Journal of Experimental Botany, Vol. 52, No. 357, pp. 791-799,
April 15, 2001
© 2001 Oxford University Press
Original Papers |
Why plants grow poorly on very acid soils: are ecologists missing the obvious?
Department of Biological Sciences, University of Stirling, Stirling FK9 4LA, UK
Received 8 May 2000; Accepted 10 October 2000
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Abstract |
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Factors associated with soil acidity are considered to be limiting for plants in many parts of the world. This work was undertaken to investigate the role of the toxicity of hydrogen (H+) which seems to have been underconsidered by ecologists as an explanation of the reduced plant growth observed in very acid soils. Racial differences are reported in plant growth response to increasing acidity in the grass Holcus lanatus L. (Yorkshire-fog) and the tree Betula pendula Roth (Silver Birch). Soils and seeds were collected from four Scottish sites which covered a range of soils from acid (organic and mineral) to more base-rich. The sites and their pH (1:2.5 fresh soil:0.01 M CaCl2) were: Flanders Moss (FM), pH 3.2±0.03; Kippenrait Glen (KP), pH 4.8± 0.05; Kinloch Rannoch (KR), pH 6.1±0.16; and Sheriffmuir (SMM), pH 4.3±0.11. The growth rates of two races of H. lanatus, FM and KP, and three races of B. pendula (SMM, KP and KR) were measured in nutrient solution cultures at pH 2.0 (H. lanatus only), 3.0, 4.0, 5.0, and 5.6. Results showed races from acid organic soils (FM) were H+-tolerant while those from acid mineral soils (SMM) were Al3+-tolerant but not necessarily H+-tolerant. These results confirmed that populations were separately adapted to H+ or Al3+ toxicity and this was dependent upon the soil characteristics at their site of collection. The fact of plant adaptation to H+ toxicity supports the view that this is an important factor in very acid soils.
Key words: Aluminium tolerance, Betula pendula Roth, Holcus lanatus L., tolerance to low pH, soil acidity.
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Introduction |
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TopAbstractIntroductionMaterials and methodsResultsDiscussionReferences |
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Soil acidity is a major growth-limiting factor for plants in many parts of the world (Foy, 1984
). Explanations of poor plant growth on acid soils have included Al3+ toxicity (Barceló et al., 1996; Kinraide, 1993), Mn2+ toxicity (Foy, 1984), low N supply (mainly NH4+-N rather than NO3--N) (Foy, 1984), P deficiency (Foy, 1984), Mo deficiency (particularly in legumes; Hafner et al., 1992) and toxic concentrations of phenolic acids (Baziramakenga et al., 1995; Vaughan and Ord, 1991; Whitehead et al., 1981). The hydrogen (H+) ion itself has been considered as the proximal cause of poor growth relatively rarely in seeming deference to Arnon and Johnson (Arnon and Johnson, 1942) who concluded that the poor growth observed in lettuce, tomato and Bermuda grass when grown in solutions of low pH was the result of a low Ca supply.
Ecological work has tended not to be explicit about the two types of acid soils (Proctor, 1999). Where there are minerals present there is usually enough aluminium to buffer the pH at around 4 and to subject non-tolerant plants to aluminium toxicity. When aluminium is not present, as in organic soils, the aluminium buffering is lost, the pH can fall to well below 4 and H+ ions dominate the composition of the soil solution. This is the situation in very acid soils which account for a high proportion of acid soils globally (for example, histosols constitute 200 million ha worldwide: Brady, 1990). The effect of H+ ions is exacerbated at low Al3+ concentrations since the latter ameliorate H+ toxicity (Kinraide, 1993).
If this interpretation is correct then the existence of plant races separately adapted to H+ toxicity or Al3+ toxicity is predicted. Experiments are reported on two species, the grass Holcus lanatus L. and the tree Betula pendula Roth to investigate such racial differences.
