January–8 April 1996). Roots, stems, and leaves were separated, washed in deionized water, and dried in an oven at 60 °C for 48 h and the dry weights recorded. Prior to drying, the lateral root growth of seedlings was observed under the binocular microscope. The number and length of lateral roots was estimated from 10 cm lengths of primary root.
Plant mineral composition
After drying, the leaves of each seedling (from each treatment) were ground and subsampled. Between 100 and 300 mg of oven-dried leaves and roots were digested in a sulphuric acid-hydrogen peroxide mixture (Allen, 1989) in a block digester at 330 °C. Digested solutions were filtered through No.44 Whatman filter paper and made up to 100 ml. Concentrations of P, K, Ca, Mg, Al, and Fe were measured using the same methods as those described above for soil solutions.
Statistical analyses
The effects of Al and race on the RER, root number, leaf area, leaf number, seedling height, number of buds, and the plant mineral composition, were analysed using a two-way analysis of variance (ANOVA). Data were log transformed where necessary to achieve homogeneity of variance. Statistical differences between Al concentrations within each race were determined using the Least Significant Difference (LSD) test at a significance level of P<0.05.
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Ionic composition of nutrient solutions
Nominal [Al] concentrations of 2, 5, 10, 15, 25, and 35 mg l-1 Al actually contained 1.01, 4.20, 7.81, 12.0, 18.6, and 29.9 mg l-1 Al. The average [Al]mono in these solutions was 1.3, 3.5, 6.9, 10.2, 21.0, and 27.0 mg l-1.
Root elongation, number of roots and root morphology
The mean rates of root elongation (RER, cm d-1) were significantly affected by the origin of the plants and by Al concentration (Fig. 1; Table 1). In FM and KP races, RER (and the number of roots) were increased at the lower Al concentrations (2 and 5 Al) compared with control plants by about 60% and 40%, and were significantly inhibited at the highest Al concentration (35 Al) by about 50% and 80%, respectively. There was a significant reduction in RER (up to 80%) and in the number of roots at Al concentrations <35 mg l-1 in the KR race. In contrast, the RER of the SMM race was not negatively affected by Al (up to 35 mg l-1 except at 2 and 5 mg l-1), in fact at the higher concentration of 25 Al root growth was stimulated. Relative root elongation (relative to control plants) was negatively correlated with leaf Al concentrations (r=-0.358, df=105, P<0.001).
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The average number (per cm primary root) of lateral roots ranged between 5–7 cm-1 primary root, with no difference between races or treatments. In FM, SMM and KP races laterals ranged from 60–80 mm in length, and lengths decreased to about 50 mm at 25–35 mg l-1 Al. In contrast, in the KR race lengths of laterals decreased to 19–22 mm at the higher Al concentrations (25–35 Al). Furthermore, the tips of primary roots of KR seedlings in these treatments were necrotic and swollen.
Leaf area and number of leaves
There were no signs of Al toxicity in the leaves of the FM, SMM or KP races of birch. In contrast, the leaves of the KR race were slightly chlorotic when grown in 35 Al solutions. Total leaf area and number of leaves were significantly affected by both the Al concentration and the seedling origin (Table 1
). Aluminium, at any concentration, increased leaf area (although not always significantly) in FM, SMM and KR races, and at most concentrations in the KP race, compared with control plants (Fig. 2). This increase was lowest in the KR race of birch, significant differences in leaf area were only found between the control and plants grown in either 2 Al or 10 Al. The number of leaves was greater in the presence of Al than in its absence in all four races (Fig. 3). Contrary to leaf area, this increase in the leaf number was most pronounced in those seedlings whose provenance was KR. In this race at 35 Al, the total number of leaves increased by 3-fold compared with controls. Figure 4 depicts the percentage of the total number of leaves which comprised leaves in the following size categories: <1 cm2, 1–2 cm2, 2–5 cm2, and >5 cm2. Prior to Al treatments there were no significant differences between plant groups in the number of leaves in each size category. The increase in leaf number in each category over the experimental period was significantly different between races and Al treatments (Table 1). An increase in Al concentration led to a concurrent increase and decrease in the percentage of the total number of leaves comprising leaves <2 cm2 and >5 cm2in area, respectively. The largest Al-induced production of small leaves occurred in the KR race (37% at 0 Al to 56% at 35 Al), and the least in the SMM race (33% to 36%). The proportion of leaves >5 cm2 in plants originating in KR was reduced from 31% (0 Al) to 10% (35 Al). In the KP and KR races, the proportion of large leaves (>5 cm2) increased at the low Al concentrations (2 and 5 Al) from 31% and 9% to 44% and 19%. There was no Al-induced reduction in the number of leaves >5 cm2 in area in seedlings from SMM. The RGR in SMM increased with Al concentration and was highest at the highest solution Al concentrations (25 and 35 mg l-1).
