the acid mineral soils of SMM have an average soil pH (152.5 fresh soil: 0.01 M CaCl. 2. ) ...... chemical conditions in a toxic serpentine soil. Journal of culture.
Journal of Experimental Botany, Vol. 51, No. 347, pp. 1057–1066, June 2000
Effects of aluminium on the growth and mineral composition of Betula pendula Roth Petra S. Kidd1 and John Proctor Department of Biological Sciences, University of Stirling, Stirling FK9 4LA, Scotland, UK Received 15 September 1999; Accepted 21 January 2000
Abstract 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 (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.
Introduction Aluminium (Al ) toxicity is widely considered to be the most important growth-limiting factor for plants in most strongly acid soils (pH 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.
Materials and methods 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 (152.5 fresh soil: 0.01 M CaCl ) of 3.2±0.03 and 4.3±0.11, and a 2 monomeric Al concentration ([Al ] ) of 3.7±0.1 and mono 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 ] of 5.3±1.9 and mono 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 mmol 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 (mM ): 776 NH OH, 350 Na SO , 74 Ca ( NO ) .4H O, 74 CaCl .6H O, 4 2 4 3 2 2 2 2 58 Mg ( NO ) .6H O, 56 NH H PO , 22 KH PO , 3 2 2 4 2 4 2 4 9.9 KFeEDDHA, 4.6 H BO , 0.91 MnSO .4H O, 0.076 3 3 4 2 ZnSO .7H O, 0.032 CuSO .5H O, and 0.0074 4 2 4 2 (NH ) Mo O .4H O. Stock solutions of 100- (macronutrients) 4 6 7 24 2 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(NO ) .9H O and at the following concentrations: 0 (con3 3 2 trol ), 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
Aluminium effects in Betula 1059 using the same analytical techniques as for soil solution extraction (described below).
of P, K, Ca, Mg, Al, and Fe were measured using the same methods as those described above for soil solutions.
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, SO and NO were measured using ion chromotography: a 4 3 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 NH were measured on a Tecator 4 FIAstar 5010 flow-injection auto-analyser. The concentration of monomeric Al species, [Al ] , in nutrient solutions and mono 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).
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