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Materials and methods |
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To establish whether or not pH-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 more base-rich: East Flanders Moss (FM) (National Grid Reference (NGR): NS 639 973), Sheriffmuir (SMM) (NGR: NN 830 029), Kippenrait Glen (KP) (NGR: NS 794 994), and Kinloch Rannoch (KR) (NGR: NN 717 574). The soil types, and their chemical analyses, are described in Table 1
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Culture solution experiments
Seeds of H. lanatus and B. pendula were collected on 1–5 August and 1–5 September 1995 from FM, SMM, KP, and KR and were germinated in acid-washed sand (H. lanatus) or on 1% agar (B. pendula). The Petri dishes were kept under a photoperiod of 16 h light and 8 h dark with a PAR of 200 µmol m-2 s-1. Temperature was 20 °C during the day and 15 °C during the night. Seeds germinated after three (H. lanatus) or five (B. pendula) days and were kept in Petri dishes for a further 7–14 d. Owing to poor germination the FM race of B. pendula was not used in the experiment. At the first leaf stage they were removed from the dishes and carefully threaded through thin glass tubes suspended from the lids of 600 ml beakers containing culture solution at pH 5.6. 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 about the possible confounding effects of NaFeEDTA in micronutrient experiments (Chaney and Bell, 1987). The use of nutrient solutions overcomes many of the complexities of field soils and allows accurate pH control.
After 14 d (H. lanatus) or 28 d (B. pendula) growth in the initial solution seedlings were grown in culture solutions at pH 2.0 (H. lanatus only), 3.0, 4.0, 5.0, and 5.6. There were five replicate seedlings per treatment per site. Solutions were buffered at pH 2.0 using citric acid (300 µM) and HCl (82 µM); at pH 3.0 using Na citrate (1000 µM) and citric acid (2000 µM); and at pH 5.0 and pH 5.6 using MES buffer (2(N-morpholino)ethanesulphonic acid, 1000 µM). Nutrient solutions at pH 4.0 were stable without the addition of a buffer. The solutions were changed every two days and their ionic speciation was predicted using the program GEOCHEM-PC (Sposito and Mattigod, 1980). Subsamples of 5 ml from each beaker, and from each treatment, were withdrawn from fresh culture solution, and from solutions 1- and 2-d-old. Solutions were analysed to monitor element concentrations using the same analytical techniques as for soil solution extraction (described below).
Measurements of root and shoot growth were recorded before seedlings were put into treatments and thereafter every 5 d until harvest. The rate of root elongation (RER) at harvest of both H. lanatus and B. pendula, and the rate of shoot elongation (SER) of H. lanatus, were determined as the increase in root/shoot length per day (cm d-1). In B. pendula, the leaf area expansion over the treatment period was determined using a relationship between actual leaf area and leaf length and breadth. 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 leaf 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). H. lanatus were harvested 10 d (19–29 September 1995) and B. pendula 28 d (17 April–15 May 1996) after pH treatments started. Roots and shoots/leaves were separated, rinsed in deionized water, and dried in an oven at 60 °C for 48 h and the dry weights recorded.
Light microscopy
Ten terminal 1 cm sections of primary roots of H. lanatus from both KP and FM, grown at each pH, were embedded in paraffin. Samples were fixed in FAA (13 ml formaldehyde: 5 ml glacial acetic acid: 200 ml 50% ethanol), dehydrated with graded ethanol (diluted with deionized water) and embedded in wax. Cross-sections (7 nm thick) were cut using an ultramicrotome and stained with safranin and light green. All sections were observed under an optical Zeiss light microscope and the diameter of the root and the stele, and the number of cortical cells were recorded. Root and stele areas were estimated from the diameter measurements, and the cortex area from the difference between the two (root area–stele area).
Plant mineral composition
After drying, the leaves/shoots 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, and Fe were measured using the same methods as those described below for soil solutions.
