SURVIVAL OF TREES IN A METAL-CONTAMINATED ENVIRONMENT
N.M. DICKINSON, A.P. TURNER AND N.W. LEPP School of Natural Sciences Liverpool Polytechnic Byrom Street, Liverpool, L3 3AF, U.K.
ABSTRACT. This study investigates the survival of sycamore (Acer pseudoplatanus) trees at a metal contaminated site where populations of herbaceous flora are known to have evolved metal tolerance~ The incidence of pollution at the site relates to a copper refinery, the existence of which is pre-dated by many trees in the area. High soil levels of Cu, Cd, Zn and Pb were recorded to depths of 40 to 90 cm. Tolerance tests on tree seedlings carried out in soils showed no evidence that the trees produce tolerant offspring. Cell culture growth experiments on explant material from shoot meristems of mature trees did show increased tolerance to Cu, compared with material originating from unpolluted sites. The significance of these findings is discussed in relation to facultative adaptations providing trees with the resilience to withstand pollution impact. i. INTRODUCTION The evolution of metal tolerance in grasses and other herbaceous flora is well documented (Bradshaw, 1984; Baker, 1987), but less is known of the impact of pollution on longer lived plants (Dickinson et al., 1988a, 1990). Possible responses of such plants to pollution are summarized in Table I. Acute toxicity which has a direct catastrophic effect on tree populations is relatively infrequent, but there is considerable current interest in the possibility of lower and perhaps continuous levels of pollution leading to chronic toxicity, particularly in relation to the effects of acid rain on European and TABLE I.
I. 2. 3. 4. 5.
Effects of pollution and potential responses of trees
Acute toxicity Chronic toxicity Evolution of tolerance Constitutional (or innate) tolerance Facultative adaptation
Water, Air, and Soil Pollution 57-58: 627-633, 1991. © 1991 Kluwer Academic Publishers. Printed in the Netherlands.
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North American forests (Blank et al., 1989; Schulze, 1989). Evolution of tolerance has been shown recently to have occurred in eastern North American forests (Karnosky et al., 1989) and in relatively short-lived colonizing tree species of mine spoils (Denny and Wilkins, 1987). In most cases though, there has been insufficient time for the evolution of tolerance to have occurred; many mature trees persist in a pollution environment which is recent relative to their own longevity. Some species nevertheless may be innately more tolerant to certain pollutants, as shown by of Platanus hybrida (Plane) which appears to flourish in polluted city atmospheres. Nevertheless, long-lived plants must possess a range of facultative adaptations, within the limits defined by their genotype, providing a varying and phenotypically plastic response to a continuously changing pollution environment. (Bradshaw and Hardwick, 1989; Pitelka, 1988). This paper aims to investigate the relative importance of these five phenomena in explaining the survival of Acer pseudoplatanus (sycamore) trees at a Cu contaminated site at Prescot in North-West England. 2. SITE AND METHODS 2.1 Sites The study relates to an area in the vicinity of a secondary copper smelter or refinery, which also houses a Cu/Cd alloying plant. The factory has been operational since 1932 in an area with soils derived from unmineralized strata with naturally low heavy metal concentrations. Copper processing currently is believed to be about half the level of i0 to 20 yr ago, but a well defined surrounding area of contamination can be identified (Hunter et al., 1987). Much work has been carried out on the evolution of tolerance in grass populations in this area (Bradshaw and McNeilly, 1981; Wu et al., 1975). There is a range of species of trees in the area; Acer pseudoplatanus (sycamore) was selected for this study, being one common species that can be found at differing ages with many specimens pre-dating the existence of the refinery. A study site 0.5 km to the south-east of the refinery contains a small relatively undisturbed copse near a churchyard which provided the main sampling site, although samples were also taken from an area of waste land directly adjacent to the factory. Reference, uncontaminated but otherwise comparable sites were located at nearby Croxteth Park and at Formby in Lancashire. 2.2 Methods Within the churchyard site, soils were carefully sampled from the sides of a trench to depths of i m. Metal determinations were carried out on these samples using an ILl51AAS, following methods of sample preparation and digestion reported previously (Dickinson et al., 1988b). Seeds were collected from the sites and germinated in the laboratory for tolerance testing of the seedlings. Seedlings from each site were grown in Prescot soil or John Innes compost in 22.5 cm
TREES IN A METAL-CONTAMINATED ENVIRONMENT
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plastic pots for 12 weeks. Explant material was also obtained from shoots of mature trees and seedlings at both sites. Following sterilization and initiation of callus on standard plant tissue culture media, shake flask cultures were established. Growth of cultures was measured at regular intervals by recording packed cell volume (PCV %) after centrifugation. Methodology relating to these studies is reported by Turner et al., (1990).