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Seedling height
Incremental height differed significantly between races (df=3, F=15.92, P<0.001; Table 1) and with increasing Al concentration (df=6, F=10.39, P<0.001; Table 1; Fig. 5). The height increase in the SMM race was significantly greater than controls at the lowest Al concentration (2 Al) and at concentrations >15 mg l-1. Although not significantly different, there was a tendency towards taller seedlings in the FM race between 2–25 mg l-1 Al, whilst at 35 Al height was reduced compared to that of the control. With the exception of 5 Al, height increase was generally stimulated (not always significantly) in KP races between 2–15 mg l-1 Al. At the higher concentration of 35 Al seedling height was significantly reduced. A similar pattern was observed in those birch with provenance KR, but height reductions were found at concentrations >10 mg l-1. At 35 Al, the mean increase in seedling height was about 17% of that in controls.
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Plant mineral composition
Overall the root Al concentrations were highest in the KR race and generally (with the exception of KP) greater in seedlings grown in 2 or 5 Al. Leaf Al concentrations were lowest in the SMM race (0.04–0.16 mg g-1, Table 2b), and highest in the KP race (0.08–0.38 mg g-1). Although inconsistent, increasing solution Al concentration often increased Al transport to the shoots (particularly in KP and KR races). Aluminium significantly reduced root K, Mg, and Fe concentrations (Table 2a: df(Al)=6, F=7.01, P<0.001 [K]; df(Al)=6, F=51.79, P<0.001 [Mg]; df(Al)=6, F=33.91, P<0.001 [Fe]). In contrast root P concentrations were not influenced by Al (with the exception of KP at 5 Al and 35 Al), and Al actually increased root Ca concentrations (particularly at 2 and 5 Al). The higher treatments (15–35 Al) reduced (often significantly) the transport of Ca, Mg and Fe to the shoots, but (with the exception of SMM) did not affect shoot P concentrations (Table 2b: df(Al)=6, F=13.10, P<0.001 [Ca]; df(Al)=6, F=16.29, P<0.001 [Mg]; df(Al)=6, F=4.23, P<0.01 [Fe]).
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Birch is regarded as an Al-tolerant plant and its growth was found to be unaffected by up to 80.9 mg Al l-1(Clegg and Gobran, 1995
). The relatively good survival of some provenances of Betula papyrifera Marsh. in highly acid mine spoils was attributed to Al-tolerance (Steiner et al., 1980). The results of this study showed race-specific differences in the growth response and mineral composition to increasing Al concentrations. The races originating from FM, SMM and KP were indeed Al-tolerant (although to different degrees), while the growth (RER, number of roots, and RGR) of the KR race was reduced at all Al concentrations (although rarely less than 50% of control plants). A continuous reduction in RER of KR was observed between 2–25 mg l-1 Al, but RER appeared to be increasing once again at 35 Al. From the results of the RGR, leaf area, and the observed leaf chlorosis and root necrosis at 35 Al, it is suggested that the overall pattern was one of a decline in growth with 35 Al (RER) as an outlier. Aluminium was shown, at low concentrations (2–5 mg l-1), to enhance the growth of the FM and KP races, which originate from soils naturally low in Al. Root elongation rate and the number of roots were significantly increased relative to control plants. Root Ca concentrations were higher in these treatments, and the translocation of both Ca and Mg to the shoots was increased. The stimulus in growth did not coincide with any promotion of either P or Fe uptake, adding discredit to the early hypothesis that Al enhances growth by promoting Fe and P uptake (Foy et al., 1978). Kinraide proposed that Al3+ amelioration of H+ toxicity explained the beneficial effects of low Al concentrations on growth (Kinraide, 1993). In separate experiments (Kidd, 1998) where races of SMM and KP were grown in solutions with no added Al, but in which the pH was varied between pH 3 and 5.6, root elongation of SMM B. pendula remained constant (2.5–3.5 cm d-1) while that of KP B. pendula increased with increasing pH (2.0–5.0 cm d-1). That is at pH 4.2 the root elongation of KP was not optimal and Al, at 2 and 5 mg l-1, could have ameliorated the adverse effects of H+ ions in the same manner as that proposed by Kinraide (Kinraide, 1993). At the same time this would explain the lack of growth stimulation seen in SMM races which were not inhibited by H+ ions. In fact RER was slower in the SMM race at 2 and 5 mg l-1 (although not significantly) Al than in controls. It has been suggested that metal-resistant plants can exhibit an increased need for metals to which they are resistant, and as a result show a less than maximal growth at ‘normal’ availability levels (Baker and Walker, 1990). The SMM race will normally experience concentrations of Al of 2–5 mg l-1 in its native soil environment.