Soil solution extraction
Twenty soil samples (to a depth of 10 cm) were collected randomly from each of the four sites on 12 February and 6 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 12000 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 chromatography: 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 (the phytotoxic Al fraction), [Al]mono, was determined by the 60 s Pyrocatechol violet method (PCV), at wavelength 585 nm, as described by Kerven et al. (Kerven et al., 1989). The analyses of soil solutions from fresh soil samples collected in February are given in Table 2.
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Statistical analyses
The effects of solution pH on the RER, SER (H. lanatus), leaf area (B. pendula), root number, element composition, and root anatomy, were analysed using a two-way analysis of variance (ANOVA). Data were log transformed where necessary to achieve homogeneity of variance. Statistical differences between solution pH values within each race were determined using the Least Significant Difference (LSD) test at a significance level of P<0.05.
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Results |
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Root elongation, number of roots, and shoot elongation of H. lanatus
Root elongation and shoot elongation of KP H. lanatus were inhibited in solutions of pH
4.0 after only 5 d but increased almost linearly with time in solutions at either pH 5.0 or 5.6 (Fig. 1; df(pH)=4, F=3.12, P<0.05 (RER) and df(pH)=4, F=8.01, P<0.001 (SER)). RER at harvest was not significantly different between either pH 5.0 or 5.6, and was up to 7-fold faster than the other treatments. H+-induced reductions of RER and SER at pH 2.0 and 3.0 were significantly lower in the FM race of H. lanatus (than the KP race) and the FM race showed best growth at pH 4.0 (Fig. 1; df(race)=1, F=23.91, P<0.001 (RER) and df(race)=1, F=43.35, P<0.001 (SER)). In the FM race shoot growth (shoot lengths, tiller number, and blade number) only decreased with time in solutions at pH 5.6 and remained constant in solutions at pH 2.0 and 3.0. At harvest, RER of the FM race was up to 2-fold faster, and SER was up to 1.3-fold faster at pH 4.0 than at pH 5.6. The same pattern was found for the number of roots: most roots were produced by the FM and KP races at pH 4.0 and pH 5.6, respectively (Fig. 2). Root numbers were 3-fold greater at pH 4.0, and 4-fold greater at pH 5.6, than numbers in the remaining treatments in FM and KP races, respectively (df(pH)=4, F=5.78, P<0.001). Flanders Moss (FM) races continued to produce roots throughout the treatment period at all solution pH values. In contrast, the KP race did not produce roots in solutions at pH 2.0 and pH 4.0 over the 10 d experimental period, and root numbers only increased slightly at pH 3.0. Root damage (suppressed lateral root development and root tip necrosis) was much less in the FM race at pH<4.0. In solutions of pH<4.0 shoots of the KP race were severely chlorotic and wilted while FM shoots showed mild chlorosis and did not wilt.
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) and shoot elongation (
) (cm d-1) at harvest in H. lanatus originating from Flanders Moss (FM) and Kippenrait Glen (KP). Seedlings were grown in nutrient solutions at pH 2.0, 3.0, 4.0, 5.0, and 5.6. Vertical bars represent±SE (n=5). Statistically different means within a race are indicated by different letters above each bar (P<0.05, ANOVA, LSD test).
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Root anatomy of H. lanatus
Root area and cortical cell number of the FM race of H. lanatus were generally constant between pH treatments, with the exception of pH 3.0 where both were significantly greater compared with the remaining treatments (df(pH)=4, F=5.04, P<0.01). In contrast, the whole-root area of the KR race progressively decreased in solutions at pH<5.0 (df(race)=1, F=19.38, P<0.001). Cortex area (as a percentage of whole-root area) was significantly greater in the FM race than the KP race irrelevant of pH (df(race)=1, F=38.84, P<0.001). This area also remained consistent in the FM race despite decreasing solution pH but significantly decreased in the KP race (by about 20%, Fig. 3). The sitexpH interaction factor was significant at P<0.01 (df=4, F=3.85).