3o RESULTS Soils were contaminated with extremely high levels of Cu, Cd, Zn and Pb to depth of 40 to 90 cm (Figure i). Growth studies on seedlings failed to show any evidence for differences in tolerance between the sites (Table II). Shake flask culture experiments did, however, show notable differences between those derived from mature trees at Prescot and at uncontaminated sites (Figure 2). Cultures derived from Prescot were considerably more tolerant of high Cu levels in the media. Cultures derived from seedlings at Prescot did not show this tolerance trait. TABLE II. Growth indices based on the measurement of 12 growth parameters of leaves, stems and roots (14 replicates per treatment). Calculation is based on a mean relative performance measurement of all parameters.
Origin of seedlings
Prescot Soil
Formby Prescot Factory Prescot Churchyard
0.468 ± 0.0549 0.476 ± 0.0591 0.446 ± 0.0747
John Innes Compost 0.885 ± 0.0592 0.822 ± 0.0539 0.767 ± 0.0678
Values are means ± s.e.
4.
DISCUSSION
The Prescot site is heavily contaminated with Cu and other metals, at levels which severely inhibit the growth of sycamore seedlings. Field observations indicate the common occurrence of toxicity in seedlings; symptoms include leaf chlorosis, reddish leaf colouration and a severely impaired root system. In studies at other metal contaminated sites using grasses, seeds from non-tolerant populations have been sown in contaminated soils and found to produce a small but significant percentage of survivors, of perhaps 2 or 3 plants for every i000 seeds, thus providing the raw material for natural selection (Gartside and McNeilly, 1974; Bradshaw, 1984). The same situation may be occurring amongst sycamore seedlings in Prescot, because plants have established in the field in recent years. It is likely that too few seeds were used in the present study to test this phenomenon however. Cell culture studies point to adaptations taking place in the
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TREES IN A METAL-CONTAMINATEDENVIRONMENT REFERENCE (UNCONTAMINATED TREE)
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N.M. DICKINSON ET AL.
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adult plant. Certainly there is evidence in the literature for the advantageous pre-conditioning of plants to pollution by previous exposure, as shown in mosses (Shaw, 1987). Also there is some evidence for induced tolerance in grasses (Baker et al., 1986). One inference from the results of the present study is that pollution stress has caused alteration of gene expression, providing a phenotypically plastic response in the adult plant. Perhaps the most surprising finding is that this adaptation is retained into cell culture. Despite the known variability of callus tissue this trait appears to hold true through repeated cell culture experiments. Whilst extrapolation of the results of cell culture experiments to the adult plant may he unrealistic, it is reasonable to infer that these findings are of direct relevance and form part of the suite of physiological traits that contribute to the syndrome of tolerance (Baker and Walker, 1989). Similar studies are continuing to examine whether this copper tolerance can be induced in cultures originating from clean sites, by repeated exposure to the metal. There appears to be increasing evidence in the literature that phenotypically plastic responses of individual plants are more significant than previously realized (Bradshaw and Hardwick, 1989). In grasses, such responses are now known to be carried over through repeated vegetative propagation for periods of at least 1 yr (Turkington, 1989). In the case of trees, further studies are required to elucidate the stability of tolerance traits in tissue culture. A variable response to a changing pollution climate over a long period of years would appear to be an essential requisite for their longevity; the processes by which this takes place are now becoming evident. REFERENCES Baker, A.J.M.: 1987, New Phytol., 106 (Suppl.), p.9. Baker, A.J.M., Grant, C.J., Martin, M.H., Shaw, S.C., and Whitebrook, J.: 1986, New Phytol., 102, 575. Baker, A.J.M., and Walker, P.L.: 1989, 'Ecophysiology of metal uptake by tolerant plants'. In: Tolerance in plants: evolutionary aspects, ed. by Shaw, A.J., C.R.C. Press, pp. 155-177. Blank, 27.
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