Aluminium, at all concentrations, induced an increase in AGR, number of leaves/buds, and total leaf area. The Al-induced change in leaf area differed between the Al-tolerant and the Al-sensitive seedlings. The classification of leaves into various size groups (<1 cm2, 1–2 cm2, 2–5 cm2, >5 cm2) showed a significant increase in the number of smaller leaves (<2 cm2), and concurrent decrease in large leaves (>5 cm2), with an increase in the solution Al concentration. This was most prominent in the Al-sensitive, KR, race, and least of all in the Al-tolerant race, SMM. At low concentrations (2–5 mg Al l-1), leaf expansion was increased and there were significantly more larger-sized leaves (>5 cm2 in area) compared with control plants. This effect of Al was not seen in SMM birch where there was also no reduction in the number of leaves >5 cm2 in area in the 25 and 35 Al treatments. Konishi et al. (Konishi et al., 1985) observed large leaves in tea plants (Camellia sinensis L.), Al accumulators, when grown in nutrient solutions with up to 172.7 mg Al l-1, but they did not quantify changes in leaf area. Janhunen et al. showed a similar reduction in the size and increase in the density of needles of Pinus sylvestris L. and Picea abies L. Karst after treatment with high concentrations of Al (150 mg l-1) (Janhunen et al., 1995).
A loss of apical dominance (and the development of soft stems) in the Al-sensitive KR provenance accompanied the reduction in leaf expansion observed with increasing Al concentration. Seedling height significantly decreased in solutions >10 Al, while the number of stems increased. This loss of upright growth habit has been attributed to the inhibition of stem lignification (Göransson, 1998). In contrast, stem height (but not number) increased in the SMM race, and remained relatively constant in both the FM and KP races.
The racial differences in the response to Al could be predicted from the soil environment of each race. Aluminium tolerance was ranked SMM >FM >KP >KR. The soil of SMM had the highest concentration of monomeric Al, an average pH of 4.3 (when Al is in its most toxic Al3+ form), and its total acidity mainly comprised exchangeable Al. It was therefore hypothesized that this race would be Al-tolerant. Surprisingly, root Al concentrations were also highest in this race. Although detection limits and spatial resolution are limited, a combination of histological staining techniques, NMR, or X-ray microanalysis may have given a more precise location of Al within the root. Godbold et al. found Al concentrated in the root cortical cell walls of P. abies (Godbold et al., 1995). It is suggested that this may have also been the case in SMM B. pendula, since shoot Al concentrations were also lowest in this race. In agreement, Steiner et al. reported lower increases in foliar Al in tolerant provenances of B. papyrifera than in intolerant provenances after exposure to Al (Steiner et al., 1980). Despite a pH at which Al would be in its most toxic form, the organic nature of the FM soil favours the formation of non-toxic organo-Al complexes and a total acidity dominated by H+ ions. The Brown Forest soil of KP has a mean pH (4.8) at which much of the monomeric Al would be present in the Al3+ form, but soil Al concentrations are low. These two races are therefore intermediate in their Al tolerance and RER was only negatively affected by Al concentrations >10–15 mg l-1. The KR race was Al-sensitive. This was the only race with clear root injuries and chlorosis of the leaves, both commonly regarded as symptoms of Al toxicity. It was also the only race where RER was reduced (relative to controls) at all Al concentrations, and where growth habit was most altered. The soil of KR is calcareous and has an average pH of 6.1, but more importantly, a pH at which Al would be non-toxic (Al).
Current research has shown an increase in soil acidification through anthropogenic effects including acid precipitation and nitrification of ammoniacal fertilizers (Hahn and Marschner, 1998). Naturally occurring species, particularly trees, have been found to show higher Al-tolerance than agricultural crop species (Clegg and Gobran, 1995; Godbold and Kettner, 1991; Göransson and Eldhuset, 1991; Janhunen et al., 1995). Crops tend to be sensitive at Al concentrations <1.0 mg l-1. In contrast at these concentrations Al stimulated growth in B. pendula. However, the mineral composition of the leaves and roots of B. pendula was significantly altered, and this occurred in all races even where there was no apparent inhibitive effect on root elongation or leaf expansion. The effects of soil acidification, through natural or anthropogenic means, may therefore have significant implications on nutrient acquisition in these plants. Further investigations of the effects of these changes in nutrient concentrations and growth habit is required to assess the implications on plant performance.
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Races of B. pendula were ranked in their tolerance to Al as: SMM <FM <KP <KR. This ranking could be predicted from their natural soils. Low Al concentrations (2 and 5 mg l-1) enhanced growth and higher Al concentrations (>10–15 mg l-1) reduced growth in the less Al-tolerant races. Aluminium significantly altered the root morphology (in Al-sensitive races), leaf expansion, and nutrient composition of birch.
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Acknowledgments |
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We thank the Scottish Wildlife Trust for allowing us to collect samples from East Flanders Moss. This research was supported jointly by a grant from The Carnegie Trust for the Universities of Scotland and a University of Stirling Studentship. We also thank C Anderson and A Duncan for technical assistance.
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1 To whom correspondence should be addressed. E-mail: psk1{at}stir.ac.uk
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