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Root elongation and leaf area expansion of B. pendula
In B. pendula, RER, root number, and total leaf area were greatly reduced in the KP and KR races, but much less so in the SMM race as acidity increased (Figs 4, 5). Mean RER was reduced from 2.90 cm2 d-1 at pH 5.6 to 1.33 cm2 d-1 at pH 3.0 in the SMM race but from 3.88 cm2 d-1 to 0.92 cm2 d-1 in the KR race. Differences among races were significant (df=2, F=3.72, P<0.05 (RER); df=2, F=6.20, P<0.01 (leaf area)). Leaf area and leaf number were highest in the SMM race, and both increased with pH. However, unlike the KP and KR races, differences between the pH treatments 4.0, 5.0, and 5.6 were not significant.
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Plant mineral composition in H. lanatus
H+-induced reductions in root K concentration (df=1, F=4.08, P<0.05), and root (df=1, F=12.62, P<0.01) and shoot Mg concentration (df=1, F=27.92, P<0.001), were more pronounced in the KP race than in the FM race of H. lanatus: root K decreased significantly only between pH 3.0 and 2.0 in the FM race (Fig. 6).
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Plant mineral composition in B. pendula
There were few differences in either root or shoot element concentrations of B. pendula. The most obvious change was a decrease in the root Ca concentration with increasing acidity in KP and KR races, but this was not observed in the SMM race (data not shown).
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Discussion |
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It must be emphasized that in this study very acid organic soils (pH<4.0) have been distinguished from acid mineral soils (pH 4.0–5.0) where the major yield-limiting constraint is generally considered to be Al toxicity or P deficiency. Numerous studies on crop plants in acid sandy soils (arenosols) in West Africa have found P deficiency to be the primary growth-limiting factor (Doumbia et al., 1993
; Rebafka et al., 1994; Yamoah et al., 1995). The high percentage Al saturation characteristic of these acid mineral soils, leads to P adsorption and a reduction in P mobility and availability for plant uptake (Hafner et al., 1993). In addition, in this study, plants, typically of the Proteaceae family, which form root clusters or proteoid roots in response to P deficiency, or plants with mycorrhizal associations which increase the host plant nutrient efficiency (Marschner, 1995) have not been considered. In both these cases, acid-soil tolerance by the plant is achieved through effective nutrient acquisition.
In this study best growth (in terms of RER, SER, root number, and tiller number) of H. lanatus originating from FM occurred at pH 4.0, with both root and shoot growth decreasing with pH <4.0 or >4.0. Examples of tolerance to low pH in crops are numerous (Arnon and Johnson, 1942; Blamey et al., 1982; Islam et al., 1980; Llugany et al., 1995; Osaki et al., 1997; Tan et al., 1993) and, out of 14 grain legume species, the growth of two species (Lupinus angustifolius L. and Lupinus albus L.) was sensitive to higher pH (pH>5–6; Tang and Thomson, 1996). However, best growth at low pH of a naturally occurring species in controlled nutrient solutions has not to-date been shown. Furthermore, demonstrations of differences in tolerance to low pH among races of naturally occurring species are rare.
Unlike the KP or KR races, the SMM race of B. pendula was tolerant to, but did not show best growth at, low pH (3.0–4.0). Both the highest dry weights and fastest RER were found at the higher solution pH values (pH 5.0/5.6) in all three races. Tolerance to low pH in B. pendula races was ranked SMM>KP>KR. The results from B. pendula, but not those for H. lanatus, corroborate the findings of Rorison (Rorison, 1986) who concluded that plants most capable of surviving acidic environments tended to have inherently slow growth rates. In contrast to the KP and KR races, the SMM race of B. pendula maintained a steady slow rate of root elongation with increasing acidity.
The observed inhibition of growth at low pH was not a symptom of low P in the nutrient solutions: solution analyses showed a maximum decrease of 24% between solution changing. GEOCHEM also predicted that over 96% of PO4 was present as H2PO4- irrelevant of solution pH.
It has been suggested (Foy, 1992) that excess H+ competes with other cations for root absorption sites, interfering with ion transport and uptake, and causes root membranes to become leaky. The close correlation between the H+-induced reduction in specific absorption rates of P, Ca, Mg, and Fe in maize led Poschenrieder et al. (Poschenrieder et al., 1995) to conclude that alterations in nutrient uptake play an important role in H+ion toxicity. H+-induced reductions in root K concentration, and root and shoot Mg concentrations, were more pronounced in the H+-sensitive KP race of H. lanatus than the FM race. In the case of these two elements, uptake occurs through operation of an energized ATPase pump at the root plasma membrane. In their most recent studies, Yan et al. (Yan et al., 1998) showed that the plasma membrane H+-ATPase plays a major role in the adaptation of maize roots to low pH. The authors suggested that the likely mechanisms involved in this adaptation included an increase in the number of H+-ATPase enzymes per unit membrane, and a shift in the coupling between ATP hydrolysis and H+ pumping. In B. pendula, a reduction in root Ca concentration with increasing acidity was observed in both KP and KR races, but not in the SMM race. Similarly, the H+-tolerant maize cultivar, BR 201F, had a higher root Ca at low pH than the H+-sensitive cultivar, HS 7777 (Llugany et al., 1995). Calcium regulates plasma membrane integrity and the functioning of the proton efflux pump (Marschner, 1995), and this may be one way in which the plant's ability to maintain higher Ca levels at low pH contributes to ion tolerance. Further investigations into the role of the ATPase activity of the FM and SMM races of H. lanatus and B. pendula, respectively, and whether or not H+ release or plasmalemma H+ permeability are less inhibited than in the (low pH-sensitive) KP and KR races are required.
A comparison of the foliar element concentrations of experimental control plants with those of plants collected from the field showed minor differences (Kidd, 1998): this gives confidence that the composition of the culture solutions used in this study were therefore representative of the conditions experienced by these plants in their native soils.
Low pH not only affected RER but also led to a change in root system architecture which, in consequence, can reduce a plants ability to absorb water (and nutrients). However, there are few studies investigating water relations in H+-stressed plants (Gunsé et al., 1997). At pH 3.0, the root tips of the KP race of H. lanatus were swollen, black and necrotic. The overall colour of the root system was brownish and new primary roots and laterals were both discoloured and stunted. In contrast, no browning was observed in the root tips of the FM race. Root browning has been attributed to enhanced suberization which may limit water uptake (Barceló and Poschenrieder, 1990). The wilting of the KP race, which occurred in solutions at pH<4.0, suggest the water relations in this race were adversely affected. In addition, the proportion of tissue occupied by cortex increased in the FM race with decreasing pH but was significantly reduced in the KP race. The cortex tissue takes part in the transport of water and nutrients, and its preservation was suggested by Ciamporová et al. (Ciamporová et al., 1995) to explain the acid-soil tolerance of Deschampsia flexuosa L. compared with Nardus stricta L. from Central Slovakia.
Estimated stele area significantly increased in FM and decreased in KP at pH 3.0 compared to pH 5.6. An increase in the number or diameter of xylem elements favours water uptake and conductivity, as does a reduction in the number or size of leaves (Barceló and Poschenrieder, 1990). At pH 3.0 and 4.0 the root:shoot ratios of the FM race were significantly higher than those of the KP race (data not shown), indicating a greater contribution to yield from roots than shoots in the former race. Through a reduction in shoot system and alteration in root morphology this race may partly tolerate low pH by water conservation. Similarly Cu-tolerant populations of Silene cucubalus Wib. have been shown to maintain their water content, while that of non-tolerant populations decreases rapidly (Lolkema and Vooijs, 1986). However there is a need for further, more detailed, investigations of the water relations of both the H. lanatus and B. pendula races. In this study it was not possible to measure changes in membrane water permeability and cell wall elasticity, structural changes in the cell ultrastructure (hypodermis, endodermis, or pericycle), or the occurrence of plasmolysed cortical cells, all of which may affect water uptake.
This work has shown that races of H. lanatus and B. pendula growing in acidic soils are adapted to high concentrations of H+. The total acidity of FM soil was represented mainly through exchangeable H+, and that of SMM soil by exchangeable Al3+ (data not shown). The FM race of H. lanatus was H+-tolerant but, in separate experiments (Kidd, 1998), this race was Al-sensitive, reflecting its adaptation to acid organic soils but not acid mineral soils. This race does not naturally experience high Al concentrations in the soil solution, while the acid mineral soil of SMM has both predominant Al and a mean solution pH at which monomeric Al ([Al]mono) exists in its most phytotoxic Al(H2O)63+ form (commonly abbreviated as Al3+) (Kochian, 1995). Root elongation and SER of FM H. lanatus grown in nutrient solutions (at pH 4.2) with 25 mg l-1 Al (21 mg l-1 [Al]mono, 300 µM {Al3+}GEOCHEM) were 54% and 38% of plants grown in solutions (at pH 4.2) with no added Al (Kidd, 1998). The SMM race of B. pendula was H+-tolerant, but best growth was not observed at low pH. In contrast, in separate experiments (Kidd and Proctor, 2000) B. pendula growth increased with an increase in Al concentration from 2–25 mg l-1 (1–21 mg l-1 [Al]mono, pH 4.2), reflecting its adaptation to acid mineral soils which experience both Al3+ and H+ at high concentrations. Growth of FM races of B. pendula decreased with an increase in Al (Kidd and Proctor, 2000), further supporting the existence of separate adaptations to either H+ or Al3+-toxicity. The soils of KP and KR have a mildly acid pH and low concentrations of Al3+, and these races, in both H. lanatus and B. pendula, were sensitive to both Al3+ and H+.
The results show that in very acid organic soils, such as those of FM, H+-tolerant as distinct from Al3+-tolerant races may occur. However, in acid mineral soils, such as those of SMM, double tolerance to H+ and Al3+ may be necessary, since Al3+ concentration increases in the soil solution with decreasing pH (Kochian, 1995). The combined effects of H+ (pH 4.0/4.3) and Al3+ (19 µM) on the growth of Bromus benekenii Lange Trimen and Hordelymus europaeus All. were no greater than the separate H+ or Al3+ effects (Brunet, 1994). In addition Kinraide (Kinraide, 1993) has shown Al3+ amelioration of H+ toxicity and vice versa in wheat. Root elongation at pH 4.0–5.0 of the KP race of B. pendula was not optimal but when grown at pH 4.2 with 2 and 5 mg Al l-1 (Kidd and Proctor, 2000) RER was significantly faster and this may be explained by Al3+ amelioration of H+ toxicity as proposed by Kinraide (Kinraide, 1993). There was no similar stimulation in growth of the SMM race when grown at these low Al concentrations (Kidd and Proctor, 2000) but then RER was not inhibited at pH 4.0 in this race.
The race-specific tolerance in FM H. lanatus to a low pH was predicted from the origin of this race from soils in which the solutions had low Al, Mn or phenolic acids (Kidd, 1998). It has been demonstrated for races of the crops maize (Zea mays L.), mung bean (Vigna radiata L. Wilczek), and rice (Oryza sativa L.), that very acid soils are so toxic that their root growth is nearly completely inhibited (Proctor, 1999). It appears that plants are faced firstly with toxic [H+] before they encounter other unfavourable factors (Al, Mn, low N, P, and Ca) which occur in very acid soils. It was concluded that the direct toxicity of the H+ ion is the proximal cause of the poor growth of non-tolerant plants on these very acid organic soils and this should be more widely recognized by ecologists.
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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 AJ Duncan for technical assistance.
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Notes |
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1 Present address and to whom correspondence should be sent: Departamento de Edafología y Química Agrícola, Facultad de Biología, Universidad de Santiago, Santiago de Compostela 15706, Spain. E-mail: edpetra{at}usc.es
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